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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).
|
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.