AUSTROADS INTERNAL REPORT
Guidelines for Road Network Condition Monitoring: Part 3 – Pavement Strength
IR-88/05
GUIDELINES FOR ROAD NETWORK CONDITION MONITORING: PART 3 — PAVEMENT STRENGTH (SEALED GRANULAR PAVEMENTS)
Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
First Published 2005
© Austroads Inc. 2005 This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without the prior written permission of Austroads.
Austroads Internal Report
Austroads Project No. AS1122 Austroads Publication No. IR–88/05
Project Manager Ron Ferguson, RTA NSW
Prepared by Tim Martin, ARRB Group Ltd L.B. Dowling & Associates
Published by Austroads Incorporated Level 9, Robell House 287 Elizabeth Street Sydney NSW 2000 Australia Phone: +61 2 9264 7088 Fax: +61 2 9264 1657 Email:
[email protected] www.austroads.com.au
Austroads believes this publication to be correct at the time of printing and does not accept responsibility for any consequences arising from the use of information herein. Readers should rely on their own skill and judgement to apply information to particular issues. This report has been produced as an Austroads Internal Report and whilst not confidential or specifically restricted, it is not intended for public release or general circulation.
GUIDELINES FOR ROAD NETWORK CONDITION MONITORING: PART 3 — PAVEMENT STRENGTH (SEALED GRANULAR PAVEMENTS)
Sydney 2005
Austroads profile Austroads is the association of Australian and New Zealand road transport and traffic authorities whose purpose is to contribute to the achievement of improved Australian and New Zealand road transport outcomes by: ♦ undertaking nationally strategic research on behalf of Australasian road agencies and communicating outcomes ♦ promoting improved practice by Australasian road agencies ♦ facilitating collaboration between road agencies to avoid duplication ♦ promoting harmonisation, consistency and uniformity in road and related operations ♦ providing expert advice to the Australian Transport Council (ATC) and the Standing Committee on Transport (SCOT).
Austroads membership Austroads membership comprises the six state and two territory road transport and traffic authorities and the Commonwealth Department of Transport and Regional Services in Australia, the Australian Local Government Association and Transit New Zealand. It is governed by a council consisting of the chief executive officer (or an alternative senior executive officer) of each of its eleven member organisations: ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦
Roads and Traffic Authority New South Wales Roads Corporation Victoria Department of Main Roads Queensland Main Roads Western Australia Department of Transport and Urban Planning South Australia Department of Infrastructure, Energy and Resources Tasmania Department of Infrastructure, Planning and Environment Northern Territory Department of Urban Services Australian Capital Territory Commonwealth Department of Transport and Regional Services Australian Local Government Association Transit New Zealand
The success of Austroads is derived from the collaboration of member organisations and others in the road industry. It aims to be the Australasian leader in providing high quality information, advice and fostering research in the road sector.
Acknowledgement Austroads wishes to acknowledge that this document is based on ARRB TR Contract Report No RC2410/1 dated March 2004 prepared by Tim Martin, and on work by Paul Robinson from 1999 to 2001 for Austroads under Project BS.AC.007 and by Tim Martin in 2002 for Austroads under Project BS.AC.025, as summarised in the Austroads Technical Report Pavement Strength in Network Analysis of Sealed Granular Roads: Basis for Austroads Guidelines (Austroads 2003b).
Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
EXECUTIVE SUMMARY ♦
Austroads recognises network level data on road pavement strength as one of a number of important inputs to a range of asset management decision tools, and in a number of corporate performance indicators used by road agencies.
♦
The purpose of these guidelines is to promote consistency and improved quality in estimating, reporting and using pavement strength in network level asset management throughout Australia and New Zealand.
♦
In these guidelines, the term “pavement strength” refers to the ability of a pavement structure to resist wheel loads that are applied to it, and is generally synonymous with structural capacity.
♦
The guidelines outline a 7-step process for estimating pavement strength parameters, starting from a decision that network level strength information is needed.
♦
Network level pavement strength parameters are estimated primarily from measurements of surface deflection using standard loading and other standard test procedure details. The guidelines describe the estimation from surface deflection data of Modified Structural Number (SNC) and Adjusted Structural Number (SNP) as the most commonly used network level pavement strength parameters.
♦
The frequency of network level deflection surveys is covered in the guidelines in Section C.
♦
The guidelines recognise the importance of the longitudinal sample spacing between deflection tests in a network survey. The proportion of a network to survey is also covered in Section C.
♦
While these guidelines are intended to be as independent as possible of the technology used for measuring surface deflection, they have been prepared in the context that three methodologies are used for measuring surface deflection in road network surveys in Australia and New Zealand, viz Benkelman Beam, Deflectograph, and Falling Weight Deflectometer.
♦
The guidelines discuss the relative merits of these devices, and contain suggested default relationships between deflection data collected by the three different devices, with a warning that the relationships should used with caution and ideally verified by experimental observations.
♦
The guidelines include preferred procedures for verification of deflection data, to ensure quality data from each network survey.
♦
Details in these guidelines for distance verification are the same as in other guidelines in this Austroads series on road condition monitoring at a network level.
♦
Recognising that current and remaining structural capacity can best be assessed in relation to a defined terminal structural condition, the guidelines suggest indicative investigation levels for deflection, rutting, roughness and cracking as an interim surrogate for terminal structural condition, as these are the common distresses most likely to be associated with structural deterioration.
♦
For information rather than for practical use, Appendix 3 in these guidelines describes an ‘Interim Model’ for predicting structural deterioration of sealed granular road pavements. The model has been postulated using preliminary information. More comprehensive and longterm pavement performance monitoring and data collection is necessary to enable the ‘Interim Model’ to be tested, confirmed or modified, and calibrated for practical use.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
♦
A glossary of terms used with information on network level assessment of road pavement strength is in Section B.
♦
The technology of road condition monitoring worldwide is continuing to develop. Austroads encourages innovation, and promotes the coordinated introduction of improved practices. These guidelines are therefore expected to be subject to ongoing review.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
TABLE OF CONTENTS Page
SECTION A
ROAD PAVEMENT STRENGTH................................................................................1
A.1 Introduction
.............................................................................................................1
A2. The Need and Application of Network Pavement Strength ...............................................3 A3 Objective
.............................................................................................................4
SECTION B
GLOSSARY OF TERMS USED IN THE NETWORK-LEVEL ASSESSMENT OF SEALED GRANULAR ROAD PAVEMENT STRENGTH ................................5
SECTION C
GUIDELINES FOR NETWORK ASSESSMENT OF ROAD PAVEMENT STRENGTH ...........................................................................................................8
C.1 What is Pavement Strength? .............................................................................................8 C1.1 Guidelines .....................................................................................................8 C1.2 Background Notes ........................................................................................9 C2 Equipment for Measuring Pavement Deflection ..............................................................12 C2.1 Guidelines ...................................................................................................12 C2.2 Deflection measuring equipment – general ................................................14 C2.3 Benkelman Beam (BB) ...............................................................................14 C2.4 Deflectograph (DEF) ...................................................................................16 C2.5 Falling Weight Deflectometer (FWD and HWD)..........................................18 C2.6 Relative merits of Benkelman Beam, Deflectograph and FWD ..................21 C3 Frequency of Pavement Deflection Surveys....................................................................23 C3.1 Guidelines ...................................................................................................23 C3.2 Background notes .......................................................................................23 C4 Scope of Pavement Deflection Surveys ..........................................................................25 C4.1 Guidelines ...................................................................................................25 C4.2 Background notes .......................................................................................25 C5 Relationships between Measures of Pavement Deflection..............................................32 C5.1 Guidelines ...................................................................................................32 C5.2 Background notes .......................................................................................33 C6 Verification of Distance Measurement .............................................................................36 C6.1 Guidelines ...................................................................................................36 C6.2 Background notes .......................................................................................36 C6.3 Recording deflection test locations .............................................................37 C7 Verification Testing for Deflection ....................................................................................39 C7.1 Guidelines ...................................................................................................39 C7.2 Background notes .......................................................................................40 C8 Repeatability and Bias .....................................................................................................42 C8.1 Guidelines ...................................................................................................42 C8.2. Background notes ................................................................................................42
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C9 Data Reporting ...........................................................................................................43 C9.1 Guidelines ...................................................................................................43 C9.2 Background notes .......................................................................................44 SECTION D
SUMMARY
...........................................................................................................46
D1 Sampling and Measurement of Pavement Deflections ....................................................46 D2 Estimation of Network Level Pavement Strength Parameters .........................................48 D3 Interim Structural Deterioration Model for Sealed Granular Pavements..........................48 REFERENCES AND BIBLIOGRAPHY ...........................................................................................49 APPENDIX 1: 1.1 1.2 1.3 1.4
ESTIMATING PAVEMENT STRENGTH PARAMETERS FROM DEFLECTION DATA................................................................................54 Modified Structural Number, SNC ...................................................................................54 Adjusted Structural Number, SNP ...................................................................................55 Structural Adequacy Indicator, SAI..................................................................................57 Relative Pavement Strength (RPS) Indicator ..................................................................58
APPENDIX 2: COST 336 PROCEDURES FOR REPEATABILITY TESTING WITH FWDs ......59 2.1 COST 336 Protocol U2-1999: FWD Short Term Repeatability Verification ....................59 2.2 COST 336 Protocol U3-1999: FWD Long Term Repeatability Verification.....................62 APPENDIX 3:
INTERIM STRUCTURAL DETERIORATION MODEL FOR SEALED GRANULAR PAVEMENTS .........................................................66 3.1 Background to ‘Interim Model’ Development ...................................................................66 3.2 Basis of Interim Structural Deterioration Model ...............................................................67 3.3 Model Calibration ...........................................................................................................71
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
TABLES Page
Table 1
Process for estimating pavement strength parameters for network level asset management planning ...................................................... 2
Table 2
Indicative investigation condition (interim surrogate for terminal structural condition) ............................................. 11
Table 3
Summary of the main features of Benkelman Beams, Deflectographs, and Falling Weight Deflectometers............................................ 13
Table 4
Target FWD test loads and corresponding surface stresses............................... 20
Table 5
Suggestions for performance indicators to undertake discrete network level sampling...................................................... 30
Table 6
Summary of deflection relationships.................................................................... 35
Table 7
COST 336 procedures for FWD calibration and verification................................ 41
Table 8
Extract from report on network level FWD survey ............................................... 44
Table 1.1
Sample values of mean characteristic maximum deflection (D0) and corresponding SNC for unbound sealed granular pavements ............................. 54
Table 3.1
Impact of granular resheeting on pavements ...................................................... 67
FIGURES Figure 1
Pavement deflection bowl (not to scale) .................................................................. 9
Figure 2
General view of Benkelman Beam (BB) with load truck and trolley, and sketch of BB arrangement .............................................................................. 15
Figure 3
Benkelman Beams (BB) with automated and manual deflection recording ........... 15
Figure 4
Host truck with loaded rear axle and Deflectograph sled in front of rear axle ....... 17
Figure 5
DEF with RWP beam shortly after starting a deflection bowl measurement.......... 17
Figure 6
Deflectograph with LWP beam positioned about the middle of a deflection bowl measurement ............................................. 18
Figure 7
Deflectograph with RWP beam positioned near the finish of a deflection bowl measurement ................................................. 18
Figure 8
Schematic diagram of a FWD................................................................................ 19
Figure 9
FWD loading plate ................................................................................................. 19
Figure 10
General view of a FWD.......................................................................................... 19
Figure 11
Rear view of a FWD showing loading plate and geophones ................................. 19
Figure 12
FWD deflection sensors (geophones) ................................................................... 20
Figure 13
FWD Sampling locations on a single carriageway two lane road .......................... 27
Figure 14
FWD Sampling locations on a dual carriageway road ........................................... 28
Figure 15
Extract from a graphical report from a Deflectograph survey ................................ 45
Figure 1.1
SNC (Paterson) vs SNP (others) ........................................................................... 56
Figure 3.1
% SNC Deterioration vs Pavement Age / Design Life (varying Pavement Design Life and constant Deterioration Factor ....................... 70
Figure 3.2
% SNC Deterioration vs Pavement Age / Design Life (fixed Pavement Design Life and varying Deterioration Factor) ............................ 71
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
ABBREVIATIONS AND ACRONYMS AADT
Annual average daily traffic (volume), measured in vehicles per day (vpd)
AASHO
American Association of State Highway Officials (forerunner of AASHTO)
AASHTO
American Association of State Highway and Transportation Officials
ACT
Australian Capital Territory
ADT
Average Daily Traffic
ALF
Accelerated Loading Facility
ARRB
ARRB Group Ltd, a research organisation based in Melbourne, Australia.
BB
Benkelman Beam
CAP
Traffic load capacity
CBR
California Bearing Ratio
COST
Cooperation in Scientific and Technical Research (Europe)
COV
Coefficient of Variation, the standard deviation of a population dived by the mean (expressed as a percentage).
D0, , D200, etc
Deflection measurements forming a deflection bowl. D0 is at the load point (the maximum deflection measurement), D200 is 200mm away in the direction of travel, etc.
DEF
Deflectograph
DGPS
Differentially Corrected Global Positioning System
DIER Tas
Department of Infrastructure, Energy and Resources, Tasmania (a member of Austroads)
DIPE NT
Department of Infrastructure, Planning and Environment Northern Territory (a member of Austroads)
DL
Design life
DUS ACT
Department of Urban Services, Australian Capital Territory (a member of Austroads)
Eqn
Equation
ESAs
Equivalent Standard Axles – a measure of traffic loading (mass)
FHWA
Federal Highway Administration (part of the USA Federal Department of Transportation)
FWD
Falling Weight Deflectometer
GPS
Global Positioning System
HDM
Highway Development and Management (formerly Highway Design and Maintenance Standards) models, software and documentation initially developed by the World Bank and released in 1979, based on the Highway Cost Model produced by the Massachusetts Institute of Technology in 1971/72. Managed by PIARC from the late 1980’s.
HDM III
A version of HDM models, software and documentation introduced in 1987. Only HDM-4 is supported since 2000.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
HDM-4
A new version of HDM models, software and documentation developed in the International Study of Highway Development and Management (ISOHDM), managed by the World Road Association (PIARC). PIARC released HDM-4 (Version 1) in 2000. As at early 2005, Version 2 is undergoing development and testing.
HDM Technology
A generic term referring to the collection of published products released by the PIARC ISOHDM project, comprising HDM-4 knowledge, model algorithms, and the HDM-4 software.
HSD
High Speed Deflectograph
HWD
Heavy Weight Deflectometer
IRI
International Roughness Index, a measure of roughness developed in the 1980s by the World Bank and adopted by the World Road Association (PIARC) and Austroads.
ISOHDM
International Study of Highway Development and Management
kN
Kilonewton
kPa
Kilopascal
LTPP
Long Term Pavement Performance (program)
LWP
Left wheel path – the wheel path nearer to the verge (because traffic in Australia and New Zealand drives on the left side of the road), sometimes referred to in the literature as the outer wheel path.
MA
(Austroads) Member Authority
MR
Resilient modulus
MRWA
Main Roads Western Australia (a member of Austroads)
NAASRA
National Association of Australian State Road Authorities (forerunner of Austroads)
NRM
NAASRA Roughness Meter, or NAASRA Roughness Measure (NRM, counts per kilometre, an alternative to IRI as a measure for roughness).
OH&S
Occupational Health & Safety
PaSE
Pavement Strength Evaluator (a Deflectograph owned and operated by VicRoads)
PMS
Pavement Management System
QDMR
Queensland Department of Main Roads (a member of Austroads)
RTA NSW
Roads and Traffic Authority, New South Wales (a member of Austroads)
RWP
Right wheel path – the wheel path nearer to the middle of the road (because traffic in Australia and New Zealand drives on the left side of the road), sometimes referred to in the literature as the inner wheel path.
SAI
Structural Adequacy Index
SHRP
Strategic Highway Research Program (USA)
SN
Structural Number
SNC
Modified Structural Number
SNP
Adjusted Structural Number
TNZ
Transit New Zealand (a member of Austroads)
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
TRL
TRL Limited, Berkshire, UK is part of the Transport Research Foundation group of companies (formerly Transport Research Laboratory (UK), and Transport and Road Research Laboratory (UK))
TSA
Transport South Australia, part of the Department of Transport and Urban Planning South Australia (a member of Austroads)
UK
United Kingdom
UNDP
United Nations Development Programme
USA
United States of America
VicRoads
Road Corporation of Victoria (a member of Austroads)
vpd
Vehicles per day
WDM
WDM Ltd, Bristol, UK, a commercial provider of services in pavement and road asset management.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
SECTION A — ROAD PAVEMENT STRENGTH A.1
INTRODUCTION
Pavement strength is considered to be one of the most important characteristics defining the general condition of a road. These guidelines provide the necessary pavement strength data and information, at a road network planning level for sealed granular pavements, for road owners and practitioners. The guidelines cover the following aspects of pavement strength: Section B
♦
A glossary of terms used in network-level assessment of strength of sealed granular road pavements.
Section C
♦
Sampling, measurement and analysis of network pavement deflection data in Australia and New Zealand with the aim of providing consistency and acceptable quality to the reported deflection and strength parameters.
♦
Descriptions of the Benkelman Beam, Deflectograph, and Falling Weight Deflectometer, being the commonly used devices for measuring pavement deflection and a discussion of their relative merits.
♦
Relationships between deflection data collected by different devices, to enable data from all three devices to be used together.
Appendix 1
♦
The estimation of pavement strength parameters for use in asset management.
Appendix 3
♦
As information only rather than for practical use, an interim structural deterioration model is documented, using the pavement strength parameter SNC to predict the deterioration in network pavement strength of sealed granular pavements. The need for testing, confirmation or amendment, and calibration of this ‘Interim Model’ is acknowledged, and methods are outlined for future improvement of the ‘Interim Model’, based on long term pavement performance monitoring.
These guidelines provide a consistent approach to pavement strength assessment and analysis. With a standard basis for recording and reporting pavement strength at a network level, road agencies, road maintenance contractors and road pavement condition monitoring service providers will have a consistent basis to collect and analyse data and specify measures for improved road asset management. This will lead to better identification of road deterioration characteristics and technical measures required to ensure stronger and longer lasting pavements are built and maintained. These guidelines distinguish deflections (that can be measured and reported using one of three main measurement devices) from pavement strength and estimates of values of pavement strength parameters. Table 1 contains a 7-step outline of the process of estimating pavement strength parameters for network level asset management planning purposes, based on data collected in pavement deflection surveys.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
Table 1: Process for estimating pavement strength parameters for network level asset management planning
Steps in the process of estimating pavement strength parameters
Reference in these guidelines
1
Decision to assess network level strength parameters, based on a deflection survey.
A strategic asset management decision, not covered by these guidelines.
2
Design sampling details by selecting a deflection measuring device, considering timing (season) of survey, selecting the lanes and wheel paths to survey, conducting trial deflection surveys, and establishing the optimal longitudinal sample spacing, or if necessary an optimal sample proportion for a Deflectograph survey.
Sections C2 and C4.
3
Conduct network survey and obtain reports of deflection data with supporting details (measuring device, operator, date, time, location referencing, weather, temperatures, etc).
Sections C2 to C9.
4
Identify and separate lengths with bound base - where pavement configurations are not known, subject to local experience and confirmation, deflection relationships can be used as filters, such as limits on maximum deflection (D0) and deflection ratios such as (D250/D0).
Section C1.2.1.1.
5
For the remaining lengths, analyse the deflection data to identify approximately homogeneous sections (short lengths, depending on the variability of the deflection data), and calculate and report the characteristic maximum deflection for each homogeneous section.
Section C4.2.4.
6
Calculate mean of the characteristic maximum deflection values for each sub-network (longer lengths, eg, management segments (PMS segments), road links, or road types).
Section C4.2.4.
7
Compute strength parameter (eg, normally SNC, and could be SNP or SAI) for each subnetwork or segment, as required for analysis purposes.
Section C1.2.1.2 and Appendix 1.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
A2.
THE NEED AND APPLICATION OF NETWORK PAVEMENT STRENGTH
Many pavement performance characteristics (roughness, cracking, rutting, etc) do not give an accurate assessment of the structural condition of a pavement because they mainly assess surface condition and not structural condition (Eijbersen and Van Zwieten 1998). These surface condition parameters are relatively inexpensive to collect and have traditionally been used to broadly identify suspect areas of the network for detailed structural testing and assessment at a project level. The need for an improved understanding of the structural condition of the whole network for strategic planning is driven by three main trends: ♦ increasing axle mass limits for heavy vehicles (NRTC 1996); ♦ relatively high rates of growth of heavy vehicle traffic on strategic freight routes (Gargett and Perry 1998); and ♦ some road agencies deciding to include the structural condition of pavements as a network performance indicator (Sapkota et al 2001). The first two of these trends potentially reduce the remaining life of pavements in the network, causing the need for earlier than expected rehabilitation treatments. These treatments are relatively costly and have a major impact on annual road agency budgets, so it is necessary to determine at a network level the likelihood and extent of any major rehabilitation, well in advance of the need. The third point above is sometimes chosen as a contractual requirement for the contractor managing a road network on behalf of the road agency. However, this does not necessarily imply that the network level structural condition of pavements is suitable as the sole input for managing the network on a structural basis. For example, complementary surface distress information is also useful for the assessment of structural condition and to represent other performance criteria (eg, ride comfort).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
A3.
OBJECTIVE
The overall objective of these Austroads Guidelines for Road Condition Monitoring is to promote and ensure a standard set of procedures are followed, so that: ♦
Only useful road condition data is collected for analysis;
♦
Collection and processing of road condition data is cost efficient;
♦
Quality of road condition data is improved; and
♦
The road condition data collected is of increased value to road owners.
These guidelines form part of the series of Pavement Condition Monitoring Guidelines and provide the basis of specifications and recommendations for collecting, analysing and reporting information on pavement strength. The document is structured around the following key sections: ♦
Section B -
♦
Section C – Guidelines for Network Assessment of Road Pavement Strength (Sealed Granular Pavements)
Glossary of Terms used in Network-Level Assessment of Road Pavement Strength (Sealed Granular Pavements)
♦
Section C1 – What is Pavement Strength
♦
Section C2 – Equipment for Measuring Pavement Deflection
♦
Section C3 – Frequency of Pavement Deflection Surveys
♦
Section C4 – Scope of Pavement Deflection Surveys
♦
Section C5 – Relationships between Measures of Pavement Deflection
♦
Section C6 – Verification of Distance Measurement
♦
Section C7 – Verification Testing for Deflection
♦
Section C8 – Repeatability and Bias
♦
Section C9 – Data Reporting
♦
Section D – Summary
♦
Appendix 1 – Estimating Pavement Strength Parameters from Deflection Data
♦
Appendix 2 – COST 336 Procedures for Repeatability Testing with FWDs
♦
Appendix 3 – Interim Structural Deterioration Model for Sealed Granular Pavements.
Each of the topics in Section C is structured to show separately: ♦
The specific Austroads guidelines; and
♦
Background notes on the basis for the guidelines.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
SECTION B — GLOSSARY OF TERMS USED IN THE NETWORK-LEVEL ASSESSMENT OF SEALED GRANULAR ROAD PAVEMENT STRENGTH Term
Interpretation
Adjusted Structural Number (SNP)
A pavement strength parameter, being an enhancement of the Modified Structural Number (SNC), developed by Parkman and Rolt (1997) to address difficulties experienced with the use of SNC for the description of pavements which incorporate lower layers of selected subgrade, or had very thick sub-base or lower sub-base layers. The SNP applies a weighting factor, which reduces with increasing depth, to the subbase and subgrade contributions so that the pavement strength for deep pavements is not over predicted. For pavements less than 700 mm thick the Modified Structural Number (SNC) and the Adjusted Structural Number (SNP) are virtually the same. (Also see ‘Modified Structural Number (SNC)’ and ‘Structural Number (SN)’.)
Benkelman Beam (BB)
An instrument for measuring the deflection of the surface of a pavement caused by the passage of a dual-tyred single-axle carrying a standard axle load (AS 1348:2002).
Bias
A statistical term to indicate whether a device is systematically measuring high or low when compared to a reference set of measures.
Condition monitoring
Continuous or periodic inspection, assessment, measurement, reporting and interpretation of resulting data to indicate the condition of a specific asset in order to determine the need for and nature and timing of maintenance. (Also see ‘condition survey’.)
Condition parameter
A quantifiable expression of a specific parameter of an asset. For example, roughness, rutting, surface texture, cracking, deflection, etc, are pavement condition parameters.
Condition survey
The process of collecting data on the condition of an asset, eg the structural or functional condition of a pavement. (Also see ‘condition monitoring’.)
Curvature
The difference between the maximum deflection (D0) at a test site and the deflection (D200) at a point 200 mm along the road from the point at which the maximum deflection was produced. Curvature gives an indication of the pavement stiffness and therefore the fatigue performance of the pavement.
Deflection
See ‘Pavement deflection’.
Deflection bowl
The depressed shape produced at the surface of a pavement when a load is applied (AS 1348:2002).
Falling Weight Deflectometer (FWD)
A device to measure the surface deflection of a pavement under a dynamic load in order to evaluate its structural adequacy (AS 1348:2002). FWDs are generally capable of imparting a load up to 150 kN. (Also see ‘Heavy Weight Deflectometer HWD)’.)
Granular pavement
A pavement which obtains its load spreading properties mainly by intergranular pressure, mechanical interlock and cohesion between the particles of the pavement material, which is gravel or crushed rock graded so as to be mechanically stable, workable and able to be compacted, and generally with a particle size no smaller than sand (adapted from AS 1348:2002).
HDM
Highway Development and Management (formerly Highway Design and Maintenance Standards) models, software and documentation initially developed by the World Bank and released in 1979, based on the Highway Cost Model produced by the Massachusetts Institute of Technology in 1971/72. Managed by PIARC from the late 1980’s.
HDM-4
A new version of HDM models, software and documentation developed in the International Study of Highway Development and Management (ISOHDM), managed by the World Road Association (PIARC). PIARC released HDM-4 in 2000.
Heavy Weight Deflectometer (HWD)
A device similar to the Falling Weight Deflectometer (FWD), but capable of imparting a greater load, up to 250 kN. (Also see ‘Falling Weight Deflectometer (FWD)’.)
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
Term
Interpretation
ISOHDM
The International Study of Highway Development and Management, an international project for the development of HDM-4, based at the University of Birmingham, UK, administered by PIARC in Paris, and funded by the World Bank, Asian Development Bank, British Department for International Development, Swedish Government and others.
Indicative investigation condition levels
A suggested interim set of condition levels (Table 2) for use as an interim surrogate to define terminal structural condition or the onset of pavement failure, for the purpose of determining the remaining structural capacity or structural life of a pavement. Planning for maintenance intervention at these condition levels is intended as a means of managing the risk of accelerating deterioration.
Lane
That portion of a carriageway occupied by a single file of traffic travelling in one direction, hence containing two wheel paths. A lane is generally between 3.0 and 3.5 m wide. A single carriageway road normally has at least one lane in each direction.
Link (or road link)
A length of road defined for strategic and reporting purposes, generally of the order of 100 km to 300 km, but can be longer in remote areas (eg, Katherine to Alice Springs (1,100km) and Port Headland to Broome (600 km)).
Management segment
A length of road pavement that is relatively uniform in treatment history, current condition, terrain, and traffic usage, with length generally between 0.5 km and 1.75 km (or up to 5 km in remote areas). (Also see ‘segment’.)
Modified Structural Number (SNC)
A pavement strength parameter, being a refinement of the AASHO Road Test estimation of pavement strength (Structural Number), which directly takes into account the subgrade contribution to pavement strength (Hodges et al 1975). The Modified Structural Number (SNC) is equal to the Structural Number (SN) that would be required if the pavement were to be designed to carry the same traffic on a subgrade with a CBR value of 3%. (Also see ‘Structural Number (SN)’ and ‘Adjusted Structural Number (SNP)’.)
Network level
A type of road condition survey or data analysis where the main purpose is to monitor network performance or assist with network asset management decisions, as distinct from project decisions.
Pavement
The portion of the road placed above the subgrade for the support of and to form a running surface for vehicular traffic. A pavement usually comprises subbase, base and wearing surface layers.
Pavement deflection
The vertical elastic (recoverable) deformation of a pavement surface between the tyres of a standard axle. (This definition is used in pavement design, and relates to Benkelman Beam and Deflectograph.) The elastic (recoverable) vertical movement at the surface of a pavement due to the application of a load (AS 1348:2002).
Pavement stiffness
The resistance to deflection of the pavement structure.
Pavement strength
The ability of a pavement structure to resist the traffic vehicle wheel loads that are applied to it. Pavement strength is often seen as synonymous with structural capacity.
Project level
A type of road condition survey or data analysis where the main purpose is to assist with decisions about proposals for a specific project on a short length of road, as distinct from network decisions.
Repeatability
A statistical term to indicate the extent of variation in outputs about the mean for a single operator using the same method. Repeatability is the standard deviation of measures obtained in repeat tests using the same measuring device and operator on a single, randomly selected road.
Reproducibility
A statistical term to indicate the extent of variation in outputs about the mean for multiple operators or measuring devices using the same method.
Road type
Road types are approximately homogenous sections of road with similar condition, carrying a similar traffic load under similar climatic and subsoil conditions. Consequently, a road network can be made up of a number of road types, the number being dependent on the accuracy required of the analysis and the available computing power to undertake the analysis.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
Term
Interpretation
Segment
The length of pavement considered as a separate entity in a pavement management analysis process. (Also see ‘Management Segment’.)
Structural capacity
A descriptive term indicating the capacity of a pavement to carry traffic before the onset of structural failure or before the pavement deteriorates to a defined terminal condition. (Also see ‘Pavement Strength’.)
Structural Adequacy Indicator
A pavement strength parameter developed by Eijberson and Van Zwieten (1998), and described in Appendix 1.3.
Structural Number (SN)
A pavement strength parameter, developed during the AASHO Road Test (Highway Research Board 1962). SN simply describes the structural capacity of a pavement in a single number, regardless of the details of the materials in the pavement. SN is related to the change in cumulative traffic loading and functional condition of the pavement (AASHTO 1993). AASHTO (1993) estimates of SN for a given traffic load and functional condition account for the contributing support of the subgrade through the use of the resilient modulus, MR, for soil support. (Also see ‘Modified Structural Number (SNC)’ and ‘Adjusted Structural Number (SNP)’.)
Surfacing
The uppermost part of the pavement or bridge deck specifically designed to resist abrasion from traffic and to minimise the entry of water. Sometimes referred to as the wearing surface.
Verification test.
A standardised procedure to test the validity of test results from a measuring device.
Wearing surface
Same as “Surfacing”.
Wheel path
That portion of the pavement that is subject to passage of and loading from vehicle wheels during trafficking. There are two wheel paths per trafficked lane – referred to in this Guidelines document as the ‘left wheel path’ (LWP), nearer to the verge, and the ‘right wheel path’ (RWP), nearer to the middle of the road (because traffic in Australia and New Zealand travels on the left side of the road).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
SECTION C – GUIDELINES FOR NETWORK ASSESSMENT OF ROAD PAVEMENT STRENGTH C.1
WHAT IS PAVEMENT STRENGTH?
C1.1 Guidelines Pavement strength is a measure of the ability of a pavement structure to resist the wheel loads that are applied to it. Pavement strength can be estimated from surface deflection data, though the deflection induced in a pavement by a wheel load is really a measure of the stiffness of a pavement – or the ability of the pavement structure to resist that deflection – rather than its strength. This difference between strength and stiffness is particularly important for flexible pavements because their mechanism of failure varies and the magnitude of the measured deflection (and hence stiffness) of bound (asphalt and cemented) pavements is usually significantly lower than unbound granular pavements. As a result, the relationship between the stiffness and structural performance (and hence strength) is very different. To estimate the strength of unbound granular pavements using deflection data, pavements with cemented bases should be excluded from the survey or the analysis. Knowing which segments should be excluded from a survey can be difficult, however, because the pavement structure of many road segments is unknown. In such cases, using maximum deflection (D0) and deflection ratios such as (D250/D0) (see Figure 1), specific deflection relationships for identifying cemented bases need to be developed and confirmed from deflection measurements at locations where pavement structures with cemented bases are known. At the network level, the strength of assumed homogeneous sections of pavement can be estimated from surface deflection data using a number of indices based on ‘Structural Number’ or ‘Structural Adequacy’ (see Appendix 1). The structural capacity of a pavement, and its remaining capacity, should be assessed in terms of a defined terminal structural condition. While terminal structural condition would be ideally defined in terms of limits on distress levels (deflection, roughness and rutting), for the interim pending a better understanding of terminal condition, these guidelines refer to the indicative investigation levels in Table 2 as the terminal structural condition. The levels in Table 2 are related to the intended level of service or functionality of the pavement. The remaining structural life (years) can then be estimated based on the difference between the existing level of distress and the terminal structural condition if reliable predictions are available for the rates of deterioration of the terminal structural condition parameters. Relationships between remaining structural capacity and traffic levels for Australasian conditions have been developed (Martin 1998, Loizos et al 2002). They can be used to predict the structural condition of pavements. These predictions are the basis for reaching informed decisions regarding the remaining life of the pavement and the necessity and timing of structural intervention (eg, rehabilitation).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C1.2 BACKGROUND NOTES C1.2.1 Derivation of pavement strength parameters from deflection data C1.2.1.1
The effect of bound pavements
Pavement strength can be estimated from surface deflection data, although Paterson (1987) notes that deflection measures stiffness rather than strength. Pavement strength is defined as the ability of a pavement structure to resist the traffic vehicle wheel loads that are applied to it, while pavement stiffness is defined as the resistance to deflection of the pavement structure (Koniditsiotis and Kosky 1996). Pavement strength is often seen as synonymous with structural capacity. The difference between strength and stiffness is particularly important when assessing flexible pavements that vary in their mechanism of failure. The magnitude of the measured deflections (and hence stiffness) of pavements with cemented and unbound bases would usually be significantly different. Pavements with cemented and unbound bases also have different relationships between stiffness and the structural performance that relates to pavement strength. To ensure the validity of assessed values of granular pavement strength parameters, which are based on deflection testing data, sections of pavement with cemented bases should be excluded from the survey or the analysis. In practice, selective network testing can be difficult when the pavement configuration details of many road segments are unknown. However, if a deflection data set is likely to include some tests conducted on pavements with cemented bases, it may be possible to identify and remove these by considering both the magnitude of the maximum deflection, D0, and the ratio of the D250 deflection to the maximum deflection, D250/D0 (see Figure 1). Relatively low maximum deflections are associated with cemented pavements and it would be unusual for these deflections to exceed 0.35 mm (using a Benkelman Beam with a 40 kN test load at a nominal surface stress of 550 kPa) even when the strengths of these configurations are rated as poor. The D250/D0 ratio for Benkelman Beam deflections is > 0.8 for cemented base or asphaltic pavements (Scala 1979). These trends should also apply to Falling Weight Deflectometer and Deflectograph deflections wherever a proportionate relationship with Benkelman Beam deflections is adopted (refer Section C5). 1500mm
1200mm 900mm 600mm 250mm
D0
D300
D200
D250
D600
Figure 1: Pavement deflection bowl (not to scale)
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D900
D1200
D1500
Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
In summary, the strength determination and performance characteristics of bound and unbound pavements differ significantly. For those road networks that include bound and unbound pavements, the deflection data relating to sealed unbound granular pavements should exclude the lengths of pavement where it has been confirmed by sample testing of cemented bases that the measured maximum deflection, D0, and the D250/D0 ratio are less than or greater than specified values. The network strength parameter Modified Structural Number (SNC) of pavements with and without cemented bases can be estimated from deflection data using the specific relationship in Appendix 1.1, which uses different coefficient values for cemented and uncemented (unbound) bases. C1.2.1.2
Network level pavement strength parameters
At the network level, the strength of assumed homogeneous sections of pavement can be estimated from surface deflection data using a number of indices based on ‘Structural Number’ or ‘Structural Adequacy’ (see Appendix 1). Initially the Structural Number, SN, was developed during the AASHO Road Test (Highway Research Board 1962) to define the structural capacity of the Road Test pavements. Pavements with different materials and layer thicknesses and built on the same subgrade and with the same remaining traffic capacity (ESAs) would have the same SN. SN has the advantage that it is related to the change in cumulative traffic loading and functional condition of the pavement (AASHTO 1993). AASHTO (1993) estimates of the SN for a given traffic load and functional condition account for the contributing support of the subgrade through the use of the resilient modulus, MR, for soil support. The AASHO Road Test estimation of pavement strength was further refined by the introduction of the Modified Structural Number, SNC, which directly took into account the subgrade contribution to pavement strength (Hodges et al 1975). The estimation of SNC was enhanced by the development of the Adjusted Structural Number, SNP (Parkman and Rolt 1997), although for pavements less than 700 mm thick the Modified Structural Number, SNC is virtually the same as the Adjusted Structural Number, SNP (Roberts 2000b). Appendix 1 outlines several approaches that can be used to estimate SNC and SNP from bowl deflection data only. Comparisons of the various means of estimating SNC and SNP using either the maximum bowl deflection, D0, or a range of bowl deflections (D0, D900 and D1500) suggest that network level assessment of SNP or SNC could be based on the D0 deflection without any significant loss in accuracy. This outcome also agrees with the findings of Martin and Crank (2001) that the bowl deflections other than D0 do not improve strength parameter estimation with the current strength and deflection relationships. Other parameters, such as the Structural Adequacy Indicator, SAI, also provide a numerical value for comparing pavements mainly based on their deflection data regardless of their initial structure or degree of deterioration (Eijberson and Van Zwieten 1998). Simple relationships, such as the Relative Pavement Strength indicator, RPS, are useful guides to preliminary intervention and testing (Roberts 2000a). Relationships between remaining structural capacity and traffic levels, such as the SNP with traffic load capacity, CAP, have been developed to predict the structural condition of pavements (Martin 1998, Loizos et al 2002). These predictions are the basis of the analysis for informed decisions regarding the remaining life of the pavement and the necessity and timing of structural intervention through rehabilitation.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C1.2.2 Assessment of structural capacity from the pavement strength parameter Current structural capacity, and therefore the remaining structural capacity, should be assessed in relation to a definition of when terminal structural condition is reached. Terminal structural condition is defined by its associated limiting distresses (deflection, roughness and rutting) that depend on the levels of service, or functionality, required of the pavement. These levels of service and their limiting distress values are based on avoiding rapid or catastrophic failure and its consequences. This means that lower distress limits are maintained for heavily trafficked pavements relative to lightly trafficked pavements. However, the limiting distresses associated with the terminal structural condition are not well defined so it is recommended, in the interim, that these distresses be defined as the indicative investigation condition levels. Table 2 outlines some possible distress limits for defining the indicative investigation condition of the pavement in a management segment. Table 2: Indicative investigation condition (interim surrogate for terminal structural condition) Typical Operating Conditions Typical Road Function
Nominal traffic ranges ESAs
Surface Deflection (D0 (mm))1
Roughness Limit (IRI (m/km))2
% Road Length with Rut Depth > 20 mm3 (1.2 m straight edge)
> 30,000
> 2 x 107
0.8
4.2
10%
100
5,000 – 30,000
3 x 106 – 2 x 107
0.9
4.2
10%
80
1,000 – 5,000
4 x 105 – 3 x 106
1.1
5.4
20%
1.6
See Note 4
30%
Speed (km/h)
AADT (vpd)
≥ 100
Highways and main roads Highways and main roads
Freeways, etc
Other sealed local roads Notes: 1 2. 3. 4.
Indicative Investigation Condition
Various
< 1,000
100 m long) where the representative strength as assessed using sampling is approaching a value where further detailed structural investigation and structural intervention may be warranted.
Strength variations occurring within a spacing of 50 to 100 m along the pavement lane are not expected to be identified by a network level deflection survey with long spacings between test points. Establishing the appropriate sample spacing along the pavement lane should be based on achieving a ‘reasonable’ value of the representative structural condition parameter of the pavement along defined links of the network. C4.2.2
Longitudinal sample spacing
A uniform longitudinal sample spacing of 50 to 100 m for deflection measurement over several kilometres of a defined road link can be adopted within the network as a trial in the first year of network strength assessment. The influence of the longitudinal sample spacing on the assessed representative structural condition of the pavement can be determined by eliminating the strength data obtained at various sample spacings. The ‘optimal’ sample spacing would be the longest interval showing an acceptable variation from the representative structural condition based on a 50 to 100 m measurement interval. Piyatrapoomi et al (2003, 2004) have described a case study in Queensland demonstrating this concept. Where ‘continuous’ deflection measurement devices are used such as a Deflectograph or a Benkelman Beam with automated recording, ‘optimal’ longitudinal sampling may be based on the sampled portion (% road length) of a defined road link (road length). A useful guide to ‘optimal’ longitudinal sampling under these conditions could be, for example, ‘100 m per 1 km’ or ‘500 m per 5 km’. However, this would need to be validated by extensive testing over long lengths of the road link, and as discussed in Section C2.4, the cost-effectiveness of this type of sampling should be carefully assessed.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
Regardless of the measurement device used, the chosen ‘optimal’ longitudinal sample spacing would be expected to vary in accordance with the variability of the deflections measured along the defined road link. The defined road link has to be sectioned into discrete lengths (segments) so that the variation in structural condition within each segment is within acceptable limits and a characteristic value can be assigned to each segment. This segmenting process aims to capture the major longitudinal variations in strength. The above process carried out in the first year of network strength assessment should be repeated on all road links of the network to establish an appropriate longitudinal sample spacing for each link. Several sampling trials within each road link may be needed if the strength variation characteristics along the link vary due to significant changes in traffic load and local climatic and geological differences. Inherent in the sampling process is an acceptance that transverse strength variations (across the pavement lane) are not sampled. Transverse variations in strength are ignored when sampling along the left wheel path in the heaviest trafficked lane where both load and environmental effects have the greatest influence on pavement deterioration. The left wheel path is considered to be weakest transverse location in the pavement and therefore is an appropriate sampling location where sampling in one wheel path only is preferred. For two lane roads, strength sampling (regardless of the testing device used) in the left and right wheel paths of both lanes or a single lane is a potential means of assessing seasonal effects if they are considered relevant. However, except with a DEF, measuring deflections in both wheel paths is not routine for a network survey, partly because of the cost. Figure 7 shows an evenly spaced sampling arrangement that is commonly used where the deflection measurement device is a FWD. It may be appropriate to sample in the wheel paths of only one lane where its traffic is significantly heavier (by mass not necessarily by volume) than in the adjacent lane. A Deflectograph or a pair of Benkelman Beams tests both wheel paths in one lane. A FWD or a single Benkelman Beam can measure deflections in the left wheel paths at the same spacing between consecutive deflection measurements along each of the two lanes when testing on twolane roads.
=
=
=
=
=
=
Equally spaced staggered left wheel path sampling locations (not to scale) Figure 13: FWD Sampling locations on a single carriageway two lane road
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
Strength sampling along multi-lane roads may be limited to the wheel paths of the known heaviest loaded lane along each carriageway (where Deflectograph testing is used). For FWD testing, strength sampling normally occurs only in the left wheel path as shown in Figure 8. However, strength sampling of other lanes in the same carriageway may be needed occasionally, to ascertain their structural condition relative to their lighter traffic loading.
=
=
=
Median
=
=
=
Equally spaced staggered left wheel path sampling locations (not to scale) Figure 14: FWD Sampling locations on a dual carriageway road
It should be noted that the above sampling process is an example of how a sampling regime can be developed. Any sampling processes must be able to identify significant differences in pavement/subgrade strength along defined road links. This will allow these links to be sectioned, or segmented, into relatively ‘homogenous’ sections for traffic loading, climate and condition (accounting for both the surface and structural state of the pavement). This requirement for ‘homogenous’ sections should be the basis for developing an appropriate sampling process. In setting a practical longitudinal sample spacing the trade-offs between sample spacing (length), the accuracy of the deflection measurement at each sample point, and the frequency of the sampling (years) need to be considered to ensure that the assessment of long term strength change at the network level is based on a consistent data series.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C4.2.3
Budgeting to establish ‘optimal’ longitudinal sample spacing
The budget for a network deflection survey should allow for initial trial testing along each road link to establish the ‘optimal’ longitudinal sample spacing. The number and extent (road length) of sampling trials along each road link for establishing the appropriate longitudinal sample spacing for each particular section within the road link may depend on the available budget for the sampling process. However, ideally all definable road links should be subject to sampling trials that cover sufficient road length to confidently assess the variation in the strength parameter along each link in establishing the ‘optimal’ sample spacing. Particularly for large networks, therefore any budget constraints on the sampling process should be reviewed in the light of the estimated total cost of the survey, the marginal cost of collecting additional data, and the expected benefits of the survey. The budget available for the ‘optimal’ sampling along each road link should also allow for different levels of sampling (test spacing length) for each road type. Higher levels of sampling (shorter test spacing length) are recommended for the higher functional road classes and the more heavily trafficked roads. C4.2.4
Assessing strength parameters
The strength parameter that represents the structural condition of the pavement along approximately homogenous segments that comprise each road type that forms the network for a strategic network analysis (strategic planning for annual maintenance and rehabilitation budgets and interventions) is usually represented by the mean strength value. This mean strength value for each road type is arrived at by taking the mean of the characteristic strength values (85th percentile) found for the defined individual segments within each road type. The characteristic strength is used to best represent the strength sampled in each defined individual segment. It is necessary to define the acceptable limits on variation of the strength parameter within each approximately homogenous segment, in order to establish the ‘optimal’ longitudinal sample spacing. The parameter for strength variation could be either the standard deviation (s) from the mean strength value or the coefficient of variation (COV, the standard deviation divided by the mean strength) as these parameters provide an understanding of the reliability of the mean. A COV of 0.25 would generally be suitable (Sapkota et al 2001). C4.2.5
Practices in deflection measurement surveys
C4.2.5.1
Current practices
Transit New Zealand and some Australian State and Territory road agencies have used some form of network strength assessment using BBs, DEFs or FWDs, which are described in Section C2. For example, Transit New Zealand (TNZ) has conducted an annual network level deflection surveys of part of the NZ State Highway network since 1999 (and plans to continue this practice) using a FWD with test points at staggered spacings of 100 m on two-lane roads (ie, 200 m spacing in each lane) where AADT exceeds 2,000 vpd, covering the network each 3 years. Similarly, MRWA has conducted network level deflection surveys every 1 or 2 years since 1998 of its road network, using a FWD with test points at staggered spacings of 400 m on both lanes of two-lane roads (ie, 800 m spacing in each lane). For the States and Territories with smaller networks (eg, Tasmania and the ACT), pavement strength assessments were made from deflection surveys using relatively close sample spacings at moderate testing cost. However for data collection, cost effectiveness is usually more important than total cost, and so project level collection methods are usually not warranted at a network level particularly for large networks, as the surveys would be both costly and time consuming. A project level approach to sampling network strength means intensive data collection at relatively low speed which can reduce the availability of strength data for a given program and budget. Austroads 2005 -- 29 --
Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C4.2.5.2
Targeting a prioritised subset of a network
Network strength surveys aim to achieve an acceptable representation of pavement strength over each road segment for a whole road network comprising many segments. Where there are limitations on testing budgets or the need for network strength assessment is not critical, network strength surveys may be based on testing a prioritised subset of segments of the road network rather than sampling all segments comprising the whole network. With the current limitations of strength assessment devices, strength surveys often only proceed where other performance indicators, such as roughness, cracking and rutting, suggest potential or actual loss of structural strength. Table 5 suggests values and progression rates for roughness, rutting and cracking, based on monitoring long term pavement performance (Tepper, Fossey and Koh 2002), as a practical guide for initiation of discrete network level deflection testing of pavement lengths for given road types or for typical functional road classifications. Table 5: Suggestions for performance indicators to undertake discrete network level sampling
Typical Road Function
Suggested Performance Indicators and Values
Typical Operating Conditions
Rutting (1.2m straight edge)
Roughness
Cracking
Speed (km/h)
AADT (v/day)
Limit (IRI (mm/km))
Rate (IRI/yr)
Limit (mm)
Rate (mm/yr)
Limit (% area)
Rate (% area/yr)
≥ 100
> 30,000
3.5
0.05
10
0.3
1
0.1
100
> 10,000
4.2
0.08
10
0.5
2
0.1
Medium trafficked arterial
80 - 100
2,000 – 10,000
4.2
0.2
15
0.6
5
0.5
Low trafficked arterial or main road
Various
< 2,000
5.4
0.3
20
0.8
10
1
Freeway Highly trafficked arterial road
Note:
1.
Deflection survey to occur when either or of any of the above performance indicator limits or rates is exceeded.
A further example of network sampling is the use of frequent strength monitoring of defined road lengths, or sites, where it may not be practical to sample at a full network level. When selecting these defined road lengths it is critical to ensure that they are representative of the road links they are located on. Historical pavement performance indicators (roughness, rutting, strength and cracking) as well as construction and maintenance records are useful in selecting such sites. C4.2.5.3
Possible future developments
As at December 2004, there is no device for continuous and robust direct monitoring of pavement strength at a network level. Such a device would be expected to reduce the current cost of network level surveys for pavement strength, thereby enabling increased survey coverage (scope). However, a High Speed Deflectograph (HSD) is under development by Greenwood Engineering to measure pavement deflection at highway speed (Rasmussen et al 2002). The HSD measures the velocity of the pavement deflection, rather than the displacement, using laser Doppler sensors mounted on a heavy vehicle. The displacement, or deflection, is estimated from the velocity which is the first derivative of the displacement. The results from early field trials with the HSD compare favourably with FWD test results.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
A representative measure of the moisture content of the pavement/subgrade system under different seasonal conditions and its relationship with the measured deflection would be useful information to pursue. However, as moisture content variation within the pavement/subgrade system is a highly variable and complex phenomenon in relation to seasonal conditions, it is unlikely that a meaningful relationship between measured deflection and moisture content is achievable. C4.2.6 Measurement details C4.2.6.1
Assessing seasonal variations in pavement strength
Ideally any potential variations in the assessed values of pavement strength parameters due to seasonal variations should be minimised or quantified. Network assessment of pavement strength should be conducted when the strength is judged to be at its expected lowest annual value. Alternatively, pavement strength could be assessed from two surveys, one when the strength of the network is judged to be at its expected highest and the other at its lowest annual values, to gain the overall variation from a mean network strength estimate. This may not be practical for reasons of cost and logistics, although these limitations may not apply to small networks. Measurement of surface deflections in both the left and right wheel paths along lanes is another approach used to gain an assessment of the seasonal influence on network strength. This approach assumes that the right wheel path strength estimate is the upper bound value that is less influenced by seasonal variation and is therefore relatively higher. On the other hand, the estimated strength along the left wheel path may not necessarily represent the lower bound value unless the survey was conducted when the expected lowest annual strength value occurred. It may also be possible to estimate the factors that give rise to potential seasonal variations in strength and account for these by a prediction model (Loizos et al 2002). C4.2.6.2
Need for standard measurement procedures
A consistent test method should be used with each device so that the resulting estimates of pavement strength are comparable between different devices and on different sections of the network. A standardised testing procedure is necessary with documentation of the deflection relationships between different devices to aid the conversion of the deflection data into a standard strength parameter (see Section C5).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C5. RELATIONSHIPS BETWEEN MEASURES OF PAVEMENT DEFLECTION
C5.1 Guidelines Deflection data from the Benkelman Beam (BB), Deflectograph (DEF), and Falling Weight Deflectometer (FWD) deflection measuring devices can be related, and can be used to estimate pavement strength parameters. Relationships between deflections from the three different measuring devices appear to vary with pavement type and structure. The following relationships are based mainly on studies on sealed granular pavements in Australia, and are suggested as default relationships where local comparative data is not available. For FWD data, the following relationships are based on using a 300 mm diameter loading plate in the FWD tests (see Table 4).
These relationships must be used with caution, and should ideally be verified by local experimental observations.
FWD and BB: Deflection FWD50 kN
= 1.25 × Deflection BB40 kN/550 kPa
Deflection FWD40 kN
= 0.91 × Deflection BB40 kN/550 kPa (when D0 < 1 mm)
FWD and DEF: Deflection FWD50 kN
= 1.5 × Deflection DEF40 kN/550 kPa
Deflection FWD50 kN
= 1.5 × Deflection DEF40 kN/750 kPa
BB and DEF: Deflection BB40 kN/550 kPa
= 1.2 × Deflection DEF40 kN/550 kPa
Deflection BB40 kN/550 kPa
= 1.2 × Deflection DEF40 kN/750 kPa
where: Deflection FWD50 kN
= Deflection (maximum value, Do) measured using a FWD with a 50 kN impact load
Deflection FWD40 kN
= Deflection (maximum value, Do) measured using a FWD with a 40 kN impact load
Deflection BB40 kN/550 kPa
= Re-bound deflection measured using a BB with a 40 kN wheel load and 550 kPa tyre pressure.
Deflection DEF40 kN/550 kPa
= Deflection measured using a DEF with a 40 kN wheel load and 550 kPa tyre pressure
Deflection DEF40 kN/750 kPa
= Deflection measured using a DEF with a 40 kN wheel load and 750 kPa tyre pressure.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C5.2 Background notes C5.2.1
Deflection relationships between Benkelman Beam, Deflectograph and Falling Weight Deflectometer
It is useful to be able to relate deflection data from different deflection devices so that the pavement strength parameters can be estimated using data from various forms of deflection collection. Relationships between deflection data from FWD, Benkelman Beam and Deflectograph devices were investigated for VicRoads (Clayton and Jameson 2001) and MRWA (Goh and Begg 2000). In summary these relationships appear to vary with pavement type and structure and other factors, such as differing ranges of accuracy for different devices. This makes the conversion of deflection from one device to another problematic (Lang 2002). These deflection relationships may be limited to specific pavement types and structures and ranges of deflection. Documented relationships are set out below. However, they must be used with caution and ideally should be verified by experimental observations. C5.2.2
Existing relationships between Falling Weight Deflectometer and Benkelman Beam
Based on the relationships found by Goh and Begg (2000) in Western Australia, DeBeer (1992) in South Africa and the interim recommendations made by Jameson (2000), the following relationship between FWD deflections and Benkelman Beam deflections is proposed: Deflection FWD50 kN = 1.25 × Deflection BB40 kN/550 kPa
(Eqn C1.1)
where: Deflection FWD50 kN
=
Deflection (maximum, D0) determined from a Falling Weight Deflectometer with a 50 kN impact load (refer to Table 4); and
Deflection BB40 kN/550 kPa
=
Rebound deflection determined from a Benkelman Beam with a 40 kN wheel load and 550 kPa tyre pressure.
Rearranging Equation C1.1 for the Benkelman Beam deflections, assuming linear elastic deflection for a reduced FWD test load of 40 kN: Deflection BB40 kN/550 kPa = 1 × Deflection FWD40 kN
(Eqn C1.2)
where: Deflection FWD40 kN
=
Deflection (maximum, D0) determined from a Falling Weight Deflectometer with a 40 kN impact load (refer to Table 4).
Tonkin and Taylor (1998) recommend the following approach be used as an interim relationship between Benkelman Beam deflections and 40 kN FWD load deflections based on New Zealand flexible pavements. Where maximum deflections are less than 1 mm, adopt the following: Deflection BB40 kN/550 kPa
=
1.1 × Deflection FWD40 kN
(Eqn C2.1)
=
0.91 × Deflection BB40 kN/550 kPa
(Eqn C2.2)
Alternatively Deflection FWD40 kN
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
where: Deflection FWD40 kN
=
Deflection (maximum, D0) determined from a Falling Weight Deflectometer with a 40 kN impact load (refer to Table 4).
The other terms are as defined above. Equation C2.1 is the same as that adopted for the Austroads Design Procedures for Flexible Overlays on Flexible Pavements (Austroads 2004c). Where maximum deflections exceed 1 mm, the following relationship applies: Deflection BB40 kN/550 kPa
=
1.1 × ( Deflection FWD40 kN )0.4
(Eqn C3.1)
=
( 0.91 × Deflection BB40 kN/550 kPa )−0.4
(Eqn C3.2)
Alternatively Deflection FWD40 kN
where the FWD and BB deflections are in mm. If a FWD load of 50 kN and plate pressure of 700 kPa is used, the relationship shown in Equation C2.2 becomes the following, assuming the deflection is linear elastic: Deflection FWD50 kN
=
1.13 × Deflection BB40 kN/550 kPa
(Eqn C4)
Equation C4 is approximately the same as Equation C1.1. Equation C2.2 and Equation C3.2 imply that there may be a tendency for non-linear deflection behaviour for New Zealand pavements when deflections exceed 1 mm. Having regard to all the possible variations and influences, Equation C1.1 appears to be the more robust for a wider range of application, and so Equation C3.1 and Equation C3.2 are not recommended. Queensland Department of Main Roads (QDMR) found the following relationship between the QDMR Benkelman Beam and FWD deflections (Baran 1994) when using a 40 kN test load for both the Benkelman Beam and FWD: Deflection BB40 kN/550 kPa
=
A × Deflection FWD40 kN
(Eqn C5)
where: A = coefficient depending on level of deflection = 0.9 (deflections > 1 mm) = 0.95 (deflections ≤ 1 mm) The other terms are as defined above. C5.2.3
Existing relationships between Falling Weight Deflectometer and Deflectograph
Research in the Netherlands confirmed the feasibility of establishing relationships between deflection bowl data from a FWD and a Deflectograph (Hoyinck et al 1992). This research was mainly conducted on asphalt pavements rather than unbound pavements. In addition, while FWD deflections were normalised to a 50 kN applied load, the axle load for the Deflectograph was 100 kN, not 80 kN as used with most Deflectographs in Australia. Consequently, more extensive fieldwork is required to establish a robust relationship for Australasian conditions.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
However, both QDMR and the RTA NSW have found the following relationship for the deflections between their similar Deflectographs and the Benkelman Beam (Baran 1994): 0.85 × Deflection BB40 kN/550 kPa
Deflection DEF40 kN/550 kPa =
(Eqn C6)
Similarly, the Austroads Design Procedures for Flexible Overlays (Austroads 2004c) used the following relationship between the Deflectograph and Benkelman Beam: Deflection BB40 kN/550 kPa = 1.2 × Deflection DEF40 kN/550 kPa
(Eqn C7)
If Equation C7 is substituted into Equation C1.1 then the following relationship between FWD and DEF deflections results: Deflection FWD50 kN = 1.5 × Deflection DEF40 kN/550 kPa
(Eqn C8)
If a higher surface stress (tyre pressure) of 760 kPa is used for the Deflectograph, Equation C8 is nominally the same for the relationship between the FWD and DEF deflections because deflection is directly proportional to load rather than surface stress, and may be expressed as: Deflection FWD50 kN = 1.5 × Deflection DEF40 kN/760 kPa
(Eqn C9)
In reviewing Equations C9, C8 and C1.1, the following relationship is proposed between the BB and DEF deflection results that is the same as Equation C7, except for the Deflectograph tyre pressure: Deflection BB40 kN/550 kPa = 1.2 × Deflection DEF40 kN/760kPa C5.2.4
(Eqn C10)
Summary of deflection relationships
On the basis of the most robust relationships, the following relationships in Table 6 are recommended to relate deflection data from the FWD, Benkelman Beam (BB) and Deflectograph (DEF). Table 6: Summary of deflection relationships Convert Deflections From Devices
Equation
Relationship
BB deflections to FWD
C1.1
FWD50 kN = 1.25 × BB40 kN/550 kPa
BB deflections to FWD (when D0 < 1 mm)
C2.2
FWD40 kN = 0.91 × BB40 kN/550 kPa
DEF deflections to FWD
C8
FWD50 kN = 1.5 × DEF40 kN/550 kPa
DEF deflections to FWD
C9
FWD40 kN = 1.5 × DEF40 kN/760 kPa
DEF deflections to BB
C10
BB40 kN/550 kPa = 1.2 × DEF40 kN/760 kPa
DEF deflections to BB
C7
BB40 kN/550 kPa = 1.2 × DEF40 kN/550 kPa
Note:
All FWD deflection parameters in this document are based on use of a 300 mm diameter loading plate, and so a 50 kN applied load corresponds to a road surface stress of 700 kPa, etc, as set out in Table 4.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C6.
VERIFICATION OF DISTANCE MEASUREMENT
C6.1 Guidelines The accuracy of distance outputs from each deflection measuring device should be verified: ♦
before each network deflection survey commences;
♦
at intervals not exceeding one month during each survey;
♦
after the completion of each survey; and
♦
immediately after any change is made to the distance measuring features of the deflection measuring device or its host vehicle during a survey.
The preferred means of checking distance outputs is by the deflection measuring device making 5 runs on a test lane between clearly marked stations where the distance between them is at least 1.0 km, and has been measured to an accuracy of within 0.005% (± 50 mm/km) using precise ground survey techniques. The distance recorded on all runs should be within 0.1% (± 1 m/km) of the distance obtained from precise ground survey techniques.
C6.2 Background notes The above guidelines are similar to corresponding advice in other Austroads guidelines in this series. However, higher accuracy is achievable than the 0.1% (± 1 m/km) in these guidelines. For example, TNZ requires distances recorded by deflection measuring devices to be within 0.05% (± 0.5 m/km) of the distance obtained from precise ground survey techniques (TNZ 2002). Accurate correlation of road condition data with physical location on the road network is pivotal to the success of almost all uses of road condition data. Australian experience indicates that almost all errors in reports from road condition surveys relate to distance measurement, particularly when comparing time-series data from a number of surveys. Reliable distance measurement is therefore an essential aspect of all road condition surveys. Road agency databases should generally be supported with robust and unambiguous physical reference points and permanent features such as roadside markers, bridge abutments, side road intersections, etc. Transit New Zealand’s State Highway Location Referencing Management System Manual (TNZ 2004) is an example of a detailed set of procedures for establishing and maintaining markers to support a road reference system. Road agency databases should also contain the distances between the various reference points and permanent features. A limited number of these distances should be measured to a high degree of accuracy (0.005% or better) using precise ground survey techniques, and clearly marked. The precisely measured sections should be sealed, relatively straight, without sags or crests, and between 1 km and 15 km long. There are a number of ongoing uses for measured sections, including calibration and checks of a range of road condition survey host vehicles and equipment. Preparation for a road condition survey should include inspection and any necessary maintenance of roadside markers at reference points.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
In general, a distance verification check should be undertaken before and after each survey, and at intervals during lengthy surveys, either monthly or after completion of specific sections within the survey. For a relatively small road condition survey, because of the cost of establishing a distance verification measured length, a road condition measuring device may be accepted for use without further verification provided: ♦
a thorough and successful distance verification has been undertaken within the past 3 months; and
♦
there have been no changes to hardware or software since the last verification.
Condition and pressure of tyres on the host vehicle can affect distance measurement outputs. Experience has shown that distance outputs are very sensitive to the temperature of tyres, and that variations in temperature are typically the most common cause of inaccurate distance measurement in road condition surveys. It is therefore desirable to operate the host vehicle at the normal operating speed for up to 15 minutes before undertaking a distance check or commencing survey operations. Currently available deflection measurement devices (BB, DEF and FWD) do not operate at high speed, as described in Section C2. Distance checking of deflection measurement devices should therefore be done from a standing start, and by coming to rest at the end. The risk of errors in distance measurement increases with high acceleration and braking. Traffic control may be necessary during distance checking to ensure safety.
C6.3 Recording deflection test locations For monitoring the change in network strength with time a practical means is necessary to identify test points (BB and FWD) and testing locations (DEF) so that subsequent testing can be conducted as near as possible to the previous testing points and locations. Especially in network surveys by a FWD and occasionally with BB testing, it is most desirable that test locations be recorded accurately, and a means should be used to accurately re-locate each actual test point on the pavement some time (years) after the test has been performed, using road referencing systems in combination with GPS coordinates. This is particularly important for FWD testing where the sample spacing can be long. In this way, each new deflection test report can more accurately indicate the change in network strength with time. In some circumstances, paint marks on the road surface may be appropriate to locate deflection tests undertaken using a FWD or a BB. However, paint marks will be lost when the next resurfacing occurs. As noted in Section C2.2, all deflection measuring devices can incorporate a Differentially Corrected Global Positioning System (DGPS), often referred to as a GPS receiver, that locates each survey test point to within a specified distance (eg, 5.0 m) of its actual location. TNZ specifies an accuracy of ± 2.5 m in this context (TNZ 2002).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
In using location referencing devices such as GPS, it is necessary to consider: 1.
the accuracy of the location fixing of each test location has to be consistent with accuracy required at a network level which could range from ± 1 m to ± 5 m, depending on the variability and structural condition of the network; and
2.
the GPS location, or reference location, must be capable of being readily and accurately converted to a linear link-node referencing system, as commonly used by the larger road agencies.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C7
VERIFICATION TESTING FOR DEFLECTION
C7.1 Guidelines These guidelines cover verification of deflection surveys on road networks with sealed granular pavements carried out by Benkelman Beams (BBs), Deflectographs (DEFs) and Falling Weight Deflectometers (FWDs). Detailed verification testing is necessary to assess a new system before it is commissioned for use, after changes to hardware and software, immediately after damage may have occurred, and as described below in association with network surveys. For all deflection measurement devices, the manufacturer’s instructions should be followed for periodic (daily, weekly, monthly, etc) checks and all necessary adjustments made to keep the device correctly calibrated. Periodic harmonisation tests for similar devices are also advisable, similar to the DEF harmonisation trials conducted in the early 1990s by RTA NSW with the four DEFs that RTA built. Verification of deflection measurements using a Benkelman Beam or a Deflectograph: Verification of deflection measurements using a BB or DEF should be undertaken monthly, and involves: ♦
checking and adjustment of the components usually in accordance with manuals provided by the supplier (eg, BB dial gauge or the equivalent automated deflection measuring equipment, DEF signal conditioner, DEF test frame, etc, and for both BB and DEF the test load, tyre pressures, temperature measuring equipment, etc); and
♦
fresh surveys of a section of road with known deflection characteristics.
Verification sections are required within each network being surveyed, for use before, during, and after each deflection survey. Verification sections for deflection surveys on sealed granular pavements: ♦
should be generally representative of the nature and condition of the pavements being surveyed;
♦
should exhibit the complete expected range of maximum pavement deflections, especially between 0.5 mm and 1.5 mm;
♦
for a DEF deflection survey, should be at least 500 m long; and
♦
for a BB survey, should be long enough for at least 10 test sites (eg 100 m with 20 m spacings, or 500 m with 50 m spacings).
Acceptance is based on judgement of the reasonableness of any differences between deflections measured either in repeat runs on the same day, or between tests at the same test locations in successive surveys, after allowing for strength changes due to seasonal and other effects. Verification of deflection measurements using a FWD (or HWD): Because of the relative complexity of a FWD (when compared to a BB or DEF), a complete calibration and verification process for a FWD involves separate calibration and verification of a number of components as well as the complete FWD unit. For FWD users, the main issues include the accuracy of the offset of deflection sensors from the load point, the repeatability of load and deflection measurements both shortterm and over time, and the relative calibration of the different sensors. These guidelines favour the calibration and verification testing protocols published by the European COST 336 documentation for FWD verification (Chapter 6 in COST 2003), as listed in Table 7.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C7.2 Background notes For any asset management planning process, it is most desirable that the input data is accurate and unbiased. Inaccurate and biased pavement deflection data can lead to incorrect conclusions about the condition of a road pavement, and poor estimates of the remaining service life. For example, a systematic error of 5% in deflection may result in an error of 25% in estimated remaining pavement life. Common sources of error in deflection surveys include errors in location referencing (see Section C6), incorrectly loaded trucks (BB and DEF), improper calibration of measuring equipment (all devices), incorrect placement of deflection sensors (FWD), inappropriate adjustments (normalisation, FWD), and inappropriate adjustments for the duration of loads (FWD). For FWD, COST 3361 prepared a set of procedures for calibration and verification of individual components as well as for the FWD as a complete unit (COST 1999). COST identified three levels of calibration and verification, viz: ♦
‘Manufacturer’ level, for testing before delivery to a purchaser and after servicing, to ensure high quality standards are met (eg, M1-1999 in Table 7);
♦
‘User’ or FWD operator level, for periodic checks during testing operations (eg, U1-1999 to U6-1999 in Table 7); and
♦
‘Calibration Station’, which is an intermediate level, involving a degree of impartiality and testing of a complexity that would usually be beyond the capability of an operator or user (eg, C1-1999 to C5-1999 in Table 7).
Table 7 lists these COST procedures, the suggested frequency of their application, the aims and brief descriptions. Appendix 2 contains the COST procedures for short term repeatability and for long term repeatability, which can be conducted together. The complete COST 336 documentation is at http://62.242.229.98/fog/fwd/cost336.htm. Corresponding FWD information from the USA2 is at http://www.tfhrc.gov/pavement/ltpp/resource.htm (Law PCS 2000). This was developed for research purposes as part of the US Long Term Pavement Performance (LTPP) program.
1
The European Commission's COST Transport Program undertook a project on the use of FWDs in pavement evaluation (COST 336), involving 20 European nations from June 1996 to December 1999. COST 336 involved three main tasks, including the applicability of FWDs at road network level, and FWD calibration. The report describes the agreed common European code of good practice for the use of FWDs in pavement evaluation. 2
The USA Long Term Pavement Performance (LTPP) program was established in 1987 as part of the Strategic Highway Research Program (SHRP). As at 2005, the the Federal Highway Administration (FHWA) manages the USA LTPP program.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements) Table 7: COST 336 procedures for FWD calibration and verification COST 336 designation and name
Suggested frequency
Aim and brief description
C1-1999
Dynamic reference calibration of FWD load cell
At least once every two years, or more often as considered necessary by the owner.
Aim is to ensure that the load cell measures the peak value of the load pulse accurately. With the load cell mounted to the loading plate, various drop heights are applied in a series of drops on the FWD plate and a reference load cell platform.
C2-1999
Laboratory reference calibration of dismounted FWD deflection sensors
At least once every two years.
Aim is to verify and calibrate the signal processing electronics of the FWD that produce deflection values. Various amplitudes and durations of deflection pulses are applied to the FWD deflection sensor and a reference instrument.
C3-1999
In-situ reference calibration of dismounted FWD deflection sensors
At least once every two years.
Aim is to verify that the FWD defection sensors produce correct peak values of deflection with the FWD as the load generator. The test uses reference instrumentation, and checks that vibrations caused by release of the falling mass do not affect the recorded deflections, by mounting the sensor on a test rig some distance form the FWD.
C4-1999
In-situ reference verification of mounted FWD deflection sensors
Optional, pending more experience.
Aim is to verify that the FWD deflection sensors at various distances from the loading plate produce correct peak values of deflection. Deflections are recorded for a range of drop heights by the FWD sensor and a reference instrument. The test is repeated for each of the FWD deflection sensors.
C5-1999
In-situ FWD harmonisation procedure
Optional, suggested at least once every two years.
Each FWD is compared with others. Aims are to verify repeatability, to derive harmonisation factors, and to improve reproducibility of deflection recording.
M1-1999
Static reference calibration of FWD load cell
At least once every two years, and if COST 336 C1 cannot be performed, and when a new load cell is mounted on the FWD.
Aim is to ensure that the load cell measures the peak value of the load pulse accurately. Can be used as a substitute for COST 336 C1. Reference load cell and the FWD load cell under test are subjected to a range of loads.
U1-1999
Verification of FWD deflection sensor position
At least once every month, and each time the deflection sensors are moved.
Sensors may be moved between FWD jobs, due to differing client needs. Aim is to ensure that sensors are positioned to a tolerance of ±4 mm + 0.5% of the radial distance (eg, ±10 mm for the 1200 mm sensor).
U2-1999
FWD short-term repeatability verification
At least once every month, or more often as considered necessary by the operator.
Aim is to verify that the FWD produces consistent results at a specific test point. Deflections and loads are recorded from all but the first two of 12 drops. Deflections are normalised and checked for consistency (SD of loads < 2% of mean load, and SD of normalised deflections < 0.2 mm). (Note: This procedure is in Appendix 2.1 herein.)
U3-1999
FWD long-term repeatability verification
At least once every month, or more often as considered necessary by the operator.
Aim is to indicate possible problems with FWD deflection results with a standard target load. Deflections and loads are recorded from all but the first two of 12 drops at an established marked test site. Deflections are normalised and checked for consistency with previous results at the same test site. Consistency is judged after taking account of seasonal and temperature conditions. (Note: This procedure is in Appendix 2.2 herein.)
U4-1999
Relative calibration of FWD deflection sensors
At least once every six months, or more often as considered necessary by the operator.
Aim is to ensure that all sensors on the FWD are in calibration with each other. Known as ‘stacking test’, all deflection sensors are stacked coaxially so that they are exposed to the same deflection in each drop in a series of 10 drops. Recorded deflections are checked for consistency (range < 0.4mm, and ratio of mean deflection for each sensor to the mean deflection for all sensors between 0.995 and 1.005 inclusive).
U5-1999
Reference calibration of FWD temperature probe
At least once every year, or more often as considered necessary by the operator.
Aim is to ensure that the FWD temperature probe measures the air and pavement temperature accurately. A reference thermometer is used. Applies to thermo-couples and other contact type temperature recording devices, but not to infrared temperature sensors.
U6-1999
Reference calibration of FWD odometer
At least once every six months, or more often as considered necessary by the operator.
Aim is to determine calibration factors for the FWD odometer or distance measuring instrument. The FWD distance reading is recorded in 2 or 3 runs over a precisely measured 500 m length travelled at low speed.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C8.
REPEATABILITY AND BIAS
C8.1 Guidelines Repeatability is a statistical term to indicate the extent of variation in outputs about the mean for a single operator using the same method. Repeatability is the standard deviation of measures obtained in repeat tests using the same measuring device and operator on a single, randomly selected road. Bias is a statistical term to indicate whether a device is systematically measuring high or low when compared to a reference set of measures. Bias error is the average of the errors from many measurements throughout the survey. Bias should be no greater than 1%. Repeatability runs and bias checks for surveys using a FWD (or HWD): Repeatability and bias for deflection surveys using Falling Weight Deflectometers (FWDs) are covered in Section C7 and Appendix 2. Repeatability runs and bias checks for surveys using a BB or a DEF: Repeatability runs and bias checks of BB and DEF deflection survey systems should be conducted over a clearly defined section of a lane of at least 10 km total length with a range of characteristic maximum surface deflections at the 100 m reporting interval level of between 0.5 mm and 3.0 mm. Five (5) repeatability runs should be conducted for each survey team undertaking the network survey at the start of the survey. Subsequent bias checks and further repeatability runs are then conducted on the test section throughout the survey at intervals of no more than 3,000 lane-km. The standard deviation of characteristic maximum surface deflections for individual 100 m reporting intervals should be less than 3%. In addition, the r2 correlation should be at least 0.95 when the individual maximum deflection values for each reporting interval are regressed against the mean maximum deflection values. Non-compliance with the above recommendations is a matter for each road agency to decide independently, as these recommendations may not always be achievable in practice.
C8.2. Background notes Repeatability and bias are important factors in ensuring that changes in strength, as derived from surface deflections, can be identified over relatively short time spans. Repeatability indicates variation in measures about the mean, whereas bias error is potentially a more serious problem that can distort perceptions of the condition of the road network and indicate that the pavement strength is better or worse than it really is. Bias error can be caused by errors in equipment calibration or physical damage to the measuring device such as may be sustained survey operations. The bias error indicates whether the measurement is systematically high or low when compared to a reference set of measurements. For a FWD, the COST U2-1999 and U3-1999 procedures cover repeatability and bias testing. Appendix 2 contains the COST procedures for short term repeatability (COST 336 U2-1999) and for long term repeatability (COST 336 U3-1999), which can be conducted together.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C9.
DATA REPORTING
C9.1 Guidelines Reporting deflection data: Deflection data can be reported in tabular or graphical form or both. Deflections are reported in mm (to 0.01 mm) from BB and DEF surveys, and in microns from FWD data. (Note: 1,000 microns = 1 mm.) For deflection surveys using a Deflectograph (DEF), summary surface deflection bowl data, as listed below, should be reported at 100 m intervals in both wheel paths, using for network purposes the deflection bowl with D0 nearest the characteristic value (85th percentile) for all bowls (with 4m spacing, there are 25) in that 100m length. In addition, charts of individual half bowls (showing deflections at 50 mm spacings) can confirm the location of the maximum deflection and the standard offsets for reporting other deflection values. For deflection surveys using a FWD or HWD or a Benkelman Beam (BB), all recorded surface deflection bowl data should be reported for each test point. As a minimum, the following surface deflection bowl data should be reported: ♦
for a BB or DEF, measured deflections at least at distances of 0, 200, 300, 600 and 900 mm from the centre of the moving test load (ie, D0, D200, D300, D600, and D900);
♦
for a FWD (or HWD), normalised deflections measured at distances of 0, 200, 300, 450, 600, 900 and 1,500 mm from the centre of the impulse test load (ie, D0, D200, D300, D450, D600, D900 and D1500);
♦
for all devices, curvature (ie, D0 - D200); and
♦
for a DEF, the characteristic maximum deflection, D0.
Deflection measurements are desirable as far as possible from the centre of the applied load and preferably beyond the 900 mm (BB and DEF) and 1,200 mm (FWD) offsets mentioned above, as deflections at large offsets increase the likelihood of recording the full extent of the bowl. Reporting other data: For deflection surveys using a Falling Weight Deflectometer (FWD or HWD), the target load and applied load should be reported for each test location. For deflection surveys using a Deflectograph (DEF) or a Benkelman Beam (BB), the load and the tyre pressures should be reported. Irrespective of the deflection measurement method, that is, for surveys using BB, DEF or FWD, the location (including road, lane and wheel path or offset distance from kerb or road edge), date, time, prevailing weather, air temperature, and pavement temperature (only where the asphalt thickness is known to be > 50 mm) should be reported.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
C9.2 Background notes Table 8 is an example of a tabular report from a FWD survey with a target surface stress of 700 mm (50 kN with a 300 mm diameter loading plate, see Table 4). While Table 8 shows deflections at nine offsets, as outlined in Section C2.5, seven are common, and six are adequate for network purposes. Table 8: Extract from report on network level FWD survey
FWD Network Level Survey Report Road name: .. SH 10 - Pacific Highway …
Test device identification: … FWD No xyz (Owner Pty Ltd) ..
Transverse location of test sites: …Left Wheel Path … Lane No 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1
Surface FWD Chaina Time of Temp Stress ge (km) Test (kPa) (oC) 0.025 0.125 0.226 0.325 0.425 0.526 0.625 0.725 0.050 0.150 0.250 0.350 0.450 0.550 0.650 0.750
21:32 21:33 21:34 21:35 21:36 21:38 21:39 21:40 23:32 23:33 23:34 23:35 23:36 23:38 23:39 23:40
29 29 28 28 28 28 28 28 15 15 15 15 15 15 15 15
696 706 702 700 701 713 701 709 699 703 699 702 689 699 703 700
Target surface stress: …. 700kPa …
Date: … 16 September 2004 … Longitudinal spacing: … 100 m …
FWD Deflection (micron) Deflection Offset (mm) 0
200
300
450
600
750
900
1200
1500
169 367 271 325 686 388 418 475 244 464 312 190 212 208 352 277
83 269 191 242 513 288 347 362 186 354 225 158 180 173 298 214
69 214 156 201 405 231 293 290 159 306 191 145 166 160 259 182
63 180 120 162 284 178 238 218 133 242 152 130 145 142 211 143
58 144 89 128 183 134 184 160 112 185 119 116 125 124 168 110
52 117 72 105 119 105 146 118 91 141 97 102 108 106 136 83
54 99 57 89 79 95 122 87 79 106 76 89 90 91 108 63
41 67 43 59 51 55 75 47 51 56 46 65 63 62 64 31
33 41 29 36 31 40 38 24 45 41 39 54 46 46 44 23
With deflection data from a Deflectograph survey, characteristic deflection is reported because the longitudinal sample spacing of between 3 m and 7 m enables a Deflectograph to be used as a project level strength assessment tool, as well as for network level purposes. In these guidelines, the 85th percentile is regarded as the characteristic value for network purposes. This contrasts with Austroads guidelines for project level deflection analysis, where the characteristic value depends on the road function (Austroads 2004b, Section 6.2.2.6). With deflection data from a Deflectograph survey, the characteristic maximum deflection and the characteristic curvature are reported at 100 m reporting intervals, as for other pavement condition parameters such as roughness, rutting and texture. With deflection data from a BB or FWD network survey, the longitudinal sample spacing is likely to be too long for meaningful reporting at 100 m intervals, and so deflection bowl data including curvature is reported specifically for each test point.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
A Deflectograph measures relative deflections at close spacings. Reporting options include: ♦
tabular form at offset distances of 0, 200, 300, 600 and 900 mm from the centre of the moving test load and referred to as D0, D200, D300, D600, and D900, respectively, so that they correspond with deflections recorded in BB and FWD testing; and
♦
graphical form at offset intervals of 50 mm between the centre of the moving test load and the end of the beam (1.2 m for Deflectographs in use in Australia as at December 2004).
Figure 15 is an example of a graphical report from a Deflectograph survey. Of the four charts in Figure 15, the top two show the LWP and RWP maximum deflections (D0) at 4.0 m longitudinal spacings (ie 250 values) over a length of 1.0 km from chainage 0.500 km to 1.500 km. The bottom two charts show the shapes of the LWP and RWP deflection bowls at chainage 1.462 km. Each bowl was created from 22 deflection readings at 50 mm intervals. Note that the Deflectograph recorded a small number of deflections beyond the maximum deflection in each wheel path, thereby ensuring that the actual maximum deflection can be identified. As with the BB, the reported deflections are relative values, each being the difference between the recorded deflection and the deflection reading furthest from D0, because the furthest reading is reported as zero.
Figure 15: Extract from a graphical report from a Deflectograph survey
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
SECTION D – SUMMARY D1.
Sampling and measurement of pavement deflections
The principles of sampling and deflection measurement in a network level context for sealed granular pavements have been outlined with a view to providing consistency and quality of outcomes, including a discussion of the relative merits of different devices. A number of new procedures need to be established, reviewed and clearly defined. These guidelines distinguish deflections (that can be measured and reported using one of three main measurement devices) from pavement strength and estimates of values of pavement strength parameters. Table 1 contains a 7-step overall outline of the process of estimating pavement strength parameters for network level asset management planning purposes, based on data collected in pavement deflection surveys. The following procedures are proposed: Sampling ♦
The strength parameter that represents the structural condition of the pavement along approximately homogenous segments suitable for a strategic network analysis, for purposes such as strategic budgetary planning for maintenance intervention, is usually the mean of the characteristic strength values (85th percentile) found for all individual segments within each road type.
♦
It is necessary to define acceptable limits on variation of the strength parameter within each assumed homogenous segment in establishing the ‘optimal’ longitudinal sample spacing as part of the sampling process. The statistical parameter to represent strength variation could be either the standard deviation, s, or the coefficient of variation, COV, as they provide an understanding of the reliability of the mean. A COV of 0.25 is a suitable limit for the variation in the strength parameter for use in establishing the ‘optimal’ longitudinal sample spacing (Sapkota et al 2001).
♦
The available budget for the sampling process will influence the number and extent (road length) of sampling trials along each road link for establishing the appropriate longitudinal sample spacing for each particular section within the road link. However, ideally all definable road links should be subject to sampling trials that cover sufficient road length to confidently assess the variation in the strength parameter along each link in establishing the ‘optimal’ sample spacing. Particularly for large networks, therefore any budget constraints on the sampling process should be reviewed in the light of the estimated total cost of the survey, the marginal cost of collecting additional data, and the expected benefits of the survey.
♦
Ideally any potential variations in the assessed values of pavement strength parameters due to seasonal variations should be minimised or quantified. Network assessment of pavement strength should be conducted when the strength is judged to be at its expected lowest annual value.
♦
Alternatively, pavement strength could be assessed from two surveys, one when the strength of the network is judged to be at its expected highest and the other at its lowest annual values, to gain the overall variation from a mean network strength estimate. This may not be practical for reasons of cost and logistics, although these limitations may not apply to small networks.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
♦
Comparison of surface deflections in the left and right wheel paths along lanes is potentially another approach used to gain an assessment of the seasonal influence on network strength. This method is not routine, partly because of the cost of a network survey of deflections in both wheel paths. This approach may assume that the right wheel path strength estimate is the upper bound value that is less influenced by seasonal variation. However, the estimated strength along the left wheel path may not necessarily represent the lower bound value unless it is judged that the survey was conducted when the expected lowest strength value occurs. It may also be possible to estimate the factors that give rise to potential seasonal variations in strength and account for these by a simple prediction model (Loizos et al 2002).
♦
Table 5 shows suggestions for the initiation of network sampling. This is based on current long term pavement performance monitoring, and is a practical guide to discrete network level sampling of pavement lengths for given road types or typical functional road classifications.
Test procedures ♦
A consistent deflection testing approach should be used for each deflection testing device so that the resulting estimates of pavement strength are comparable between different devices and on different sections of the network. Where a standardised testing procedure is not possible, documentation of the testing procedure needs to be undertaken to aid the conversion of the deflection data into a standard strength parameter. An approved Test Method is the preferred approach. Deflection testing should be performed using standard applied loads and pressures to simulate the field conditions of the actual axle loads and tyre pressures experienced.
♦
In network level surveys, at each test point, at least the D0, D200, D300, D600 and D900 deflection values should be recorded. Where possible, the D450 and D1500 deflections should also be recorded. The D0 value is used in estimating network pavement strength. The D200 and D300 values are used to estimate D250 which is used in the ratio D250/D0 to filter out cemented base and other bound pavements from the estimate of network pavement strength. The D600, D900 and D1500 values can be used in estimating the Adjusted Structural Number (SNP) as described in Appendix 1. Deflections are desirable as far as possible from the centre of the applied load and preferably beyond the 900 mm offset mentioned above (eg, D1500), because deflections at large offsets increase the likelihood of recording the full extent of the bowl. The additional cost of recording and storing D450 can mostly be warranted on the basis of the potential to use this data in more detailed analysis.
♦
In setting a practical longitudinal sample spacing the trade-offs between sample spacing (length), the accuracy of the deflection measurement at each sample point and the frequency of the sampling (years) need to be considered to ensure that a consistent long term assessment of strength change at the network level occurs.
Location referencing ♦
For monitoring the change in network strength with time a practical means is necessary to identify test points (BB and FWD) and testing locations (DEF) so that subsequent testing be conducted as near as possible to the previous testing locations. In network surveys by a BB or FWD, a means should be used to accurately re-locate each actual test point on the pavement some time (years) after the test has been performed at each specified location using road referencing systems in combination with GPS coordinates. This is particularly important for FWD testing where the sample spacing can be long.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
♦
For location referencing devices such as GPS, the accuracy of the location fixing of each test location has to be consistent with accuracy required at a network level which could range from ± 1 m to ± 5 m, depending on the variability of the structural condition of the network.
♦
The GPS location, or reference location, must be capable of being readily and accurately converted to a linear link-node location referencing system that is commonly used by MAs.
D2.
Estimation of network level pavement strength parameters
Various means of estimating network level pavement strength parameters have been outlined. As shown by comparative testing (see Appendix 1.2.4), bowl deflections other than the maximum deflection, D0, do not significantly improve estimates of the Modified Structural Number, SNC, with currently used relationships. Quantification of network strength potentially allows assessment and prediction of structural deterioration and estimation of the remaining traffic load capacity of the pavement. Network strength parameters can be effectively used in conjunction with the following considerations: ♦
Current structural capacity, and therefore the remaining structural capacity, should be assessed in relation to a definition of when terminal structural condition is reached. The actual terminal structural condition and its associated distresses will depend on the levels of service, or functionality, required of the pavement. As the defined terminal structural condition is difficult to quantify, indicative investigation condition levels are suggested for use in the interim (see Table 2). These levels of service and their limiting distress values are based on avoiding rapid or catastrophic failure and its consequences.
♦
From a defined terminal structural condition, with the distress limits as shown in Table 2, the remaining structural life can be estimated based on the difference between the existing distresses and the limiting distresses. The deterioration rate, the time rate of distress from the current distress level to the limiting distresses, is needed to estimate the remaining life (years). The deterioration estimation needs to account for the factors influencing it (traffic loading, climate, construction quality, maintenance, etc).
♦
Deterioration prediction needs to be made with reference to the limiting distresses and the existing distresses. Table 2 shows that the limiting distresses for the suggested indicative investigation levels depend on the level of service required from the road. The variation of the limiting distresses with level of service also provides some reserve structural capacity needed to avoid catastrophic failure.
D3.
Interim structural deterioration model for sealed granular pavements
An interim structural deterioration model, using a pavement strength parameter, SNC, as the dependent variable has been postulated (see Appendix 3). The ‘Interim Model’ is included for information only. This postulation has been based on the following assumptions that need substantiation, or modification, based on further information: ♦
The terminal structural condition of the pavement has been inferred from Austroads (2004b) predicted deflection reductions due to the impact of granular resheeting.
♦
The above assumption is also based on the view that the deflection reductions due to strength recovery are equal to the deflection increases due to the loss of strength.
♦
The network strength parameter, SNC, is based only on the measurement of maximum deflection (D0). However, previous studies have shown that deflection (bowl shape) is a more appropriate estimate of pavement stiffness rather than strength.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
REFERENCES AND BIBLIOGRAPHY AASHTO (1993). AASHTO Guide for Design of Pavement Structures, (AASHTO: Washington D.C., USA). AUSTROADS (1992). Pavement Design – A Guide to the Structural Design of Road Pavements. AP-17/92. (Austroads: Sydney, NSW, Australia). AUSTROADS (2003a). Comparison of Project Level and Network Level Pavement Strength Assessment. AP-T21/03. (Austroads: Sydney, NSW, Australia). (The output of Austroads Project T&E.P.N.528) AUSTROADS (2003b). Pavement Strength in Network Analysis of Sealed Granular Roads: Basis for Austroads Guidelines. AP-R233/03. (Austroads: Sydney, NSW, Australia). (An output of Austroads Project BS.A.C.025) AUSTROADS (2004a). Pavement Design – A Guide to the Structural Design of Road Pavements. AP-G17/04. (Austroads: Sydney, NSW, Australia). AUSTROADS (2004b). Pavement Rehabilitation - A Guide to the Design of Rehabilitation Treatments for Road Pavements. AP-G78/04. (Austroads: Sydney, NSW, Australia). (The output of Austroads Project T&E.P.N.502) AUSTROADS (2004c). Technical Basis of the 2004 Austroads Design Procedures for Flexible Overlays on Flexible Pavements. AP-T T34/04. pp 149 (Austroads: Sydney, NSW, Australia). (An output of Austroads Projects T&E.P.C.029 and PUB.PT.C.007) BARAN, E. (1994). Developments in pavement testing and interpretation, Queensland Technology Workshop on Low Volume Roads, March 1994. BLAKE, P., MILAZZO, A. and ROSE, S. (1996). Rural Road Replacement Projections – A Probabilistic Method using Roughness, Proc. Roads 96 Conference, Combined 18th ARRB Transport Research Conference, New Zealand, Part 4, pp 215-228, (ARRB TR: Vermont South, Victoria, Australia). CLAYTON, B. and JAMESON, G. (2001). Correlation between Benkelman Beam, PaSE Deflectograph and FWD, ARRB Transport Research Contract Report RC2007, (ARRB TR, Vermont South, Victoria Australia). CLAYTON, B. and STYLES, E. (2001). Long Term Pavement Performance Study – Summary of Data Collection Activities: 1994 – 2001, ARRB Transport Research Contract Report RC1525-2, June 2001, pp32, (ARRB TR, Vermont South, Victoria Australia). COST 336 (2003). Use of Falling Weight Deflectometers in Pavement Evaluation, viewed 13 January 2005, http://62.242.229.98/fog/fwd/cost336.htm. DE BEER, M. (1992). Developments in the Failure Criteria of the South African Mechanistic Design Procedure for Asphalt Pavements. Proceedings, 7th International Conference on Asphalt Pavements, Vol 3 (ISAP: Austin, Texas, USA). DEPARTMENT OF MAIN ROADS NSW (1982). Benkelman Beam Deflection Test. Test Method T160. (Roads and Traffic Authority NSW, Sydney, Australia). EIJBERSEN, M.J. and VAN ZWIETEN, J. (1998). Application of FWD-measurements at the network level, 4th International Conference on Managing Pavements, Vol 1, pp 438-450 (Dept. Civil Engineering, University of Pretoria, South Africa).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
FERNE, B.W. (1997). Use of deflections at network level in England for programming and other purposes, Cost 336 Workshop, 4-5 June at LNEC, Lisbon, Portugal. GARGETT, D. and PERRY, R. (1998). Interstate non-bulk freight, Proc. 22nd Australian Transport Research Forum, Vol 22 pp 19-28 (Transport Data Centre of New South Wales Department of Transport: Sydney, Australia). GOH, A.L. and BEGG, S. (2000). Development of a relationship between Falling Weight Deflectometer and Benkelman Beam Deflection, Goh and Begg Consultants, MWRA Panel Contract 1035C98. GREENWOOD ENGINEERING (2002). High Speed Deflectograph, Denmark (www.greenwood.dk). HIGHWAY RESEARCH BOARD, (1962). The AASHO Road Test, Report 5, Pavement Research, Highway Research Board Special Report 61E, pp 56 (National Academy of Sciences: Washington, D.C., USA). HODGES, J.W., ROLT, J. and JONES, T.E. (1975). The Kenya Road Transport Cost Study: Research on Road Deterioration. Transport and Road Research Laboratory Report 673, pp 56 TRRL: Crowthorne, Berkshire, United Kingdom). HOYINCK, W.T., VAN DER LOO, J.M.M., MULDERIJ, J. and WEGMEETDIENST, R.K. (1992). Comparative Tests of FWD and Lacroix Deflectograph. Proc. 7th International Conference on Asphalt Pavements. Vol 3 pp179 – 193. ISAP. ISOHDM (International Study of Highway Development Management Tools) (1998). HDM-4 Technical User Guide Part D: Models, D-1: Road Deterioration (Flexible Pavements) (HDM-4\Vol2\D-1a) (ISOHDM: University of Birmingham, United Kingdom). JAMESON, G.W. (1993). Development of procedures to predict structural number and subgrade strength from falling weight deflectometer deflections, (ARRB TR, Vermont South, Victoria, Australia, unpublished). JAMESON, G.W. (2000). Towards improved Austroads guidelines for the design of flexible overlays on flexible pavements, Contract Report RC91039D-2 (ARRB TR: Vermont South, Victoria, Australia). KONIDITSIOTIS. C. and KOSKY, C. (1996). National Uniformity in Pavement Condition Data Definition, Proceedings Roads 96 Conference, Combined 18th ARRB Transport Research and Transit New Zealand Land Transport Symposium, Part 4, pp 425-46 (ARRB TR: Vermont South, Victoria, Australia). LANG, J. (2001). Personal Communication, October 2001, Department of Infrastructure, Energy and Resources, Tasmania, Australia. LANG, J. (2002). Personal Communication, July 2002, Department of Infrastructure, Energy and Resources, Tasmania, Australia. LAW PCS. (2000). LTPP Manual for Falling Weight Deflectometer Measurements - Operational Field Guidelines, Version 3.1. Prepared for the FHWA LTPP. (LAW PCS, Beltsville, Maryland, USA). LOIZOS, A., ROBERTS, J. and CRANK, S. (2002). Asphalt Pavement Deterioration Models for Mild Climatic Conditions, Proceedings 9th International Conference on Asphalt Pavements, Copenhagen, Denmark (not yet published).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
MCLEAN, J.R. and MARTIN, T.C. (1999). In search of the missing model, Road and Transport Research, 8(3) (ARRB TR: Vermont South, Victoria, Australia). MARTIN, T. (1998). State-of-the-art pavement performance modelling at a network and project level, ARRB Transport Research Special Report SR56, pp 39 (ARRB TR: Vermont South, Victoria, Australia). MARTIN, T. (2002). Interim Network Level Structural Deterioration Model for Sealed Granular Pavements, ARRB Transport Research Contract Report RC1702/3, pp 14 (ARRB TR: Vermont South, Victoria). MARTIN, T.C. and RAMSAY, E. (1996). Rural pavement improvement prediction due to rehabilitation. ARRB TR, Research Report ARR 283, pp 23 (ARRB TR: Vermont South, Victoria, Australia). MARTIN, T. and GLEESON, B. (1999). The Effect of Maintenance on Pavement Performance: The Accelerated Load Pilot Test, ARRB Transport Research Contract Report RC7094B, pp 42 (ARRB TR: Vermont South, Victoria, Australia). MARTIN, T., GLEESON, B., JOHNSON-CLARKE, J., TREDREA, P. and FOSSEY, D. (2000). The Effect of Maintenance on Pavement Performance: Accelerated Load Testing in 1999/2000, ARRB Transport Research Contract Report RC90264, pp 44 (ARRB TR: Vermont South, Victoria, Australia). MARTIN, T., GLEESON, B., JOHNSON-CLARKE, J., TREDREA, P., LUKE, R. and FOSSEY, D. (2001). The Effect of Maintenance on Pavement Performance: Accelerated Load Testing in 2000/2001, ARRB Transport Research Contract Report RC1739, pp 62 (ARRB TR: Vermont South, Victoria, Australia). MARTIN, T. and CRANK, S. (2001). Use of Pavement Strength Information in Network Asset Management, ARRB Transport Research Contract Report RC1702, Austroads Project No BS.A.C.025, October 2001, pp 32 (ARRB TR: Vermont South, Victoria, Australia). MOROSUK, G., RILEY, M.J. and ODOKI, J.B. (2001). HDM-4 Highway Development and Management, Vol 6, Modelling Road Deterioration and Works Effects, Review Draft Version 1.1, pp 306 (World Road Association (PIARC): Paris, France). NAASRA (National Association of Australian State Road Authorities) (1979). Interim Guide to Pavement Thickness Design, pp 169 (NAASRA: Sydney, Australia). NRTC (National Road Transport Commission) (1996). Mass Limits Review: Report and Recommendations, July 1996, pp 64 (NRTC: Melbourne, Australia). PAINE, D. (1998). The incorporation of structural data in a pavement management system, Proc. 4th International Conference on Managing Pavements. Department of Civil Engineering, University of Pretoria, South Africa. PARKMAN, C.C. and ROLT, J. (1997). Characterisation of pavement strength in HDM-III and possible changes for HDM-4, Transport Research Laboratory Unpublished Report, PR/ORC/587/97 (TRRL: Crowthorne, Berkshire, United Kingdom). PATERSON, W.D.O. (1987). Road Deterioration and Maintenance Effects: models for planning and management, The Highway Design and Maintenance Standards Series (John Hopkins University Press: Baltimore, USA).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
PCS/LAW ENGINEERING, (1993), Manual for FWD testing in the Long-Term Pavement Performance Program. SHRP-P-661. Strategic Highway Research Program (SHRP) pp 129 (National Academy of Sciences: Washington, D.C., USA). PIYATRAPOOMI, N., KUMAR, A., ROBERTSON, N., WELIGAMAGE, J. (2003). A ProbabilityBased Analysis for Identifying Pavement Deflection Test Intervals for Road Data Collection. International Conference on Highway Pavement Data, Analysis and Mechanistic Design Application, Volume II, pp. 291-302, Columbus, Ohio, USA. PIYATRAPOOMI, N., KUMAR, A., ROBERTSON, N., WELIGAMAGE, J. (2004). Reliability of Optimal Intervals for Pavement Strength Data Collection at the Network Level. Proceedings of the 6th International Conference on Managing Pavements (ICMP6), Brisbane, Australia. QDMR (Queensland Department of Main Roads) (1993). Pavement Design Manual, (QDMR: Brisbane, Australia). RASMUSSEN, S., KRARUP, J.A., HILDEBRAND, G. (2002). Non-contact Deflection Measurement at High Speed, Proceedings 6th International Conference on the Bearing Capacity of Roads, Railways and Airfields (BCRA 2002), 24-26th June, 2002, Lisbon, Portugal. ROBERTS, J.D. (1995). Pavement Management System: Operation Guide and System Description. 5th ADB Road Improvement Project, Philippines, Kampsax in association with SMEC and OPCV of Australia, (SMEC International Pty Ltd: Cooma, NSW, Australia). ROBERTS, J.D. (2000a). A Pavement Structural Deterioration Model for HDM-4, Proceedings First European Conference on Pavement Management Systems, Budapest, Hungary. ROBERTS, J.D. (2000b). Project level pavement performance modelling, Road and Transport Research, 9(4), pp. 29-47 (ARRB TR Ltd: Vermont South, Victoria, Australia). ROBERTS, J. and MARTIN, T. (1996). Recommendations for monitoring pavement performance. ARRB TR, Research Report ARR 293, pp 69 (ARRB TR: Vermont South, Victoria, Australia). SALT, G. and STEVENS, D. (2001). Pavement performance prediction: determination and calibration of structural capacity (SNP), Proceedings, 20th ARRB Conference, Part 4 pp 99-116 (ARRB TR: Vermont South, Victoria, Australia). SAPKOTA, B., BUTKUS, F., NORRIS, B. and GOH, A.L. (2001). Main Roads Western Australia’s experience in the use of the falling weight deflectometer for network pavement strength assessment, Proceedings, 20th ARRB TR Conference, (ARRB TR: Vermont South, Victoria, Australia). SCALA, A. J. (1979). An Analysis of Deflection Bowls in Pavements Measured by the Benkelman Beam Test. Internal Report No AIR 032-2. (Australian Road Research Board: Vermont South, Victoria, Australia). SMITH, R.B. (1985). Preliminary Evaluation of the Dynatest 8000 Falling Weight Deflectometer, Australian Road Research, 15(4), pp. 229-238 (ARRB: Vermont South, Victoria, Australia). TEPPER, S. and MARTIN, T. (2001). Long Term Pavement Performance Maintenance (LTPPM) – Progress Report, ARRB TR Contract Report RC1561, pp 77. (ARRB TR, Vermont South, Victoria, Australia). TEPPER, S., FOSSEY, D. and KOH, S.L. (2002). Long-Term Pavement Performance Study Data Report: 2001/2002 Season. Contract Report RC2008-3. (ARRB TR, Vermont South, Victoria, Australia).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
TONKIN and TAYLOR (1998). Pavement deflection measurement and interpretation for the design of rehabilitation treatments, Transfund New Zealand Research Report No.117, (Transfund New Zealand, Wellington, New Zealand). TRANSIT NEW ZEALAND (1977). Standard Test Procedure for Benkelman Beam Deflection Measurements, T/1 June 1977, (Transit New Zealand, Wellington, New Zealand). TRANSIT NEW ZEALAND (2002). Data Collection Specification, Auckland North Network, PSMC 005, September 2002, (Transit New Zealand, Wellington, New Zealand). TRANSIT NEW ZEALAND (2004). State Highway Location Referencing Management System (LRMS) Manual, SM051. (Transit New Zealand, Wellington, New Zealand). TRL (Transport Research Laboratory) (1993). A Guide to the structural design of Bitumen Surfaced Roads in Tropical and Sub-Tropical Countries. Overseas Road Note 31, 4th Ed. (TRL: Crowthorne, Berkshire, United Kingdom). VICROADS (1986). Pavement Strength Evaluation and Rehabilitation, Technical Bulletin No. 33 (VicRoads: Kew, Victoria, Australia). YODER, E.J. and WITZCAK, M.W. (1975). Principles of Pavement Design, 2nd Edition, (J. Wiley and Sons: New York, USA).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
APPENDIX 1:
1.1
ESTIMATING PAVEMENT STRENGTH PARAMETERS FROM DEFLECTION DATA
Modified Structural Number, SNC
A relationship between deflection and modified structural number, SNC, developed by Paterson (1987) from Brazilian pavement performance data, is defined as follows for sealed granular pavements: SNC = Ao × ( D0 )− 0.63
(Eqn 1.1)
where: D0 = Maximum deflection (mm) determined from a Benkelman Beam Ao = Model coefficient = 3.2 (uncemented base) and 2.2 (cemented base). As Paterson (1987) notes, Equation 1.1 is not a good fit to the observed data (r2 = 0.56) because SNC assesses pavement/subgrade strength and deflection measures the stiffness of the pavement. This is the fundamental limitation of using deflection data to estimate pavement strength. Equation 1.1 has been applied to the maximum deflections from the Deflectograph and the FWD, often without applying corrections to account for the differing test plate pressures. Notionally the Benkelman Beam and the Lacroix Deflectograph test loads apply a surface stress of 550 kPa while the FWD applies a surface stress of 700 kPa. Provided the deflection testing is within the usual range of elastic behaviour, then the relationships in Equation C1.1 and Equation C1.2 in Section C5.2 can be used in the interim to convert FWD deflection to Benkelman Beam deflection. Table 1.1 gives sample values of SNC for unbound sealed granular pavements, derived using Equation 1.1. Table 1.1: Sample values of mean characteristic maximum deflection (D0) and corresponding Modified Structural Number (SNC) for unbound sealed granular pavements
D0 (mm)
SNC
D0 (mm)
SNC
D0 (mm)
SNC
D0 (mm)
SNC
D0 (mm)
SNC
0.20
8.82
0.60
4.41
1.00
3.20
1.40
2.59
1.80
2.21
0.25
7.66
0.65
4.20
1.05
3.10
1.45
2.53
1.85
2.17
0.30
6.83
0.70
4.01
1.10
3.01
1.50
2.48
1.90
2.14
0.35
6.20
0.75
3.84
1.15
2.93
1.55
2.43
1.95
2.10
0.40
5.70
0.80
3.68
1.20
2.85
1.60
2.38
2.00
2.07
0.45
5.29
0.85
3.54
1.25
2.78
1.65
2.33
2.05
2.04
0.50
4.95
0.90
3.42
1.30
2.71
1.70
2.29
2.10
2.01
0.55
4.66
0.95
3.31
1.35
2.65
1.75
2.25
2.15
1.98
Note:
For most sealed granular pavements in Australia and New Zealand (viz, sealed granular pavements less than 700 mm thick), SNC and SNP have the same values (see Section C1.2.1.2 and Appendix 1.2).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
1.2
Adjusted Structural Number, SNP
A number of methods are available for estimating the adjusted structural number, SNP, from deflection data (Morosiuk et al 2001). The methods outlined below are those that are considered appropriate for conditions in Australia and New Zealand. 1.2.1
Jameson’s Method
The structural number, SN, is calculated using the following relationship derived from Jameson (1993) from a range of pavements: SN =
1.69 +
−
842.8
42.94
(D0 − D1500)
(Eqn 1.2)
D900
where: D0 = deflection (mm) at the centre of FWD test plate D900 = deflection (mm) 900 mm from centre of FWD test plate D1500 = deflection (mm) 1500 mm from centre of FWD test plate. The other terms in Equation 1.2 are as defined previously. The deflections are all normalised to a stress of 700 kPa. The California Bearing Ratio (CBR) of the subgrade is determined using the following equation (Jameson 1993): Log10(CBR) = 3.264 − 1.018 × Log10(D900)
(Eqn 1.3)
The CBR is then used to calculate the structural contribution of the subgrade, SNsg, as shown below (Hodges et al 1975):
SNsg = 3.51 × Log10(CBR) − 0.85 × ( Log10(CBR) )2 − 1.43
(Eqn 1.4)
The adjusted structural number, SNP, is determined from the sum of the structural number, SN, and the contribution of the subgrade, SNsg, as shown below (Hodges et al 1975), assuming that the modified structural number, SNC, is approximately the same as SNP for most pavements in Australia and New Zealand (see Section C1.2.1.2): SNP = SN + SNsg 1.2.2
(Eqn 1.5)
Robert’s method
Roberts (1995) developed the following relationship for the structural number, SN, based on FWD data collected in Australia and the Philippines:
SN = 12.992 − 4.167 × Log10(D0) + 0.936 × Log10(D900)
(Eqn 1.6)
with the structural contribution of the subgrade estimated using Equation 1.3 and Equation 1.4. This allows the adjusted structural number, SNC, to be estimated from Equation 1.5. Loizos, Roberts and Crank (2002) derived the following relationship to estimate SNP for asphalt pavements in Greece using FWD deflections: SNP = 167 × ( D0 )− 0.57
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(Eqn 1.7)
Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
1.2.3
Salt’s method
Salt and Stevens (2001) developed the following relationship for the Adjusted Structural Number, SNP, based on FWD deflections on sealed granular pavements in New Zealand: SNP = 112 × D0-0.5 + 47 × (D0 − D900)-0.5 + 56 × (D0 − D1500)-0.5 − 0.4
(Eqn 1.8)
where D0, D900 and D1500 are as defined above, except that the test plate pressure is normalised to a stress of 566 kPa. This means that if the test plate pressure is 700 kPa (more precisely 700 kPa – See Table 4), then the deflections are adjusted by a ratio of 566/707 (0.80). 1.2.4
Comparison of SNP and SNC estimation for granular pavements
A comparison was undertaken between the Adjusted Structural Number, SNP, estimated by Jameson (1993), Roberts (1995) and Salt and Stevens (2001) using deflections D0, D900 and D1500, and the Modified Structural Number, SNC, estimated by Paterson (1987) using the deflection D0. The comparison was based on a sealed granular pavement network with 688 FWD tests. Figure 1.1 compares the SNC and SNP estimates and the relationships found between SNC and SNP for the various SNP relationships. 8.5 SNP (Jameson 1993) SNP (Roberts 1995)
7.5
SNP (Salt and Stevens 2001) Line of Equality (SNC = SNP) SNC (Paterson 1987 using D0)
6.5
5.5
4.5
3.5
Sealed Granular Pavements (688 Test Samples) SNC = -2.76 + 1.61 x SNP (Jameson: r 2 = 0.96; SE = 0.21) SNC = -0.72 + 1.05 x SNP (Roberts: r 2 = 0.94; SE = 0.24) SNC = -0.32 + 1.08 x SNP (Salt & Stevens: r 2 = 0.999; SE = 0.05)
2.5
1.5 1.5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
SNP Prediction (using D0, D900 & D1500)
Figure 1.1: SNC (Paterson) vs SNP (others)
Figure 1.1 shows that all the relationships between SNC and SNP are closely related for this network data set (r2 = 0.94 to 0.999), particularly the Salt and Stevens (2001) estimate of SNP. This outcome shows that the additional bowl deflection terms D900 and D1500 do not provide a significantly different estimate of SNP from that based on only the D0 deflection. On face value the above data set appears to be reasonably representative of a sealed granular pavement network as it has a wide range of SNC and SNP values. Assuming this data set is representative, network level estimates of SNP, or SNC, could therefore be based on estimates using the D0 deflection without significant loss in accuracy.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
The correlation of the Salt and Stevens (2001) estimate of SNP with the Paterson (1987) estimate of SNC is remarkable. This is particularly so because the Paterson (1987) estimate of SNC was based on Benkelman Beam deflections on Brazilian pavements while Salt and Stevens (2001) used FWD deflections on New Zealand pavements to estimate SNP.
1.3
Structural Adequacy Indicator, SAI
The use of the Structural Adequacy Factor, SAI, developed by Eijberson and Van Zwieten (1998) is described below. To apply this model in practice three levels were developed as follows: ♦
SAI at Level 1 = SCI600 below lower level;
♦
SAI at Level 3 = SCI600 above highest level; and
♦
SAI at Level 2 = SCI600 between highest and lowest level.
The Surface Curvature Index, SCI600, at the lower level is defined as follows:
SCI600 =
+ a1
a2
(Eqn 1.9)
( 1 + a3 × Trucks ) where: Trucks = Number of trucks per day per lane a1, a2 and a3 = Constants defined in tabulation below. Constant
Full Depth Construction
Granular Base Construction
a1
53.9
60.52
a2
134.1
269.6
a3
0.002576
0.003841
The parameters, SCI600, lower level and highest level are defined as follows: ♦
SCI600 = D0 − D600;
♦
SCI600, lower level = see Equation 1.9; and
♦
SCI600, higher level = 1.8 × lower level.
If the SCI600 is above the highest level then the pavement is regarded as being structurally inadequate (SAI at level 3) and if SCI600 is below the lower level then the pavement is regarded as being structurally sound (SAI at level 1). When SCI600 falls between the higher and lower levels, the structural condition of the pavement is uncertain (SAI at level 2). The above was further refined by developing a Structural Distress Indicator matrix which, included the following parameters: 1.
the SAI level (1, 2 or 3);
2.
the parameter, Diff. SCI, defined below in Equation 1.10; and
3.
the amount of visible cracking (percentage of surface area cracked) in the pavement (% cracking).
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
Diff. SCI =
( SCI600,traffic − SCI600,untraffic )
× 100%
(Eqn 1.10)
SCI600,untraffic where:
SCI600,traffic = SCI measured on trafficked area of pavement SCI600,untraffic = SCI measured on untrafficked area of pavement. The three parameters were then combined to develop the Structural Distress Indicator matrix of values from 1 to 10, where 1 indicates a poor structural condition and 10 is a very good structural condition. The layout of this matrix is below. Visible Cracking
30%
1.4
1
Relative Pavement Strength (RPS) Indicator
The Relative Pavement Strength (RPS) indicator is a useful guide for assigning the timing of structural intervention (Roberts 2000b). The RPS is a dimensionless unit that estimates the current structural adequacy of a pavement. The RPS is defined as the percentage of remaining life due to traffic load relative to the user defined analysis period. Consequently, a pavement with adequate structural strength for the analysis period (and only the analysis period) has an RPS value of 100%. To use this indicator each pavement is assigned an RPS value, generally based on a 20 year analysis period, however, the analysis period can be revised as required. Assuming the 20 year analysis period, a pavement with only two years of remaining life has an RPS of 10%, while a pavement with 50 years remaining life has an RPS of 250%. The timings of investigative and structural intervention works are assigned according to the outcomes of this simple analysis. For instance an RPS of less than 10% requires immediate site investigation for pavement strengthening, while an RPS ranging from 10% to 40% may need a future strengthening and a more focussed monitoring of strength within one to two years, and an RPS ranging from 40% to 80% does not require any detailed attention in the short to medium term other than some strength monitoring within the next two to three years.
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APPENDIX 2
2.1
COST 336 PROCEDURES FOR REPEATABILITY TESTING WITH FWDs
Cost 336 Protocol U2–1999: Fwd Short Term Repeatability Verification NOTES:
This protocol is issued under the fixed designation 'Protocol U2'. The number immediately following the designation indicates the year of original adoption or, in case of revision, the year of last revision. Testing in accordance with this protocol should be conducted at least once every month, or more often as considered necessary by the operator.
1.
Scope
1.1
This protocol covers the determination of the repeatability of magnitude of load and deflections generated by the Falling Weight Deflectometer (FWD).
1.2
This protocol provides the means for any FWD user for periodic verification of repeatability of the FWD equipment.
2.
Referenced documents Protocol U3 - FWD Long-term Repeatability Verification.
3.
Significance and use
3.1
The objective of this procedure is to verify whether the FWD under test is capable of producing consistent results on a specific test site. In this procedure the short-term repeatability of a FWD is verified by using a series of twelve successive drops without lifting the loading plate. The first two drops are omitted from the analysis. The deflections are all normalised to the mean of the load imparted. The standard deviation of the load and normalised deflections should agree with the specified limits. When the results do not meet the requirements, the test should be repeated. Cases of persistent non-compliance, invalidate data collected by the instrument under test.
3.2
This protocol must be applied as often as specified in the Calibration Scheme or more frequently as considered necessary by the FWD user. The FWD user should keep records of the verification as conducted using this protocol.
3.3
This procedure may be combined with Protocol U3.
4.
Apparatus •
Falling Weight Deflectometer including control and signal processing electronics.
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5.
Procedure
5.1
Enter the following data on the calibration sheet: • FWD user; • FWD manufacturer; • FWD type/serial/ID number; • FWD deflection sensor serial numbers; • FWD loading plate diameter; • FWD deflection sensor offsets; • Current calibration factors for FWD deflection sensors; • Repeatability verification operator name; • Location of repeatability verification; • Date and time of last repeatability verification; and • Date and time of repeatability verification.
5.2
Position the FWD on a smooth, level, sound asphalt pavement structure with no visible cracks, where a peak deflection in the order of 250 to 600 µm can be produced in the load centre when using the selected target load level.
5.3
Warm up the FWD rubber buffers and condition the test point by repeating a sequence of ten drops until the loads and deflections that are registered are nearly uniform. The deflections in this sequence of ten drops should not be showing a steadily increasing or decreasing trend. If liquefaction or compaction is indicated by the warm-up data, or when the required deflection level cannot be achieved, relocate the FWD to another pavement.
5.4
Set the drop height and drop mass to generate the selected target load level. Apply two seating drops, for which no data is recorded, followed by ten replicate drops, for which peak values of load and deflection are recorded. Only these last ten drops will be used in the analysis. Do not raise the FWD loading plate during the test.
6.
Analysis
6.1
Normalise all deflections with the use of linear interpolation techniques to a reference load level that does not depart more than ten percent from the actually applied load (Equation 8.1). Determine the mean deflection of each deflection sensor for the set of ten drops (Equation 8.2).
6.2
Determine the standard deviation of all loads (Equation 8.3), and the standard deviation of all normalised deflections of each deflection sensor (Equation 8.4).
6.3
The standard deviation of the load recorded in the series of ten drops shall be less than, or equal to two percent of the mean of the recorded values. If the actual standard deviation exceeds the requirement, then the repeatability verification should be repeated at another pavement.
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6.4
The standard deviation of the normalised deflections, recorded in the series of ten drops shall be less than, or equal to 2 µm in case the mean of normalised deflections is less than, or equal to 40 µm. The standard deviation of the normalised deflections, recorded in the series of ten drops shall be less than, or equal to the sum of 1.5 µm and 1.25 percent of the mean of the recorded normalised values, in case this mean is greater than 40 µm. If the actual standard deviation of one or more deflection sensors exceeds the specified values, then the repeatability verification should be repeated at another pavement. Failure to satisfy the repeatability criteria again, necessitates closer investigation of the deflection sensors and their holders on the raise/lower bar. The non-compliance invalidates data collected by the FWD under test.
7.
Symbols i j NK Fi Fref sF uij dij dj
= = = = = = = = =
Drop label Deflection sensor label Number of drops (= 10) Magnitude of load at Drop i (kN) Preselected target reference load (kN) Standard deviation of load over all drops Unnormalised deflection measured by Deflection Sensor j at Drop i Normalised deflection measured by Deflection Sensor j at Drop i Mean of normalised deflections measured by Deflection Sensor j over NK
sdj
=
Standard deviation of normalised deflections measured by Deflection Sensor
drops j over NK drops 8.
Equations
8.1
Normalise deflections to target reference load level:
dij =
8.2
Fref Fi
⋅ uij
Calculate arithmetic mean of normalised deflections per deflection sensor: NK
∑d dj =
8.3
ij
i =1
NK
Calculate standard deviation of loads:
⎛ NK NK ⋅ ∑ F - ⎜⎜ ∑ Fi i =1 ⎝ i=1 NK ⋅ ( NK - 1 ) NK
2 i
sF =
⎞ ⎟⎟ ⎠
2
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8.4
Calculate standard deviation of normalised deflections:
⎛ NK NK ⋅ ∑ d - ⎜⎜ ∑ dij i =1 ⎝ i=1 NK ⋅ ( NK - 1 ) NK
2 ij
sd j =
9.
⎞ ⎟⎟ ⎠
2
Report The report should contain at least: • Data of the calibration sheet; • Principal test data used in analysis; • Analysis results; and • Declaration whether FWD or FWD component under test complies with the specifications.
2.2
Cost 336 Protocol U3–1999: Fwd Long Term Repeatability Verification Note:
This protocol is issued under the fixed designation 'Protocol U3'. The number immediately following the designation indicates the year of original adoption or, in case of revision, the year of last revision. Testing in accordance with this protocol should be conducted at least once every month, or more often as considered necessary by the operator.
1.
Scope
1.1
This protocol covers the determination of the long-term repeatability of the Falling Weight Deflectometer (FWD) on a carefully selected test site producing deflection data with limited annual and/or seasonal variation.
1.2
This protocol provides a procedure for easy verification of accuracy of FWD deflection output at the home base of the FWD user.
2.
Referenced documents Protocol U1 - Verification of Deflection Sensor Position; and Protocol U2 - FWD Short-term Repeatability Verification.
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3.
Significance and use
3.1
In this procedure the long-term repeatability of a FWD is verified by using a series of twelve successive drops. The first two drops are omitted from the analysis. The deflections are all normalised to the target load level. This target load level may be freely chosen at the first time of performing this repeatability verification action. In all later replicates, the same target load level must be used. The mean of the deflections is compared to the results previously collected at the same location. This location should preferably be selected close to the FWD home base and shielded from climatic influences as much as possible. The objective of this test is to detect any anomalies in the deflection output. Deflection results will not be a constant over the year due to temperature and seasonal changes. For that reason the test provides only indicative data. It reveals whether unexpected absolute changes of deflection have occurred. If suspicion has risen over the output, load cell and deflection sensors should be investigated to identify the source of the problem.
3.2
This protocol must be applied as often as specified in the Calibration Scheme or more frequently as considered necessary by the FWD user. The FWD user should keep records of the verification as conducted using this protocol.
3.3
This procedure may be combined with Protocol U2.
4.
Apparatus • • •
Falling Weight Deflectometer including control and signal processing electronics Thermometer Clock
5.
Preparation
5.1
In the first application of this protocol, the FWD under test should be equipped, and drop height and deflection sensor offset should be set as used in normal operation situations. These settings will be termed as default settings. In any future use of this protocol, settings should be identical to the default settings. Keep a record of the default settings.
5.2
Select a smooth, level, and sound pavement structure with no visible cracks on which deflections may be measured which will hardly change with time of the year. This test site should preferably be shielded from direct solar radiation and other climatic influences. Deflections measured in sequences of multiple drops should not be showing a steadily increasing or decreasing trend. If so, relocate the FWD to another pavement.
5.3
Mark the position where the loading plate of the FWD rests so that it can be relocated precisely on the same spot at another day of testing. This may be done by paint, or by marking a small divot in the pavement with a chisel. Also mark the direction in which the deflection sensor beam points. Include a description of the test position in the record of the default settings.
6.
Procedure
6.1
Enter the following data on the calibration sheet: • FWD user; • FWD manufacturer; • FWD type/serial/ID number;
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
• • • • • • • •
FWD deflection sensor serial numbers; FWD loading plate diameter; FWD deflection sensor offsets; Default FWD settings; Current calibration factors for FWD deflection sensors; Repeatability verification operator name; Location of repeatability verification; and Date and time of repeatability verification.
6.2
Verify if deflection sensors are mounted at the default offsets (use COST 336 Protocol U1). In case of non-compliance, reposition deflection sensors to the correct offset. Check whether the FWD is equipped according to the default settings.
6.3
Warm up the FWD rubber buffers and condition the test point by repeating a sequence of ten drops until the loads and deflections that are registered are nearly uniform.
6.4
Use the selected target load level. Apply two seating drops, for which no data is recorded, followed by ten replicate drops, for which peak values of load and deflection is recorded. Only these last ten drops will be used in the analysis. Do not raise the FWD loading plate during the test.
6.5
Measure the pavement temperature at mid-depth of the asphalt concrete layer or cement concrete slab. Register the temperature in °C with one digit placed beyond the decimal point. The accuracy of the temperature-measuring device should be ±0.5°C. Record day of the year and clock time in hours (24 hour system) and minutes (eg, 14:35).
7.
Analysis
7.1
Normalise all deflections with the use of linear interpolation techniques to a reference load level that does not depart more than ten percent from the actually applied load (Equation 9.1). Determine the mean deflection of each deflection sensor for the set of ten drops (Equation 9.2).
7.2
Plot the deflections against the measured pavement temperature. Examination of the variations of these parameters with time will then provide an indication of any anomalies in the consistency of the FWD equipment, which need to be further investigated.
8.
Symbols i j NK Fi Fref uij dij dj
= = = = = = = =
Drop label Deflection sensor label Number of drops (= 10) Magnitude of load at Drop i (kN) Preselected target reference load (kN) Unnormalised deflection measured by Deflection Sensor j at Drop i Normalised deflection measured by Deflection Sensor j at Drop i Mean of normalised deflections measured by Deflection Sensor j over NK drops
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
9.
Equations
9.1
Normalise deflections to reference load level:
dij =
9.2
Fref Fi
⋅ uij
Calculate mean deflection per deflection sensor: NK
∑d dj =
10.
ij
i =1
NK
Report The report should contain at least: • Data of the calibration sheet; • Principal test data used in analysis; • Analysis results; and • Declaration whether FWD or FWD component under test depart from the expected pattern.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
APPENDIX 3
INTERIM STRUCTURAL DETERIORATION MODEL FOR SEALED GRANULAR PAVEMENTS
This Appendix outlines information about an interim structural deterioration model for sealed granular pavements. The information provided is preliminary, and substantial observational data is needed to support and confirm the adaptation of the ‘Interim Model’ for practical use. The primary aim of the interim structural deterioration model is to provide a quantitative assessment of the remaining structural pavement life in a network context. It is not intended as a detailed pavement structural capacity prediction tool at the sub-network level.
3.1.
Background to ‘Interim Model’ development
3.1.1
Strength parameter and terminal condition
The selection and definition of a suitable pavement performance indicator(s) or strength parameter, to assess an approaching terminal structural condition, is needed to objectively assess the remaining structural pavement life in a network context. The only Austroads (1992) defined terminal condition limiting pavement life is roughness, which is a surface serviceability criterion not a structural criterion. There is no current recommended structural failure limit in terms of the pavement performance indicators. Current practice, at a project level, tends to use indicators such as rut depth and surface deflection in combination with roughness to assess either the end of pavement life, with subsequent rehabilitation, or the need and extent of surface intervention. These indicators are useful for targeting deflection surveys. The magnitude and severity of these indicators for terminal structural condition could also vary depending upon the consequences of structural failure for the pavement. Therefore these indicators may need to be lower in magnitude and severity for more heavily trafficked roads compared with those used for lightly trafficked roads. For example, this is reflected in Table 2 and Table 5 in these guidelines. A 1996 study of 30 arterial rural road samples in Victoria (Martin and Ramsay 1996, McLean and Martin 1999) showed that a majority (67%) of the study’s road samples were rehabilitated partly or fully for excessive roughness reasons rather than for excessive surface deflection alone. For these samples, excessive roughness was defined when the roughness exceeded 4.3 IRI (112 NRM). Excessive deflection was defined when the maximum deflection, D0, from FWD testing exceeded 1.4 mm. 3.1.2
Inferred terminal conditions from Austroads (2004b)
Section 6.2.5 of Austroads (2004b) states that for a granular resheet there is a 6% reduction in surface deflection for every 25 mm of granular resheet thickness. These rates of deflection reduction are assumed to apply to the usual range of granular resheet thickness from 100 mm to 250 mm. The impact of this reduction in deflection is a commensurate increase in the pavement’s modified structural number, SNC, using Equation 1.1 in Appendix 1 relating SNC to deflection. Table 3.1 summarises this estimated impact on deflection reduction and increase in SNC.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
If the increases in SNC shown in Table 3.1 were assumed to be those needed to restore the pavement to its original zero pavement age structural condition, then a simple deterioration model for pavements needing rehabilitation could be postulated. The assumption underlying this approach is that strength recovery is equal to strength deterioration, that is, linear elastic theory is appropriate. For most in service conditions of granular pavements, the elastic behaviour assumption is reasonable (Yoder and Witzcak 1975), although it is not likely to be appropriate for pavements with marginal materials subject to excessive loading. It also may not be appropriate for lightly trafficked thin pavements where the majority of structural deterioration may be within the supporting subgrade. Table 3.1: Impact of granular resheeting on pavements
Note:
Resheet Thickness (mm)
Reduction in Deflection1 (%)
Increase in SNC2 (%)
100
24
12.7
150
36
17.6
200
48
21.9
250
60
25.6
1. 2.
Austroads 2004b (Section 6.2.5 and Figures 6.5 and 6.7). Based on Equation 1.1 in Appendix 1.
3.2.
Basis of interim structural deterioration model
3.2.1
Model postulation
The deterioration of pavement/subgrade strength can be described either by the reduction in an appropriate strength parameter, or increases in measured pavement deflection over the pavement’s life-cycle. An examination of various deflection and traffic load capacity relationships (Martin 2002) showed that the variations between the deflection and traffic load capacity relationships are greater than the variations between the pavement strength parameter (such as SNC) and traffic load capacity relationships (NAASRA 1979, Paterson 1987, Martin 1998, QDMR 1993, TRL 1993). This outcome suggests that deflection deterioration may be more difficult to predict than strength parameter deterioration. It is proposed to describe the deterioration of network pavement strength in terms of the Modified Structural Number, SNC, which is defined in Appendix 1.1 and is the same as the Adjusted Structural Number, SNP, for sealed granular pavements less than 700 mm thick, as noted in Section C1.2.1.2. SNP is used by the Highway Development Management model, HDM-4 (ISOHDM 2000), as a pavement strength deterioration indicator. The most practical means of currently estimating SNC at a network level is to measure surface deflection by the various means and devices discussed earlier and as shown in Appendix 1. This means using the relationship between SNC and deflection shown in Equation 1.1 and Table 1.1 in Appendix 1, with all of its attendant problems of matching strength (SNC) with pavement performance in terms of deflection that measures pavement stiffness better than strength. Equation 1.1 in Appendix 1 uses only the maximum deflection, D0, to estimate SNC. As implied by Martin and Crank (2001) and noted in Section C1.2.1.2, the other bowl deflections (D900 and D1500) do not significantly improve the estimation of SNC with the relationships used.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
Prediction of network level pavement deterioration is an analysis with a long term time frame. Although a significant number of Australia’s sealed granular pavements have lives in excess of 40 years (Blake et al 1996) there are often instances of premature structural failure due to local defects in the pavement base or subgrade that reduce pavement design life. In other cases reduced pavement life is due to the use of marginal materials. Nevertheless, network pavement deterioration cannot reasonably account for local defects and marginal materials if they are just a local problem. 3.2.2
Model definition
The following Equation 3.1 is postulated on the basis of expected sealed granular pavement performance for use as the ‘Interim Model’ for the time dependent deterioration of SNC: SNC =
Kc × SNCd × { 2 − EXP ( km × b/DL × AGE) }
(Eqn 3.1)
where: SNC = modified structural number of the pavement/subgrade at time = pavement age, AGE SNCd = modified structural number of the pavement/subgrade as required for the design traffic load Kc = calibration factor for climatic and construction quality factors that influence the initial value of SNC (default = 1; possible range 0.8 [wet climate, poor construction] to 1.2 [dry climate, very good construction] ) km = Calibration factor for maintenance factors (drainage and surfacing) that influence the value of SNC during pavement life (default = 1; possible range 1.2 [poor maintenance] to 0.5 [excellent maintenance]) EXP = e raised to the power DL = design life of the pavement (currently, 20 to 25 years) which can change with altered traffic loading conditions b = Deterioration factor AGE = Pavement age (years). Equation 3.1 meets the main boundary conditions, that is, when the pavement age, AGE, is zero, SNC is at its highest expected value, and when the pavement age is at its terminal value (AGE ≥ DL), SNC has decreased to its lowest expected value (actual value not specified at present). The reducing value of SNC can be used to estimate the pavement’s remaining traffic load capacity by means of existing SNC relationships with traffic load capacity (NAASRA 1979, Paterson 1987, Martin 1998, QDMR 1993, TRL 1993) at any time (AGE) during the pavement’s life-cycle. However, as noted in Section 3.2.1, there is considerable variation in the estimated remaining traffic capacity given by these equations. Limitations of the ‘Interim Model’ The ‘Interim Model’ (Equation 3.1) predicts structural deterioration of sealed granular pavements assuming that ongoing distress only occurs in the upper portion of the pavement base with the majority of the remaining pavement in relatively good condition. This assumes a stable subgrade with good drainage and no deterioration in the subgrade. Where distress occurs in lower pavement layers, such as in very thin pavements with distress in the supporting subgrade, Equation 3.1 may not be a useful predictor of structural deterioration. However, it still may be possible to calibrate Equation 3.1 for these conditions.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
The Equation 3.1 model form for deterioration does not provide for potential improvements to pavement/subgrade strength that may occur during a pavement’s life-cycle as observed by the previous data review (Martin 2002). The reasons behind these observed strength improvements need to be established to discover whether additional independent variables need to be included in Equation 3.1. The longer term prognosis for all pavement/subgrade strength is for its eventual deterioration, unless maintenance intervention markedly improves structural conditions. The impact of increased traffic loading above the original design load estimate in Equation 3.1 could be accounted for by a pro rata reduction in the design life, DL, as follows: Reduced DL =
DL ×
Design Traffic Load Increased Traffic Load
(Eqn 3.2)
Equation 3.2 is a simplified, but practical, approach that could be used in conjunction with Equation 3.1 until improved structural deterioration models become available. Equation 3.1 can be refined when further evidence is available, such as the typical pavement terminal life (years) and the prevailing terminal structural conditions (roughness, rutting and deflection). This refinement needs a reasonable assessment of what would also be an adequate reserve of traffic load capacity for the pavement to avoid excessive surface and structural deterioration prior to rehabilitation. This reserve could be expressed as a percentage of the initial value of SNC (or initial traffic load capacity), which means that the magnitude of the reserve capacity increases in line with pavements that have an increasing value of SNC. In the interim, Equation 3.1 can serve as a potential means of predicting the loss of SNC for pavements at a network level over their life-cycle. Sectio 3.2.3 provides two alternative means of defining the calibration factors in Equation 3.1. The assumption underlying both means of defining the calibration factors is that strength recovery is equal to strength deterioration, that is, linear elastic theory is appropriate, as noted in Section 3.1.2. The limitations of this approach are also discussed in Sectio 3.1.2. Deterioration prediction Structural deterioration prediction using Equation 3.1 needs to be made with reference to the terminal structural condition and the existing structural condition. Table 2 in Section C1.2.2 shows that the terminal condition varies depending on the level of service required from the road. The varying terminal condition with level of service indirectly accounts for some reserve structural capacity needed to avoid catastrophic failure. 3.2.3
Potential use of the ‘Interim Model’
The estimation of the deterioration in SNC can be demonstrated under two extreme possible conditions as shown below. Deterioration model with a varying pavement design life (constant deterioration factor, b) The deterioration predicted by Equation 3.1 is shown in Figure 3.1 for a varying pavement design life. Equation 3.1 deterioration factor, b, in this case is based on using the increases in SNC with various levels of rehabilitation thickness in Table 3.1 as being equivalent to reductions in SNC during the deterioration phase.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
Assume initially pavement design life is reached when the minimum rehabilitation thickness of 100 mm is needed to restore the pavement and the surface deflection has decreased by 24%. Equation 3.1 estimates a potential extension to pavement life of some 91% if pavement life is based on the longest intervention level when the rehabilitation thickness to restore the pavement is 250 mm and the surface deflection has decreased by 60%. Equation 3.1 uses a constant deterioration factor, b, of 0.12 in Figure 3.1 to meet the above conditions. The predicted 91% increase in pavement life means that a design life of 20 to 25 years could become 30 to 48 years which is similar to the range observed in some parts of Australia (Blake et al 1996). Deterioration model with a fixed pavement design life (varying deterioration factor, b) Alternatively, if it is assumed that the pavement design life is fixed and that rehabilitation coincides with the end of the design life, Equation 3.1 needs different values of the deterioration factor, b, to predict the different rates of deterioration in SNC shown in Table 3.1. Figure 3.2 shows the range of values for ‘b’ which could also represent the varying influences like maintenance which could be included in the calibration factor for maintenance, km, if “b’ was held to a constant value. The value of ‘b’ in Figure 3.2 varies from 0.12 to 0.23, a range of twice the lowest value of ‘b’. This means that the calibration factor for maintenance effects, km, could range from 1.0 to 0.5. 1 100% 12.7% 17.6%
SNC (Paterson 1987) as a percent of the as-constructed value
SNC = Kc x SNCd x [ 2 - EXP
( km x 0.12 / DL x AGE)
21.9%
]
25.6%
Deterioration restored by 100 mm overlay Deterioration restored by 150 mm overlay Deterioration restored by 200 mm overlay Deterioration restored by 250 mm overlay
AGE / Initial Design Life (DL)
1.36
1.66
1.91
0% 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
(Pavement Age, AGE) / (Design Life, DL)
Figure 3.1: % SNC Deterioration vs Pavement Age / Design Life (varying Pavement Design Life and constant Deterioration Factor (b = 0.12)
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2
Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
1 100% 12.7
17.6 21.9
SNC (Paterson 1987) as a percent of the as-constructed value
SNC = Kc x SNCd x [ 2 - EXP
( km x b / DL x AGE)
25.6
]
b = 0.12 (restoration by 100 mm overlay) b = 0.16 (restoration by 150 mm overlay) b = 0.2 (restoration by 200 mm overlay) b = 0.23 (restoration by 250 mm overlay)
0% 0
0.2
0.4
0.6
0.8
1
(Pavement Age, AGE) / (Design Life, DL)
Figure 3.2: % SNC Deterioration vs Pavement Age / Design Life (fixed Pavement Design Life and varying Deterioration Factor)
3.3.
Model calibration
3.3.1
Use of performance data
There is currently performance data that can aid the calibration and the possible refinement of the ‘Interim Model’. This performance data includes the accelerated load testing data aimed at quantifying the influence of maintenance on pavement performance which covers the full pavement life-cycle (Martin and Gleeson 1999, Martin et al 2000, 2001). Additional independent variables may need to be included in the ‘Interim Model’ if they can rigorously account for long term changes in structural condition, including strength improvements and seasonal changes. These are not readily available and some will have to come from the performance data for the long term pavement performance monitoring (LTPP) sites and the long term pavement performance maintenance monitoring (LTPPM) sites (Clayton and Styles 2001, Tepper and Martin 2001), and road agency historical databases. 3.3.2
Model calibration
It is possible that the model form finally used for the deterioration of pavement/subgrade strength, SNC, will allow for variation of both the pavement life and the rate of deterioration as observed from the deterioration of deflection measured at a network level. It is important that the model allows for ongoing refinement and calibration from observed performance.
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Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements)
In the interim, if the rate of SNC deterioration with time is observed it can be expressed in terms of Equation 3.1 as follows: d(SNC) d(AGE)
= km × b DL
× Kc × SNCd × { 2 − EXP ( km × b/DL × AGE) }
(Eqn 3.3)
Equation 3.3 can be used to re-calibrate either Kc or b in Equation 3.1 for a given value of pavement age, AGE, where the pavement maintenance regime has not changed. This approach assumes that the maintenance factor calibration, km, does not need adjustment. However, the design life, DL, may also need adjustment under these conditions. The DL may also need reduction or an increase if there is a significant change to the design traffic loading using the approach shown in Equation 3.2. Where the maintenance regime has changed, the maintenance factor, km, will need calibration. Unexpected and abrupt changes in the observed rate of SNC deterioration may demand a change in the form of the deterioration model.
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INFORMATION RETRIEVAL
Austroads (2005), Guidelines for Road Network Condition Monitoring: Part 3 — Pavement Strength (Sealed Granular Pavements), Sydney, A4, 86pp, IR-88/05
KEYWORDS: Benkelman Beam, deflection, Deflectograph, Deflectometer, monitoring, network level, pavement, pavement management, pavement performance, pavement strength, performance prediction, road management, road network, structural analysis
ABSTRACT: This document contains guidelines for and background notes on network level measurement and reporting of deflection data, and analysis of the structural capacity of sealed granular pavements, for road network management purposes in Australia and New Zealand. The guidelines discuss the frequency and scope of network deflection surveys, including issues such as selection of longitudinal sampling intervals or sampling proportions for deflection surveys, and choice of parameters and estimation of parameter values to represent network level pavement strength in asset management analysis. The guidelines are intended as a basis for a consistent approach in Australia and New Zealand. Verification procedures for deflection measurement devices are covered, and repeatability and bias are discussed. A glossary of terms used in network level assessment of pavement strength is also included. The guidelines include for information rather than use, an ‘Interim Model’ that has been postulated for prediction of structural deterioration of sealed granular pavements, depending on influences such as traffic loading, climate, construction quality, maintenance circumstances, and age.
