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BỘ GIAO THÔNG VẬN
TẢI
CỤC HÀNG KHÔNG VIỆT NAM
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CỘNG HÒA XÃ HỘI CHỦ
NGHĨA VIỆT NAM
Độc lập - Tự do - Hạnh phúc
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Số: 1046/QĐ-CHK
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Hà Nội, ngày 10
tháng 5 năm 2024
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QUYẾT
ĐỊNH
VỀ VIỆC BAN HÀNH HƯỚNG DẪN VIỆC THỰC HIỆN QUY
ĐỊNH, KHUYẾN CÁO THỰC HÀNH CỦA ICAO (DOC 9157 - PART 3) VỀ SÂN ĐƯỜNG KHU BAY
CỤC
TRƯỞNG CỤC HÀNG KHÔNG VIỆT NAM
Căn cứ Luật Hàng không
dân dụng Việt Nam ngày 29/6/2006 và Luật sửa đổi, bổ sung một số điều của Luật Hàng
không dân dụng Việt Nam ngày 21/11/2014;
Căn cứ Nghị định số
66/2015/NĐ-CP ngày 12/8/2015 của Chính phủ quy định về Nhà chức trách hàng
không;
Căn cứ Nghị định số
05/2021/NĐ-CP ngày 25/01/2021 của Chính phủ về quản lý, khai thác cảng hàng không,
sân bay, Nghị định số 20/2024/NĐ-CP ngày 23/02/2024 sửa đổi Nghị định số
05/2021/NĐ-CP ;
Căn cứ Nghị định số
64/2022/NĐ-CP ngày 15/9/2022 của Chính phủ về sửa đổi, bổ sung một số điều của
các Nghị định liên quan đến hoạt động kinh doanh trong lĩnh vực hàng không dân
dụng;
Căn cứ Thông tư số
29/2021/TT-BGTVT ngày 30/11/2021 của Bộ Giao thông vận tải quy định chi tiết về
quản lý, khai thác cảng hàng không, sân bay;
Căn cứ Thông tư số
19/2017/TT-BGTVT ngày 06/6/2017 của Bộ Giao thông vận tải về quản lý, bảo đảm
hoạt động bay, Thông tư số 32/2021/TT- BGTVT ngày 14/12/2021 của Bộ Giao thông vận
tải sửa đổi Thông tư số 19/2017/TT-BGTVT ;
Căn cứ Quyết định số
651/QĐ-BGTVT ngày 29/5/2023 của Bộ trưởng Bộ Giao thông vận tải quy định chức
năng, nhiệm vụ, quyền hạn và cơ cấu tổ chức của Cục Hàng không Việt Nam; Quyết
định số 371/QĐ-BGTVT ngày 02/4/2024 của Bộ trưởng Bộ Giao thông vận tải về việc
sửa đổi Điều 3 Quyết định số 651/QĐ-BGTVT;
Xét đề nghị của Trưởng
phòng Quản lý cảng hàng không, sân bay.
QUYẾT
ĐỊNH:
Điều 1. Ban
hành kèm theo Quyết định này Hướng dẫn việc thực hiện quy định, khuyến cáo thực
hành của ICAO (Doc 9157 - Part 3) về Sân đường khu bay (Số tham chiếu: GM 2.3).
Điều 2. Quyết
định này có hiệu lực kể từ ngày ký.
Điều 3. Các
ông/bà Tổng giám đốc Tổng công ty Cảng hàng không Việt Nam - CTCP, Tổng giám
đốc Tổng công ty Quản lý bay Việt Nam, Tổng giám đốc Cảng hàng không quốc tế
Vân Đồn, Giám đốc các Cảng vụ hàng không miền Bắc, miền Trung, miền Nam, Trưởng
phòng Quản lý cảng hàng không, sân bay và Thủ trưởng các cơ quan, đơn vị liên
quan chịu trách nhiệm thi hành Quyết định này./.
|
Nơi nhận:
-
Như Điều 3;
- Cục trưởng (để b/c);
- Các Phó Cục trưởng;
- Các phòng: QLC, QLHĐB, TCATB, ANHK, KHCNMT, PC-HTQT, TTHK;
- Lưu: VT, QLC (TD 10bn).
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KT. CỤC TRƯỞNG
PHÓ CỤC TRƯỞNG
Phạm Văn Hảo
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AERODROME
DESIGN MANUAL
Pavements
Hướng dẫn việc thực hiện quy định, khuyến cáo
thực hành của ICAO (Doc 9157 - Part 3) về
Sân đường khu bay
(GM 2.3)
Issued
herewith Decision No. 1046/QD-CHK dated 10th May 2024
VERSION
RECORDS
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Date
of issue
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Subject(s)/
Amendment
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Source
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Effective
date
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Note
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10th May 2024
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First
Edition
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ICAO
Doc 9157 - Part 3 (2022)
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10th May 2024
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TABLE
OF CONTENTS
1. BACKGROUND
2. SCOPE AND PURPOSE
3. FOREWORD
4. DEFINITIONS
5. ABBREVIATIONS AND
ACRONYMS
CHAPTER 1
PROCEDURES FOR
REPORTING AERODROME PAVEMENT STRENGTH
1.1 PROCEDURE FOR
PAVEMENTS MEANT FOR HEAVY AIRCRAFT (AIRCRAFT CLASSIFICATION RATING-PAVEMENT
CLASSIFICATION RATING (ACR-PCR) METHOD)
CHAPTER 2
GUIDANCE FOR OVERLOAD
OPERATIONS
2.1 CRITERIA
SUGGESTED IN ANNEX 14, VOLUME I, ATTACHMENT A
CHAPTER 3
STRUCTURAL EVALUATION
OF PAVEMENTS
3.1 GENERAL
3.2 ELEMENTS OF
PAVEMENT EVALUATION
3.3 ELEMENTS OF THE
ACR-PCR METHOD
3.4 ASSESSING THE
MAGNITUDE AND COMPOSITION OF TRAFFIC
3.5 TECHNIQUES FOR
“USING AIRCRAFT” EVALUATION
CHAPTER 4
STATE PRACTICES FOR
DESIGN AND EVALUATION OF PAVEMENTS
4.1 PURPOSE
4.2 PRACTICE OF
FRANCE
4.3 PRACTICE OF THE
UNITED KINGDOM
4.4 PRACTICE OF THE
UNITED STATES
CHAPTER 5
Page intentionally
left blank
CHAPTER 6
Page intentionally
left blank
CHAPTER 7
CONSIDERATIONS FOR
CULVERTS, BRIDGES AND OTHER STRUCTURES
7.1 PURPOSE
7.2 GENERAL
7.3 DESIGN
CONSIDERATIONS
CHAPTER 8
Page intentionally
left blank
CHAPTER 9
STRUCTURAL CRITERIA
FOR NATURAL GROUND
9.1 INTRODUCTION
9.2 DESIGN
BACKGROUND
9.3 DESIGN DETAILS
9.4 GUIDANCE FOR BEARING
STRENGTH OF PREPARED NATURAL GROUND AREAS
APPENDIX 1
AIRCRAFT CHARACTERISTICS
AFFECTING PAVEMENT BEARING STRENGTH
1. GENERAL
2. AIRCRAFT CHARACTERISTICS
FOR DESIGN AND EVALUATION OF PAVEMENTS
APPENDIX 2
USER INFORMATION FOR
THE ICAO-ACR COMPUTER PROGRAMME
1. GENERAL
2. DYNAMIC-LINK
LIBRARY (DLL) TECHNICAL INFORMATION
3. PROGRAMME ICAO-ACR
APPENDIX 3
DAMAGE MODEL FOR
FLEXIBLE ACR
1. ELEMENTARY DAMAGE
LAW
2. MULTIPLE-AXLE GEAR
LOADS
3. CONTINUOUS
INTEGRAL FORM OF THE DAMAGE LAW
4. DAMAGE MODEL FOR
FLEXIBLE ACR
APPENDIX 4
Page intentionally
left blank
APPENDIX 5
Page intentionally
left blank
APPENDIX 6
PAVEMENT OPERATIONS
AND MAINTENANCE RELATED GUIDANCE
1. PAVEMENT
MANAGEMENT PROGRAMME (PMP)
2. METHODS FOR IMPROVING/MAINTAINING
RUNWAY SURFACE TEXTURE AND PAVEMENT MAGNETIC CHARACTERISTICS
3. PROTECTION OF
ASPHALT PAVEMENTS
4. CONSTRUCTION OF ASPHALT
OVERLAYS DURING OPERATIONS CLOSURES
1. BACKGROUND
1.1. Pursuant to
Article 11, Clause 2 of the Decree No. 66/2015/ND-CP dated 12th August 2015 on
the Civil Aviation Authority stipulating the function and responsibility of the
Civil Aviation Authority of Viet Nam (CAAV) in guiding the implementation of
Standards and Recommended Practices of ICAO.
1.2. Pursuant to
Article 8, Clause 10 of the Decree No. 05/2021/ND-CP dated 25th January 2021 on
the management and operation of airport and aerodrome stipulating the function
and responsibility in providing guidance on and issue professional instructions
related to management and operation of airports/aerodromes in line with
regulations of law and regulations of ICAO (including: documents providing
guidelines for airport/aerodrome design; documents providing guidelines for
operation and assurance of security and safety at airports/aerodromes).
1.3. Pursuant to
Article 4, Clause 6 of the Decree No. 0/5/2021/ND-CP dated 25th January 2021
stipulating details of airport/aerodrome infrastructure shall be designed and
operated according to the standards of International Civil Aviation Organization
(ICAO).
1.4. This document is
issued by CAAV specifying the details of provisions to be met by the AO
organizations, originators related. The details in this Manual are based on
those stipulated in ICAO Doc 9157- Part 3 [as in force and amended from time to
time by the Council of the International Civil Aviation Organization (ICAO)]
and with such modifications as may be determined by CAAV to be applicable in
Viet Nam.
1.5. Amendments to
this document are the responsibility of the Airport Management Department -
CAAV. Readers should forward advice of errors, inconsistencies or suggestions
for improvement to this Manual to the addressee stipulated below.
Airport Management
Department - CAAV
No. 119, Nguyen Son
Street, Gia Thuy ward, Long Bien district, Ha Noi.
Email Address:
and@caa.gov.vn.
2. SCOPE AND PURPOSE
2.1 This part of the
Airport Design Manual includes guidance on the pavements. Much of the material included
herein is closely associated ith the specifications contained in Annex 14 -
Aerodromes. The main purpose of this manual is to encourage the uniform
application of those specifications and to provide information and guidance in
accordance with ICAO.
3. FOREWORD
This document
provides guidance on the design of pavements including their characteristics,
and on evaluation and reporting of their bearing strength. The material
included herein is closely associated with the specifications contained in MAS
1. The purpose of this manual is to encourage the uniform application of those
specifications and to provide information and guidance to States. The manual
has been substantially rewritten, resulting in the following major evolutions
from the second edition (1983):
a) updated
information on the ACR-PCR method for reporting pavement bearing strength
(Chapter 1);
b) updated material
on regulating overload operations in accordance with the use of the ACR-PCR
method (Chapter 2);
c) updated material
on the evaluation of pavements (Chapter 3) ;
d) updated material
on States’ practices for the design and evaluation of pavements provided by
France, the United Kingdom and the United States (Chapter 4), subject to change
when the ACR-PCR method becomes applicable (2024);
e) updated material
on runway surface texture and drainage characteristics (Chapter 5 moved to
Appendix 6-B);
f) guidance on protection
of asphalt pavements (Chapter 6 moved to Appendix 6-C);
g) updated material
on structural design considerations for culverts and bridges (Chapter 7);
h) updated material on
the construction of pavement overlays during operations closures (Chapter 8
moved to Appendix 6-D);
i) new material on
the bearing strength of natural ground areas (Chapter 9);
j) new landing gear designation
system and updated main aircraft characteristics affecting pavement bearing strength
(Appendix 1);
k) user information
for the ICAO-ACR computer programme (Appendix 2);
l) details on damage
model for flexible ACR (Appendix 3);
m) removal of
Appendix 4, all needed information being provided in Chapter 4;
n) removal of
Appendix 5, aircraft ACRs are available at any mass, CG and tire pressure by
using ICAO-ACR computer programme;
o) pavement
operations and maintenance oriented guidance (new Appendix 6).
Chapter 4 of this
manual is based on material on pavement design and evaluation submitted by
States and is, therefore, believed to be current. Should a State, at any time,
consider that the material included therein is out-of-date, it should inform
the Secretary General of this and, if possible, provide appropriate revised
material.
Chapters 5, 6 and 8
of the second edition covering non-design subjects have been updated and moved
to new Appendix 6, which comprises the updated operations and maintenance
oriented materials augmented with guidance on mitigation of magnetic field
distortions. Appendix 6 is provided to accommodate identified non-design
materials until such time they can be moved to more appropriate documents, such
as the Aerodrome Services Manual (Doc 9137) and the Procedures for
Air Navigation Services - Aerodromes (PANS-Aerodromes, Doc 9981).
In order to maintain
the consistency of possible references made to the 2nd edition in other
guidance and documents, all deleted chapters and appendices (e.g. deleted
chapters which have been gathered in the new Appendix 6) have been replaced by
pages “intentionally left blank”.
4. DEFINITIONS
Aggregate. General term for
the mineral fragments or particles which, through the agency of a suitable
binder, can be combined into a solid mass, e.g., to form a pavement.
Aircraft
classification number (ACN). A number expressing the relative effect of
an aircraft on a pavement for a specified standard subgrade strength.
Aircraft
classification rating (ACR). A number expressing the relative effect of
an aircraft on a pavement for a specified standard subgrade strength.
All-up mass. Aircraft maximum
ramp or taxi mass, also referred as gross weight.
Asphalt. Highly viscous
binder occurring as a liquid or semi-solid form of petroleum, also referred as
bitumen.
May be found in
natural deposits or may be a refined product.
Asphalt concrete. A graded mixture of
aggregate, and filler with asphalt or bitumen, placed hot or cold, and rolled,
also referred as asphaltic concrete or bitumen concrete.
Base course (or base). The layer or layers
of specified or selected material of designed thickness placed on a sub-base or
subgrade to support a surface course.
Bearing strength. The measure of the
ability of a pavement to sustain the applied load, also referred as bearing
capacity or pavement strength.
California Bearing
Ratio (CBR).
The bearing ratio of soil determined by comparing the penetration load of the
soil to that of a standard material. The method covers evaluation of the
relative quality of subgrade soils but is applicable to sub-base and some base
course materials.
Note: The Standard Test Method
for CBR of Laboratory-Compacted Soils is an ASTM standard (ASTM D1883).
Composite pavement. A pavement
consisting of both flexible and rigid layers with or without separating
granular layers.
Flexible pavement. A pavement
structure that maintains intimate contact with and distributes loads to the
subgrade and depends on aggregate interlock, particle friction, and cohesion
for stability.
Lateral wander. The path of a given
aircraft will deviate relative to the path centred on the longitudinal axis of
the pavement in question in a statistically predictable pattern. This
phenomenon is referred to as lateral wander.
Mean aerodynamic
chord (MAC). The
MAC is a two-dimensional representation of the whole wing. The pressure distribution
over the entire wing can be reduced to a single lift force on and a moment
around the aerodynamic centre of the MAC. Centre of gravity position is
expressed as percentage of MAC.
Modulus of elasticity
(E). The
modulus of elasticity of a material is a measure of its stiffness. It is equal
to the stress applied to it divided by the resulting elastic strain.
Overlay. An additional
surface course placed on existing pavement either with or without intermediate
base or sub-base courses, usually to strengthen the pavement or restore the
profile of the surface.
Pavement
classification number (PCN). A number expressing the bearing strength of
a pavement.
Pavement
classification rating (PCR). A number expressing the bearing strength of
a pavement for unrestricted operations.
Pavement structure
(or pavement).
The combination of sub-base, base course, and surface course placed on a
subgrade to support the traffic load and distribute it to the subgrade.
Poisson’s ratio. The ratio of
transverse to longitudinal strains of a loaded specimen.
Portland cement
concrete (PCC).
A mixture of graded aggregate with Portland cement and water.
Rigid pavement. A pavement
structure that distributes loads to the subgrade having as its surface course a
Portland cement concrete slab of relatively high bending resistance, also
referred as concrete pavement.
Sub-base course. The layer or layers
of specified selected material of designed thickness placed on a subgrade to
support a base course.
Subgrade. The upper part of
the soil, natural or constructed, which supports the loads transmitted by the
pavement, also referred as the formation foundation.
Surface course. The top course of a
pavement structure, also referred as wearing course.
5. ABBREVIATIONS AND ACRONYMS
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2D
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Dual tandem
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ACN
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Aircraft
classification number
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ACR
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Aircraft
classification rating
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AIP
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Aeronautical
information publication
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ASTM
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American Society
for Testing and Materials
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CBR
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California bearing
ratio
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CDF
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Cumulative damage
factor
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CG
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Centre of gravity
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cm
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Centimeter
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D
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Dual
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FAA
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Federal Aviation
Administration
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FOD
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Foreign object
debris
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FWD
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Falling weight
deflectometers
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GPR
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Ground penetrating
radar
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HFWD
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Heavy falling
weight deflectometer
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HWD
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Heavy weight
deflectometer
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kN
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Kilonewton
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LRFD
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Load and resistance
factor design
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MPa
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Megapascal
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MRGM
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Maximum ramp gross
mass
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NDT
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Non-destructive
testing
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PCA
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Portland Cement
Association
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PCC
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Portland cement
concrete
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PCN
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Pavement
classification number
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PCR
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Pavement
classification rating
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PMP
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Pavement management
programme
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RESA
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Runway end safety
area
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S
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Distance between
centres of contact areas of dual wheels
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SD
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Distance between
centres of contact areas of diagonal wheels
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ST
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Distance between
axes of tandem wheels
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SARP
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Standard and
Recommended Practice
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STAC
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Service technique
de l'Aviation civile
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TSD
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Traffic speed
deflectometer
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CHAPTER 1
PROCEDURES FOR REPORTING AERODROME PAVEMENT
STRENGTH
1.1 PROCEDURE FOR PAVEMENTS MEANT FOR HEAVY
AIRCRAFT
(AIRCRAFT CLASSIFICATION RATING-PAVEMENT CLASSIFICATION
RATING (ACR-PCR) METHOD)
1.1.1
Annex 14 - Aerodromes, Volume I - Aerodrome Design and Operations, specifies
that the bearing strength of a pavement intended for aircraft of mass greater
than 5 700 kg should be made available using the aircraft classification rating-pavement
classification rating (ACR-PCR) method. To facilitate a proper understanding
and usage of the ACR-PCR method the following material explains:
a) the concept of the method;
b) how the aircraft classification ratings
(ACRs) of an aircraft are determined; and
c) how the pavement classification ratings
(PCRs) of a pavement can be determined using the cumulative damage factor (CDF)
concept.
The key parameters of the determination of
the pavement classification rating (PCR) are summarized in Figure 1-1.
1.1.2
Concept of the ACR-PCR method
1.1.2.1 The ACR-PCR
method is meant only for the publication of pavement strength data in
aeronautical information publications (AIPs). It is not intended for the design
or evaluation of pavements, nor does it contemplate the use of a specific
method by the aerodrome operator either for the design or evaluation of
pavements. In fact, the ACR- PCR method does permit States to use any
design/evaluation method of their choice. To this end, the method shifts the
emphasis from the evaluation of pavements to the evaluation of load rating of
aircraft (ACR) and includes a standard procedure for the evaluation of the load
rating of aircraft. The strength of a pavement is reported under the method in
terms of the load rating of the aircraft, which the pavement can accept on an
unrestricted basis. When referring to unrestricted operations, it does not mean
unlimited operations but refers to the relationship of PCR to the aircraft ACR
and it is permissible for an aircraft to operate without weight restriction
(subject to tire pressure limitations) when the PCR is greater than or equal to
the ACR. The term unlimited operations does not take into account pavement
life. The PCR to be reported is such that the pavement strength is sufficient
for the current and future traffic analysed and should be re- evaluated if
traffic changes significantly. A significant change in traffic would be
indicated by the introduction of a new aircraft type or an increase in current
aircraft traffic levels not accounted for in the original PCR analysis. The
airport authority can use any method of its choice to determine the load rating
of its pavement, provided it uses the CDF concept. The PCR so reported would
indicate that an aircraft with an ACR equal to or less than that figure can
operate on the pavement, subject to any limitation on the tire pressure.
1.1.2.2 The ACR-PCR
method contemplates the reporting of pavement strengths on a continuous scale.
The lower end of the scale is zero and there is no upper end. Additionally, the
same scale is used to measure the load ratings of both aircraft and pavements.
1.1.2.3 To
facilitate the use of the method, aircraft manufacturers will publish, in the
documents detailing the characteristics of their aircraft, ACRs computed at two
different masses (the maximum apron mass and a representative operating mass
empty) both on rigid and flexible pavements, and for the four standard subgrade
strength categories. The ICAO-ACR computer programme, which is available to all
stakeholders, provides any aircraft ACRs at any mass and centre of gravity (CG)
position for both flexible and rigid pavement and for the four standard
subgrade strength categories. It is to be noted that the mass used in the ACR
calculation is a “static” mass and that no allowance is made for an increase in
loading through dynamic effects.
1.1.2.4 The ACR-PCR
method also envisages the reporting of the following information in respect of
each pavement:
a) pavement type;
b) subgrade category;
c) maximum allowable
tire pressure; and
d) pavement
evaluation method used.
The data obtained
from the characteristics listed above are primarily intended to enable aircraft
operators to determine the permissible aircraft types and operating masses, and
the aircraft manufacturers to ensure compatibility between airport pavements
and aircraft under development. There is, however, no need to report the actual
subgrade strength or the maximum allowable tire pressure. Consequently, the
subgrade strengths and tire pressures normally encountered have been grouped
into categories as indicated in 1.1.3.2 below. It is sufficient for the airport
authority to identify the categories appropriate to its pavement (see also the
examples included in Annex 14, Volume I, 2.6). The airport authority should
whenever possible, report pavement strength based on a technical evaluation of
the pavement. Details of the technical evaluation process are included in 3.6.
If due to financial or engineering constraints a technical evaluation is not feasible,
then using the aircraft method must be used for reporting pavement strength.
Details on the aircraft method are contained in 3.5.
1.1.2.5 The ACR/PCR
method permits States to use the design/evaluation procedure of their choice
when determining the PCR for their pavements. However, in many instances, the
State may lack expertise in this area or wish to incorporate a standard
methodology for performing the technical evaluation of their pavements. Refer
to Chapter 4 for State practices.
1.1.2.6 In some
cases, culverts, bridges, and other surface and subsurface structures can be
the critical or limiting element necessitating the reporting of a lower PCR for
the pavement. Considerations, which permit use of the ACR-PCR method to limit
pavement overloading, are not necessarily adequate to protect these structures.
Evaluation of, and consideration for these structures are discussed in Chapter
7.
1.1.3
How ACRs are determined
1.1.3.1 ACRs of
aircraft are computed under the ACR-PCR method as shown in Figure 1-2.
Figure
1-2. ACR Computation[1]
Relevant
documents/software:
1. Aircraft
characteristics for airport planning (published by the aircraft manufacturers).
2. ICAO-ACR computer
programme (current version).
1.1.3.2 The
following are standard values used in the method and description of the various
terms:
Subgrade category
1.1.3.2.1 In the
ACR-PCR method four standard subgrade values (modulus values) are used, rather
than a continuous scale of subgrade moduli. The grouping of subgrades with a
standard value at the mid-range of each group is considered to be entirely
adequate for reporting. Subgrade categories apply to both flexible and rigid
pavements.
1.1.3.2.2 The
subgrade categories are identified as high, medium, low and ultra-low and
assigned the following numerical values:
Code A - High
strength; characterized by E = 200 MPa and representing all E values equal to
or above 150 MPa, for rigid and flexible pavements.
Code B - Medium
strength; characterized by E = 120 MPa and representing a range in E equal to
or above 100 MPa and strictly less than 150 MPa, for rigid and flexible
pavements.
Code C - Low
strength; characterized by E = 80 MPa and representing a range in E equal to or
above 60 MPa and strictly less than 100 MPa, for rigid and flexible pavements.
Code D - Ultra-low
strength; characterized by E = 50 MPa and representing all E values strictly
less than 60 MPa, for rigid and flexible pavements.
Concrete working
stress for rigid pavements
1.1.3.2.3 For rigid
pavements, a standard stress for reporting purposes is stipulated (σ = 2.75
MPa) only as a means of ensuring uniform reporting. The working stress to be
used for the design and/or evaluation of the pavements has no relationship to
the standard stress for reporting.
Mathematically
derived single wheel load
1.1.3.2.4 The
concept of a mathematically derived single wheel load has been employed in the
ACR-PCR method as a means to define the aircraft landing gear/pavement
interaction without specifying pavement thickness as an ACR parameter. This is
done by equating the thickness given by the mathematical model for an aircraft
landing gear to the thickness for a single wheel at a standard tire pressure of
1.50 MPa. The single wheel load so obtained is then used without further
reference to thickness; this is so because the essential significance is
attached to the fact of having equal thicknesses, implying “same applied stress
to the pavement”, rather than the magnitude of the thickness. The foregoing is
in accord with the objective of the ACR-PCR method to evaluate the relative
loading effect of an aircraft on a pavement.
Aircraft
Classification Rating (ACR)
1.1.3.2.5 The ACR of
an aircraft is numerically defined as two times the derived single wheel load,
where the derived single wheel load is expressed in hundreds of kilograms. As
noted previously, single wheel tire pressure is standardized at 1.50 MPa.
Additionally, the derived single wheel load is a function of the subgrade
modulus. The aircraft classification rating (ACR) is defined only for the four
standard subgrade categories (i.e. high, medium, low and ultra-low). The factor
of two in the preceding numerical definition of ACR is used to achieve a
suitable ACR versus gross mass scale, so that whole number values of ACR may be
used with reasonable accuracy.
1.1.3.2.6 Because an
aircraft operates at various mass and CG conditions, the following conventions
have been used in ACR computations (see Figures 1-3 and 1-4):
a) the maximum ACR of
an aircraft is calculated at the mass and CG. that produces the highest main
gear loading on the pavement, usually the maximum ramp mass and corresponding
aft CG The aircraft tires are considered as inflated to the tire manufacturer’s
recommendation for the condition;
b) relative aircraft
ACR charts and tables show the ACR as a function of aircraft gross mass with
the aircraft CG as a constant value corresponding to the maximum ACR value
(i.e. usually, the aft CG for maximum ramp mass) and at the maximum ramp mass
tire pressure; and
c) specific condition
ACR values are those ACR values that are adjusted for the effects of tire
pressure and/or CG location, at a specified gross mass for the aircraft.
Mathematical models
1.1.3.3 The sole
mathematical model used in the ACR-PCR method is the layered elastic analysis
(LEA). The LEA model assumes that several homogeneous, elastic, isotropic
layers arranged as a stack, whether flexible or rigid, can represent the
pavement structure. Each layer in the system is characterized by an elastic
modulus Ei, Poisson’s ratio νi and uniform layer thickness ti. Layers are
assumed to be of infinite horizontal extent and the bottom or subgrade layer is
assumed to extend vertically to infinity (i.e. the subgrade is modeled as an
elastic half-space). Due to the linear elastic nature of the model, individual
wheel loads can be summed to obtain the combined stress and strain responses
for a complex, multiple-wheel aircraft gear load. The use of the LEA model
permits the maximum correlation to worldwide pavement design methods.
Computer programmes
1.1.3.4 The computer
programme LEAF was developed using the above LEA mathematical model by the
United States Federal Aviation Administration (FAA). LEAF is an open-source
computer programme whose source code is available from the FAA, Airport
Technology R&D Branch, William J. Hughes Technical Center, USA. In
addition, a second LEA programme, Alize-Aeronautics, was developed by the
French Institute of Science and Technology for Transport, Development and
Networks (IFSTTAR) in partnership with AIRBUS SAS, and has been found to give
nearly identical results for equal inputs. The ICAO-ACR computer programme
incorporates the LEAF programme and was developed to implement the ACR
computational procedures for rigid and flexible pavements. ICAO-ACR is distributed
in compiled form as a Visual Basic.NET dynamic-link library (DLL), and may be
linked to other programmes that either compute ACR directly or that use the ACR
computation to evaluate PCR. By default, the ICAO-ACR programme takes as
inputs: the maximum ramp mass for ACR calculations; percent of maximum ramp
mass acting on the main gear (equivalent for this purpose to the aft c.g.
corresponding to maximum ramp mass); the number of wheels; the geometric
coordinates of all wheels; and the type of pavement (rigid or flexible). The
output is the ACR at each subgrade category; and the pavement reference
thickness t corresponding to ACR at each subgrade category. Appendix 2
of this manual contains information on linking to the ICAO-ACR library.
Graphical procedures
1.1.3.5 Graphical
procedures should not be used for determination of ACR. Instead, use the
computer programmes as described above.
Rigid pavements
1.1.3.6 The rigid
pavement ACR procedure relates the derived single wheel load at a constant tire
pressure of 1.50 MPa to a reference concrete slab thickness t. It takes into
account the four subgrade categories detailed in 1.1.3.2 a) above, and a
standard concrete stress of 2.75 MPa. Note that, because a standard concrete
stress is used, no information concerning either pavement flexural strength or
number of coverages is needed for rigid ACR computation. The steps below are
used to determine the rigid ACR of an aircraft.
Reference pavement
structure
1.1.3.6.1 Using the
aircraft data published by the manufacturer, obtain the reference thickness t
for the given aircraft mass, E-value of the subgrade, and standard concrete
stress for reporting, i.e. 2.75 MPa. For all four subgrade categories, assume
the following cross-section for the LEA model:
Table
1-1. Reference pavement structure for rigid ACR
|
Layer
Description
|
Designation
|
Thickness,
mm
|
E, MPa
|
ν
|
|
Surface course
(PCC)
|
Layer
1
|
variable
|
27
579
|
0.15
|
|
Base course
(crushed aggregate)
|
Layer
2
|
200
|
500
|
0.35
|
|
Subgrade
|
Layer
3
|
infinite
|
Par.
1.1.3.2 a)
|
0.40
|
The minimum allowable
thickness of Layer 1 in the LEA model is 50.8 mm. LEA computations further assume
that the horizontal interface between Layer 1 and Layer 2 is not bonded (full
slip), and that the horizontal interface between Layer 2 and Layer 3 is full
bond. Within the LEA model, stress σ is the maximum horizontal stress computed
on the bottom of Layer 1 (the Portland cement concrete layer).
Evaluation gear
1.1.3.6.2 The ACR
value is computed for a single truck in the main landing gear assembly (i.e.
for two wheels in a dual, or D assembly, four wheels in a dual-tandem, or 2D
assembly, etc.). For more complex landing gear types with more than two trucks
(i.e. having a designation in FAA Order 5300.7, “Standard Naming Convention for
Aircraft Landing Gear Configurations” consisting of more than two characters),
the individual truck in the main gear assembly with the largest rigid ACR
determines the rigid ACR for the aircraft. All trucks are evaluated at the mass
and CG that produce the highest total main gear loading on the pavement.
Stress evaluation
points
1.1.3.6.3 The number
of LEA evaluation points is equal to the number of wheels in the evaluation
gear. The evaluation points are located at the bottom of Layer 1, below the
centre point of each wheel. The thickness t of Layer 1 is adjusted until
the maximum stress evaluated over all evaluation points is equal to 2.75 MPa.
The resulting t is the reference thickness for ACR.
DSWL calculation
1.1.3.6.4 Using the
above reference thickness and the same LEA model as in a), obtain a derived
single wheel load for the selected subgrade. Maintaining the constant tire
pressure of 1.50 MPa, the single wheel load magnitude is adjusted until the
maximum horizontal stress at the bottom of Layer 1 is equal to 2.75 MPa. For
evaluation of stresses under the single wheel load, use one evaluation point
located at the bottom of Layer 1, directly below the centre of the wheel.
Modified DSWL
calculation for lightweight aircraft
1.1.3.6.5 For some
lightweight aircraft, the required reference thickness t is less than
the minimum allowable thickness. Use the following modified steps to compute
DSWL only when the theoretical thickness of Layer 1 that makes the maximum
stress equal to 2.75 MPa is less than 50.8 mm:
a) determine the
value of stress (less than 2.75 MPa) corresponding to the minimum allowable
concrete thickness (50.8 mm); and
b) calculate DSWL for
the selected subgrade using the minimum thickness of the reference structure.
Maintaining the constant tire pressure of 1.50 MPa, the single wheel load
magnitude is adjusted until the maximum horizontal stress at the bottom of
Layer 1 is equal to the value determined in (a).
ACR calculation
1.1.3.6.6 The
aircraft classification rating, at the selected mass and subgrade category, is
two times the derived single wheel load in hundreds of kilograms. The numerical
value of ACR may be rounded to the nearest multiple of ten for reporting.
Flexible Pavements
1.1.3.7 The flexible
pavement ACR procedure relates the derived single wheel load at a constant tire
pressure of 1.50 MPa to a reference total thickness t computed for 36 500
passes of the aircraft. It takes into account the four subgrade categories
detailed in 1.1.3.2 a) above.
Reference Pavement
Structures
1.1.3.8 The ACR-PCR
system must cover a wide range of aircraft weighing from a few to several
hundreds of tons. Reference structures have been chosen to produce appropriate
thicknesses for the standard subgrade categories for the range of aircraft
weights used. Determining the reference structures for the flexible ACR
computation consists in defining the materials and constitutive properties of
the several layers. All layers are defined by: Elastic modulus E, Poisson’s
ratio ν, and (except for the design layer) thickness. LEA computations assume
that all horizontal interfaces between layers are fully bonded. The tables
below define the reference structures to be used in calculating flexible ACR.
1.1.3.9 In the LEA
model, the minimum allowable thickness of the variable (base course) layer is
25.4 mm. Because of the intentionally limited number of reference structures,
computed layer thicknesses may not be realistic at the extremes of the aircraft
weight range. However, this does not invalidate the ACR concept, in which t is
a relative indicator rather than the basis for a practical design.
Base Layer Modulus
1.1.3.10 All
flexible reference pavement structures include a variable thickness layer above
the subgrade, representing a crushed aggregate base layer. The modulus of the
variable thickness layer is not fixed in the ACR procedure, but is a function
of the thickness and of the subgrade modulus. Within the LEA model, the base
layer is subdivided into smaller sub-layers and a modulus value is then
assigned to each sub- layer using a recursive procedure as explained below.
Modulus values are assigned to the sub-layers following the procedure in the
FAA computer programme FAARFIELD (version 1.42), for item P-209 (crushed aggregate).
The steps in the procedure are as follows:
Step 1. Determine the number
of sub-layers N. If the base layer thickness tB is less than 381
mm, then N= 1 and sub-layering is not required. If tB is greater than or
equal to 381 mm, the number of sub-layers is:
where tB is in mm, and the
integer function returns the integer part of the argument (i.e. rounds down to
the next whole number).
Step 2. Determine the
thickness of each sub-layer. If N = 1, then the sublayer thickness is
equal to the base layer thickness tB. If N > 1,
then the thickness of the bottom N - 1 sub-layer is 254 mm, and the
thickness of the top sub-layer is tB N 1 254 mm. Note that,
in general, the N sublayers do not have equal thickness. For example, if
the thickness of the base layer is 660 mm, then from Step 1, the number of
sub-layers is three. The bottom two sub-layers are each 254 mm, while the top
sub-layer is 660 - 2 × 254 = 152 mm.
Step 3. Assign a modulus
value E to each sub-layer. Modulus values increase from bottom to top,
reflecting the effect of increasing confinement of the aggregate material.
Modulus values are given by the following equation:
where En = the modulus of the
current sub-layer in MPa;
En-1=
the modulus of the sub-layer immediately below the current sub-layer; or the
modulus of the subgrade layer when the current sub-layer is the bottom
sub-layer;
tn = the thickness of
the current sub-layer in mm;
c = 10.52 (constant);
and
d = 2.0 (constant).
The above equation is
applied recursively beginning with the bottom sub-layer.
Step 4. The modulus
assignment procedure in Step 3 must be modified for the top two sub-layers
whenever tB is between 127 and 254 mm greater than an integer multiple
of 254 mm. This modification ensures that the modulus of all sub-layers is a
continuous function of the layer thickness, with no gaps. If N > 1
and tB exceeds an integer multiple of 254 mm by more than 127 mm, but
less than 254 mm, then:
a) The top sub-layer
(sub-layer N) is between 127 and 254 mm thick, and all sub-layers below
it (sub-layers 1 to N-1) are 254 mm thick.
b) Using the equation
in Step 3, compute the modulus E254 that would be obtained for sub-layer
N for an assumed top sub-layer thickness tn equal to 254 mm.
c) Compute the
modulus of sub-layer N-1 (i.e. the sub-layer immediately below the top
sub-layer) using the equation in Step 3, but substituting tn = 508 mm - tN,
where tN is the actual thickness of the top sub-layer in mm.
d) Compute the
modulus of sub-layer N by linear interpolation between EN-1 (the
modulus of sub-layer N-1) and E254:
Evaluation gear
1.1.3.11 The ACR
value is computed using all wheels in the main landing gear (wheels in the nose
landing gear are not included). Main landing gears are evaluated at the mass
and c.g. that produces the highest total main gear loading on the pavement.
Strain Evaluation
Points
1.1.3.12 Within the
LEA model, strain ε is the maximum vertical strain computed on the top surface
of the subgrade (lowest) layer. In the ICAO-ACR computer programme, strains are
computed at specific evaluation points based on the geometry of the evaluation
gear. Evaluation points are placed directly below the centre point of each
wheel, and at the points defined by a regular rectangular grid spaced at 10-cm
intervals, and oriented parallel to the principal axes of the gear.
1.1.3.12.1 For
simple main landing gears consisting of two trucks (i.e. for two wheels in a
dual, or D assembly, four wheels in a dual-tandem, or 2D assembly, etc.) the
grid origin is set at the geometric centre of one truck. The limits of the grid
extend 30 cm beyond the maximum wheel coordinates on all sides of the truck (Figure
1-5).
1.1.3.12.2 For more
complex gear types with more than two trucks comprising the main landing gear
assembly (i.e. all aircraft whose gear designation consists of more than two
characters in FAA Order 5300.7, “Standard Naming Convention for Aircraft
Landing Gear Configurations”), the origin of the grid is at the geometric
centre of the entire landing gear assembly. The limits of the grid extend 30 cm
beyond the maximum wheel coordinates on all sides (Figure 1-6). For the purpose
of computing the geometric centre coordinates, all included wheels should be
weighted equally, regardless of different wheel loads or tire pressures.
1.1.3.12.3 Strain ε
is the maximum of the strains computed for all evaluation points.
Note. ICAO-ACR
automatically detects symmetries within the evaluation point grid to reduce the
number of required computations. In the case of B787-9, only one half of the
evaluation point grid may actually be computed due to the transverse symmetry.
Damage Model
1.1.3.13 The
flexible ACR procedure relies on the subgrade failure criterion associated with
the elementary damage law:
This elementary
damage law is based on the notion of loading cycle (single-peak strain profile
with maximum value ), which cannot be applied to arrangements with axles in
tandem producing complex strain profiles, possibly with multiple strain peaks
and no return to zero-strain between peaks. Therefore the elementary damage law
is extended to a continuous integral form:
where x refers to the
longitudinal position along the landing gear and <y> to the positive part
of y. Details of the integral formulation are described in Appendix 3.
DSWL calculation
1.1.3.14 Using the
pavement requirement data published by the manufacturer, calculate the
reference thickness t for the given aircraft mass, E-value of the
subgrade, and 36 500 passes of the aircraft. Use the appropriate reference
pavement structure from 1.1.3.7 a) with evaluation points as described in
1.1.3.7 d). The thickness of the variable (design) layer is adjusted until the
damage as computed from 1.1.3.7 e) is equal to 1.0. The resulting thickness t
is the reference thickness for ACR.
1.1.3.15 Using the
above reference thickness and the same LEA model as in 1.1.3.7 e), obtain a
derived single wheel load for the selected subgrade. Maintaining the constant
tire pressure of 1.50 MPa, the single wheel load magnitude is adjusted until
the damage is equal to 1.0 for 36 500 passes. For evaluation of strains under
the single wheel load, use one evaluation point located at the top of the
subgrade, directly below the centre of the wheel.
Modified DSWL
calculation for lightweight aircraft
1.1.3.16 For some
lightweight aircraft, the required reference thickness t is less than
the minimum allowable thickness. Use the following modified steps to compute
DSWL only when the theoretical thickness of the variable design layer that
makes the damage equal to 1.0 for 36 500 aircraft passes is less than 25.4 mm:
a) determine the
value of maximum vertical strain at the top of the subgrade corresponding to
the minimum allowable variable design layer thickness (25.4 mm); and
b) calculate DSWL for
the selected subgrade using the minimum thickness of the reference structure.
Maintaining the constant tire pressure of 1.50 MPa, the single wheel load
magnitude is adjusted until the maximum vertical strain at the top of the
subgrade is equal to the value determined in (a).
ACR Calculation
1.1.3.17 The
aircraft classification rating, at the selected mass and subgrade category is
two times the derived single wheel load in hundreds of kilograms. The numerical
value of ACR may be rounded to the nearest multiple of ten for reporting.
Tire pressure
adjustment to ACR
1.1.3.18 Aircraft
normally have their tires inflated to the pressure corresponding to the maximum
gross mass without engine thrust, and maintain this pressure regardless of the
variation in take-off masses. There are times, however, when operations at
reduced masses, modified CG and/or reduced tire pressures are productive and
reduced ACRs need to be calculated. To calculate the ACR for these conditions,
the adjusted tire inflation pressure should be entered in the ICAO-ACR
dedicated input field.
1.1.3.19 Worked
examples:
Example 1: Find the ACR of
B747-400 at 397 800 kg on a rigid pavement resting on a medium-strength
subgrade. The tire pressure of the main wheels is 1.38 MPa. From manufacturer
data, it is known that, at the aft c.g. for maximum ramp mass, 93.33 per cent
of the aircraft mass is on the main gear.
Solution: The ACR is found
based on steps as described in 1.1.3.6. These steps are automatically
implemented in the ICAO-ACR programme referenced in 1.1.3.4.
Step 1. Use the main gear
characteristics and the standard rigid pavement structure to determine the
reference ACR thickness t. Compute ACR for one four-wheel truck of the
16-wheel B747-400 main gear. All trucks in the B747-400 main gear have the same
load, tire pressure and wheel configuration; therefore, the selection of the
single truck to be used for evaluation is arbitrary. The LEAF programme
described in 1.1.3.4 was used to determine stresses at evaluation points at the
bottom of the concrete layer under each of the four wheels in one truck. The
LEAF input data for the B747-400 aircraft are shown in Figure 1-7. In Figure
1-7, the force acting on the single truck, 910.22 kN, is the gross weight of
the aircraft times 93.33 per cent, divided by four. From this analysis, for the
given load and gear geometry, a concrete thickness 381 mm produces a maximum
horizontal concrete stress of 2.75 MPa. Due to symmetry, the maximum horizontal
stress under all four wheels is the same. Therefore, the reference thickness t
is 381 mm.
Step 2. Determine the derived
single wheel load corresponding to reference ACR thickness t.
Use the same layered
elastic structure as in Step 1 with Layer 1 thickness equal to 381 mm.
Apply a single wheel
load with constant tire pressure Ps equal to 1.50 MPa. Vary the
magnitude
of the derived single
wheel load until the horizontal stress computed at a single evaluation point
located at the bottom of the concrete layer is 2.75 MPa. Figure 1-8 shows the
LEAF programme output for this case. From Figure 1-8, the tire load producing
the standard stress σ = 2.75 MPa at the standard tire pressure = 1.50
MPa is 336.17 kN, corresponding to a single wheel load of 34 280 kg.
Step 3. The numerical value
of ACR is two times the single wheel load in kg determined in Step 2, divided
by 100. Therefore, the ACR on medium- strength (“B”) subgrade is 2 × 343 = 686.
The ACR on subgrade category “B” will be reported as 690.
Figure
1-7. Input data for B747-400 evaluation aircraft in programme LEAF in ACR
example 1
Figure
1-8. Data for derived single wheel load in programme LEAF in ACR example 1
(stresses are in MPa)
Example 2: Find the ACR of
B787-9 at 254 692 kg on a flexible pavement resting on a low-strength subgrade.
The tire pressure of the main wheels is 1.56 MPa. From manufacturer data, it is
known that, at the aft c.g. for maximum ramp mass, 92.46 per cent of the
aircraft mass is on the main gear.
Solution: The ACR is found
using the steps as described in 1.1.3.7. These steps are automatically
implemented in the ICAO-ACR programme referenced in 1.1.3.4.
Step 1. Use the main gear
characteristics and the standard flexible pavement structure for aircraft with
more than two wheels to determine the reference ACR thickness t. From
FAA Order 5300.7, “Standard Naming Convention for Aircraft Landing Gear
Configurations”, the B787-9 has main gear designation 2D. As a simple landing
gear (the gear designation does not exceed 2 characters), the strain evaluation
points for ACR are based on a single truck. Use programme ICAO-ACR to find the
reference thickness t = 796 mm for 36 500 passes of the evaluated
aircraft. The layered elastic structure for subgrade category C (low-strength) with
moduli assigned according to paragraph 1.1.3.7 b) is:
|
Layer
|
Thickness,
mm
|
E, MPa
|
ν
|
|
Asphalt
|
127
|
1379
|
0.35
|
|
Sub-layer
3
|
161
|
769.52
|
0.35
|
|
Sub-layer
2
|
254
|
680.85
|
0.35
|
|
Sub-layer
1
|
254
|
271.27
|
0.35
|
|
Subgrade
|
infinite
|
80
|
0.35
|
Step 2. Determine the DSWL
corresponding to reference ACR thickness t. Use the same layered elastic
structure as in Step 1. Apply a single wheel load with constant tire pressure Ps
equal to 1.50 MPa. Vary the magnitude of the DSWL until damage is 1.0 for
36 500 passes. From ICAO-ACR, the computed value of the DSWL is 37 522.2 kg,
corresponding to a maximum vertical strain on the top of the subgrade of
0.001325. Note that for the single wheel load there is no multi-axle effect,
therefore the maximum strain can be found directly by substituting 36 500
passes in the elementary damage law equation in Appendix 3, Section 1.
Step 3. The numerical value
of ACR is two times the single wheel load in kg determined in Step 3, divided
by 100. Therefore, the ACR on low-strength (“C”) subgrade is 2 × 375 = 750. The
ACR on subgrade category “C” will be reported as 750.
Example 3: Find the ACR of
A380-800 at 562 000 kg on a flexible pavement resting on a medium-strength
subgrade. The tire pressure of the main wheels is 1.50 MPa. From manufacturer
data, it is known that, at the aft c.g. for maximum ramp mass, 95.13 percent of
the aircraft mass is on the main gear (57.08 per cent on the body landing gear
and 38.05 per cent on the wing landing gear).
Solution: The ACR is found
using the steps as described in 1.1.3.7. These steps are automatically
implemented in the ICAO-ACR programme referenced in 1.1.3.4.
Step 1. Use the main gear
characteristics and the standard flexible pavement structure for aircraft with
more than 2 wheels to determine the reference ACR thickness t. From FAA
Order 5300.7, “Standard Naming Convention for Aircraft Landing Gear
Configurations”, the A380-800 has main gear designation 2D/3D2. As a complex
landing gear (the gear designation exceeds two characters), the strain
evaluation points for ACR are based on the entire landing gear assembly. Use
the ICAO-ACR programme to find the reference thickness t = 616 mm. The
layered elastic structure for subgrade category B (medium-strength) with moduli
assigned according to paragraph 1.1.3.7 b) is:
|
Layer
|
Thickness,
mm
|
E, MPa
|
ν
|
|
Asphalt
|
127
|
1379
|
0.35
|
|
Sub-layer
2
|
235
|
698.75
|
0.35
|
|
Sub-layer
1
|
254
|
372.29
|
0.35
|
|
Subgrade
|
infinite
|
120
|
0.35
|
Step 2. Determine the DSWL
corresponding to reference ACR thickness t. Use the same layered elastic
structure as in Step 1. Apply a single wheel load with constant tire pressure Ps
equal to 1.50 MPa. Vary the magnitude of the DSWL until CDF = 1.0 for 36
500 passes. From ICAO-ACR, the computed value of the DSWL is 28 902.4 kg,
corresponding to a maximum vertical strain on the top of the subgrade of
0.001325. Note that for the single wheel load there is no multi-axle effect,
therefore the maximum strain can be found directly by substituting 36 500
passes in the elementary damage law equation in Appendix 3, Section 1.
Step 3. The numerical value
of ACR is two times the single wheel load in kg determined in Step 3, divided
by 100. Therefore, the ACR on low-strength (“B”) subgrade is 2 × 289 = 578. The
ACR on subgrade category
1.1.4
How PCRs are determined
1.1.4.1 The section
is intended to provide a model procedure for PCR determination and publication,
using the CDF concept. States may develop their own methods for PCR
determination, consistent with the overall parameters of the ACR-PCR method.
1.1.4.2 CDF concept
1.1.4.2.1 The CDF is
the amount of the structural fatigue life of a pavement which has been used up.
It is expressed as the ratio of applied load repetitions to allowable load
repetitions to failure, or, for one aircraft and constant annual departures where
a coverage is one application of the maximum strain or stress due to load on a
given point in the pavement structure:
Note 1. When CDF = 1,
the pavement subgrade will have used all of its fatigue life.
Note 2. When CDF <
1, the pavement subgrade will have some remaining life and the value of CDF
will give the fraction of the life used.
Note 3. When CDF >
1, all of the fatigue life will have been used and the pavement subgrade will
have failed.
1.1.4.2.2 In these
definitions, failure means failure according to the assumptions and definitions
on which the design procedures are based. A value of CDF greater than one does
not mean that the pavement will no longer support traffic, but that it will
have failed according to the definition of failure used in the design
procedure. The thickness design is based on the assumption that failure occurs
when CDF = 1.
1.1.4.2.3 Multiple
aircraft types are accounted for using Miner's Rule:
= 1+ 2+...+
where CDFi is the CDF
for each aircraft in the traffic mix and N is the number of aircrafts in the
mix.
1.1.4.2.4 Since the
PCR relates to the structural pavement life, the CDF is based on the subgrade
failure mode.
1.1.4.3 Lateral
wander
1.1.4.3.1 The
distribution of aircraft passes for a given aircraft type over the life of the
pavement is described by a Gaussian (or normal) distribution function, with a
standard deviation s that depends on several factors: the type of
aircraft, its ground speed and the maneuvering area. Another term that is
frequently used is the amplitude of lateral wander,
1.1.4.3.2 High-speed
sections (e.g. runways) are associated with higher values of s than moderate-speed
sections (e.g. taxiways), while wander may be considered negligible (
0) on low-speed sections
(e.g. aprons).
1.1.4.3.3 The
following values of standard deviation may be used independently of the type of
aircraft:
|
Pavement
section
|
Standard
Deviation s (meters)
|
|
High-speed sections
(runway, rapid exit taxiway)
|
0.75
|
|
Moderate-speed
sections (taxiways)
|
0.5
|
|
Aprons and
low-speed sections
|
0
|
1.1.4.3.4 The FAA
design procedure assumes s = 0.776 m (30.54 inches) independently of the
type of aircraft or feature.
1.1.4.3.5 The effect
of lateral wander may be considered indirectly by computing a pass-to-coverage
(P/C) ratio from the normal aircraft distribution. Alternatively, the
distribution function can be discretized (mapped to a calculation grid) and the
wandered damage computed numerically. A more closely spaced grid results in
higher calculation times but greater accuracy. A grid spacing of 5 cm has been found
to give good results. Discretization on a grid with transverse pitch Δy results
in the distribution of the paths on nw lines yw of the grid,
which are associated with percentages of the traffic Pw.
1.1.4.3.6 The effect
of including lateral wander is to reduce the theoretical damage that would be
caused by having all aircraft traverse a single path, i.e. Dwander <
Dzero wander. Zero wander implies that the number of passes equals the
number of coverages (P/C = 1).
Calculation of damage
assuming lateral wander
1.1.4.3.7 When the
grid method is used, it is necessary to obtain the total damage (for one
aircraft) by summing the individual damage contributions from each of the nw
profiles. This step consists of adding up the damage profiles Dno wander(y,
z), offset by the value yw and weighted by probability of occurrence
Pw in the lateral wander law:
where nw = total
number of damage profiles.
Determination of the
cumulative damage for a traffic mix
1.1.4.3.8 The
cumulative damage for all aircraft comprising an aircraft mix is given by the
following equation, which treats the additive effect of damage according to
Miner’s law:
where m =
total number of aircraft in the traffic mix; i = aircraft within the
aircraft mix; and = Number of aircraft passes.
1.1.4.3.9 The
resulting curve represents the variation of the CDF in the transverse direction
(relative to the longitudinal centreline).
1.1.4.3.10 If the
P/C ratio is computed for each aircraft i, an equivalent expression
giving CDF at lateral offset j is:
where Di is
the damage contributed by a pass of aircraft i, including any effects of
interaction between wheels in tandem.
1.1.4.4 Pavement
strength reporting
1.1.4.4.1 PCR shall
be reported using the following codes:
a) Rigid pavement = R
b) Flexible pavement
= F
Note. If the actual
pavement construction is composite or non-standard, include a note to that
effect.
1.1.4.4.2 Subgrade
category
1.1.4.4.3 The
subgrade categories are:
a) High strength:
Characterized by E=200 MPa, and representing all E values equal to or above 150
MPa for rigid and flexible pavements = Code A
b) Medium strength:
Characterized by E=120 MPa and representing a range in E equals to or above 100
and strictly less than 150 MPa, for rigid and flexible pavements = Code B
c) Low strength:
Characterized by E=80 MPa and representing a range in E equals to or above 60
and strictly less than 100 MPa, for rigid and flexible pavements = Code C
d) Ultra-low
strength: Characterized by E=50 MPa and representing all E values strictly less
than 60 MPa, for rigid and flexible pavements = Code D
1.1.4.4.4 For existing
pavements initially designed with the CBR design procedure, subgrade modulus
values can be determined in a number of ways. The procedure which will be
applicable in most cases is to use available CBR values and substitute the
relationship:
1.1.4.4.5 This
method provides designs compatible with the earlier flexible design procedure
based on subgrade CBR, but other accepted equivalencies can also be used
(Shell, APSDS etc.). Subgrade modulus values for PCR determination may also be
determined from direct soil testing (e.g., light weight deflectometer, plate
test).
1.1.4.4.6 Similarly,
for rigid pavement design, the foundation modulus can be expressed as the
modulus of subgrade reaction k or as the elastic (Young’s) modulus E. However,
all structural computations are performed using the elastic modulus E. If the
foundation modulus is input as a k-value it can be converted to the equivalent
E value using the following equations:
where ESG = Elastic
(Young’s) modulus of the subgrade, psi; and K = Modulus of subgrade reaction,
pci.
1.1.4.4.7 For new pavement
construction, the subgrade modulus value for PCR determination should be the
same value used for pavement thickness design.
1.1.4.4.8 The
maximum allowable tire pressure categories are:
a) Unlimited: no
pressure limit = Code W
b) High: pressure
limited to 1.75 MPa = Code X
c) Medium: pressure
limited to 1.25 MPa = Code Y
d) Low: pressure
limited to 0.5 MPa = Code Z
1.1.4.4.9 There are
two types of evaluation methods, mainly:
a) Technical evaluation:
representing a specific study of the pavement characteristics and its
capability of supporting the aircraft mix that is intended to serve, using the
CDF concept through a mechanistic design/evaluation method calibrated against
observed pavement behavior = Code T
b) Using aircraft
experience: representing a knowledge of the specific type and mass of aircraft
satisfactorily being supported under regular use = Code U
1.1.4.5 PCR
recommended procedure for technical evaluation (T)
1.1.4.5.1 The
following recommended PCR procedure reduces to the computation of an aircraft
ACR. The steps below can be used to convert the mix of using aircraft traffic
to an equivalent critical, or reference aircraft at maximum allowable gross
weight, which will then produce a CDF of 1.0 on the evaluated pavement. The ACR
calculation follows the ACR procedure described in 1.1.3.
1.1.4.5.2 The PCR
procedure considers the actual pavement characteristics at the time of the
evaluation - considering the existing pavement structure, and the aircraft
traffic forecast to use the pavement over its design structural life (for new
pavement construction) or estimated remaining structural life (for in service
pavements). The PCR should be valid only for this usage period. In case of
major pavement rehabilitation or significant traffic changes compared to the
initial traffic, a new evaluation should be performed.
1.1.4.5.3 The PCR
procedure involves the following steps:
a) Collect all
relevant pavement data (layer thicknesses, elastic moduli and Poisson’s ratio
of all layers, using or projected aircraft traffic) using the best available
sources.
b) Define the aircraft
mix by aircraft type, number of departures (or operations consistent with
pavement design practices), and aircraft weight that the evaluated pavement is
expected to experience over its design or estimated remaining structural life
(according to the manoeuver area (runway, taxiway, apron, ramp), the traffic
can be assigned a lateral wander characterized by a standard deviation as
detailed in 1.1.4.2.1).
c) Compute the ACRs
for each aircraft in the aircraft mix at its operating weight and record the
maximum ACR aircraft (ACR computations must follow the procedure in 1.1.3).
d) Compute the
maximum CDF of the aircraft mix and record the value (the CDF is computed with
any damage/failure model consistent with the procedure used for pavement
design).
e) Select the
aircraft with the highest contribution to the maximum CDF as the critical
aircraft. This aircraft is designated AC(i), where i is an index value
with an initial value 1. Remove all aircraft other than the current critical
aircraft AC(i) from the traffic list.
f) Adjust the number
of departures of the critical aircraft until the maximum aircraft CDF is equal
to the value recorded in d). Record the equivalent number of departures of the
critical aircraft.
g) Adjust the
critical aircraft weight to obtain a maximum CDF of 1.0 for the number of
departures obtained at f). This is the maximum allowable gross weight (MAGW)
for the critical aircraft.
h) Compute the ACR of
the critical aircraft at its MAGW. The value obtained is designated as PCR(i).
(ACR computations must follow the procedure in 1.1.3).
i) If AC(i) is the
maximum ACR aircraft from c) above, then skip to m).
j) Remove the current
critical aircraft AC(i) from the traffic list and re- introduce the other
aircraft not previously considered as critical aircraft. The new aircraft list,
which does not contain any of the previous critical aircraft, is referred to as
the reduced aircraft list. Increment the index value (i = i+1).
k) Compute the
maximum CDF of the reduced aircraft list and select the new critical aircraft
AC(i).
l) Repeat steps 5-9
for AC(i). In step 6, use the same maximum CDF as computed for the initial
aircraft mix to compute the equivalent number of departures for the reduced
list.
m) The PCR to be
reported is the maximum value of all computed PCR(i). The critical aircraft is
the aircraft associated with this maximum value of PCR(i).
1.1.4.5.4 A
flowchart of the above procedure is shown in Figure 1-9. The purpose of j) to
l) is to account for certain cases with a large number of departures of a
short/medium-range aircraft (such as the B737) and a relatively small number of
departures of a long-range aircraft (e.g. the A350). Without this step, the
smaller aircraft would generally be identified as critical, with the result
that the PCR would require
unreasonable operating weight restrictions on larger aircraft
(unreasonable because
the design traffic already included the large aircraft). Note that if
the initial critical
aircraft is also the aircraft in the list with the maximum ACR at
operating weight, then the
procedure is completed in one iteration, with no subsequent reduction to the traffic list.
1.1.4.5.5 The above
procedure returns a uniquely determined PCR numerical value on the identified
critical aircraft.
1.1.4.6 Applicability
1.1.4.6.1 The
technical evaluation should be used when pavement characteristics and aircraft
mix are consistently known and documented.
1.1.4.6.2 The PCR
procedure does not dictate the use of a preferred subgrade failure/damage model
nor a method for treating the multi-axle loading. Therefore, States can use
their existing pavement design and evaluation methodologies. The use of the initial
pavement design parameters will ensure consistency between what the actual
pavement is able to withstand and the PCR assignment.
PCR procedure - Using
aircraft experience (U)
1.1.4.6.3 Whenever
possible, reported pavement strength should be based on a “technical
evaluation”. When, for economic or other reasons a technical evaluation is not
feasible, evaluation can be based on experience with “using aircraft”. A
pavement satisfactorily supporting aircraft using it, can accept other aircraft
if they are no more demanding than the using aircraft. This can be the basis
for an evaluation
1.1.4.6.4 Techniques
for “using aircraft” evaluation are given in 3.5.
1.1.4.6.5 Worked
examples:
Example 1 (Flexible)
Steps 1 and 2: Data collection
a) Pavement characteristics
The pavement
description consists in providing for each layer its thickness, modulus of
elasticity (E) and Poisson’s ratio (ν). For new pavement construction, the data
should be those which served for the pavement design.
For in-service
pavement, it may be necessary to determine these input values by non-
destructive testing (core sampling, Heavy-Weight Deflectometer, etc.). Due to
loading or environmental conditions, the pavement material characteristics may
change over time. In the following example, the pavement was designed according
to the French pavement design procedure, using standard French material
specification found in NF EN 13 108-1, for a period of usage of ten years. For
PCR consistency, and to determine precisely the individual contribution of each
aircraft in the mix to the maximum CDF, the same parameters which were used for
the original pavement design (subgrade failure model, treatment of multi-axle loads,
etc.) are also used to determine PCR. The evaluated pavement is a runway.
|
PAVEMENT
CHARACTERISTICS
|
|
Layers
|
Designation
|
E-Modulus
(MPa)
|
Poisson’s
ratio
|
Thickness
(cm)
|
|
Surface
course
|
EB-BBSG3
|
E=f
( , freq.)
|
0.35
|
6
|
|
Base
course
|
EB-GB3
|
E=f
( , freq.)
|
0.35
|
18
|
|
Sub-base
(1)
|
GNT1
|
600
|
0.35
|
12
|
|
Sub-base
(2)
|
GNT1
|
240
|
0.35
|
25
|
|
Subgrade
|
|
80
|
0.35
|
∞
|
b) Aircraft mix data
For new pavement
construction, the aircraft mix for PCR determination is the same aircraft list
used for the pavement design.
For in-service
pavement, the PCR analysis considers aircraft usage over the remaining pavement
(structural) life. If the mixture of aircraft types using the pavement is known
to have changed significantly from the design forecast, an updated aircraft
list should be used. This example used the following list of aircraft with
maximum operating weights and annual departures:
|
AIRCRAFT
MIX ANALYSED
|
|
No.
|
Aircraft
model
|
Maximum
Taxi Weight (t)
|
Annual
departures
|
|
1
|
A321-200
|
93.9
|
14600
|
|
2
|
A350-900
|
268.9
|
5475
|
|
3
|
A380-800
|
571
|
1825
|
|
4
|
B737-900
|
79.2
|
10950
|
|
5
|
B787-8
|
228.4
|
3650
|
|
6
|
B777-300ER
|
352.4
|
4380
|
Note. The evaluated
pavement being a runway, each aircraft is assigned a lateral wander of 1.5 m
(standard deviation of 0.75 m). Each aircraft is centred on the pavement centre
line (CL) and modelled with its real main landing gear coordinates.
Step 3: Aircraft ACR at
operating weight
|
|
B777-300ER
|
A321-200
|
A350-900
|
B787-8
|
B737-9
|
A380-800
|
|
Operating
Weight (t)
|
352.4
|
93.9
|
268.9
|
228.4
|
79.2
|
571
|
|
ACR
|
790
|
550
|
720
|
680
|
450
|
650
|
Step 4: CDF of the entire
aircraft mix:
The CDF is computed
for the entire fleet by summing the individual aircraft CDF contributions along
a transverse axis perpendicular to the runway centreline. Figure 1-10 shows the
individual aircraft contributions to CDF and the resulting total CDF of the
mix. The maximum value of CDF is 1.153, located at an offset 4.9 m from the
runway centreline. The contribution of each aircraft in the mix to the maximum
CDF is plotted in Figure 1-10.
The maximum CDF is
greater than 1.0, indicating that the pavement is under-designed for the
traffic analysed.
Note. It is important
to distinguish the CDF contributions of each aircraft to the maximum CDF at the
critical offset from the maximum damage due to individual aircraft (which may
or may not occur at the critical offset). For instance, the A321-200 damage
contribution to the maximum CDF at the critical offset is 0.153 while its
maximum damage is equal to 0.341. Similarly, the A350-900 produces a max damage
of 0.306, lower than the A321, but its contribution to the max CDF is of 0.302,
higher than the A321 contribution. The difference is due to different track
dimensions (distance of the landing gear from the centreline) of the various
aircraft.
The aircraft with the
highest CDF contribution (to the maximum CDF) becomes the most demanding
aircraft within the mix. In this example, the highest contribution to the
maximum CDF (0.399 - see Figure 1-10) is produced by the B777-300ER.
Step 5: The B777-300ER is
selected as the most contributing aircraft to the maximum CDF. All other
aircraft are removed.
Step 6: The contribution of
the B777-300ER to the maximum CDF at its initial annual departure level is
0.457. The number of annual departures is adjusted until CDF equals 1.153. This
step is performed by simple linear extrapolation, giving 11,050 equivalent
annual departures of the B777-300ER (110 500 total departures).
Step 7: The gross weight of
the B777-300ER is adjusted to obtain a maximum CDF of 1.0. In other words, the
pavement is now correctly designed to accommodate the single equivalent
aircraft at its adjusted weight and equivalent annual departure level. The MAGW
is 341.3t.
Step 8: The B777-300ER ACR at
its MAGW is 740/F/C.
Step 9: Checking against the
list in Step 3, the B777-300ER is the maximum ACR aircraft. Therefore, the
procedure is stopped. The PCR to be reported is equal to the B777-300ER ACR at
its MAGW:
PCR 740 FCWT.
For the tire pressure
code, the letter W is selected since the evaluated pavement is new
construction, and the surface asphalt mix has been designed to resist the
imposed tire pressures.
Example 2 (Flexible)
Steps 1 and 2: Data collection
a) Pavement
characteristics
In this example, a
flexible runway was designed according to the FAA pavement design procedure,
using standard US material specifications found in FAA AC 150/5370-10. For PCR
consistency, and to determine precisely the individual contribution of each
aircraft in the mix to the maximum CDF, the procedure should consider the
design parameters which served the original pavement design (subgrade failure
model, treatment of multi-axle loadings etc.). This is achieved in accordance
with FAA AC 150/5320-6F, Airport Pavement Design and Evaluation.
|
PAVEMENT
CHARACTERISTICS
|
|
Layers
|
Designation
|
E-Modulus
(MPa)
|
Poisson’s
ratio
|
Thickness
(cm)
|
|
Surface
course
|
P-401/P-403
HMA Surface
|
1379
|
0.35
|
10.2
|
|
Base
course
|
P-401/P-403
(flex)
|
2758
|
0.35
|
12.7
|
|
Sub-base
|
P-209
|
467
|
0.35
|
17.5
|
|
Subgrade
|
|
200
|
0.35
|
∞
|
b) Aircraft mix data
In this example, the
traffic data represent a regional hub, in which there is a large number of
departures of mid-range jet aircraft (A320, A321, B737) combined with a smaller
number of operations of long range or large aircraft (A330, B777 and A380). The
design life is 20 years.
|
AIRCRAFT
MIX
|
|
No.
|
Aircraft
model
|
Maximum
Taxi Weight (t)
|
Annual
departures
|
|
1
|
A330-300
|
233.9
|
52
|
|
2
|
B777-300ER
|
352.4
|
52
|
|
3
|
A380-800
|
571
|
52
|
|
4
|
B737-900ER
|
85.4
|
10950
|
|
5
|
A320-200
|
77.4
|
10950
|
|
6
|
A321-200
|
93.9
|
1560
|
|
|
|
|
|
|
Note. Consistent with
FAA design standards, the assumed standard deviation of aircraft wander is
0.776 m (30.54 inches).
Step 3: Aircraft ACR at
operating weight:
|
|
A321-200
|
B737-900ER
|
B777-300ER
|
A320-200
|
A330-300
|
A380-800
|
|
Operating
weight (t)
|
93.9
|
85.4
|
352.4
|
77.4
|
233.9
|
571
|
|
ACR
|
460
|
420
|
570
|
360
|
570
|
550
|
Step 4: CDF of the entire
aircraft mix
The CDF is computed
for the entire fleet by summing the individual aircraft CDF contributions along
a transverse axis perpendicular to the runway centreline. In this example, the
computation was done using the FAA programme FAARFIELD 1.42.
Figure 1-11 shows the
individual aircraft CDF and the resulting total CDF for the design. The maximum
CDF is 0.99, located at a lateral offset 3.7 m from the runway centreline. The contribution
of each aircraft in the mix to the maximum CDF is plotted in Figure 1-11. Note
that the CDF values plotted in Figure 1-11 are based on aircraft
characteristics for thickness design, according to which 95 per cent of the
aircraft gross weight acts on the main gear. The maximum CDF is slightly less
than 1.0, indicating that the pavement thickness is properly designed for the
traffic analysed. When the characteristics are adjusted to reflect the mass and
c.g. values that produce the highest main gear loads on each aircraft (see
1.1.3.2 e), then the maximum CDF is reduced to 0.898; however, the relative
contributions of the aircraft are the same. In contrast to example 1, the
maximum CDF is concentrated around single-aisle aircraft, while the contribution
of the long- range aircraft is less, due to the small number of annual
departures.
Step 5: Based on Figure 1-11,
the B737-900ER is selected as the most contributing aircraft to the maximum
CDF. All other aircraft are removed.
Step 6: The contribution of
the B737-900ER to the maximum CDF at its initial annual departure level is
0.405. The programme adjusts the number of annual departures iteratively until
CDF equals 0.898, giving 21 837 equivalent annual departures of the B737-900ER.
Step 7: The gross weight of
the B737-900ER is adjusted to obtain a maximum CDF of 1.0. The pavement is now
correctly designed to accommodate the single equivalent aircraft at its
adjusted weight and equivalent annual departure level. The MAGW is 85.77 t.
Step 8: The ACR of the
B737-900ER at its MAGW is 425 FA = PCR1.
Step 9: Checking against the
table in Step 3, it is found that the B737-900ER is not the maximum ACR
aircraft.
Therefore, the
procedure continues to Step 10.
Step 10: The B737-900ER is
removed from the aircraft list, and all other aircraft are reintroduced.
Step 11: In the reduced
aircraft mix, the new most contributing aircraft is the A321-200, since the
location of the maximum CDF has now changed by removing the B737-900ER.
Step 12: Steps 5 to 9 are
repeated until the aircraft that is the highest contributor to CDF at the
critical offset is also the maximum ACR aircraft.
In this example, the
recursive procedure is stopped at the third potential critical aircraft. The
resulting PCRi values are:
a) PCR1 425 FAWT
(first critical aircraft, B737-900ER)
b) PCR2 465 FAWT
(second critical aircraft of the reduced aircraft mix, A321-200)
c) PCR3 580 FAWT
(third critical aircraft and maximum ACR aircraft, B777-300ER)
Retained PCR= Max
(PCR1, PCR2, PCR3) = 580 FAWT
Because the reported
PCR is higher than the maximum operating weight ACR of any of the mix aircraft,
there are no operating weight restrictions.
Example 3 (Rigid)
Steps 1 and 2: Data collection
a) Pavement
characteristics
In this example, a
rigid taxiway is evaluated for PCR reporting. Material properties are assigned
to layers following standard the United States’ material specifications found
in FAA AC 150/5370-10 and FAA AC 150/5320-6F. Assume that based on laboratory
tests, the concrete flexural strength of the concrete is 4.5 MPa.
|
PAVEMENT
CHARACTERISTICS
|
|
Layers
|
Designation
|
E-Modulus
(MPa)
|
Poisson’s
ratio
|
Thickness
(cm)
|
|
Surface
course
|
P-501
Portland cement concrete
|
27579
|
0.15
|
45.0
|
|
Base
course
|
P-401/P-403
(flex)
|
2758
|
0.35
|
12.5
|
|
Sub-base
|
P-209
|
311
|
0.35
|
30.0
|
|
Subgrade
|
P-152
|
90
|
0.40
|
[infinite]
|
b) Aircraft mix data
The applied traffic
for this example is given in the table below. The design life is 20 years. For
this traffic mix, the FAA standard thickness design requirement (FAARFIELD
1.42) is 45.6 cm of concrete. Therefore, the existing pavement thickness is
slightly under-designed for the given traffic and operating weight restrictions
may be required for some of the heavier aircraft.
|
AIRCRAFT
MIX ANALYSED
|
|
No.
|
Aircraft
model
|
Max.
Taxi Weight (t)
|
Percent
Weight on Main Gear
|
Annual
departures
|
|
1
|
B747-8
|
440.0
|
94.7
|
365
|
|
2
|
A350-900
|
268.9
|
94.8
|
5475
|
|
3
|
B787-8
|
228.4
|
91.3
|
3650
|
|
4
|
A321-200
|
93.9
|
94.6
|
14600
|
|
5
|
B737-900
|
79.2
|
94.6
|
10950
|
|
6
|
EMB-190
|
48.0
|
95.0
|
10950
|
Note. Consistent with
FAA design standards, the assumed standard deviation of aircraft wander is
0.776 m (30.54 inches).
Step 3: Aircraft ACR at
operating weight:
|
|
B747-8
|
A350-900
|
B787-8
|
A321-200
|
B737-900
|
EMB-190
|
|
Operating
weight (t)
|
440.0
|
268.9
|
228.4
|
93.9
|
79.2
|
48.0
|
|
ACR/R/C
|
910
|
920
|
870
|
660
|
550
|
290
|
Step 4: CDF of the entire
aircraft mix
The CDF is computed
for the entire fleet by summing the individual aircraft CDF contributions along
a transverse axis perpendicular to the runway centreline. In this example, the
computation was done using the FAA programme FAARFIELD 1.42.
Using the aircraft
data in Step 2, the maximum CDF for the given traffic mix is found to be 1.24,
which is higher than the design target value 1.0. The maximum CDF is located at
a lateral offset 4.7 m from the runway centreline. The aircraft that is the
largest contributor to CDF at this critical offset is the A350-900.
Step 5: The A350-900 is
selected as the most contributing aircraft to the maximum CDF. All other
aircraft are removed.
Step 6: The contribution of
the A350-900 to the maximum CDF at its initial annual departure level is 0.935.
The programme adjusts the number of annual departures iteratively until CDF
equals 1.24, giving 7 227 equivalent annual departures of the A350-900.
Step 7: The gross weight of
the A350-900 is adjusted to obtain a maximum CDF of 1.0 for 7227 annual
departures. The MAGW is 270.4 t.
Step 8: The ACR of the
A350-900 at its MAGW is 906/R/C = PCR1.
Step 9: Checking against the
table in Step 3, it is found that the A350-900 is also the maximum ACR
aircraft. Therefore, the procedure jumps to Step 13 (end). Rounding the PCR numerical
value to the nearest multiple of 10, the PCR to be reported is 910/R/C/W/T.
If the airport
publishes this PCR, then minor operating weight restrictions will be required
on the A350-900. Alternatively, the A350-900 could be allowed to operate under
the overload provisions (see 2.1.1), as its ACR exceeds the PCR by less than
the 10 per cent allowance.
CHAPTER 2
GUIDANCE FOR OVERLOAD OPERATIONS
2.1 CRITERIA SUGGESTED IN ANNEX 14, VOLUME
I, ATTACHMENT A
2.1.1 Overloading of
pavements can result either from loads too large or from a substantially
increased application rate, or both. Loads larger than the defined (design or
evaluation) load shorten the design life whilst smaller loads extend it. With
the exception of massive overloading, pavements in their structural behaviour
are not subject to a particular limiting load above which they suddenly or
catastrophically fail. Behaviour is such that a pavement can sustain a
definable load for an expected number of repetitions during its design life. As
a result, occasional minor overloading is acceptable, when expedient, with only
limited loss in pavement life expectancy and relatively small acceleration of
pavement deterioration. For those operations in which magnitude of overload
and/or the frequency of use do not justify a detailed analysis the following
criteria are suggested:
a) for flexible and
rigid pavements, occasional movements by aircraft with ACR not exceeding 10 per
cent above the reported PCR should not adversely affect the pavement; and
b) the annual number of
overload movements should not exceed approximately 5 per cent of the total
annual movements excluding light aircraft.
2.1.2 Such overload
movements should not normally be permitted on pavements exhibiting signs of
distress or failure. Furthermore, overloading should be avoided during any
periods of thaw following frost penetration or when the strength of the
pavement or its subgrade could be weakened by water. Where overload operations
are conducted, the appropriate authority should review the relevant pavement
condition regularly and should also review the criteria for overload operations
periodically since excessive repetition of overloads can cause severe
shortening of pavement life or require major rehabilitation of pavement.
Overload Technical
Analysis
2.1.3 Overloads in
excess of 10 per cent may be considered on a case by case basis when supported
by a more detailed technical analysis. When overload operations exceed allowances
described in 2.1.1, a pavement analysis is required for granting the proposed
additional loads, which was not scheduled in the initial pavement design. In
those cases, the pavement analysis should determine how the overload operation
contributes to the maximum CDF when it is mixed with the actual aircraft mix.
Indeed, the ACR as a relative indicator, even if exceeding the reported PCR
cannot predict how the overload aircraft will affects the pavement structural
behaviour and/or its design life, since it will be strongly dependant of its
offset to the location of the maximum CDF produced by the aircraft mix
(critical offset).
2.1.4 The pavement
analysis would then consist in determining the number of permitted overload
operations so that the CDF of the entire aircraft mix, including the overload
aircraft, remains in the tolerances agreed by the relevant authority.
CHAPTER 3
STRUCTURAL EVALUATION OF PAVEMENTS
3.1 GENERAL
3.1.1 The purpose of
this chapter is to present guidance on the evaluation of pavements to those
responsible for evaluating and reporting pavement bearing strength. Recognizing
that responsible individuals may range from experienced pavement engineers to
airfield managers not enjoying the direct staff support of pavement behaviour
experts, information included in this chapter attempts to serve the various
levels of need.
3.2 ELEMENTS OF PAVEMENT EVALUATION
3.2.1 The behaviour
of any pavement depends upon the native materials of the site, which after
levelling and preparation is called the subgrade, its structure including all
layers up through the surfacing, and the mass and frequency of using aircraft.
Each of these three elements must be considered when evaluating a pavement.
The
subgrade
3.2.2 The subgrade
is the layer of material immediately below the pavement structure, which is
prepared during construction to support the loads transmitted by the pavement.
It is prepared by stripping vegetation, levelling or bringing to planned grade
by cut and fill operations, and compacting to the needed density. Strength of
the subgrade is a significant element and this must be characterized for
evaluation or design of a pavement facility or for each section of a facility
evaluated or designed separately. Soil strength and therefore, subgrade strength
is very dependent on soil moisture and must be evaluated for the condition it
is expected to attain in situ beneath the pavement structure. Except in cases
with high water tables, unusual drainage, or extremely porous or cracked
pavement conditions, soil moisture will tend to stabilize under wide pavements
to something above 90 per cent of full saturation. Seasonal variation
(excepting frost penetration of susceptible materials) is normally small to
none and higher soil moisture conditions are possible even in quite arid areas.
Because materials can vary widely in type, the subgrade strength established
for a particular pavement may fall anywhere within the range indicated by the
four subgrade strength categories used in the ACR- PCR method. See Chapter 1 of
this manual and Annex 14, Volume I, Chapter 2.
The
pavement structure
3.2.3 The terms
“rigid” and “flexible” have come into use for identification of the two
principal types of pavements. The terms attempt to characterize the response of
each type to loading. The primary element of a rigid pavement is a layer or
slab of Portland cement concrete (PCC), plain or reinforced in any of several
ways. It is often underlain by a granular layer which contributes to the
structure both directly and by facilitating the drainage of water. A rigid
pavement responds “stiffly” to surface loads and distributes the loads by
bending or beam action to wide areas of the subgrade. The strength of the
pavement depends on the thickness and strength of the PCC and any underlying
layers above the subgrade. The pavement must be adequate to distribute surface
loads so that the pressure on the subgrade does not exceed its evaluated
strength. A flexible pavement consists of a series of layers increasing in
strength from the subgrade to the surface layer. A series such as select
material, lower sub-base, sub-base, base and wearing course is commonly used.
However, the lower layers may not be present in a particular pavement. The
pavements meant for heavy aircraft usually have a bituminous bound wearing
course. A flexible pavement yields more under surface loading merely
accomplishing a widening of the loaded area and consequent reduction of
pressure, layer by layer. At each level from the surface to subgrade, the
layers must have strength sufficient to tolerate the pressures at their level.
The pavement thus depends on its thickness over the subgrade for reduction of
the surface pressure to a value which the subgrade can accept. A flexible
pavement must also have thickness of structure above each layer to reduce the
pressure to a level acceptable by the layer. In addition, the wearing course
must be sufficient in strength to accept without distress, tire pressures of
using aircraft.
Aircraft
loading
3.2.4 The aircraft
mass is transmitted to the pavement through the undercarriage of the aircraft.
The number of wheels, their spacing, tire pressure and size determine the
distribution of aircraft load to the pavement. In general, the pavement must be
strong enough to support the loads applied by the individual wheels, not only
at the surface and the subgrade but also at intermediate levels. For the
closely spaced wheels of dual and dual-tandem legs, and for adjacent legs of
aircraft with complex undercarriages, the effects of distributed loads from
adjacent wheels overlap at the subgrade (and intermediate) level. In such
cases, the effective pressures are those combined from two or more wheels and
must be attenuated sufficiently by the pavement structure. Since the
distribution of load by a pavement structure is over a much narrower area on a
high strength subgrade than on a low strength subgrade, the combining effects
of adjacent wheels is much less for pavements on high strength than on low
strength subgrades. This is the reason why the relative effects of two aircraft
types are not the same for pavements of equivalent design strength, and this is
the basis for reporting pavement bearing strength by subgrade strength
category. Within a subgrade strength category the relative effects of two aircraft
types on pavements can be uniquely stated with good accuracy.
Load
repetitions and composition of traffic
3.2.5 It is not
sufficient to consider the magnitude of loading alone. There is a fatigue or
repetitions of load factor that should also be considered. Thus magnitude and
repetitions must be treated together, and a pavement, which is designed to
support one magnitude of load at a defined number of repetitions can support a
larger load at fewer repetitions and a smaller load for a greater number of
repetitions. It is thus possible to establish the effect of one aircraft mass
in terms of equivalent repetitions of another aircraft mass (and type).
Application of this concept permits the determination of a single (selected)
magnitude of load and repetitions level to represent the effect of the mixture
of aircraft using a pavement.
Pavement
condition survey
3.2.6 A particularly
important adjunct to or part of evaluation is a careful condition survey. The pavement
should be closely examined for evidences of deterioration, movement, or change
of any kind. Any observable pavement change provides information on effects of
traffic or the environment on the pavement. Observable effects of traffic along
with an assessment of the magnitude and composition of that traffic can provide
an excellent basis for defining the bearing capacity of a pavement.
3.3 ELEMENTS OF THE ACR-PCR METHOD
Pavement
classification rating (PCR)
3.3.1 The pavement
classification rating (PCR) is an index rating (1/50th) of the mass, expressed
in kilograms, which an evaluation shows can be borne by the pavement when
applied by a standard (1.50 MPa tire pressure) single-wheel. The PCR rating
established for a pavement indicates that the pavement is capable of supporting
aircraft having an aircraft classification rating (ACR) of equal or lower
magnitude. The ACR for comparison to the PCR must be the aircraft ACR
established for the particular pavement type and subgrade category of the rated
pavement as well as for the particular aircraft mass and characteristics.
Pavement
type
3.3.2 For purposes
of reporting pavement strength, pavements must be classified as either rigid or
flexible. A rigid pavement is that employing a PCC slab whether plain,
reinforced, or pre-stressed and with or without intermediate layers between the
slab and subgrade. Large precast slabs, which require crane handling for
placement can be classified as rigid when used in pavements. A flexible
pavement is that consisting of a series of layers increasing in strength from the
subgrade to the wearing surface. Composite pavements resulting from a PCC
overlay on a flexible pavement or an asphaltic concrete overlay on a rigid pavement
or those incorporating chemically (cement) stabilized layers of particularly
good integrity require care in classification. If the “rigid” element remains
the predominant structural element of the pavement and is not severely
distressed by closely spaced cracking, the pavement should be classified as
rigid. Otherwise the flexible classification should apply. Where classification
remains doubtful, designation as flexible pavement will generally be
conservative. Since PCR is relative to paved surfaces it should not be
considered appropriate for unpaved surfaces (compacted earth, gravel, laterite,
coral, etc.), surfaces built with bricks or blocks, and surfaces covered with
landing mat and membrane surfaces. If some type of PCR is determined for these
types of surfaces, only the “using aircraft” method should be followed. In such
a case, the “pavement type” field should indicate the actual surface type.
Alternatively, it may be classified as flexible for reporting; however, it must
include a designation indicating that the surface is not actually a paved
surface (Note to 2.6.6 a), Annex 14, Volume I refers). If the actual
construction is composite or non- standard, a note should be included to that
effect.
Subgrade
category
3.3.3 Since the
effectiveness of aircraft undercarriages using multiple-wheels is greater on
pavements founded on strong subgrades compared to those on weak subgrades, the
problem of reporting bearing strength is complicated. To simplify the reporting
and permit the use of index values for aircraft and pavement classification
ratings (ACR and PCR) the ACR-PCR method uses four subgrade strength
categories. These are termed high, medium, low and ultra-low with prescribed
ranges for the categories. It follows that for a reported evaluation (PCR) to
be useful, the subgrade category to which the subgrade of the reported pavement
belongs must be established and reported. Normally, subgrade strength will have
been evaluated in connection with original design of a pavement or later
rehabilitation or strengthening. Where this information is not available the
subgrade strength should be determined as part of pavement evaluation. Subgrade
strength evaluation should be based on testing wherever possible. Where
evaluation based on testing is not feasible, a representative subgrade strength
category must be selected based on soil characteristics, soil classification,
local experience or judgement. Commonly, one subgrade category may be
appropriate for an aerodrome. However, where pavement facilities are scattered
over a large area and soil conditions differ from location to location, several
categories may apply and should be assessed and so reported. The subgrade
strength evaluated must be that in situ beneath the pavement. The subgrade
beneath an aerodrome pavement will normally reach and retain a fairly constant
moisture and strength despite seasonal variations. However, in the case of
severely cracked surfacing, porous paving, high ground water or poor local
drainage, the subgrade strength can reduce substantially during wet periods.
Unpaved surfaces will be especially subject to moisture change. In areas of
seasonal frost, a lower reduced subgrade strength can be expected during the
thaw period where frost susceptible materials are involved.
Tire
pressure category
3.3.4 Directly at
the surface, the tire contact pressure is the most critical element of loading
with little relation to other aspects of pavement strength. This is the reason
for reporting permissible tire pressure in terms of tire pressure categories.
Except for rare cases of spalling joints and unusual surface deficiencies,
rigid pavements do not require tire pressure restrictions. However, pavements
categorized as rigid which have overlays of flexible or bituminous construction
must be treated as flexible pavements for reporting permissible tire pressure.
Flexible pavements, which are classified in the highest tire pressure category
must be of very good quality and integrity, while those classified in the
lowest category need only be capable of accepting casual highway type traffic.
While tests of bituminous mixes and extracted cores for quality of the
bituminous surfacing will be most helpful in selecting the tire pressure
category, no specific relations have been developed between test behaviour and
acceptable tire pressure. It will usually be adequate, except where limitations
are obvious, to establish category limits only when experience with high tire
pressures indicates pavement distress.
Evaluation
method
3.3.5 Wherever
possible, reported pavement strength should be based on a “technical
evaluation”. Commonly, evaluation is an inversion of a design method. Design
takes into consideration the aircraft loading to be sustained and the subgrade
strength resulting from preparation of the local soil, which then provides the
necessary thicknesses and quality of materials for the needed pavement
structure. Evaluation inverts this process. It begins with the existing
subgrade strength, finds thickness and quality of each component of the
pavement structure, and uses a design procedure pattern to determine the
aircraft loading which the pavement can support. Where available, the design,
testing and construction record data for the subgrade and components of the
pavement structure can often be used to make the evaluation. Or, test pits can
be opened to determine the thicknesses of layers, their strengths and subgrade
strength for the purpose of evaluation. A technical evaluation can also be made
based on measurement of the response of pavement to load. Deflection of a
pavement under static plate or tire load can be used to predict its behaviour.
Also there are various devices for applying dynamic loads to a pavement (e.g.
heavy falling weight deflectometers (H/FWD), potential adaptation of traffic
speed deflectometers (TSD) and other emerging techniques to airport pavements),
observing its response and using this to predict its behaviour. When, for
economic or other reasons, a technical evaluation is not feasible, evaluation
can be based on experience with “using aircraft”. A pavement satisfactorily
supporting aircraft using it can accept other aircraft if they are no more
demanding than the using aircraft. This can be the basis for an evaluation.
Pavements
for light aircraft
3.3.6 Light aircraft
are those having a mass of 5 700 kg or less. These aircraft have pavement
requirements less than that of many highway trucks. Technical evaluations of
those pavements can be made but an evaluation based on using aircraft is
satisfactory. It is worth noting that at some airports service vehicles such as
fire trucks, fuel trucks or snow ploughs may be more critical than aircraft.
Since nearly all light aircraft have single-wheel undercarriage legs there is
no need for reporting subgrade categories. However, since some helicopters and
military trainer aircraft within this mass range have quite high tire pressures,
limited quality pavements may need to have tire pressure limits established.
3.4 ASSESSING THE MAGNITUDE AND COMPOSITION
OF TRAFFIC
General
3.4.1 Pavement
bearing strength evaluations should address not merely an allowable load but a
repetitions use level for that load. A pavement that can sustain many
repetitions of one load can sustain a larger load but for fewer repetitions.
Observable effects of traffic, even those involving careful measurements in
situ or on samples in controlled laboratory tests, unfortunately do not (unless
physical damage is apparent[2]) permit a determination of the
portion of pavement’s repetitions life that has been used or, conversely, is
remaining. Thus an evaluation leading to bearing capacity determination is an
assessment of pavement’s total expected repetitions (traffic/load) life. Any
projection of remaining useful life of the pavement will depend on a
determination of all traffic sustained since construction or reconstruction.
Mixed
loadings
3.4.2 Normally, it
will be necessary to consider a mixture of loadings at their respective
repetitions use levels. There is a strong tendency to rate pavement bearing
strength in terms of some selected loading for the allowable repetitions use
level and to rate each loading applied to a pavement in terms of its equivalent
number of this basic loading. To do this, a relation is first established
between loading and repetitions to produce failure. Such relations are
variously established using combinations of theory or design methods and
experience behaviour patterns or laboratory fatigue curves for the principal
structural element of the pavement. Not all relations are the same, but the
repetitions parameter is not subtly effective. It needs only to be established
in general magnitude and not in specific value. Thus fairly large variations
can exist in the loading-repetitions relation without serious differences in
evaluation resulting.
3.4.3 Using the
curve for loading versus repetitions to failure, the failure repetitions for
each loading can be determined and compared to that for the basic selected
loading. From these comparisons, the equivalent number of the basic selected
loading for single applications of any loading are determined (i.e. factors
greater than one for larger loadings and less than one for smaller loadings).
An explanatory example of this process follows:
a)
relate loading to failure repetitions, as illustrated in Figure 3-1;
Figure
3-1. Curve for loading versus repetitions to failure
b) L4 - r4 for
selected loads L, read repetitions r from curve;
L1
- r1
L2
- r2
L3
- r3
L4
- r4
c) choose L3 as the basic load; and
d) compute equivalent repetitions factor f for each load (see
Table 3-1).
By use of these factors, the accumulated
effect of any
combination of loads experienced or contemplated can be compared to the bearing strength evaluation
in terms of a selected
loading at its evaluated allowable repetitions use level.
3.5 TECHNIQUES FOR “USING AIRCRAFT”
EVALUATION
3.5.1 While
technical evaluation should be accomplished wherever possible, it is recognized
that financial and circumstantial constraints will occasionally prevent it.
Since it is most important to have completely reported bearing strength
information and since using aircraft evaluation is reasonably direct and
readily comprehensible, it is being presented first.
Heaviest
using aircraft
3.5.2. A pavement
satisfactorily sustaining its using traffic can be considered capable of
supporting the heaviest aircraft regularly using it and any other aircraft that
has no greater pavement strength requirements. Thus, to begin an evaluation
based on using aircraft, the types and masses of aircraft and number of times
each operates in a given period must be examined. Emphasis should be on the
heaviest aircraft regularly using the pavement. Support of a particularly heavy
load, but only rarely, does not necessarily establish a capability to support
equivalent loads on a regular repetitive basis (see 3.4).
Pavement
condition and behaviour
3.5.3. There must
next be a careful examination of what effect the traffic of using aircraft is
having on the pavement. The condition of the pavement in relation to any
cracking, distortion or wear, and the experience with needed maintenance are of
first importance. Age must be considered since overload effects on a new
pavement may not yet be evident while some accumulated indications of distress
may normally be evident in a very old pavement. In general, however, a pavement
in good condition can be considered to be satisfactorily
carrying the using
traffic, while indications of advancing distress show the pavement is being
overloaded. The condition examination should take note of relative pavement
behaviour in areas of intense versus low usage such as in and out of wheel
paths or most and least used taxiways, zones subject to maximum braking (e.g.
taxiway turn-off, etc.). Note should also be taken of behaviour of any known or
observable weak or critical areas such as low points of pavement grade, old
stream crossings, pipe crossings where initial compaction was poor,
structurally weak sections, etc. These will help to predict the rate of
deterioration under extant traffic and thereby indicate the degree of
overloading or of underloading. The condition examination should also focus on
any damage resulting from tire pressures of using aircraft and the need for
tire pressure limitations.
Reference
aircraft
3.5.4 Study of the
types and masses of aircraft will indicate those which must be of concern in
establishing a reference aircraft and the condition survey findings will
indicate whether the load of the reference aircraft should be less than that
being applied or might be somewhat greater. Since load distribution to the
subgrade depends somewhat on pavement type and subgrade strength, the particular
reference aircraft and its mass cannot be selected until those elements of the
ACR-PCR method, which are reported in addition to the PCR have been established
(see 3.3.2 and 3.3.3).
Determination
of the pavement type, subgrade strength and tire pressure categories
3.5.5. The pavement
type must be established as rigid or flexible. If the pavement includes a PCC
slab as the primary structural element it should be classified as rigid even
though it may have a bituminous overlay resurfacing (see 3.3.2). If the
pavement includes no such load-distributing slab it should be classified as
flexible.
3.5.6 The subgrade
category must be determined as high, medium, low, or ultra-low strength. If
modulus of elasticity test data are available for the subgrade, these can be
used directly to select the subgrade category. Such data, however, must
represent in situ subgrade conditions. Similar data from any surrounding
structures on the same type of soil and in similar topography can also be used.
Soil strength data in almost any other form (such as CBR data) can be used to
project an equivalent modulus of elasticity E for use in selecting the subgrade
category. Information on subgrade soil strength may be obtainable from local
road or highway agencies, or local agricultural agencies. A direct, though
somewhat crude or approximate determination of subgrade strength can be made
from classification of the subgrade material and reference to any of many
published correlations such as that shown in Figure 3-2. (Also see 3.3.3 and
3.2.2). Chapter 1.1.4.4 b) gives equivalencies between CBR or module of
subgrade reaction k and modulus of elasticity E.
3.5.7 The tire
pressure category must be determined as unlimited, high, medium or low. PCC
surfacing and good to excellent quality bituminous surfacing can sustain the
tire pressures commonly encountered and should be classified as unlimited
pressure category with no limit on pressure. Bituminous surfacing of inferior quality
and aggregate or earth surfacing will require the limitation of lower
categories (see 3.3.4). The applicable pressure category should normally be
selected based on experience with using aircraft. The highest tire pressure
being applied, other than rarely, by using aircraft, without producing
observable distress should be the basis for determining the tire pressure
category.
3.5.8 The most
significant element of the using aircraft evaluation is determination of the
critical aircraft and the equivalent PCR for reporting purposes. Having
determined the pavement type and the subgrade category the next step would be
the determination of the ACRs of aircraft using the pavement. For this purpose,
needed information can be obtained by analysis using the prescribed ACR-PCR
methods (see the ICAO-ACR programme). Comparison of aircraft regularly using
the pavements - at their operating masses - with the above-mentioned programme
or the relevant aircraft characteristics documents will permit determination of
the most critical aircraft using the pavement. If the using aircraft are
satisfactorily being sustained by the pavement and there are no known factors
which indicate that substantially heavier aircraft could be supported, the ACR
of the most critical aircraft should be reported as the PCR of the pavement.
Thus any aircraft having an ACR no higher than this PCR can use the pavement
facility at a use rate (as repetitions per month) no greater than that of
presently supported aircraft without shortening the use-life of the pavement.
3.5.9 In arriving at
the critical aircraft, only aircraft using the pavement on a continuing basis
without unacceptable pavement distress should be considered. The occasional use
of the pavement by a more demanding aircraft is not sufficient to ensure its
continued support even if no pavement distress is apparent.
3.5.10 As indicated,
a PCR directly selected based on the evaluated critical aircraft loading
contemplates an aircraft use intensity in the future similar to that at the
time of evaluation. If a substantial increase in use (wheel load repetitions)
is expected, the PCR should be adjusted downward to accommodate the increase. A
basis for the adjustment, which relates load magnitude to load repetitions, is
presented in 3.4.
Pavements
for light aircraft
3.5.11 In evaluating
pavements meant for light aircraft - 5 700 kg mass and less - it is unnecessary
to consider the geometry of the undercarriage of aircraft or how the aircraft
load is distributed among the wheels. Thus, subgrade class and pavement type
need not be reported, and only the maximum allowable aircraft mass and maximum
allowable tire pressure need be determined and reported. For these, the
foregoing guidance on techniques for “using aircraft” evaluation should be
followed.
3.5.12 Because the 5
700 kg limit for light aircraft represents pavement loads only two- thirds or
less of common highway loads, the assessment of traffic using pavements should
extend to consideration of heavy ground vehicles such as fuel trucks, fire
trucks, snow ploughs, service vehicles and the like. These must also be
controlled in relation to load limited pavements.
3.6 TECHNIQUES AND EQUIPMENT FOR “TECHNICAL”
EVALUATION
Technical evaluation
is the process of defining or quantifying the bearing capacity of a pavement
through measurement and study of the characteristics of the pavement and its
behaviour under load. This can be done either by an inversion of the design
process, using design parameters and methods, but reversing the process to
determine allowable load from existing pavement characteristics, or by a direct
determination of response of the pavement to load by one of several means.
3.6.1 Pavement behaviour concepts for design
and evaluation
Concepts of behaviour
developed into analytical means by which pavements can be designed to
accommodate specific site and aircraft traffic conditions are commonly referred
to as design methods. There are a variety of concepts and many specific design methods.
For example, several design and evaluation methods are presented in Chapter 4
of this manual.
The
early methods
3.6.1.1 The early
methods for design and evaluation of flexible pavements were experience-based
and theory-extended. They made use of index type tests to assess the strength
of the subgrade and commonly to also assess the strength or contributing strength
of base and sub-base layers. These were tests such as the CBR, plate bearing,
and many others, especially in highway design.
3.6.1.2 Early
methods for design and evaluation of rigid pavements virtually all made use of
the Westergaard model (elastic plate on a Winkler foundation) but included
various extensions to treat fatigue, ratio of design stress to ultimate stress,
strengthening effects of sub-base (or base) layers, etc. Westergaard developed
methods for two cases, loading at the centre of a pavement slab (width
unlimited) and loading at the edge of a slab (otherwise unlimited). While most
rigid pavement methods use the centre slab load condition, some use the edge
condition. These consider load transfer to the adjacent slab but means of treating
the transfer vary. Plate bearing tests are used to characterize subgrade (or
subgrade and sub-base) support which is an essential element of these design
methods. Here again the early methods, further developed, remained the primary
basis for aerodrome pavement design before the introduction of the linear
elastic analysis and finite element method (FEM). The method previously adopted
for ACN determination is an example of these methods.
The
newer - more fundamental - methods
3.6.1.3 Continuing
efforts to base pavement design on more fundamental principles has led to the
development of methods using the stress-strain response of materials and
rational theoretical models. The advances in computer technology have made
these previously intractable methods practical and led to computer-oriented
developments not otherwise possible.
3.6.1.4 The most
popular theoretical model for the newer design methods is the elastic layered
system. Layers are of finite thickness and infinite extent laterally except
that the lowest layer (subgrade) is also of infinite extent downward. Response
of each layer is characterized by its modulus of elasticity and Poisson's
ratio.
Values for these
parameters are variously determined by laboratory tests of several types, by
field tests of several types with correlations or calculated derivations, or
merely by estimating values where magnitudes are not critical. These methods
permit the stresses, strains, and deflections from imposed loads to be
computed. Multiple loads can be treated by superimposition of single loads.
Commonly, the magnitude of strain at critical points (top of subgrade beneath
load, bottom of surface layer, etc.) is correlated with intended pavement
performance for use in design or evaluation. While these methods have been
applied mostly to flexible pavements there have also been applications to
design of rigid pavements.
3.6.1.5 While the
elastic layered models are currently popular, it is recognized that the
stress-strain response of pavement materials is non-linear. The layering
permits variation of elastic modulus magnitude from layer to layer, but not
laterally within each layer. There are developments which establish a stress
dependence of the modulus of elasticity and use this dependence in finite
element models of the pavement, through iterative computational means, to
establish the effective modulus - element by element in the grid - and thereby
produce a more satisfactory model. Here also strains are calculated for
critical locations and compared with correlations to expected behaviour. Finite
element models are also being used to better model specific geometric aspects
of rigid pavements as has been incorporated in the FAA’s rigid pavement design
procedure.
Direct
load response methods
3.6.1.6 Theories
applied earlier to pavement behaviour indicated a proportionality between load
and deflection, thus implying that deflection should be an indicator of
capacity of a pavement to support load. This also implied that pavement
deflection determined for a particular applied load could be adjusted
proportionately to predict the deflection which would result from other loads.
These were a basis for pavement evaluation. Field verification both from
experience and research soon showed strong trends relating pavement behaviour
to load magnitude and deflection and led to the establishment of limiting deflections
for evaluation. There have since been many controlled tests and much carefully
analysed field experience which confirm a strong relation between pavement
deflection and the expected load repetitions (to failure) life of the pavement
subject to the load which caused that deflection. However, this relation,
though strong, is not well represented by a single line or curve. It is a
somewhat broad band within which many secondary factors appear to be impacting.
3.6.1.7 This
established strong relation has been and is being used as the basis for
pavement evaluation, but predominantly - until recently - applications have
been to flexible pavements. Methods based on static and dynamic plate loading
tests using plates up to 75 cm diameter for static and from 30 to 45 cm for
H/FWD dynamic tests have been used. One source for guidance on evaluation and
non-destructive testing using falling weight is presented in FAA Advisory Circular
150/5320-6 Airport Pavement Design and Evaluation, and [3]FAA
Advisory Circular 150/5370-11 Use of Non- destructive Testing in the Evaluation
of Airport Pavements.
3.6.1.8 Deflections
under actual wheel loads (or between the duals of two and four wheel gear) are
the basis of some expedient methods which closely parallel the plate methods.
The Benkelman Beam methods, as well as other highway methods, are applicable to
evaluation of pavements designed for light aircraft.
3.6.1.9 There are a
number of reasons why dynamic pavement loading equipment became popular. Static
plate loads of wheel load magnitude are neither transportable nor easily
repositioned. Dynamic loading applies a pulse load better simulating the pulse
induced by a passing wheel. But most important was the development of sensors
which could merely be positioned on the pavement or load plate and would
measure deflection (vertical displacement). H/FWDs with a falling mass apply
loads in excess the static mass and vary force magnitude by controlling the
height of fall. Pulses induced are repetitive but not steady, and the frequency
is that which is adjusted to the device and pavement combination. The dynamic
devices are applied in much the same manner as the static methods discussed in
3.6.1.7. They can also be used to generate data on the stress-strain response
of the pavement materials, as will be discussed later in this section.
Essential inputs to pavement design methods
3.6.1.10 The
parameters which define behaviour of elements (layers) of a particular pavement
within the model upon which its design is based vary from the CBR and other
index type tests of the earlier flexible pavement methods and plate load tests
of rigid pavement and some flexible pavement methods to the stress-strain, modulus
values employed in the newer more fundamental methods.
3.6.1.11 CBR
tests for determining the strengths of subgrades and of other unbound pavement
layers for use in design or evaluation should be as described in the particular
method employed (France, United States, other), but generally will be as
covered in ASTM D1883 or EN 13286, "Bearing Ratio of Laboratory Compacted
Soils for Laboratory Test Determinations". Commonly, field in-place CBR
tests are preferable to laboratory tests whenever possible, and such tests
should be conducted in accordance with the following guidance (from United
States Military Standard 621A).
Field in-place CBR tests
3.6.1.12 a)
These tests are used for design under any one of the following conditions:
1) when
the in-place density and water content are such that the degree of saturation
(percentage of voids filled with water) is 80 per cent or greater;
2) when
the material is coarse-grained and without cohesion so that it is not affected
by changes in water content; or
3) when
construction was completed several years before. In the last- named case, the
water content does not actually become constant but appears to fluctuate within
rather narrow ranges, and the field in-place test is considered a satisfactory
indicator of the load-carrying capacity. The time required for the water
content to become stabilized cannot be stated definitely, but the minimum time
is approximately three years.
b)
Penetration
Level the
surface to be tested, and remove all loose material. Then follow the procedure
described in ASTM D-1883.
c) Number
of tests
Three
in-place CBR tests should be performed at each elevation tested in the base
course and at the surface of the subgrade. However, if the results of the three
tests in any group do not show reasonable agreement, additional tests should be
made at the same location. A reasonable agreement between three tests where the
CBR is less than 10 permits a tolerance of 3; where the CBR is from 10 to 30, a
tolerance of 5; and where the CBR is from 30 to 60, a tolerance of 10. For CBRs
above 60, variations in the individual readings are not of particular
importance. For example, actual test results of 6, 8 and 9 are reasonable and
can be averaged as 8; results of 23, 18, and 20 are reasonable and can be averaged
as 20. If the first three tests do not fall within the specified tolerance,
three additional tests are made at the same location, and the numerical average
of the six tests is used as the CBR at that location.
d)
Moisture content and density
After completion
of the CBR test, a sample shall be obtained at the point of penetration for
moisture-content determination, and 10 to 15 cm away from the point of
penetration for density determination.
3.6.1.13 Plate
load tests for determination of the modulus of subgrade reaction (k) to be used
for evaluation or design of rigid pavements should be made in accordance with
procedures of the method employed, or can be as presented in ASTM D1196,
"Non- Repetitive Static Plate Load Tests of Soils and Flexible Pavement
Components, for use in Evaluation and Design of Airport and Highway
Pavements" or in ASTM D1195, "Repetitive Static Plate Load Tests of
Soils and Flexible Pavement Components, for Use in Evaluation and Design of
Airport and Highway Pavements". The procedures also relate to flexible
pavement design, as indicated by ASTM standards’ titles.
3.6.1.14 Conventional
methods and values pertaining to determination of modulus of elasticity, E, and
Poisson’s ratio, ν, are used in depicting structural behaviour of the concrete
layer in analyses of rigid pavement. Commonly, ν is taken to be 0.15. The
modulus, E, should be determined by test of the concrete and will normally be
in the range of 25 000 to 30 000 MPa.
3.6.1.15 Modulus
of elasticity and Poisson’s ratio values are needed for each layer of an
elastic layered system and these can be determined in a variety of ways.
Poisson’s ratio is not a sensitive parameter and is commonly taken to be 0.3 to
0.33 for aggregate materials and 0.4 to 0.5 for fine grained or plastic
materials. Since means of determining modulus of elasticity vary and because
the stress-strain response of soil and aggregate materials is non-linear (not
proportional), the values found for a particular material, by the various
means, are not the same singular quantity which ideal theoretical
considerations would lead one to expect. Following are some of the ways in
which modulus of elasticity values can be determined for use in theoretical
models (such as elastic layered) of pavement behaviour.
a) Modulus
of elasticity values for subgrade materials particularly, but for other
pavement layers as well - excepting bituminous or cemented materials - can be
determined from correlations with index type strength tests. Most common has
been correlation with CBR where: E = 10 CBR MPa.
b) Stress-strain
response (modulus) can be determined by direct test of prepared or field
sampled specimens, but these are nearly always unsatisfactory. Response is too
greatly affected by either preparation or sampling disturbance to be
representative.
c) It has
been found that prepared specimens, and in some cases specimens from field
samples, can be subjected to repeated loading to provide - after several to
many load cycles - a reasonably representative modulus or stress-strain
response curve. Modulus of elasticity so determined is referred to as resilient
modulus and is currently strongly favoured - in some form - for layered elastic
analyses. Tests can be conducted as triaxial tests, indirect tensile tests,
even unconfined compression tests, and there may be others. Loadings can be
regular wave forms (sinusoidal, etc.) but are commonly of a selected load pulse
shape with delays between pulses to better represent passing wheels. Resilient
modulus can be determined for bituminous materials by some of these tests and
other similar tests, but temperature is most significant both for testing and
application of the modulus for bituminous layers. Moduli for the various
pavement layers are taken from these type tests and used directly in layered system
analyses, but there are frequently problems or questions of validity.
d) When dynamic
plate load testing is carried out on existing pavements it is possible to
instrument to measure the velocity of propagation of stress waves within the
pavements. Means have been developed for deducing the modulus of elasticity of
each layer - generally excepting the top layer or layers - of the pavement from
these velocity measurements. While moduli so determined are sometimes used
directly in layered analyses the determinations are for such small strains that
values represent tangent moduli for curved stress-strain relations while the
moduli for higher (working strain) stress levels should be lower.
Determinations by this means adjusted by judgement or some established
analytical means are used.
e) The subgrade
modulus is the most significant parameter and some analyses use one of the
above methods to determine a modulus for the subgrade and choose the moduli of
other layers either directly on a judgement basis or by some simple numerical
process (such as twice the underlying layer modulus or one-half the overlying
layer modulus) since precise values are not critical.
f) By
using selected or simplistically derived moduli for all layers except the
subgrade, it is possible to compute a value for subgrade modulus using elastic
layered analysis and plate or wheel load deflections. This is done for some
analyses.
g) There
is great interest currently in using elastic layered theory and using field
determined deflections from dynamic load pavement tests for points beneath the
centre of load and at several off-set positions from the load centre. By
iterative computer means the moduli of the subgrade and several overlying
layers can be computed. Such computed moduli are then used in the layered model
to compute strains at critical locations as predictors of pavement performance.
3.6.1.16 Finite
element methods permit formulation of pavement models which not only can
provide for layering but can treat non-linear (curved) stress-strain responses
found for most pavement materials. Here again there is a requirement for
Poisson’s ratios and moduli of elasticity but these must now be determined for
each pavement layer as a function of the load or stress condition existing at
any point in the model (on any finite element). Moduli relations are
established from laboratory tests and most commonly by repeated triaxial load
tests. Generally, these are of the following form but there are variants.
3.6.2 Evaluation by inversion of
design
To design
a pavement one must select a design method. Then determine the thicknesses and
acceptable characteristics of materials for each layer and the wearing surface
taking into account the subgrade upon which the pavement will rest and the
magnitude and intensity of traffic loading which must be supported. For
evaluation, the process must be inverted since the pavement is already in
existence. Character of the subgrade and thickness and character of each
structural layer including the surfacing must be established, from which the
maximum allowable magnitude and frequency of allowable aircraft loading can be
determined by using a chosen design method in reverse. It is not necessary that
the design method selected for evaluation be the method by which the pavement
was designed, but the essential parameters, which characterize behaviour of the
various materials (layers) must be those which the chosen design method
employed.
The method and elements of design
3.6.2.1 The
design method to be inverted for evaluation must first be chosen. Next the
elements of design inherent in the existing pavement must be evaluated in
accordance with the selected design method.
a) Thickness
of each layer must be determined. This may be possible from construction
records or may require the drilling of core holes or opening of test pits to
permit measuring thickness.
b)
Subgrade strength and character must be determined. Here also construction
records may supply the needed information either directly or by a translation
of the information to the form needed for the selected design method. Otherwise
it will be necessary to obtain the needed information from field studies.
Reference to 3.6.1.9 to 3.6.1.14 will show the wide variety of ways in which
subgrade behaviour is treated in the various design methods. Test pits may be
necessary to permit penetration or plate testing or sampling of subgrade
material for laboratory testing. Sampling or penetration testing in core holes
may be possible. Dynamic or static surface load-deflection or wave propagation
testing may be required. Specific guidance must be gained from details of the
design method chosen for use in evaluation.
c) The
strength and character of layers between the subgrade and surface must also be
determined. Problems are much the same as for the subgrade (see b) above) and
guidance must come from the chosen design method.
d) Most
procedures for the design of rigid pavements require a modulus of elasticity
and limiting flexural stress for the concrete. If these are not available from
construction records they should be determined by test on specimens extracted
from the pavement (see ASTM C469 - modulus of elasticity and ASTM C683 -
flexural strength). For reinforced or pre- stressed concrete layers dependence
must be placed on details of the individual selected design method.
e) Bituminous
surfacing (or overlay) layers must be characterized to suit the selected design
method and to permit determination of any tire pressure limitation which might
apply. Construction records may provide the needed information otherwise
testing will be required. Pavement temperature data may be required to help
assess the stress-strain response or tire pressure effects on the bituminous
layer.
f) Any
special consideration of frost effects by the selected design method or for the
climate of the area need to be treated and the impact upon the evaluation
determined.
g) The
load repetition factor to which the pavement is subject is an important element
of design and both past traffic sustained and future traffic expected may be
factors in evaluation. See 3.4 in relation to assessing traffic. For some
design methods it is sufficient to consider that the traffic being sustained
adequately represents future traffic and the limiting load established by
evaluation is for this intensity of traffic. This assumption is inherent in the
translations between aircraft mass and ACR (or the reverse) of the ACR-PCR
method.
Many
methods, however, require a load or stress repetitions magnitude as a basis for
selection of a limiting deflection or strain which is needed for load limit
evaluation.
From the
chosen design method and established quantities for the design elements,
limiting load or mass can be established for any aircraft expected to use the
pavement.
3.6.3 Direct or non-destructive
evaluation
Direct
evaluation involves loading a pavement, measuring its response (usually in
terms of deflection under the load and sometimes also at points offset from the
load to show deflection basin shape) and inferring expected load support
capacity from the measurements. Concepts were discussed in 3.6.1.6, 3.6.1.7 and
3.6.1.8
Static methods
3.6.3.1 Static
methods involve positioning plates or wheels, applying load and measuring
deflections. Plate loads require a reaction against which to work in applying
load while wheels can be rolled into position and then away. These direct
methods depend on a correlation between pavement performance and deflection
resulting from loading of the type indicated in Figure 3-3. A warning comment
may be needed here, since such correlations can be misinterpreted. They do not
indicate the deflection which will be measured under the load after it has been
applied for some number of repetitions as might be interpreted. Deflections of
a pavement are essentially the same when measured early or late (following
initial adjustment and before terminal deterioration) in its life. These
correlations indicate the number of repetitions that can be applied to the
pavement by the load which caused the deflection before failure of the
pavement. Correlations are established by measuring the deflections of satisfactory
pavements and establishing their traffic history. The expeditious deflection
methods for evaluation described below are a good example of static methods.
Expeditious deflection methods
3.6.3.2 Studies
and observations by many researchers have shown a strong general correlation
between the deflection of a pavement under a wheel load and the number of
traffic applications (repetitions) of that wheel load which will result in
severe deterioration (failure) of the pavement (see Figure 3-3). These provide
the basis for a simple expeditious means of evaluating pavement strength. Few
references from studies and observations conducted in the early 1970s may still
be possible to locate; but since the practice of measuring deflections under
the landing gear of an actual aircraft towed on the pavement has practically
disappeared with the advent of falling weight deflectometers in pavement
testing, references to these more recent studies and observations typically
discuss correlations relative to dynamic methods of measurement (see 3.6.3.8).
3.6.3.3 While
the pattern of these relations is quite strong, the scatter of specific points
is considerable. Thus either the conservatisms of a limiting curve or the low
confidence engendered by the broad scatter of points or some combination must
be accepted in using these relations for expeditious pavement evaluations. They
do provide a simple relatively inexpensive means of evaluation. The procedure
for such evaluation is as follows:
a) measure
deflection under a substantial wheel load in a selected critical pavement
location. Single or multiple wheel configurations can be used;
1)
position aircraft wheel in critical area;
2) mark
points along pavement for measurement as indicated in Figure 3-4 a);
3) using
“line of sight” method, take rod readings at each point;
4) move
aircraft away and repeat rod readings;
5) plot
difference in rod readings as deflections as illustrated in Figure 3-4 b);
6)
connect points to gain an estimate of maximum deflection beneath tire;
b) plot
load versus maximum deflection as illustrated in Figure 3-4 c);
c) combine
the deflection versus failure repetitions curve with the above curve to provide
an evaluation of pavement bearing strength for the gear used to determine
deflection;
1)
determine the repetitions of load (or equivalent repetitions as explained in
3.4) which it is intended must use the pavement before failure;
2) from a
correlation of the type shown in Figure 3-3 determine the deflection for the
repetitions to failure;
3) from
the established relation of load to deflection of the type shown in Figure 3-4
determine the pavement bearing strength in terms of the magnitude of load
allowable on the wheel used for the deflection measurements; and
d) use
the procedure described in Chapter 1 to find how the evaluated pavement bearing
strength relates to the PCR. Aircraft with ACR no greater than this PCR can use
the pavement without overloading it. See ICAO-ACR computer programme for ACR
versus mass information.
3.6.3.4 A
similar procedure can be followed using a jack and loading plate working
beneath a jacking point of an aircraft wing or some equally suitable reaction
load. The complete pattern of load versus deflection can be determined and dial
gauges mounted on a long reference beam can be used instead of optical survey
methods. With provision of a suitable access aperture the deflection directly
beneath the centre of the load can be measured. Results can be treated on the
same lines as those for a single wheel load.
3.6.3.5 Methods
used for highway load deflection measurements, such as the Benkelman Beam
methods, can be used to develop deflection versus load patterns. Results are
treated as indicated in Figure 3-4 to extrapolate loads to those of aircraft
single-wheel loads, which with a relation as in Figure 3-3, permits evaluation
of pavement bearing strength for single-wheel loads. From this the limiting
aircraft mass on pavements for light aircraft can be determined directly and
reported in accordance with Chapter 1, 1.2. If unusually large loading plate or
tire pressures are involved it may be necessary to adjust between the single
load characteristics used in the determination of the type indicated in Figure
3-4 (3.6.3.3 a)) and the reported limiting aircraft mass allowable or critical
vehicle loads being compared to the limiting mass. Such adjustments can follow
the procedures in any selected pavement design method. Limits on pavements for
heavier aircraft can be determined as indicated in 3.6.3.3 d). It should be
noted that extrapolation of load deflection relations (as in Figure 3-4 c))
from small load data taken on high strength pavements do not give good results.
Unfortunately, the limits of extrapolation for good results are not
established.
Dynamic methods
3.6.3.6 These
methods involve a dynamic loading device which is mounted for travel on a
vehicle or trailer and which is lowered, in position, onto the pavement.
Devices make use of counter rotating masses, hydraulically actuated
reciprocating masses, or falling weights (masses) to apply a series of pulses
either in steady state by the reciprocating or rotating masses or attenuating
by the falling mass. Most apply the load through a loading plate but some
smaller devices use rigid wheels or pads. All methods make use of inertial
instruments (sensors) which when placed on the pavement surface or on the
loading plate can measure vertical displacement (deflection). The dynamic
loading is determined, usually by a load cell through which the load is passed
on to the load plate. Comparison of the load applied and displacements measured
provides load-deflection relations for the pavement tested. Displacements are
always measured directly under the load but are also measured at several
additional points at specific distances from the centre of the load. Thus
load-deflection relations are determined not only for the load axis (point of
maximum deflection) but also at offset points which indicate the curvature or
shape (slope) of the deflection basin. The devices vary in size from some highly
developed, highway oriented, units which apply loadings of less than 1 000 kg
to the large unit described in the United States FAA non-destructive test
method referenced in 3.6.1.7. Some of the counter-rotating and reciprocating
mass systems can vary the frequency of dynamic loading and some of these and
the falling weight units can vary the applied load.
3.6.3.7 It
is possible to measure the time for stress waves induced by the dynamic loading
to travel from one sensor to the next, and to compute the velocity from this
time and distance between sensors. Some dynamic methods make use of these
velocity measurements to evaluate the strength or stress-strain response of the
subgrade and overlying pavement layers for use in various design methods. Shear
wave velocity, v, is related to modulus of elasticity, E, by the relation:
Where
Poisson’s ratio, ν, can satisfactorily be estimated (see 3.6.1.13 and
3.6.1.14), and volumic mass, ρ, of the subgrade or pavement layer (sub-base to
base) can be determined by measurement or satisfactorily estimated. Modulus
values thus determined are used, either directly or with modification, in
theoretical design models, or they are used with correlations to project
subgrade and other layer strengths in terms of CBR, subgrade coefficient k, and
similar strength index quantities. Sensors used in the velocity measurements
may need to be located at greater distances from the load than when used to
determine deflection basin shape. Also, the dynamic device must be capable of
frequency variation since the various pavement layers respond at preferred frequencies,
these must be found and dynamic load energy induced at the preferred frequency
for determination of each layer’s velocity of wave energy propagation.
Application of dynamic methods measurements
3.6.3.8 Falling
weight deflectometers (FWD) and heavy weight deflectometers (HWD) provide an
application for determining areas of a pavement system with consistent response
to load. H/FWDs are quick to use, relatively economical, and have become widely
available. Note that some will offer “PCR” or other type results directly from
the deflection data and a nominal pavement structure; however, the reliability
of this practice is questionable due to the significantly different modulus
assigned to each layer as a result of only minor differences in deflections. The
central and offset positions deflections and stress-wave velocities variously
determined by the variety of dynamic equipment and methods in use are being
applied for pavement evaluation in a number of ways:
a) Direct
correlations are made between the load-deflection in response of pavement to
dynamic loading and pavement behaviour. The correlations are developed from
dynamic load testing of pavements for which behaviour can be established.
b)
Measurements from dynamic methods, either directly or with extrapolation, can
provide plate load information. This can serve as input - with suitable plate
size or other conversions - to various methods. Used directly on subgrades or
on other layers with established correlations subgrade coefficients they can be
determined for rigid pavement analyses.
c) Shape
of the deflection basin established from sensors placed at offsets from the
load axis are used in some methods to reflect over-all stiffness, and thereby
load distributing character, of the pavement structure. But direct use in
establishing evaluation of load capacity has not found success.
d)
Measured deflection under dynamic load is used to establish the effective
modulus of elasticity of the subgrade in theoretical pavement models. The
elastic constants (modulus and Poisson’s ratio) for other layers are
established by assumption or test and the subgrade modulus calculated using the
load, the deflection measured and the pavement model, commonly the elastic
layered theory.
e) More
recent developments involve the use of the elastic layered computer programmes.
With an appropriate load applied, deflections are measured in the centre and at
several offset locations. Then iterative computation means are used to
establish elastic moduli for all layers of the pavement modelled.
f) Theoretical
models with elastic constants as in d) and e) above are used to calculate
strain in flexure of the top layer beneath the load or vertical strain at the
top of subgrade beneath the load, which locations are considered critical for flexible
pavements. Stress or strain in flexure of a rigid pavement slab can be
similarly calculated. These are compared to values of strain (or stress) from
established correlations with pavement performance. Examples of these
correlations have been documented since the late 1970s and can be found in
pavement literature from the last 20 years.
g)
Stress-wave velocity measurements are used to establish pavement layer
characteristics without sampling. Moduli of elasticity of pavement layers are
derived from these measurements and used directly in theoretical models or
adjusted to better represent moduli at larger strains and used in the models.
CBR values are derived from correlations between CBR and derived elastic
moduli, commonly from E=10 CBR, in MPa. Modulus of subgrade reaction, k, and
other such strength values could be similarly derived.
CHAPTER 4
STATE PRACTICES FOR DESIGN AND
EVALUATION OF PAVEMENTS
Note.
This chapter will be updated as individual States update their guidance
regarding pavement evaluation during the implementation period of the ACR-PCR
pavement strength reporting protocol.
4.1 PURPOSE
4.1.1 Aerodrome
pavements are designed and constructed to provide adequate support for the
loads imposed by aircraft and to produce a firm, stable, smooth, year-round,
all- weather surface free of debris or other particles that can be blown or
picked up by propeller wash or jet blast. To fulfil these requirements, the
quality and thickness of the pavement must not fail under the imposed loads.
The pavement must also possess sufficient inherent stability to withstand,
without damage, the abrasive action of traffic, adverse weather conditions, and
other deteriorating influences. This requires coordination of many design
factors, construction, and inspection to assure the best combination of
available materials and workmanship.
4.1.2 The
purpose of this chapter is to provide various State practices for pavement
design and pavement evaluation; and the reporting of pavement strength.
4.2 PRACTICE OF FRANCE
4.2.1 General
All the documents and
guidance quoted in this section are available at the Service technique de
l’Aviation civile (DGAC/STAC) website
https://www.stac.aviation-civile.gouv.fr/fr/publications, as well as other
pavement design information, documentation, and software at www.stac.aviation-civile.gouv.fr.
4.2.2 Pavement
design
4.2.2.1 It
is well agreed among the international airfield pavement community that
thickness design methods based only on empirical considerations have shown
their limitations. Therefore, practices of France - as in many other States -
have moved towards more sophisticated and rational tools than the CBR design
procedure for flexible pavements and PCA method for rigid pavements (Portland
Cement Association).
4.2.2.2 These
new structural design tools include accurate descriptions of pavement structure
layers in terms of thickness, material properties and binding conditions
between layers. Intrinsic mechanical properties of pavement materials are used
for both bituminous and cement treated mixtures according to standards of
Europe or France. Moreover, variability and evolution of aircraft landing gears
is addressed in the pavement design process through a full description of
aircraft wheel gears (geometry, load, tire pressure).
4.2.2.3 This
type of methodology is available for flexible pavement structures in the
technical guidance “Rational design method for flexible airfield pavements -
STAC”, associated to the Alizé-Airfield Pavement software for its application.
Such guidance is also being developed for the design of rigid pavements and
overlays. Meanwhile, in the transition period, the CBR and PCA procedures
adapted to the context of France are still in use, as described in the
“Instruction sur le dimensionnement des chaussées d’aérodrome et la
détermination des charges admissibles - STBA” (Circular on Airfield Pavement
Structural Design) and available for the application of the current ICAO
ACN-PCN reporting method.
4.2.3 Pavement
evaluation
4.2.3.1 Evaluating
the condition of existing pavements is a necessary task for airfield pavement
managers so as to define rehabilitation programmes as accurately as possible.
The civil aviation authority, Direction Générale de l'Aviation Civile (DGAC),
provides a methodology for assessing airfield pavement condition based on
visual surveys, allowing determining the “Indice de service”, IS of the
pavement, ranging from 0 (out of order) to 100 (no damage). Two sub-indices
enable quantifying the surface condition as well as the structural condition of
the pavement. These indices are associated to threshold values defined so as to
provide pavement managers with guidance on considering rehabilitation as soon
as possible, in a near future, or at a later time. The complete methodology is
described in the guide “Méthode indice de service - STBA” (Pavement condition
index method).
4.2.3.2 The
use of heavy weight deflectometer (HWD) for structural evaluation of pavements
is the trend that is fostered by the DGAC as well as pavement managers. Indeed,
this equipment associated to an appropriate analysis tool gives an accurate
description of the structural pavement condition. The “Guide to the
evaluation of flexible airfield pavements with an HWD” provides guidance
for the evaluation of flexible pavements. Similar guidance is under development
for rigid pavements.
4.2.4 Reporting
pavement strength
Note.
This chapter will be further updated by the relevant States to in accordance
with the implementation of ACR-PCR.
4.3 PRACTICE OF THE UNITED
KINGDOM
4.3.1 Pavement
design and evaluation
4.3.1.1 It
is the practice in the United Kingdom to design for unlimited operational use
by a given aircraft taking into account the loading resulting from interaction
of adjacent landing gear wheel assemblies where applicable. The aircraft is
designated "the design aircraft" for the pavement. The support
strength classification of the pavement is represented by the design aircraft's
pavement classification number identifying its level of loading severity. All
other aircraft ranked by the United Kingdom standards as less severe may
anticipate unlimited use of the pavement though the final decision rests with
the aerodrome authority.
4.3.1.2 While
there are now available a number of computer programmes based on plate theory,
multilayer elastic theory and finite element analysis, for those wishing to
have readily available tabulated data for pavement design and evaluation, the
Reference Construction Classification (RCC) system has been developed from the
British Load Classification Number (LCN) and Load Classification Croup (LCG) systems.
Pavements are identified as dividing broadly into rigid or flexible
construction and analysed accordingly.
4.3.1.3 For
the reaction of aircraft on rigid pavements, a simple two-layer model is
adopted. To establish an aircraft's theoretical depth of reference construction
on a range of subgrade support values equating to the ICAO ACN/PCN reporting
method, the model is analysed by Westergaard centre case theory. Account is
taken of the effect of adjacent landing gear wheel assemblies up to a distance
equal to three times the radius of relative stiffness. This is considered
essential in any new system in view of the increasing mass of aircraft,
complexity of landing gear layouts and the possible interaction of adjacent
wheel assemblies on poor subgrades especially.
4.3.1.4 To
resolve practical design and evaluation problems, a range of equivalency
factors appropriate to the relative strengths of indigenous construction
materials is adopted to convert between theoretical model reference
construction depths and actual pavement thickness.
4.3.1.5 Aircraft
reaction on flexible pavements follows the same basic pattern adopted for rigid
pavement design and evaluation. In this case a four pavement model is analysed
using the United States Corps of Engineers' development of the California
Bearing Ratio (CBR) method. This includes Boussinesq deflection factors and
takes into account interaction between adjacent landing gear wheel assemblies
up to 20 radii distance. Practical design and evaluation problems are resolved
using equivalency factors to relate materials and layer thicknesses to the
theoretical model on which the reference construction depths for aircraft are
assessed.
4.3.2 Reporting
pavement strength
4.3.2.1 The
ICAO ACN/PCN reporting method for aircraft pavements described in Annex 14,
2.6. The critical aircraft is identified as the one which imposes a severity of
loading condition closest to the maximum permitted on a given pavement for unlimited
operational use. Using the critical aircraft's ACN individual aerodrome
authorities decide on the PCN to be published for the pavement concerned.
4.3.2.2 Though
not revealed by the ICAO ACN/PCN reporting method, when interaction between
adjacent landing gear wheel assemblies affects the level of loading imposed by an
aircraft, United Kingdom aerodrome authorities may impose restrictions on
operations by a mass limitation or a reduction in the number of permitted
movements. This is unlikely to occur, however, with aircraft currently in
operational use except where subgrade support values are poor.
4.4 PRACTICE OF THE UNITED
STATES
4.4.1 General
All the documents and
guidance quoted in this section and relative to airport pavement design and
construction are available through the FAA website
http://www.faa.gov/airports/engineering/pavement_design and airport design
software at https://www.faa.gov/airports/engineering/design_software/.
4.4.2 Pavement
design
4.4.2.1 Pavement
design guidance is presented in FAA Advisory Circular (AC)
150/5320-6
Airport Pavement Design and Evaluation[4]. Design practice implements layered
elastic theory for flexible pavement design and three-dimensional finite
element theory for rigid pavement design. The FAA adopted these methodologies
to address the impact of landing gear configurations and increased pavement
load conditions on airport pavements. These procedures are robust and can
address future gear configurations without modifying their underlying design
procedures. The failure curves have been calibrated with full scale pavement
tests at the FAA National Airport Pavement Test Facility (NAPTF).
4.4.2.2 The
design methods are computationally intense, so the FAA developed a computer
programme called FAARFIELD to help pavement engineers implement it. FAARFIELD may
be downloaded at no cost from http://www.airporttech.tc.faa.gov/Products/Airport-Pavement-Software-Programs.
4.4.3 Pavement
evaluation
4.4.3.1 Pavement
evaluation guidance is presented in FAA AC 150/5320-6 Airport Pavement Design
and Evaluation[5]. Airport pavement evaluations are
used to assess the ability of an existing pavement to support different types,
weights, or volumes of aircraft traffic. The load carrying capacity of existing
bridges, culverts, storm drains, and other structures are to be considered in
these evaluations. Evaluations may also assist to determine the condition of
existing pavements for use in the planning or design of improvements to the
airport.
4.4.3.2 Evaluation
procedures are essentially the reverse of design procedures. This AC covers the
evaluation of pavements for all weights of aircrafts.
4.4.4 Reporting
pavement strength
4.4.4.1 Guidance
for reporting pavement strength is presented in FAA AC 150/5335-5 Standardized
Method of Reporting Airport Pavement Strength - PCN[6]. The
AC provides guidance for use of the standardized method of reporting pavement
strength which applies only to pavements with bearing strengths of 12,500
pounds (5 700 kg) or greater. Determination of the numerical PCN value for a
particular pavement can be based upon one of two procedures, the “Using”
aircraft method or the “Technical” evaluation method. Guidance on both methods
is provided and either may be used to determine a PCN, but the methodology used
must be reported as part of the posted rating. The posted rating of the PCN
system uses the coded format described in Annex 14, Volume I, 2.6.
CHAPTER 5
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CHAPTER 6
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CHAPTER 7
CONSIDERATIONS FOR CULVERTS,
BRIDGES AND OTHER STRUCTURES
7.1 PURPOSE
7.1.1 The
purpose of this chapter is to provide basic information and guidance for design
and concerns of aerodrome structures. Aerodrome structures such as culverts and
bridges are usually designed to last for the foreseeable future of the
aerodrome. Information concerning the landing gear arrangement of future heavy
aircraft is speculative, however it may be assumed with sufficient confidence
that strengthening of pavements to accommodate future aircraft can be performed
without undue problems. Strengthening of structures, however, may prove to be
extremely difficult, costly, and time-consuming.
7.2 GENERAL
7.2.1 Structures
for drainage or access commonly cross pavements which support aircraft. Such
facilities are subject to the added direct loading imposed by the aircraft,
where point loadings may be increased; such as on bridges, overpasses, and
subsurface terminal facilities where the entire aircraft weight may be imposed
on a deck span, pier, or footing. However, more often, loading is indirectly
transmitted to culverts and buried pipes through the soil layer beneath the
pavement. These subsurface structures must be considered in connection both
when designing and with evaluation of pavement strength. The patterns of
stresses induced by surface wheel loads as they are transmitted downward are
not the same on the subsurface structures as on the subgrade. This is not only
because these structures are not at subgrade level but also because the
presence of the structure distorts the patterns.
7.2.2 In
the design of new facilities care must be given to the structural adequacy of
pipes, culverts, and bridged crossings, not only for the contemplated design
loadings but for possible future loadings to avoid a need for very costly
corrective treatments made necessary by a growth in aircraft loadings.
7.3 DESIGN CONSIDERATIONS
7.3.1 For
many structures the design is highly dependent upon the aircraft landing gear
configuration therefore design should be for the largest aircraft at maximum
gross weight that could use the aerodrome over the life of the aerodrome.
Consider all loading conditions, both dead and live loads, similar to those
used by State and local Load and Resistance Factor Design methodology and programmes;
such as the AASHTO LFRD used in the United States or the Structural Eurocodes
(standards EN-1990 through EN-1999) used in
Europe.
Suggested design parameters are provided in the following paragraphs.
Foundation Design
7.3.2 Foundation
design will vary with soil type and depth. No departure from accepted
methodology should be anticipated; except that for shallow structures, such as
inlets and culverts, the concentrated loads may require heavier and wider
spread footings than those presently provided by the structural standards in
current use. For subsurface/buried structures, such as culverts, the following
guidance is recommended:
a) When
the depth of fill is less than 0.6 m (2 ft), the wheel loads will be treated as
concentrate loads.
b) When
the depth of fill is 0.6 m (2 ft) or more, wheel loads should be considered as
uniformly distributed over a square with sides equal to 1.75 times the depth of
the fill. When such areas from several concentrations overlap, the total load
should be uniformly distributed over the area defined by the outside limits of
the individual areas, but the total width of distribution should not exceed the
total width of the supporting slab.
c) For
maximum wheel loads exceeding 11 400 kg (25 000 lbs), perform a structural analysis
to determine the distribution of wheel loads at the top of the buried
structure. Consider the maximum wheel loads, tire pressures and gear
configuration that will act on top of the buried structure. The load
distributions may be assumed conservatively in lieu of performing a detailed
structural analysis.
Loads
7.3.3 All
loads are to be considered as dead load plus live loads. The design of
structures subject to direct wheel loads should also anticipate braking loads
as high as 0.7 G (for no-slip brakes).
Direct Loading
7.3.4 Direct
Loading. Decks and covers subject to direct heavy aircraft loadings such as
manhole covers, inlet grates, utility tunnel roofs, bridges, etc., should be
designed for the following loadings:
a) Manhole
covers for 45 000 kg (100 000 lb) wheel loads with 1.72 MPa (250 psi) tire
pressure. Higher tire pressures should be assumed if using aircraft will be
greater than 1.72 MPa (250 psi).
b) For
spans of 0.6 m (2 ft) or less in the least direction, a uniform live load of
the larger of 1.72 MPa (250 psi) or the maximum tire pressure assumed for
manhole cover design.
c) For
spans of 0.6 m (2 ft) or greater in the least direction, the design will be
based on the number of wheels which will fit the span. Design for the maximum
wheel load anticipated.
7.3.5 Special
consideration should be given to structures that will be required to support
both in-line and diagonal traffic lanes, such as diagonal taxiways or apron
taxi routes. If structures require expansion joints, load transfer may not be
possible.
CHAPTER 8
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CHAPTER 9
STRUCTURAL CRITERIA FOR NATURAL
GROUND
9.1 INTRODUCTION
9.1.1 In
some cases physical attributes of an aerodrome may not be paved but must still
be capable of supporting the occasional passage of an aircraft. The natural
ground in these instances may not have sufficient bearing strength to handle
the aircraft, and therefore special preparation may be necessary. Adequate
strength is required in order to ensure that no structural damage is sustained
by an aircraft veering off onto the unpaved surface. The unpaved surface must
also be capable of supporting any ground vehicles that may occasionally operate
on the area.
9.1.2 The
guidance provided in this section is geared towards the physical attributes
most commonly left unpaved at an aerodrome. Specifically these are, runway and
taxiway shoulders, runway end safety areas (RESAs) and runway strips outside
the runway-shoulder area. The guidance does not apply to unpaved runways
themselves, since the strength requirements for a runway are much more
stringent.
9.1.3 For
any unpaved surface the ingestion or jet blast of foreign object debris by
aircraft turbine engines is an important consideration. The protection of the
surface to ensure no loose material is allowed is the responsibility of the
aerodrome. Some type of chemical treatment or the use of turf may be required
for the unpaved surface, along with visual inspections, to ensure that foreign
object debris is not present.
9.1.4 Attention
is drawn to the fact that any bearing strength related guidance provided in
this chapter should in no way be interpreted as a design requirement. Such
guidance is only to support the judgement of the engineer when no specific data
is available.
9.2 DESIGN BACKGROUND
9.2.1 In
order to design for an unpaved area several parameters must be known. The type
of aircraft expected to operate at the aerodrome must be known since the
equivalent single wheel load of the main landing gear wheel arrangement is
critical as well as the tire pressure of the main gear tires. Knowing the
expected aircraft coverages along with the aforementioned aircraft parameters
will allow for the natural soil CBR required to support the aircraft loads
without failure to be determined.
9.2.2 In
most cases the natural soil CBR is not sufficient to handle the larger aircraft
wheel loads. The design methodology assumes a single layer of high quality
granular cover material can be added on top of the natural soil of low CBR in
order to support the aircraft. The minimum CBR required for the granular cover
layer is CBR 20. This CBR is typical for most granular sub-base materials used
in flexible pavement construction and should be readily available. If the
existing grade of the unpaved area needs to be maintained, the required
thickness of low CBR natural soil can be removed and replaced by the high CBR
cover layer. It is recommended to verify the natural soil CBR up to at least a
depth of 60 cm.
9.3 DESIGN DETAILS
9.3.1 The
design methodology is based on previous work done by the United States Corps of
Engineers, Waterways Experiment Station, for semi-prepared military airfields.
A reformulation of the original CBR equation used in the design of flexible
pavements can be used for determining the thickness of the required cover layer
needed to support the natural soil in the unpaved area. The equation is:
9.3.2 The
design methodology is conservatively assumed to be ten passes of an aircraft.
The failure criteria is three inches of rutting (8 cm), but is based on rolling
loads without aircraft braking effects. The ten pass level was set to allow a
slightly stronger cover layer to counteract the aircraft braking loads. If
heavy braking were to occur then the expected rutting would be higher than the
8 cm failure criteria. However, an unpaved surface can be easily repaired by
regrading and recompacting of the disturbed surface if required.
9.3.3 In
order to provide broad technical guidance for aerodromes in the design of
unpaved areas, a method similar to the PCN system was adopted. The same four
subgrade categories used in the PCN system were adopted since these are typical
for most natural soil conditions to be expected at aerodromes worldwide.
Additionally, three aircraft categories were assumed to be representative of
the traffic that exists at the majority of aerodromes and are noted as follows:
Group 1:
Regional aircraft with less than 13 600 kg wheel loads
Group 2:
Narrow Body aircraft with wheel loads between 13 600 to 20 410 kg
Group 3:
Wide-body aircraft with wheel loads greater than 22 680 kg
9.3.4 The
cover thicknesses noted in Table 9-1 are that of the layer of CBR 20 material
or higher needed to protect the subgrade type listed. For natural soil
conditions with CBR values that fall between the four subgrade types, use
linear interpolation to arrive at the required cover thickness. A layer of
seeded top soil could be placed on top of the cover layer where needed to
provide protection against erosion and foreign object debris (FOD) risk.
9.4 GUIDANCE FOR BEARING
STRENGTH OF PREPARED NATURAL GROUND AREAS
Runway
and taxiway shoulders
9.4.1 The
purpose of the shoulder is to provide adequate strength in order to support an
aircraft in the event of an aircraft veer-off. The design guidance provided in
table 9-1 is sufficient for the design of an unpaved shoulder.
Runway
end safety areas
9.4.2 The
purpose of the RESA is to provide adequate strength in the event of an aircraft
undershooting or overrunning the runway. If there is a need to provide a better
resistance and facilitate aircraft deceleration, adding a layer of lesser
strength granular material may be an option. However, consideration should be
given to the preparation and maintenance of such a granular layer.
Runway
strips
9.4.3 The
graded runway strip out to the shoulder should be of adequate strength to
provide drag to an aircraft and facilitate deceleration in the event of an
aircraft leaving the runway. The upper surface may be 15 cm of lesser strength
material and the underlying natural soil of adequate strength to support the
aircraft for one pass, such that structural damage does not occur.
APPENDIX 1
AIRCRAFT CHARACTERISTICS
AFFECTING PAVEMENT BEARING STRENGTH
1. GENERAL
1.1 This
appendix describes those characteristics of aircraft which affect pavement
strength design, namely: aircraft weight; percentage load on nose wheel; wheel
arrangement; main leg load; tire pressure; and contact area of each tire. Table
A1-1 contains these data for most of the commonly used aircraft.
1.2 Aircraft
loads are transmitted to the pavement through the landing gear which normally
consists of two main legs and an auxiliary leg, the latter being either near
the nose (now the most frequent arrangement) or near the tail (older system).
1.3 The
portion of the load imposed by each leg will depend on the position of the CG
with reference to the three supporting points. The static distribution of the
load by the different legs of a common tricycle landing gear may be illustrated
as follows:
Where W
is the aircraft weight; P1 the load transmitted by the auxiliary leg; P2
the load transmitted by both main legs; L1 and L2 the distance measured along
the plane of symmetry from the CG to P1 and P2 respectively,
Then:
W = P1
+ P2
P1L1
= P2L2
Therefore:
1.4 The
ratio L1/L2 is usually around 9 (i.e. the auxiliary leg accounts for
approximately 10 per cent of the aircraft gross weight). Therefore, each main
leg imposes a load equal to about 45 per cent of that weight. Wheel base and
track width have not been included, since these dimensions are such that there
is no possibility of interaction of the stresses imposed by the different legs
of the landing gear.
1.5 From
the above considerations, it will be seen that the characteristics of each main
leg provide sufficient information for assessing pavement strength
requirements. Accordingly, the table confines itself to providing data thereon.
1.6 The
load supported by each leg is transmitted to the pavement by one or several
rubber-tired wheels. The wheel arrangements shown in Figure A1-2 will be found
on the main legs landing gear of civil aircraft at presently in service.
1.7 For
pavement design and evaluation purposes the following wheel spacing are
significant and therefore listed in the table:
S - distance
between centres of contact areas of dual wheels
ST -
distance between axes of tandem wheels
SD -
distance between centres of contact areas of diagonal wheels and is given by
the following expression:
Note:
Tire pressures given are internal or inflation pressures.
1.8 It
should be noted that throughout the table figures refer to the aircraft at its
maximum take-off weight. For lesser operational weights, figures quoted for
“load on each leg”, “contact area” should be decreased proportionally.
2. AIRCRAFT CHARACTERISTICS FOR
DESIGN AND EVALUATION OF PAVEMENTS
2.1 The
aircraft listed in Table A1-1 are representative of the aircraft manufacturers’
most current commercial aircraft types, typically carrying 70 passengers or
greater or having All-up mass exceeding 40 tonnes. Aircraft in this weight
range are the most demanding in terms of pavement loading. The table in most
cases lists the heaviest version of an aircraft model, and more detailed
information can be found in the aircraft manufacturers’ aircraft
characteristics for airport planning documents.
2.2 Wheel arrangement nomenclature (used in Table A1-1)
2.2.1 Basic
name for aircraft gear geometry. Under the naming convention,
abbreviated aircraft gear designations may include two variables, the main gear
configuration and the body/belly gear configuration, if body/belly gears are
present. Figure A1-3 illustrates the two primary variables.
2.2.2 Basic
gear type. Gear type for an individual landing strut is determined by the
number of wheels across a given axle (or axle line) and whether wheels are
repeated in tandem. There may exist, however, instances in which multiple
struts are in close proximity and are best treated as a single gear, e.g.
Antonov AN-124 (see Figure 14). If body/belly gears are not present, the second
portion of the name is omitted. For aircraft with multiple gears, such as the
B747 and the A380, the outer gear pair is treated as the main gear.
2.2.3 Basic
gear codes. This naming convention, as shown in Figure A1-4 below, uses the
following codes for gear designation purposes: single (S); dual (D); triple
(T); and quadruple (Q).
2.2.4 Main
gear portion of gear designation. The first portion of the aircraft gear
name comprises the main gear designation. This portion may consist of up to
three characters.
a) First
character indicates the number of tandem sets or wheels in tandem (e.g.
“3D” represents three dual gears in tandem). If a tandem configuration is not
present, the leading value of “1” is omitted. Typical codes are: “S” representing
single; “2D” representing two dual wheels in tandem; “5D” representing five
dual wheels in tandem; and “2T” representing two triple wheels in tandem.
b) Second
character of the gear designation indicates the gear code (S, D, T or Q).
c) Third
character of the gear designation is a numeric value that indicates
multiples of gears. For the main gear, the gear designation assumes that the
gear is present on both sides (symmetrical) of the aircraft and that the
reported value indicates the number of gears on one side of the aircraft. A
value of 1 is used for aircraft with one gear on each side of the airplane. For
simplicity, a value of 1 is assumed and is omitted from the main gear
designation. Aircraft with more than one main gear on each side of the aircraft
and where the gears are in line will use a value indicating the number of gears
in line.
2.2.5 Body/belly
gear portion of gear designation. The second portion of the
aircraft gear name is used when body/belly gears are present. If body/belly
gears are present, the main gear designation is followed by a forward slash
(/), then the body/belly gear designation. For example, the B-747 aircraft has
a two dual wheels in tandem main gear and two dual wheels in tandem body/belly
gears. The full gear designation for this aircraft is 2D/2D2. The body/belly
gear designation is similar to the main gear designation except that the
trailing numeric value after the gear type (S, D, T or Q) denotes the total
number of body/belly gears present (e.g. 2D1 = one dual tandem body/belly gear;
2D2 = two dual tandem body/belly gears). Because body/belly gear arrangement
may not be symmetrical, the gear code must identify the total number of gears
present and a value of 1 is not omitted if only one gear exists.
2.2.6 Unique
gear configurations. The Lockheed C-5 Galaxy has a unique gear type and is difficult to
name using the proposed method. This aircraft will not be classified using the
new naming convention and will continue to be referred to directly as the C5.
Gear configurations such as those on the Boeing C-17, Antonov AN-124, and
Ilyshin IL-76 might also cause some confusion. In these cases, it is important
to observe the number of landing struts and the proximity of the struts. In the
case of the AN-124, it is more advantageous to address the multiple landing
struts as one gear (i.e. 5D or five duals in tandem) rather than use D5 or dual
wheel gears with five sets per side of the aircraft. Due to wheel proximity,
the C-17 gear is more appropriately called a 2T as it appears to have triple
wheels in tandem. In contrast, the IL-76 has considerable spacing between the
struts and should be designated as a Q2.
2.2.7 Examples
of gear geometry naming convention. Figure 2 provides examples of
generic gear types in individual and multiple tandem configurations.
APPENDIX 2
USER INFORMATION FOR THE ICAO-ACR
COMPUTER PROGRAMME
1. GENERAL
1.1 The
ICAO-ACR computer programme is maintained by the FAA, William J. Hughes
Technical Center (WJHTC). The programme implements the ACR computational
procedures for rigid and flexible pavements. ICAO-ACR incorporates LEAF
(Layered Elastic Analysis - FAA), a computer programme that computes the
structural responses of a layered pavement system according to Burmister’s
theory (layered elastic model). ICAO-ACR is distributed in compiled form as a
Visual Basic.NET dynamic-link library (DLL). Programme files may be downloaded
from the FAA WJHTC website: Airport Pavement Software Programs (faa.gov).
1.2 The
following files are available for download from the above website:
a)
ICAO-ACR is an executable (stand-alone) computer programme that executes the
DLL ACRClassLib.dll, and returns standard ACR values.
b) ACRClassLib.dll
is a Visual Basic.NET DLL that can be linked directly to other programmes
that either compute ACR directly, or that use the ACR computation to evaluate
PCR. ACRClassLib.dll is not a stand-alone computer programme. Rather, it
is intended to be run from within a separate calling programme such as
ICAO-ACR. Information on linking the library to a calling programme is given
below.
1.3 ICAO-ACR
is an open-source programme. The source codes for ICAO-ACR, ACRClassLib.dll and
LEAF may be obtained from:
Federal
Aviation Administration
William
H. Hughes Technical Center
Airport Technology
R&D Branch., ANG-E26
Atlantic
City International Airport, NJ 08405
United
States
2. DYNAMIC-LINK LIBRARY (DLL) TECHNICAL INFORMATION
The DLL ACRClassLib.dll
was compiled using Microsoft Visual Basic 2013 in the Microsoft Visual
Studio programming environment. Its target framework is Microsoft.NET Framework
4.5.
2.1 Input
Data. The ACRClassLib.dll class library accepts the following data inputs:
2.1.1 Aircraft
gross weight (in tonnes or pounds).
2.1.2 Percent
of aircraft gross weight acting on the main gear, expressed as a decimal value.
2.1.3 Number
of wheels in the aircraft gear to be analysed.
2.1.4 Tire
pressure (in MPa or pounds per square inch).
2.1.5 Horizontal
coordinates (x, y) of each wheel (in mm or inches).
2.1.6 For
each wheel, a value 0 or 1, indicating whether the wheel is within the limits
of the evaluation point grid. (The value 1 indicates that it is included.)
2.1.7 Pavement
type. This value can only be “Flexible” or “Rigid.”
2.1.8 System
of Units (Metric or US).
2.2 Microsoft
Visual Studio.NET programming environment, the procedure for linking to the DLL
is as follows:
2.3.1 In
the project properties, add ACRClassLib.dll to References.
2.3.2 Declare
all variables that will be passed between the calling programme and ACRClassLib.dll.
The following input variables are declared as Single type: aircraft gross
weight, percent gross weight, tire pressure, x-coordinate (array), y-coordinate
(array). The following input variables are declared as Integer type: number of
wheels, wheel selection variable (array) (see 2.1.6).
Certain
variables have special definitions. The pavement type is specified as an
enumerate variable type:
Public
Enum PavementType
Flexible
= 1
Rigid = 2
End Enum
ACR data
are stored in a Visual Basic data structure ACR data:
Public
Structure ACRdata
Dim libACR() As Single
Dim libACRthick()
As Single
Dim libSubCat()
As String
Dim libSubCatMPa()
As String
End
Structure
The four
elements in data structure ACR data are:
1. ACRdata.libACR()
stores ACR numerical values following execution.
2. ACRdata.libACRthick()
stores ACR thickness values following execution.
3. ACRdata.libSubCat()
stores subgrade category letter designations.
4. ACRdata.libSubCatMPa()
stores subgrade category standard modulus values in MPa.
Each of
the elements 1-4 above is an array of length 5, of declared data type as
indicated above. Within each array, the first element in the array ACRData.array(0)
is not used, while the last four elements ACRData.array(1) through
ACRData.array(4) correspond to standard subgrade categories “D” through
“A” respectively. For reference, the following snippets of Visual Basic code
are examples of function signatures used in the DLL. Executing the function
CalculateACR from the calling programme returns ACR values in the array
ACRdata.libACR(). The first function signature applies for the majority of gear
types where all wheels in the main gear have equal tire pressure and load. The
second function signature is used specifically to compute the flexible ACR for
certain gear configurations (e.g., the Airbus A340 series) where the centre
landing gear has a different tire pressure/wheel load combination than the wing
landing gear.
Public
Overloads Function CalculateACR(ByVal PavementType As clsACR.PavementType, _
ByVal gross_weight
As Single, _
ByVal percent_gw
As Single, _
ByVal wheels_number
As Integer, _
ByVal tire_pressure
As Single, _
ByVal CoordX() As Single, _
ByVal CoordY() As Single, _
ByVal SW() As Integer, _
ByVal Metric As Boolean) As ACRdata
Public
Overloads Function CalculateACR(ByVal PavementType As clsACR.PavementType, _
ByVal gross_weight
As Single, _
ByVal percent_gw
As Single, _
ByVal wheels_number
As Integer, _
ByVal tire_pressure
As Single, _
ByVal CoordX() As Single, _
ByVal CoordY() As Single, _
ByVal percent_gw2
As Single, _
ByVal wheels_number2
As Integer, _
ByVal tire_pressure2
As Single, _
ByVal CoordX2()
As Single, _
ByVal CoordY2()
As Single, _
ByVal Metric As Boolean) As ACRdata
'ACR for two gears
The
following is sample Visual Basic code for declaring variables in the calling
programme:
Dim ACRData As ACRClassLib.clsACR.ACRdata
Dim PavementType
As ACRClassLib.clsACR.PavementType
Dim Gross_Wt,
Percent_GW, Tire_Pressure As Single
Dim No_Wheels
As Integer
Dim X1( ),
Y1( ) As Single
Dim SW( ) As Integer
Dim Metric As Boolean
2.3.3 Assign
numerical values to declared input variables. The Boolean variable Metric is
True for metric units, False for US units. Figure A2-1 explains how to use
variable SW(), which tells the programme whether to include a given wheel in
the limits of the evaluation point grid (see 1.1.3.7 d). In ICAO-ACR, a value
of 1 assigned to SW means the wheel is included in the strain evaluation point
grid area; any value other than 1 is treated as 0. Note that ACRClassLib.dll
does not determine the correct number of wheels to include in the strain
evaluation point grid. This determination should be made by the calling
programme with reference to the guidance in 1.1.3.7 d. Also note that variable
SW only controls which sub-group of wheels in the main gear assembly defines
the strain evaluation point grid, not the number of wheels used to determine
ACR. ACRClassLib.dll determines the ACR value for all wheels passed to
it (No_Wheels). (In Figure A2-1, if all wheels 1-8 were assigned SW = 1, the
difference in computed ACR values would be insignificant. However, the
computation would take much longer.)
2.3.4 Call
Function CalculateACR. The following snippet of Visual Basic code calls the
function CalculateACR.
Dim RunACR As ACRClassLib.clsACR
RunACR = New ACRClassLib.clsACR()
ACRData =
RunACR.CalculateACR(PavementType, Gross_Wt, Percent_GW, No_Wheels,
Tire_Pressure, X1, Y1, SW, Metric)
3. PROGRAMME ICAO-ACR
3.1 Programme
ICAO-ACR functions as a stand-alone programme that computes flexible and rigid
ACR values for arbitrary aircraft gear configurations, using the
ACRClassLib.dll DLL. For convenience, the programme includes a library of
aircraft types commonly in use. For library aircraft, programme ICAO-ACR
automatically selects the correct number of wheels for ACR evaluation, i.e. all
wheels in the main landing assembly for flexible ACR and all wheels in the most
demanding single truck for rigid ACR.
APPENDIX 3
DAMAGE MODEL FOR FLEXIBLE ACR
1. ELEMENTARY DAMAGE LAW
1.1 The
flexible ACR procedure relies on the subgrade failure criterion associated with
the elementary damage law:
where is
the number of traffic coverages producing subgrade failure, for a given
subgrade vertical strain.
1.2 The
elementary damage De is then defined as:
2. MULTIPLE-AXLE GEAR LOADS
2.1 Modern
landing gears often feature multi-axle wheel groups that produce complex strain
profiles in the pavement, possibly with multiple strain peaks and no return to
zero between peaks. As an example, Figure A3-1 describes the strain history
profile for a pavement structure (i.e. the trace of all strain values along a
longitudinal profile below the landing gear).
2.2 Due
to the interaction between axles in tandem, the strain that makes the CDF equal
to 1.0 for 36,500 passes of the evaluation aircraft will generally be different
from that given by the above elementary damage law, which is based on the
concept of load cycles. Therefore, the above equation cannot be used directly.
3. CONTINUOUS INTEGRAL FORM OF
THE DAMAGE LAW
3.1 In
order to account for complex strain profiles induced by multiple-axle gear
loads, the elementary damage law is extended to a continuous integral form
thanks to Miner’s rule and the Equivalent Single Peak (ESP) factor introduced
by Jean Maurice Balay and Cyril Fabre (2009).
3.2 The
damage D1 for a single aircraft pass producing a longitudinal strain
profile ε(x) is then calculated as:
where x
refers to the longitudinal position along the strain profile and <z>
is the positive part of z:
3.3 The
longitudinal position x does not play an explicit role in this equation,
therefore any other monotonically increasing parameterization (e.g., time)
would lead to the same result
3.4 For
the specific case of a single-peak strain profile with maximum amplitude εmax,
D1 reduces to the elementary damage law D1 = De (εmax).
For an arbitrary strain profile, it is therefore possible to compute an
equivalent single-peak strain εeq that would produce the same damage as
the entire profile: D1 = De (εeq).
3.5 Based
on this equivalence, the Equivalent Single Peak (ESP) ratio is then defined as
the number of passes that would be required by a virtual aircraft producing a
single- peak strain profile with maximum value εeq to produce the same
damage as one pass of a virtual aircraft producing a single-peak strain profile
with maximum value εmax, as shown below.
3.6 The
total damage D produced by N aircraft passes is now given by the following
equation:
3.7 If
the longitudinal component of the Pass-to-Coverage (P/C) ratio is used, an
equivalent expression of D is:
3.8 ESP
and P/C ratios are therefore functionally equivalent; they both represent the
load repetition effect due to wheels in tandem in absence of lateral wander.
3.9 Substituting
the elementary damage law in D1, the integral form can be expressed as:
3.10 It
can be verified that the above integral form is equivalent to:
where εk
are the strain extremums of the longitudinal profile and sk is a factor characterizing
the type of extremum:
3.11 It
should be noted that for the specific case of a single-peak strain profile with
maximum value εmax, the integral form reduces to the elementary damage
law: D1=De(εmax)
4. DAMAGE MODEL FOR FLEXIBLE ACR
4.1 The
continuous integral form of the damage law is adopted for the computation of
pavement damage in the flexible ACR procedure.
4.2 This
procedure is implemented in the ICAO-ACR computer programme.
APPENDIX 4
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APPENDIX 5
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APPENDIX 6
PAVEMENT OPERATIONS AND
MAINTENANCE RELATED GUIDANCE
Note: Doc
9157 has been dedicated to design matters. However, the 2nd edition (1983) of
its Part 3 - Pavements included several sections of operation and
maintenance oriented guidance material. Due to the non-design nature of these
subjects, they more appropriately pertain to documents such as the PANS-Aerodromes
or Aerodrome Services Manual (Doc 9137). However, in order to facilitate its
future relocation, the identified guidance has been updated and compiled as
this dedicated appendix with the following content:
1)
pavement management programme (PMP) (new - possible relocation to Doc 9137,
Part 9, Chapter 1.6);
2)
methods for improving/maintaining runway surface texture (2nd edition, Chapter
5) and pavement magnetic characteristics (new);
3)
protection of asphalt pavements (2nd edition, Chapter 6); and
4)
construction of pavement overlays during operations closures (2nd edition,
Chapter 8).
1. PAVEMENT MANAGEMENT PROGRAMME
(PMP)
1.1 In
Annex 14, Volume I, Chapter 10, a requirement for a maintenance programme has
been established as a Standard, including preventive maintenance, making, by
inference, the implementation of a pavement maintenance programme (PMP)
mandatory.
1.2 Extending
the pavement life through a regular programme, for a constantly changing
aircraft fleet, requires more sophisticated maintenance techniques such as a
PMP. As preventive maintenance, it will be desirable to implement a PMP where
appropriate, to maintain aerodrome pavements/facilities in a condition which
does not impair the safety, regularity or efficiency of air navigation.
1.3 A
PMP is a set of defined procedures for collecting, analysing, maintaining and
reporting pavement data, to assist decision makers in finding optimum
strategies for maintaining pavements in safe serviceable condition over a given
period of time for the least cost. A PMP should take into account:
a) inspection
procedures and condition assessment;
b)
maintenance protocols and procedures;
c)
management and oversight of completed works; and A6-1
d) staff
competence needed (human factors).
1.4 Depending
on the complexity of the paved areas in an aerodrome, a PMP would contain as a
minimum the following functionalities:
a) pavement
inventory (pavement condition evaluation, pavement history, traffic, costs);
and
b)
pavement condition assessment (e.g. ASTM D5340-12 Standard Test Method for
Airport Pavement Condition Index Surveys - PCI).
1.5 Additional
functionalities could include:
a) modelling
to predict future conditions - analysis (serviceability rating, performance
predictions, economic analyses-budgeting/programming);
b) pavement
performance report (past and future);
c) pavement
maintenance and repair (planning, scheduling, budgeting and analysing
alternatives); and
d)
project planning.
2. METHODS FOR
IMPROVING/MAINTAINING RUNWAY SURFACE TEXTURE AND PAVEMENT MAGNETIC CHARACTERISTICS
2.1 Purpose
2.1.1 Annex
14 requires that the surface of a paved runway be so constructed or resurfaced
as to provide surface friction characteristics at or above the minimum friction
level set by the State. Additional provisions include recommended
specifications for the configuration of runway surfaces in terms of transverse
and longitudinal slopes, surface evenness and texture. The purpose of this
chapter is to provide guidance on proven methods for improving runway surface
texture and drainage.
2.1.2 This
chapter also identifies means to mitigate potentially hazardous magnetic field
distortions induced by metallic masses in or below aerodrome pavements and
magnetisation of steel reinforcement meshes, tie-bars and dowels by the
repeated use of magnetic devices to clean the surface of the pavement.
2.2 Basic considerations
Historical
background
2.2.1 One
of the most significant and potentially dangerous operational issues during wet
weather conditions is the aquaplaning phenomenon, which has been responsible
for a number of aircraft incidents and accidents.
2.2.2 Efforts
to alleviate aquaplaning have resulted in the development of runway pavements
with improved surface texture and drainage characteristics. Experience has
shown that these types of pavements, apart from successfully minimizing
aquaplaning risks, provide a substantially higher friction level in all degrees
of wetness (i.e. from damp to flooded surfaces).
2.2.3 The
methods discussed in this section for the enhancement of runway surface texture
and drainage have been proven effective in improving the surface performance of
runways.
Functional
requirements
2.2.4 A
runway pavement, considered as a whole, is required to fulfil the following
three basic functions, which is to:
a) provide
adequate bearing strength;
b)
provide good riding qualities; and
c) provide
good surface friction characteristics.
The first
criterion (a) above, addresses the structure of the pavement, the second (b),
the geometric shape of the top of the pavement and the third (c), the texture
of the actual surface.
2.2.5 All
three criteria are considered essential to achieve a pavement which will
functionally satisfy the operational requirements. From the operational aspect,
however, providing good surface friction characteristics (third criterion) is
considered the most important because it has a direct impact on the safety of
aircraft operations. Regularity and efficiency may also be affected. Thus, the
friction criterion may become a decisive factor for the selection and the form
of the most suitable finish of the pavement surface.
2.2.6 Aerodrome
pavement designers and managers attention is drawn to the trend monitoring of
surface friction characteristics contained in Annex 14, Volume I, Chapter 10. A
trend monitoring concept of the runway surface friction characteristics is
shown in Figure A6-1
2.2.7 The
objective is to ensure that the surface friction characteristics for the entire
runway remain at or above a minimum friction level specified by the State.
2.2.8 The
State-set criteria for surface friction characteristics and output from
State-set or agreed assessment methods establish the reference from which trend
monitoring are performed and evaluated.
2.2.9 Aerodrome
pavement designers and managers attention is drawn to the magnetic field
distortions possibly induced by aerodrome pavements, which could interfere with
aircraft navigation systems.
Problem
identification
2.2.10 When
in a dry and clean state, individual runways generally provide comparable
friction characteristics with operationally insignificant differences in
friction levels, regardless of the type of pavement (asphalt/cement concrete)
and configuration of the surface. Moreover, the friction level available is
relatively unaffected by the speed of the aircraft. Hence, the operation on dry
runway surfaces is satisfactorily consistent and no particular engineering
criteria for surface friction are needed for this case.
2.2.11 In
contrast, when the runway surface is affected by water to any degree of wetness
(i.e. from a damp to a flooded state), the situation is entirely different. For
this condition, the friction levels provided by individual runways drop
significantly from the dry value and there is considerable disparity in the
resulting friction level between different surfaces. This variance is due to
differences in the type of pavement, the form of surface finish (texture) and
the drainage characteristics (shape). Degradation of available friction, which
is particularly evident when aircraft operate at high speeds, can have serious
implications on safety, regularity or efficiency of operations. The extent will
depend on the friction actually required versus the friction provided.
2.2.12 The
typical reduction of friction when a surface is wet and the reduction of
friction as aircraft speed increases are explained by the combined effect of
viscous and dynamic water pressures to which the tire/surface is subjected.
This pressure causes a partial loss of “dry” contact the extent of which tends
to increase with speed. There are conditions where the loss is practically
total and the friction drops to negligible values. This is identified as
viscous, dynamic or rubber-reverted aquaplaning. The manner in which these
phenomena affect different areas of the tire/surface interface and how they
change in size with speed is illustrated in Figure A6-2.
Painted
areas on pavement surfaces
2.2.13 Painted
areas on wet runway pavement surfaces can be very slippery. Additionally,
aircraft with one main gear on a painted surface, and the other on an unpainted
surface, may experience differential braking. It is important to keep the skid
resistance properties of painted surfaces to that of the surrounding
non-painted surfaces. This usually involves adding a small amount of silica
sand to the paint mix.
Design
objectives
2.2.14 In
the light of the foregoing considerations, the objectives for runway pavement
design, which are similarly applicable for maintenance, can be formulated as
follows:
A runway
pavement should be so designed and maintained as to provide a runway surface
which meets adequately all functional requirements at all times throughout the
anticipated lifetime of the pavement, in particular:
a) to
provide in all anticipated conditions of wetness, high friction levels and
uniform friction characteristics; and
b) to
minimize the potential risk of all forms of aquaplaning (i.e. viscous, dynamic
and rubber-reverted aquaplaning). Information on these types of aquaplaning is
contained in Doc 9137, Part 2 - Pavement Surface Conditions.
2.2.15 The
provision of adequate wet runway friction is closely related to the drainage
characteristics of the runway surface. The drainage demand in turn is
determined by local precipitation rates. Drainage demand, therefore, is a local
variable which will essentially determine the engineering efforts and
associated investments/costs required to achieve the objective. In general, the
higher the drainage demand, the more stringent the interpretation and
application of the relevant engineering criteria will become.
Physical
design criteria
2.2.16 The
problem of friction on runway surfaces affected by water can be interpreted as
a generalized drainage problem consisting of three distinct criteria:
a) surface
drainage (surface shape);
b)
tire/surface interface drainage (macrotexture); and
c) penetration
drainage (microtexture).
The three
criteria can significantly be influenced by engineering measures and must all
be satisfied to achieve adequate friction in all possible conditions of wetness
(i.e. from a damp to a flooded surface).
Surface
drainage
2.2.17 Surface
drainage is a basic requirement. It serves to minimize water depth on the
surface, particularly in the area of the wheel path. The objective is to drain
water off the runway in the shortest path possible and out of the area of the
wheel path. Adequate surface drainage is provided primarily by an appropriately
sloped surface (in both the longitudinal and transverse directions) and surface
evenness. Drainage capability can, in addition, be enhanced by other measures
such as providing closely spaced transverse grooves or by draining water
initially through the voids of a specially treated wearing course (porous
friction course). It should be clearly understood, however, that other measures
(such as the provision of runway grooving) are not a substitute for poor runway
shape, but is due to inadequate slopes or lack of surface evenness. This may be
an important consideration when deciding on the most effective method for
improving the surface performance of an existing runway.
Tire/surface
interface drainage (macrotexture)
2.2.18 The
purpose of interface drainage (under a moving tire) is twofold:
a) to
prevent as far as feasible residual surface bulkwater from intruding into the
forward area of the interface; and
b) to
drain intruding water to the outside of the interface.
The
objective is to achieve high water discharge rates from under the tire with a
minimum of dynamic pressure build-up. It has been established that this can be
achieved by providing a surface with an open macrotexture.
2.2.19 Interface
drainage is actually a dynamic process highly correlated to the square of
speed. Macrotexture is therefore particularly important for the provision of
adequate friction in the high speed range. From the operational aspect, this is
most significant because it is in this speed range where lack of adequate
friction is most critical with respect to stopping distance and directional
control capability.
2.2.20 In
this context it is worthwhile to make a comparison between the textures applied
in road construction and runways. The smoother textures provided by road
surfaces can achieve adequate drainage of the footprint of an automobile tire
because of the patterned tire treads which significantly contribute to
interface drainage. Aircraft tires, however, cannot be produced with similar
patterned treads and have only a number of circumferential grooves which
contribute substantially less to interface drainage. Their effectiveness
diminishes relatively quickly with tire wear. The more vital factor, however,
which dictates the macrotexture requirement, is the substantially higher speed
range in which aircraft operate. This may explain why some conventional runway
surfaces, which were built to specifications similar to road surfaces
(relatively closed- textured) show a marked drop in wet friction with
increasing speed and often a susceptibility to dynamic aquaplaning at
comparatively small water depths.
2.2.21 With
cement concrete pavement surfaces, the provision of macrotexture may be
achieved by using one of a number of available methods to apply macrotexture
characteristics to the surface during the finishing process such as brush or
broom finish or burlap drag finish. With asphalt surfaces, the provision of
macrotexture may be achieved by the selection of the appropriate aggregate (in
terms of size, gradation, resistance to polish and wear, and shape) or by
providing open graded surfaces.
2.2.22 A
further design criteria calls for best possible uniformity of surface texture.
This requirement is important to avoid undue fluctuations in available friction
since these fluctuations would degrade antiskid braking efficiency or may cause
tire damage.
2.2.23 Methods
which enhance macrotexture and/or are effective in reducing the risk of
aquaplaning are described in 2.3.
Penetration
drainage (microtexture)
2.2.24 The
purpose of penetration drainage is to establish “dry” contact between the
asperities of the surface and the tire tread in the presence of a thin viscous
water film. The viscous pressures which increase with speed tend to prevent
direct contact except at those locations of the surface where asperities
prevail, penetrating the viscous film. This kind of roughness is defined as
microtexture.
2.2.25 Microtexture
refers to the fine-scale roughness of the individual aggregate of the surface
and is hardly detectable by the eye, however, assessable by the touch.
Accordingly, adequate microtexture can be provided by the appropriate selection
of aggregates known to have a harsh surface. This excludes in particular all
polishable aggregates.
2.2.26 Macro-
and microtexture are both vital components for wet surface friction (i.e. both
must adequately be provided to achieve acceptable friction characteristics in
all different conditions of wetness). The combined effect of micro- and
macrotexture of a surface on the resulting wet friction versus speed is
illustrated in Figure A6-3 indicating also that the design objective formulated
in 2.2.14 can be achieved by engineering means.
2.2.27 A
major problem with microtexture is that it can change within short time periods
(unlike macrotexture), without being easily detected. A typical example of this
is the accumulation of rubber deposits in the touchdown area which will largely
mask microtexture without necessarily reducing macrotexture. The result can be
a considerable decrease in the wet friction level. This problem is addressed by
periodic friction measurements which provide a measure of existing microtexture.
If it is determined that low wet friction is caused by degraded surface
microtexture, there are methods available to effectively restore adequate
microtexture for existing runway surfaces (see 2.3).
Minimum
specifications
2.2.28 The
basic engineering specifications for the geometrical shape (longitudinal
slope/transverse slope/surface evenness) and for the texture (macrotexture) of
a runway surface are contained in Annex 14, Volume I.
Slopes
2.2.29 All
new runways should be designed with uniform transverse profile in accordance
with the value of transverse slope recommended in Annex 14, Volume I, and with
a longitudinal profile as nearly level as possible. A cambered transverse
section from a centre crown is preferable but if for any reason this cannot be
provided then the single runway crossfall should be carefully related to
prevailing wet winds to ensure that surface water drainage is not impeded by
the wind blowing up the transverse slope (in the case of single crossfalls it
may be necessary at certain sites to provide cut-off drainage along the higher
edge to prevent water from the shoulder spilling over the runway surface).
Particular attention should be paid to the need for good drainage in the
touchdown zone since aquaplaning induced at this early stage of the landing,
once started, can be sustained by considerably shallower water deposits further
along the runway.
2.2.30 If
these ideal shape criteria are met, aquaplaning incidents will be reduced to a
minimum, but departures from these ideals will result in an increase of
aquaplaning probability, no matter how good the friction characteristic of the
runway surface may be. These comments hold true for major reconstruction
projects and, in addition, when old runways become due for resurfacing, the
opportunity should be taken, wherever possible, to improve the levels to assist
surface drainage. Every improvement in shape helps, no matter how small.
Surface
evenness
2.2.31 This
is a constituent of runway shape which requires equally careful attention.
Surface evenness is also important for the riding quality of high speed jet
aircraft.
2.2.32 Requirements
for surface evenness are described in Annex 14, Volume I, Attachment A, Section
5, and reflect good engineering practices. Failure to meet these minimum
requirements can seriously degrade surface water drainage and lead to ponding.
This can be the case with aging runways as a result of differential settlement and
permanent deformation of the pavement surface. Evenness requirements apply not
only for the construction of a new pavement but throughout the life of the
pavement. The maximum tolerable deformation of the surface should be specified
as a vital design criterion. This may have a significant impact on the
determination of the most appropriate type of construction and type of
pavement.
2.2.33 With
respect to susceptibility to ponding when surface irregularities develop,
runway shapes with maximum permissible transverse slopes are considerably less
affected than those with marginal transverse slopes. Runways exhibiting ponding
will normally require a resurfacing and reshaping to effectively alleviate the
problem.
Surface
texture
2.2.34 Surface
macrotexture requirements are specified in Annex 14, Volume I, in terms of
average surface texture depth, which should not be less than 1 mm for new
surfaces. The minimum value for average texture depth has been empirically
derived and reflects the absolute minimum required to provide adequate
interface drainage. Higher values of average texture depth may be required
where rainfall rates and intensities are a critical factor to satisfy interface
drainage demand. Surfaces which fall short of the minimum requirement for
average surface texture depth will show poor wet friction characteristics,
particularly if the runway is used by aircraft with high landing speeds.
Remedial action is, therefore, imperative. Methods for improving the surface
performance of runways are described in 2.3.
2.2.35 As
outlined earlier, uniformity of the texture is also an important criterion. In
this respect, there are several specific types of surfaces which meet this
requirement (see 2.3). These surfaces will normally achieve average texture
depths higher than 1 mm.
2.2.36 The
macrotexture of a surface does not normally change considerably with time,
except for the touchdown area as a result of rubber deposits. Therefore,
periodic control of available average surface texture depth on the
uncontaminated portion of the runway surface will only be required at long
intervals.
2.2.37 With
respect to microtexture there is no direct measure available to define the
required fine scale roughness of the individual aggregate in engineering terms.
Accordingly, there are no relevant specifications in Annex 14, Volume I.
However, from experience it is known that good aggregate must have a harsh
surface and sharp edges to provide good water film penetration properties. It
is also important that the aggregate be actually exposed to the surface and not
coated entirely by a smooth material. Since microtexture is a vital component
of wet friction regardless of speed, the adequacy of microtexture provided by a
particular surface can be assessed generally by friction measurements. Lack of
microtexture will result in a considerable drop in friction levels throughout
the whole speed range. This will occur even with minor degrees of surface
wetness (e.g. damp). This rather qualitative method may be adequate for
detecting lack of microtexture in obvious cases.
2.2.38 Degradation
of microtexture caused by traffic and weathering may occur, in contrast to
macrotexture, within comparatively short time periods and can also change with
the operational state of the surface. Accordingly, short-termed periodic checks
by friction measurements are necessary, in particular with respect to the
touchdown areas where rubber deposits quickly mask microtexture.
Runway
surface friction measurement
2.2.39 Annex
14, Volume I, requires runway surfaces to be measured periodically with a
continuous friction measuring device using self-wetting features to verify
their friction characteristics when wet. These friction characteristics must
not fall below minimum levels specified by the State. Table 3-1 of Doc 9137,
Part 2 shows the criteria in use in some States for specifying the friction
characteristics of new or resurfaced runway surfaces, for establishing
maintenance planning levels and for setting minimum friction levels.
2.2.40 For
the design of a new runway, the optimum application of the basic engineering
criteria for runway shape and texture will normally provide a fair guarantee of
achieving levels well in excess of the applicable specified minimum wet
friction level. When large deviations from the basic specifications for shape
or texture are planned, it will then be advisable to conduct wet friction
measurements on different test surfaces in order to assess the relative
influence of each parameter on wet friction, prior to deciding on the final
design. Similar considerations apply for surface texture treatment of existing
runways.
2.3 Improvement of surface
performance of runways
2.3.1 The
methods described in this section are based on the experiences of several
States. It is important that a full engineering evaluation of the existing
pavement be made at each site before any particular method is considered, and
that, once selected, the method is suitable for the types of aircraft
operating. It should be noted that repairs or resurfacing of the pavement may
be required in certain cases prior to the application of the improvement method
in order for it to be effective.
Grooving
of pavements
2.3.2 There
are no operational objections to the grooving of existing surfaces. Experience
of operating all types of aircraft from grooved surfaces over a number of years
indicates that there is no limit within the foreseeable future to the aircraft
size, loading or type for which such surfaces will be satisfactory. There is
inconclusive evidence of a slightly greater rate of tire wear under some
operational conditions.
2.3.3 Methods
of grooving include the sawing of grooves in existing or properly cured
asphalt, shown in Figure A6-4 or PCC pavements, and the grooving or wire
combing of PCC while it is in the plastic condition. Based on current
techniques, sawed grooves provide a more uniform width, depth, and alignment.
This method is the most effective means of removing water from the
pavement/tire interface. However, plastic grooving and wire combing are also
effective in enhancing pavement surface drainage. They are cheaper to construct
than the sawed grooves, particularly where very hard aggregates are used in
pavements. Therefore the cost-benefit relationship should be considered in
deciding which grooving technique should be used for a particular runway
2.3.4 The
following factors should be considered in determining the need for runway
grooving:
a) historical
review of aircraft accidents/incidents related to aquaplaning at airport
facility;
b)
wetness frequency (review of annual rainfall rate and intensity);
c) transverse
and longitudinal slopes, flat areas, depressions, mounds or any other
abnormalities that may affect water run-off;
d)
surface texture quality as to slipperiness under dry or wet conditions. Polishing
of aggregate, improper seal coating, inadequate microtexture/macrotexture and
contaminant build-up are some examples of conditions which may affect the loss
of surface friction;
e) terrain
limitations such as drop-offs at the ends of runway end safety areas;
f) adequacy
of number and length of available runways;
g)
cross-wind effects, particularly when low friction factors prevail; and
h) the
strength and condition of existing runway pavements.
Evaluation
of existing pavement
2.3.5 Asphalt
surfaces must be examined to determine that the existing wearing course is
dense, stable and well-compacted. If the surface exhibits ravelling or where
large particle fractions of coarse aggregate are exposed on the surface itself,
then other methods will need to be considered or resurfacing will have to be
undertaken before grooving is conducted. Rigid pavement must be examined to
ensure that the existing surface is sound, free of scaling or extensive spalls,
or “working cracks”. Apart from the condition of the surface itself, the ratio
between transverse and longitudinal slopes becomes important. If the
longitudinal slopes are such that the water run-off is directed along the
runway instead of clearing quickly to the runway side drains, then a condition
could arise when the grooves would fill with free water, fail to drain quickly
and possibly encourage aquaplaning. For the same reason, surfaces with
depressed areas should be repaired or replaced before grooving.
Effectiveness
of treatment
2.3.6 Transverse
grooving improves the macrotexture of the runway pavement surface, reduces
water film thicknesses during rainfall and provides an escape channel for water
that may become trapped between the pavement surface and an aircraft tire.
These effects reduce the potential for aircraft aquaplaning under wet
conditions. Grooving may also improve aircraft braking performance on a wet
runway as compared to a wet non- grooved runway. Grooving does however have
limits with respect to coping with deep standing water due to heavy rainfall.
In addition, the build-up of rubber deposits in the grooves will reduce the
effectiveness of the grooving, and rubber removal should be performed as
necessary. The improvement related to grooving applies to both asphalt and
concrete pavement surfaces. For asphalt pavements, the duration of this
improvement will depend on the properties of the asphalt wearing course,
climate and traffic.
Technique
2.3.8 The
grooves may be terminated within 3 m of the runway pavement edge to allow
adequate space for the operation of the grooving equipment. Tolerances should
be established to define groove alignment, depth, width and spacing. Suggested
tolerances are ± 40 mm in alignment for 22 m, and average depth or width ± 1.5
mm. Grooves should not be cut closer than 75 mm to transverse joints. Diagonal
or longitudinal saw kerfs where lighting cables are installed should be
avoided. Grooves may be continued through longitudinal construction joints.
Extreme care must be exercised when grooving near in-runway lighting fixtures
and subsurface wiring. A 60 cm easement on each side of the light fixture is
recommended to avoid contact by the grooving machine. Contracts should specify
the contractor's liability for damage to light fixtures and cable. Clean-up is
extremely important and should be continuous throughout the grooving operation.
The waste material collected during the grooving operation must be disposed of
by flushing with water, sweeping or vacuuming. If waste material is flushed,
the specifications should state whether the airport owner or contractor is
responsible for furnishing water for clean-up operations. Waste material
collected during the grooving operation must not be allowed to enter the
airport storm or sanitary sewer, as the material will eventually clog the
system. Failure to remove the material can create conditions that will be
hazardous to aircraft operations.
Groove
deterioration
2.3.9 Periodic
inspections of the grooves by the airport operator should be conducted to
measure the depth and width to check for wear and damage. When 40 per cent of
the grooves in the runway are less than a half their design depth (either 1.5
or 3 mm) and or width for a distance of 500 m, the grooves effectiveness for
preventing aquaplaning has been reduced and corrective action to reinstate the
3 mm or 6 mm groove depth is recommended. Re-grooving of a worn asphalt
pavement may not be feasible without causing an FOD risk; it may be necessary
to resurface and groove full width.
Plastic
grooves and wire comb
2.3.10 Grooves
can be constructed in new PCC pavements while in the plastic condition. The
“plastic grooving” or wire comb, as depicted in Figure A6-6, technique can be
included as an integral part of the paving train operation. A test section
should be constructed to demonstrate the performance of the plastic grooving or
wire combing equipment and set a standard for acceptance of the complete
product.
Technique
2.3.11 Tolerances
for plastic grooving should be established to define groove alignment, depth,
width and spacing. Suggested tolerances are:
a) ± 7.5
mm in alignment for 22 m;
b)
minimum depth 3 mm, maximum depth 9.5 mm;
c) minimum
width 3 mm, maximum width 9.5 mm; and
d)
minimum spacing 28 mm, maximum spacing 50 mm centre to centre.
Tolerances
for wire combing should result in an average 3 mm x 3 mm x 12 mm configuration.
2.3.12 The
junction of groove face and pavement surface should be squared or rounded or
slightly chamfered. Hand-finishing tools, shaped to match the grooved surface,
should be provided. The contractor should furnish a “bridge” for workmen to
work from to repair any imperfect areas. The equipment should be designed and
constructed so that it can be controlled to grade and be capable of producing
the finish required. If pavement grinding is used to meet specified surface
tolerances, it should be accomplished in a direction parallel to the formed
grooves.
Grooving
runway intersections
2.3.13 Runway
intersections require a decision as to which runway’s continuous grooving is to
be applied. The selection of the preferred runway will normally be dictated by
surface drainage aspects, except that if this criterion does not favour either
runway, consideration will be given to other relevant criteria.
Criteria
2.3.14 The
main physical criterion is surface drainage. Where drainage characteristics are
similar for the grooving pattern of either runway, consideration should be
given to the following operational criteria:
a) aircraft
ground speed regime;
b)
touchdown area; and
c) risk
assessment.
Surface
drainage
2.3.15 The
primary purpose of grooving a runway surface is to enhance surface drainage.
Hence, the preferred runway is the one on which grooves are aligned closest to
the direction of the major downslope within the intersection area. The major
downslope can be determined from a grade contour map.
2.3.16 The
above aspect is essential because intersection areas involve, by design, rather
flat grades (to satisfy the requirement to provide smooth transition to
aircraft travelling at high speeds) and, therefore, are susceptible to water
ponding.
2.3.17 Where
appropriate, consideration may be given to additional drainage channels across
the secondary runway where the groove pattern terminates in order to prevent
water from this origin from affecting the intersection area.
Aircraft
speed
2.3.18 Since
grooving is particularly effective in the high ground speed regime, preference
should be given to that runway on which the higher ground speeds are frequently
attained at the intersection.
Touchdown
area
2.3.19 Provided
the speed criterion does not apply, the runway on which the intersection forms
part of the touchdown area should be preferred because grooving will provide
rapid wheel spin-up on touchdown in particular when the surface is wet.
Risk
assessments
2.3.20 Eventually,
the selection of the primary runway can be based on an operational judgement of
risks for overruns (rejected take-off or landing) taking into account:
a) runway
use (take off/landing);
b) runway
lengths;
c) available
runway end safety areas;
d)
movement rates; and
e) particular
operating conditions.
Diamond
grinding of cement concrete
2.3.21 There
do not appear to be any operational objections to the diamond grinding of
existing PCC surfaces, see Figure A6-7, and this method of treatment seems to
be suitable for all types of aircraft.
2.3.22 It
would be difficult to grind uniformly concrete surfaces which are “rough”.
Pavements with damaged or poorly formed joints, or on which laitance has led to
extensive spalling of the surface, would be equally difficult to grind. If the
existing surface is reasonably free of these defects, there are no other
engineering limitations to grinding.
Effectiveness
of treatment
2.3.23 Transverse
grinding of concrete considerably improves the friction characteristics of
pavements initially textured at the time of construction with burlap or brooms.
The useful life of the treatment depends on the frequency of traffic but in
general the grinding remains effective for the life of the concrete.
Runway
ends
2.3.24 Grinding
should not be performed on the runway ends to make it easier to wash down and
clean off fuel and oil droppings. Moreover, engine blast can be more damaging
on a surface on which grinding has been performed than on an untextured
surface. The directional control of an aircraft moving from the taxiway on to
the runway can become reduced, presumably because of a tendency of the tires to
track in the transverse texture of the runway. In addition, a possibility of an
increase in tire wear in turning cannot be totally discounted.
Technique
2.3.25 An
acceptable “trial” area should be available for inspection and it is
recommended that this be provided at the aerodrome to determine a precise
texture depth requirement, as this will tend to vary with the quality of the
concrete. Grinding is to be performed transversely by a single pass of a
cutting drum, as shown in Figure A6-8, incorporating not less than 50 circular
segmented diamond saw blades per 30 cm width of drum. The drum is to be set at
3 mm setting on a multi-wheeled articulated frame with outrigger wheels, fixed
to give a uniform depth of grinding over the entire surface of the runway to
ensure the removal of all laitance and the exposure of the aggregate. It should
be noted that grinding generates a great deal of dust during treatment and it
is necessary to sweep and wash down the surface before operations restart.
Porous
friction course
2.3.26 The
porous friction course consists of an open-graded, bituminous surface course
composed of mineral aggregate and bituminous material, mixed in a central
mixing plant, and placed on a prepared surface, as shown in Figure A6-9. This
friction course is deliberately designed not only to improve the
skid-resistance but to reduce aquaplaning incidence by providing a
"honeycomb" material to ensure a quick drainage of water from the
pavement surface direct to the underlying impervious asphalt. The porous
friction course is able to maintain over a long period a constant and
relatively high wet friction value due to its porosity and durability.
Limitations
of porous friction course
2.3.27 Friction
courses of this kind should only be laid on new runways of good shape or on
reshaped runways approaching the criteria expected for new runways. They must
always be over densely graded impervious asphalt wearing courses of high stability.
Both of these requirements are necessary to ensure a quick flow of the water
below the friction course and over the impervious asphalt to the runway
drainage channels. In addition, special consideration has to be given to
periodic cleaning of the surface to maintain its porosity and care needs to be
taken during snow and ice removal not to damage the surface.
Runway
ends
2.3.28 The
porous friction course is not recommended at the runway ends. Oil and fuel
droppings would clog the interstices and soften the bitumen binder and jet
engine heat would soften the material which blast would then erode. Erosion
would tend to be deeper than on normal dense asphalt and the possibility of
engine damage through ingestion of particles of runway material should not be
discounted. Scuffing might occur in turning movements during the first few
weeks after laying. For these reasons, it is recommended that runway ends be
constructed of brushed or grooved concrete, or of dense asphalt.
Aggregate
2.3.29 The
aggregate consists of crushed stone, crushed gravel or crushed slag, with or
without other inert finely divided mineral aggregate. The aggregate is composed
of clean, sound, tough, durable particles, free from clay balls, organic matter
and other deleterious substances. The type and grade of bituminous material is
to be based on geographical location and climatic conditions. The maximum
mixing temperature and controlling specification is also to be specified.
Weather
and seasonal limitations
2.3.30 The
porous friction course is to be constructed only on a dry surface when the
atmospheric temperature is 10 °C and rising (at calm wind conditions) and when
the weather is not foggy or rainy.
Preparation
of existing surfaces
2.3.31 Rehabilitation
of an existing pavement for the placement of a porous friction course includes
construction of bituminous overlay, joint sealing, crack repair, reconstruction
of failed pavement and cleaning of grease, oil, and fuel spills. Immediately
before placing the tack-coat, it is critical the underlying course is cleared
of all loose or deleterious material and cleaned with power blowers, power
brooms or hand brooms as directed. A tack coat is to be placed on those
existing surfaces where a tack coat is necessary for bonding the porous friction
course to the existing surface. If emulsified asphalt is used, placement of the
porous friction course can be applied immediately. However, if cutback asphalt
is used, placement of porous friction course must be delayed until the tack
coat has properly aired.
2.4 State practices
2.4.1 Practice of France - bituminous concrete
(Béton Bitumineux pour Chaussées Aéronautiques (BBA)) for surface course
2.4.1.1 Wet
runway surface friction can be obtained from an adequate combination of macro-
and microtexture. Crushing aggregates or specifying high polished stone value
(PSV) aggregates are effective ways of providing and maintaining high
microtexture. High runway surface macrotexture can be obtained from an
appropriate choice of aggregate gradation and mortar mix. European standards
define the composition, performance characteristics and test conditions for
skid resistant bituminous products and mixtures (i.e. NF EN 13 108-1).
2.4.1.2 Standard
NF EN 13 108-1 describes eight types of materials that can be used as aerodrome
pavement surface courses. Four of these, designated as béton bitumineux
aéronautique (BBA), have proven high surface characteristics. BBA can be
continuous or discontinuous grading, each grade with 0/10 mm and 0/14 mm
aggregate sizes and can be used as surface courses in new construction and
overlay. Figure A6-10 shows a typical surface texture of ungrooved
discontinuous graded BBA 0/14.
2.4.1.3 BBA
0/14 achieves mean texture depth (MTD) specifications for runways and rapid
exit taxiways. For any other aerodrome pavement surfacing, BBA 0/10 is also
suitable. The surface characteristics inherent to these products may negate the
operational need for special treatment such as grooving (without qualifying for
specific credit for aircraft performance). They are therefore ready for
trafficking as soon as the material cools down to ambient temperature. BBA
runways are easier to maintain compared to grooved asphalt runways.
2.4.1.4 BBA
is an effective way of meeting the requirements regarding friction and texture
values as constructed, without any supplementary surface treatment. Moreover,
the friction tends to increase during the first year of service due to the wear
of excess binder by traffic. It is less prone to rubber build-up than grooved
materials. Nevertheless, BBA retains moisture for a longer period than grooved
runways. In winter conditions, it means de-icing agents may have to be applied
on a more frequent basis during periods of cold, damp weather.
2.4.2 China - stone matrix asphalt (SMA) for surface course
2.4.2.1 The
main features of a stone matrix asphalt, also called stone mastic asphalt
(SMA), wearing course is its rough surface and large texture depth. The texture
depth of an SMA-16 surface course is not less than 1.2 mm and SMA-13 is not
less than 1.0 mm. The skid resistance of an SMA wearing course is better than a
conventional surface course and is suitable for paving of a new construction
wearing course or wearing course overlaid on existing pavement. In 1996, the
runway 18L/36R at Beijing Capital International Airport was the first major
airport pavement to be overlaid with SMA on deteriorated concrete. Given the
benefits and practicality of this surface course, almost all asphalt runways
used SMA pavement in China, in addition to other countries such as the United
Kingdom, France, Germany, Norway and Singapore. Figure A6-11 shows an SMA
wearing course.
2.4.2.2 SMA
is differentiated from dense-graded mixes by its coarse aggregate skeleton,
consisting of a limited number of particle sizes, which carries the load.
Mastic, consisting of mineral filler, fibres and asphalt binder, fills the
voids between the coarse aggregate skeleton. In addition to good skid
resistance, SMA pavement also has the following advantages:
a) its
high coarse aggregate content interlocks to form a stone skeleton that resists
permanent deformation and fuel spillage, which is able to adapt to the needs of
heavy traffic;
b) its improved
pavement performance includes low-temperature crack resistance, anti-aging
capability, water damage resistance and durability, due to its fibrous
characteristics, higher bitumen content, thicker bitumen film and lower air
voids; and
c) higher
content of mineral filler enhances the bonding capability between bitumen and
aggregate.
2.4.2.3 Figure
A6-12 presents the different composition of dense graded asphalt concrete
(conventional asphalt), SMA and porous asphalt (open graded friction course).
Table A6-1 presents comparative performance results of the three surfaces.
2.4.2.4 The
excellent performance of SMA mixture makes it suitable for most climates,
taking into account the variation of performance requirements with different
environmental conditions. Some performances should be considered emphatically,
such as high-temperature performance in hot climates, low-temperature crack
resistance in cold climates and anti-aging capability against ultraviolet (UV)
in plateaus.
2.4.2.5 SMA
can be paved on runways, taxiways and apron taxiways. Under the conditions of
high temperature or heavy traffic, reinforcing measures, such as polymer
modified bitumen, high-modulus bitumen, lake asphalt and anti-rutting agent,
should be implemented at heavy load zones (i.e. holding positions, runway-end
taxiways and runway
turn pads).
2.4.2.6 In order to keep a good skid
resistance of an SMA wearing course, hard stone (such as basalt, diabase, etc.)
should be chosen as SMA coarse aggregate. The aggregate crushing value should
not be greater than 20 per cent, Los Angeles abrasion loss should not be greater
than 30 per cent, and PSV should not be less than 42 per cent. To bear heavy
traffic, it is necessary to use modified bitumen in hot or cold climates. The
reasonable dosage of modifier should be within the following scope: 4 to 6 per
cent of bitumen weight for polymer modifier bitumen. Asphalt content should be
5.7 to 6.0 per cent in cold climates, and 5.5 to 5.7 per cent in hot climates.
Fibre stabilizers of SMA include cellulose, mineral and chemical fibre, which
adsorb bitumen to improve water resistance and anti-aging resistance of asphalt
mixture. Usually fibre content of SMA is 0.3 to 0.5 per cent of mixture weight.
The recommended SMA mixture gradation is shown in Table A6-2.
2.4.2.7 SMA
mixture should meet the technical requirements listed in Table A6-3.
2.4.2.8 For
SMA-16, the optimum thickness after compaction is 6 cm (maximum is 7 cm and
minimum is 5 cm). For SMA-13, optimum thickness after compaction is 5 cm
(maximum is 6 cm and minimum is 4 cm). The SMA mixture should be made under high
temperature conditions meeting the requirements shown in Table A6-4.
2.4.2.9 When
paving an SMA wearing course, the placement width of one paver should not be
more than eight meters. In order to avoid the cold joint, six to seven pavers
are arranged in echelon form for one-time paving full-width pavement on a 45
meter wide runway. Compaction should be accomplished with steel wheel rollers
of a minimum weight of 12 tonnes. Rubber rollers are restricted since the rich
bitumen tends to adhere to the rubber tires and causes excessive grinding of
the mastic. However, when pavement temperature drops between 80 to 100 °C, it
is permissible that rubber rollers are used for additional compaction to heal
pavement voids and prevent water penetration.
2.5 Runway contaminant removal
techniques
2.5.1 Depending
on the type and frequency of the air traffic received by the runway, the runway
pavement surface can accumulate contaminants such as oil, fuel and rubber
deposits from aircraft tires. As the rubber deposits accumulate in the
microtexture and macrotexture, runway surface friction can fall below the
specification established by the State, especially when wet. The methods bellow
are used in the removal of runway contaminants.
High
pressure water
2.5.2 A series
of high pressure water jets is aimed at the pavement to blast the contaminants
from the surface. The contaminants can either be flushed off the runway or
picked up by a vacuum truck. Water pressures can vary greatly depending on the
contractor’s equipment. As there are many parameters in high pressure water
removal and the potential to damage the pavement from prolonged treatment, the
treatment being particularly aggressive for cracked bituminous wearing courses,
care must be taken in selecting a contractor with experience, demonstrated
expertise and references.
Chemical
2.5.3 Chemical
solvents have been used successfully for the removal of rubber on both concrete
and asphalt runways. Effective chemicals used on concrete runways have a base
of cresylic acid and a blend of benzene with a synthetic detergent for a
wetting agent. Alkaline chemicals are generally used on asphalt pavements. As
these chemicals are volatile and toxic in nature, extreme care must be
exercised during and after the application. If the chemicals remain on the
pavement too long, the painted areas and the surface could possibly be damaged.
It is also important to properly dilute the chemical solvent that is washed off
the pavement surface so the effluent will not harm surrounding vegetation,
drainage systems or pollute nearby streams and wildlife habitats.
High
velocity impact
2.5.4 Abrasive
particles (usually steel shot) are sent at a very high velocity at the runway
pavement surface which can be adjusted to produce a desired surface texture.
The abrasive is propelled mechanically from the peripheral tips of radial
blades in a high speed, fan-like wheel. The entire operation can be
environmentally clean as it is self-contained; collects and recycles the
abrasive particles and collects any loose contaminants and abrasive dust. The
equipment can be self-contained and mobile and thus can be removed rapidly from
the runway if required by airport operations. In any instance, a trial run
should be carried out in a non-critical area in order to check the suitability
of the equipment settings, the effectiveness of the cleaning, the absence of
damage to the pavement surface texture and the complete removal of the steel
shot, including from any openings/joints/cracks, etc. in the pavement. It is
critical to remove all steel shots to prevent FOD risks.
Mechanical
Removal
2.5.5 Mechanical
grinding that employs the corrugating technique has been successfully used to
remove heavy rubber deposits for both asphalt and concrete runways. This method
(or thin milling) removes a surface layer in depth (3.2 to 4.8 mm) and improves
the surface friction properties.
2.6 Mitigation of magnetic field
distortions
2.6.1 The
presence of massive steel elements in or below the pavements and the use of
magnetic devices to remove metallic elements from the surface have been causing
local distortions to the Earth’s magnetic field. Such distortions, also known
as local magnetic anomalies, can interfere with the aircraft navigation systems
and have been identified as a potential hazard for aircraft operations.
2.6.2 With
reference to the Technical Instructions for the safe Transport of Dangerous Goods
by Air (Doc 9284, paragraph 9.2.1.d) and Packing Instructions 953 for goods
UN 2807 - Magnetised masses, magnetic field local distortions deviating a
compass 4.6 m above the pavement by more than two degrees (equivalent to 0.418
A/m or 0.00525 gauss) may have a significant effect on the direct-reading
magnetic compasses or on the master compass detector units.
2.6.3 There
are four possible methods of removing or attenuating the effect of airport
infrastructure on the Earth’s magnetic field:
a) Each magnetic
anomaly is individually demagnetised making it magnetically neutral. This is a
short term solution as the magnetic anomaly would return over a period of a few
years.
b) Each
individual magnetic anomaly has a permanent demagnetising system installed with
an individual magnetic field sensor to monitor the change in the magnetic
effect of the anomaly over time, and have the demagnetising system adjusted
accordingly.
c) A
sheet of magnetically opaque material (e.g. magnetic shield foil) is placed
over the area of the magnetic anomalies.
d)
Removal of the items, such as steel reinforced mesh or massive steel elements,
that cause the magnetic anomalies.
3. PROTECTION OF ASPHALT
PAVEMENTS
3.1 Purpose
3.1.1 Maintenance
includes preventive and any regular or recurring work necessary to preserve
existing aerodrome pavements in good condition. Replacing individual parts and
mending portions of a pavement are considered minor repair. Typical preventive
and regular or recurring pavement maintenance includes: routine cleaning,
filling and/or sealing of cracks; patching pavement; seal coating; grading
pavement edges; maintaining pavement drainage systems; and restoring pavement
markings. Timely maintenance and repair of pavements is essential in
maintaining adequate load-carrying capacity, preserving sufficient surface
friction under all weather conditions and providing good ride quality necessary
for the safe operation of aircraft and minimizing the potential for FOD.
3.1.2 The
purpose of this chapter is to discuss protection of asphalt pavements from two
types of pavement distress: weathering and ravelling from environmental
oxidation; and oil (fuel and lubricants) spillage, which can be prevented or
minimized by the proactive engagement of the aerodrome and staff in protecting
their asphalt pavement surface.
3.2 Weathering and ravelling
from environmental oxidation
3.2.1 Weathering
and ravelling of the pavement surface is caused by oxidation due to exposure to
the environment, which leads to the problem of pavement produced FOD. Locations
of concern are all pavement where aircraft traffic occurs (runways, taxiways
and aprons) as well as pavement immediately adjacent such as shoulders and
vehicle traffic lanes.
3.2.2 Seventy
to ninety percent of asphalt pavement deterioration and failure are the result
of exposure to the environment and degradation of the asphalt binder
(oxidation). Oxidation occurs when the pavement surface is exposed to oxygen in
the air and water, which attacks the asphalt binder causing it to harden and
become brittle or “breaks down”. Ultraviolet rays from the sun exacerbate this
process, which is often referred to as “ageing”.
3.2.3 Asphalt
(also known as bitumen) comes from the left over fractions in the crude oil
refining process. Technological advances in the refining process extract and
separate increasingly more high value resins and oils, leaving less of these
resins and oils for asphalt. Therefore, left over fractions are fortified and
designed to meet necessary physical properties for asphalt binders,
consequently performing in a reduced capacity when exposed to environmental
effects.
3.2.4 The
asphalt binder (the “glue” that binds the aggregate together) breaks down by
oxidation from the weather, resulting in pavement surface deterioration. The
aggregate literally comes “unglued”; first the fine aggregate, which is
considered “weathering” distress, then the coarse aggregate, which is
considered “ravelling” distress. These distresses produce loose aggregate and
pieces of pavement, which is considered pavement generated FOD.
Treatment
3.2.5 This
problem can be substantially reduced if the oxygen in the air and water are not
allowed to
come in
contact with the asphalt binder in the pavement surface. Protective coatings
have accordingly been developed to provide a barrier between the environment
and the pavement surface, which assists to minimize the effects of oxidation.
Protective
coatings
3.2.6 Liquid
coatings (“seal coat”) to assist against oxidation have been developed by
incorporating at least 20 per cent natural asphalt (known as gilsonite,
uintaite, rock asphalt, etc.) with refined asphalt, which is used as an emulsified
asphalt sealer; typically described as emulsified asphalt seal coat. This
material should be applied by using asphalt distributor equipment. In small
areas, the material can be applied by using hand sprayers or by pouring it on
the surface and spreading it using squeegees or brushes.
3.2.7 The
emulsified refined asphalt can be substituted with a solvent-based asphalt by
incorporating at least 40 per cent natural asphalt.
Note:
Local environmental regulations should be taken into consideration if
solvent-based asphalt products are considered to be used.
3.2.8 Other
modified asphalt emulsified seal coats such as a polymer modified without
incorporating natural asphalt have also been used; however to achieve desirable
results, this material must be applied using at least two applications, the
second coat being applied immediately after the first coat has dried to the
touch (and the third, if used, after the second is dried to the touch).
3.2.9 Coating
materials in emulsion form can be extended and premixed with fine aggregate to
form a slurry and applied as a slurry seal.
Protection
gains and concerns
3.2.10 Seal
coats will reduce skid resistance immediately when applied but will improve and
typically obtain acceptable friction test results within the first 24 to 48
hours; which needs to be considered when the application is on a runway or high
speed exit taxiway. Solvent-based treatments will typically obtain acceptable
friction test results within two to three hours. Skid resistance will continue to
improve and obtain similar skid resistance as prior to application typically
within about one week to three months.
3.2.11 For
the application of a seal coat surface treatment on the runway and high speed
exit taxiway, the application of a suitable aggregate to maintain initial
adequate surface friction for the first few hours or days must be included.
When aggregate is to be applied, it must be spread by an asphalt distributor
truck equipped with an aggregate spreader mounted to the distributer truck that
can apply sand to the emulsion in a single pass operation without driving
through wet emulsion.
3.2.12 Application
rates of seal coats will vary from location to location and with various
pavement conditions, age and so on. Therefore, test areas or sections should be
performed for each location to provide the contractor and the engineer an
opportunity to determine the quality of the mixture in place, the quantity
actually needed as well as the performance of the equipment. This is also used
to document skid resistance acceptance if the application is to be on a runway
or high speed exit taxiway.
3.2.13 The
decision to apply a treatment, or not, particularly to a runway, is a balance
between the risk of adversely impacting skid resistance versus the risk of FOD
being generated from the surface. Factors to consider include the existing
asphalt surface condition, climatic conditions, aircraft aborted take-off
distance required versus accelerate-stop distance available (ASDA) and timing
of asphalt resurfacing. The residual from past multiple treatments, even many
years after their application, must be considered as they may contribute to
friction concerns.
3.3 Oil spillage distress from
fuel and lubricants spillage
3.3.1 Fuels
and lubricants contain solvents which will effectively dissolve the asphalt
binder and temporarily reduce its hardness when in contact with an asphalt
pavement surface. Locations of concern are those areas where aircraft are
regularly fuelled, parked or serviced. The areas for landing and taxiing
operations will not be of concern, since even spillages due to aircraft
accidents will be minimized by clean-up and a single spillage will cure without
permanent damage.
3.3.2 The
severity of problems is related to the degree of exposure to the penetrating
solvents, therefore, concern is with the frequency of spillage repeated on one
location, the length of time the spilled fuel or oil remains on or in the
pavement, and the location and extent of spillage on the pavement. A single
spillage of jet fuel and even several spillages in the same location when there
is time for evaporation and curing between spillages do not normally have a
significant adverse effect on the pavement. However, some staining and a tender
(temporarily softened) pavement are to be expected while the solvents evaporate
and the asphalt re-hardens.
3.3.3 Spillages
can result from routine operations such as engine shut-down, fuel tank sediment
draining, consistent use of solvents for cleaning of engine or hydraulic system
elements, etc. More commonly spillage is the result of fuel handling
operations, of spilled oil or hydraulic fluid, or accumulated drippings from
engine oil leakage or mishandling.
3.3.4 In
areas where spillage occurs repeatedly or spilled fuel or oil remains for long
periods of time on the pavement the solvent action softens the asphalt and
reduces adhesion to the surface aggregate. The result of the spillage can be
shoving of the asphalt mix, tire tread printing, tracking of asphalt to
adjacent areas or production of loose material, and pavement abrasion also
producing loose material on the pavement surface. In maintenance and work
areas, asphalt and grit picked up by tools, shoes and clothing can be
transferred to mechanical systems.
Treatment
3.3.5 The
best treatment is avoidance of spillage, which is possible in many cases of
operational spillage and some accidental spillage. Fuel tank sediment drainage
can be caught and need not be allowed on the pavement. Drip pans can be used
for oil drip locations and for bleeding or servicing of hydraulic systems.
Trays may be practical to catch engine shut-down spillage or small quantities
of refuelling spillage.
3.3.6 Removal
of the spilled fuel or oil and reduction of exposure through clean-up is the
next aspect of treatment. There are a number of ways that spilled fuel or oil
can be cleaned off and removed from the pavement; ranging from wiping-up a
small spill with detergents, to a vacuuming process, with suitable equipment
which can be used to remove spilled fuel and possibly some fuel recovery, to
absorbent materials which can also pick up fuel and oil with suitable
arrangement for disposal.
Note:
Local environmental regulations should be taken into consideration for both
removal and disposal of contaminants.
Protective
coatings
3.3.7 Spillage
problems cannot develop if spilled fuel or oil is not allowed to come in
contact with the asphalt in the pavement surface. Protective coatings have
accordingly been developed to provide a barrier between the fuel or oil and the
pavement, which assists to minimize the effects by the spilled fuel or oil.
3.3.8 Thin
overlays of fuel-resistant hot mix asphalt pavement or other materials that may
not be affected by spillage can be applied to protect asphalt pavements.
Conventional construction methods are applicable unless some very
unconventional materials are employed.
3.3.9 Some
States or local vicinities may still allow a coal tar pitch liquid coating,
which is used as an emulsified sealer and is the basic ingredient in various
commercially offered “coal tar” sealers. Coating materials in emulsion form can
be extended and premixed with fine aggregate to form a slurry and applied as a
slurry seal.
Note:
Local environmental regulations or prohibitions should be taken into consideration
if coal tar products are considered to be used on pavement surfaces.
Protection
gains and concerns
3.3.10 Durability
and wear can vary with the materials and applications, the surface cleaning and
preparation, maintenance of the protective coating and of course exposure to
spillage and traffic.
3.3.11 For
coal tar-based liquid coating some material formulations and application
methods, either individually or in concert, can result in imperfect coverage by
the seal coating. Bubbles can exist at application leaving holes in the
coating, or bubbles can form beneath a coating after cure and on “breaking”
leave holes. Coal tar coatings can shrink and crack. Improper surface cleaning
can result in a poor bond and peeling of the coating. Pre-existing cracks in
the coated pavement will tend to come through the protective surface coating.
3.3.12 When
fuel can gain access through holes or cracks in the coating, through peeled
areas, or through cracks reflected from the lower pavement, or when fuel
saturated pavement has not been removed and is covered by the coating,
conditions are worsened rather than improved by the seal since, in addition to
not preventing access of the spilled fuel or oil to the asphalt, the coating
greatly inhibits the evaporation and curing-out of the spillage.
3.3.13 Overlays
and slurry seals give spillage protection and are not subject to bubble holes,
peeling or wear through. Coal tar slurry seals are subject to shrinkage,
cracking and to crack reflection from underlying pavements. Overlays described
previously do not have these inherent issues as long as they are properly
compacted and having void content of about two per cent.
4. CONSTRUCTION OF ASPHALT
OVERLAYS DURING OPERATIONS CLOSURES
4.1 Introduction
4.1.1 The
volume and frequency of operations at many airports makes it virtually
mandatory to repair movement areas portion by portion during short periods of
traffic operation closure. The purpose of this chapter is to provide guidance
for the procedures to be used by those associated with such short term repair,
namely the airport manager, project manager, designer, inspector, material
testing technicians and contractors to ensure that the work is carried out
safely and most efficiently without loss of revenues, inconvenience to
passengers or delays to the air traffic systems.
4.2 Airport authority's role
Project
coordination
4.2.1 Off-peak
construction is, by its very nature, a highly visible project requiring close
coordination with all elements of the airport during planning and design and
virtually daily during construction. Once a runway paving project has been
identified by the airport, it is important that the nominees of the airport
authority, users and the civil aviation authority of the State meet to discuss
the manner in which construction is to be implemented. The following key
personnel should be in attendance at all planning meetings:
a) from
the airport authority: the project manager, the operations, planning,
engineering and maintenance directors;
b) from
the airlines: local station managers and head office representatives where
appropriate; and
c) from
the civil aviation authority: representatives from air traffic services and
aeronautical information services.
4.2.2 The
agenda should include:
a) determination
of working hours. Since time is of the essence in off-peak construction, the
contractor should be given as much time as possible to overlay the pavement each
work period. A period of eight hours is generally considered. Work should be
scheduled for a time period that will displace the least amount of scheduled
flights. The selection of a specific time period should be developed and
coordinated with airline and other representatives during the initial planning
meetings. Early identification of the hours will allow the airlines to adjust
future schedules, as needed, to meet construction demands. It is essential that
the runway be opened and closed at the designated time without exception, as
airline flight schedules, as well as the contractor's schedules, will be
predicated on the availability of the runway at the designated time;
b)
identification of operational factors during construction and establishment of
acceptable criteria should include:
1)
designation of work areas;
2)
aircraft operations;
3)
affected navigation aids (visual and non-visual aids);
4)
security requirements and truck haul routes;
5)
inspection and requirements to open the area for operational use;
6)
placement and removal of construction barricades;
7)
temporary aerodrome pavement marking and signing;
8)
anticipated days of the week that construction will take place; and
9)
issuance of NOTAM and advisories;
c) lines
of communication and coordination elements. It is essential that the project
manager be the only person to conduct coordination of the pavement project. The
methods and lines of communication should be discussed for determining the
availability of the runway at the start of each work period and the condition
of the runway prior to opening it for operations;
d)
special aspects of construction including temporary ramps and other details as
described herein;
e) contingency
plan in case of abnormal failure or an unexpected disaster;
f) coordination
of necessary pre-investigation measures for determining the thickness and
quality of the respective pavement sample; and
g) ensure
that design is carried out by competent staff, including identification of
tasks and workflow and production of a quality assurance plan.
Role of
project manager and resident engineer
4.2.3 It
is essential that the airport authority select a qualified project manager to
oversee all phases of the project, from planning through final inspection of
the completed work. The qualification of the project manager could be assessed
via international organizations. This individual should be experienced in
design and management of aerodrome pavement construction projects and be
familiar with the operation of the airport. The project manager should be the
final authority on all technical aspects of the project and be responsible for
its coordination with airport operations. All contact with any element of the
airport authority should be made only by the project manager so as to ensure
continuity and proper coordination with all elements of aerodrome operations.
Responsibilities should include:
a) planning
and design;
1)
establishment of clear and concise lines of communications;
2)
participation as a member of the design engineer’s selection team;
3)
coordination of project design to meet applicable budget constraints;
4)
coordination of airport and airlines with regards to design review, including
designated working hours, aircraft operational requirements, technical review
and establishment of procedures for coordinating all work; and
5)
chairmanship of all meetings pertaining to the project;
b)
construction;
1)
complete management of construction with adequate number of inspectors to
observe and document work by the contractor;
2)
checking with the weather bureau, airport operations and air traffic control
prior to starting construction and confirming with the contractor’s
superintendent to verify if weather and air traffic conditions will allow work
to proceed as scheduled;
3)
conferring with the contractor’s project superintendent daily and agreeing on
how much work to attempt, to ensure the opening of the runway promptly at the
specified time each morning. This is especially applicable in areas where
pavement repair and replacement are to take place; and
4)
conducting an inspection with airport operations of the work area before
opening it to aircraft traffic to ensure that all pavement surfaces have been
swept clean, temporary ramps are properly constructed and marking is available
for aircraft to operate safely.
The
project manager has to monitor during the entire construction that
country-specific regulations and minimum standards are met (i.e. in Germany,
ZTV-Asphalt, Chapter 5 or ZTV-Concrete, Chapter 3.5 or United States’ practice
mentioned in Chapter 4 of this manual).
4.2.4 The
designation of a resident engineer, preferably a civil engineer, will be of
great benefit to the project and of great assistance to the project manager.
Duties of the resident engineer should include:
a) preparation
of documentation on the work executed during each work period;
b)
ensuring all tests are performed and results obtained from each work period;
c) scheduling
of inspection to occur each work period;
d)
observing contract specifications compliance and reporting of any discrepancies
to the project manager and the contractor; and
e) maintaining
a construction diary.
Testing
requirements
4.2.5 At
the end of each work phase and prior to the start of operation, an acceptance
test must be carried out and the results must be checked before the start of
operation. These procedures normally will require additional personnel to
ensure that tests are performed correctly and on time.
4.2.6 The
review of the quality of raw materials is carried out according to country-
specific regulations. Here, minimum standards should be respected. (i.e. in
Germany, TL-asphalt or TL-concrete). For the installation of fast-hardening concrete
and a monitoring by means of sensors (maturity computer), refer to Dutch
standard NEN 5970.
Inspection
requirements
4.2.7 One
of the most important aspects of successful completion of any kind of paving
project is the amount and quality of inspection performed. Since the airport
accepts beneficial occupancy each time the runway is open to traffic,
acceptance testing must take place each work period. In addition to the project
manager and resident engineer, the following personnel are recommended as a
minimum to observe compliance with specifications:
a) Material
plant inspector (asphalt, concrete). A plant inspector with a helper whose
primary duty it will be to perform quality control tests, including aggregate
gradation, hot bin samples and Marshall tests (e.g. the taking of samples from
the gas reservoir and the Marshall tests), slump, reading maturity computer,
compressive strength test.
b) Inspector
for paver or manual installation. Per machine and/or hand box. There should
be two paving inspectors with each paving machine. Their duties should include
collection of delivery tickets, checking temperatures of delivered material,
inspection of grade, control methods, and inspection of asphalt or concrete
lay-down techniques and joint construction smoothness.
Note: It
is recommended to carry out a field test run in advance of the construction
work to test the installation conditions and the material.
c) Compaction
inspector. The compaction inspector should be responsible for observing
proper sequencing of rollers and for working with a field density meter to
provide the contractor with optimum compaction information.
d) Survey
crew. Finished grade information from each work period is essential to
ensuring a quality job. An independent registered surveyor and crew should
record levels of the completed pavement at intervals of at least 8 m
longitudinally and 4 m transversely, and report the results to the project
manager at the completion of each work period.
e) Pavement
repair inspector. Shall be responsible for inspection of all pavement
repairs and surface preparation prior to paving.
f) Electrical
inspector. Ensures compliance with specifications.
4.3 Design considerations
4.3.1 Plans
and specifications for pavement repair and overlay during off-peak periods
should be presented in such detail as to allow ready determination of the
limits of pavement repair, finish grades and depths of overlay. Plans and
specifications are to be used for each work period by the contractor and
inspection personnel, and should be clear and precise in every detail.
Pavement
survey
4.3.2 A
complete system of bench marks must be set on the side of the runway or taxiway
to permit a ready reference during cross-sectioning operations. The bench marks
should be set at approximately 125 m intervals. Pavement cross-sectioning
should be performed approximately at 8 m intervals longitudinally, and 4 m
intervals transversely. Extreme care should be exercised in level operations,
since the elevations are to be used in determining the depth of asphalt
overlay. The designer should not consider utilizing grade information from
previous as-built drawings or surveys that were run during the winter months,
as it has been shown that elevations can vary from one season to the next. This
is especially critical for single lift asphalt overlays.
4.3.3 After
finish grades and transverse slope of the runway are determined, a tabulation
of grades should be included in the plans for the contractor to use in bidding
the project and for establishment of erected stringline. The tabulation of
grades should include a column showing existing runway elevation, a column
showing finish overlay grade and a column showing depth of overlay. Grades
should be shown longitudinally every 8 m and transversely every 4 m. This item
is considered essential in the preparation of plans for contracting off-peak
construction.
Special
details
4.3.4 Details
pertaining to the items below should be included in the plans.
4.3.4.1 Temporary
ramps. At the end of each hot mix asphalt concrete overlay work period, it
will be necessary to construct a ramp to provide a transition from the new
course of overlay to the existing pavement.
The only
exception to construction of a ramp is when the depth of the overlay is 4 cm or
less. In multiple lift overlays, these transitions should be no closer than 150
m to one another. As far as possible, the overlay should proceed from one end
of the runway toward the other end in the same direction as predominant
aircraft operations so that most aircraft encounter a downward ramp slope. See
Figures A6-13 and A6-14.
4.3.4.2 In-pavement
lighting. Details depicting the removal and reinstallation of in- pavement
lighting are to be included on the plans where applicable. The details should
depict the removal of the light fixture and extension ring, placement of a
target plate over the light base, filling the hole with hot mix dense graded
asphalt until overlay operations are complete, accurate survey location
information, core drilling with a 10 cm core to locate the centre of the target
plate and final coring with an appropriate sized core machine. The light and
new extension ring can then be installed to the proper elevation.
4.3.4.3 Runway
markings. During the course of off-peak construction of a runway overlay,
it has been found acceptable, if properly covered by a NOTAM, to mark only the
centre line stripes and the runway designation numbers on the new pavement
until the final asphalt lift has been completed and final striping can then be
performed. In some cases where cold planing of the surface or multiple lift
overlays are used, as many as three consecutive centre line stripes may be
omitted to enhance the bond between layers.
4.3.4.4 Construction
with concrete: In addition to the renewal of the pavement by asphalt
concrete material can be used. Through the development of concrete mix, fast-
hardening concrete manufacturing is now applicable. For this purpose, an
advance concrete formulation research should take place to enable complete
construction within the available timeframe (minimum ten hours). During
installation, the required minimum compressive strength can be determined by
means of sensors (maturity computer).
4.3.4.5 Scope
of the pre-investigation: the pre-investigation of structure and subsoil
should include the parameters sustainability, forestry sensitivity and
water-sensitivity. The extent depends on the country-specific requirements (for
example, in Europe, Euro Code 7). It is also recommended to perform a PCR
calculation ahead of the pavement repair.