Australean Guidelines for Road Network Condition Monitoring Part 3 – Pavement Strength

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AUSTROADS INTERNAL REPORT

Guidelines for Road Network Condition

Monitoring: Part 3 – Pavement Strength

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GUIDELINES FOR ROAD NETWORK CONDITION

MONITORING: PART 3 — PAVEMENT STRENGTH

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(Sealed Granular Pavements)

First Published 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: austroads@austroads.com.au 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.

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GUIDELINES FOR ROAD NETWORK CONDITION

MONITORING: PART 3 — PAVEMENT STRENGTH

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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).

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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 long-term pavement performance monitoring and data collection is necessary to enable the

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♦ 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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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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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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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

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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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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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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

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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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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 - Glossary of Terms used in Network-Level Assessment of Road Pavement Strength (Sealed Granular Pavements)

♦ Section C – Guidelines for Network 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

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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.

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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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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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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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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).

Figure 1: Pavement deflection bowl (not to scale)

1200mm 250mm 900mm 1500mm 600mm D0 D250 D300 D200 D900 D1500 D600 D1200

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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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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 Indicative Investigation Condition Nominal traffic ranges

Typical Road Function _Speed

(km/h) _{AADT (vpd)} _ESAs Surface Deflection (D0 (mm))1 Roughness Limit (IRI (m/km))2

% Road Length with Rut Depth > 20 mm3

(1.2 m straight edge)

Freeways, etc ≥ 100 > 30,000 > 2 x 107_{0.8 4.2} _10%

Highways and main roads 100 5,000 – 30,000 3 x 106_{– 2 x 10}7_0.9 _4.2 _10%

Highways and main roads 80 1,000 – 5,000 4 x 105_{– 3 x 10}6_1.1 _5.4 _20%

Other sealed local roads Various < 1,000 < 4 x 105 _1.6 _{See Note 4} _30%

Notes: 1 Based on Figure 6.5, Austroads (2004b).

2. Based on Table 3.1, Austroads (2004b). 3. Based on Table 3.2, Austroads (2004b).

4. In accordance with the relevant local asset management strategy.

From the 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 (or the indicative investigation condition levels). 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 deterioration such as traffic loading, climate, construction quality, maintenance, etc.

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C2. EQUIPMENT FOR MEASURING PAVEMENT DEFLECTION

C2.1 Guidelines

There are three main types of testing devices for measuring surface deflection at a network level:

♦ Benkelman Beam (BB);

♦ Deflectograph (DEF); and

♦ Falling Weight Deflectometer (FWD) or Heavy Weight Deflectometer (HWD).

These devices all produce half deflection bowls. The devices and the associated test methodologies are described in Sections C2.2 to C2.6 and summarised in Table 3.

Benkelman Beam (BB)

Benkelman Beams measure the rebound deflection of a pivot beam relative to a base beam at a point on the pavement while the test wheel load (from a loaded truck, with standard wheel configuration, axle load and tyre pressure) moves away at creep speed along the pavement surface. A Benkelman Beam is a lightweight, low cost tool that is widely used for project level deflection testing, and is suitable for network level deflection surveys of small networks. Deflection recording is usually manual, but automated recording is available.

Deflectograph (DEF)

A Deflectograph consists of two beams mounted on a sled under the chassis of a host truck. The DEF beams measure the downward deflection in each wheel path while the test wheel load approaches. A DEF measures and records deflections in 50 mm increments up to 900 mm from the centre of the test load (referred to as D900), in a series of tests at longitudinal spacings of 3 to 7 m,

at a constant speed of 3 to 4 km/h.

Two DEF versions are commonly used in Australia. Both are based on the La Croix Deflectograph that was developed and proven in Europe. One version built by RTA NSW operates in New South Wales, Queensland and Tasmania. The other version, built and operated by VicRoads, is called the Pavement Strength Evaluator (PaSE). TSA expects to take delivery in mid-2005 of a Deflectograph constructed by WDM Ltd (UK).

DEFs initially applied a nominal surface stress of 550 kPa (tyre pressure). They now operate mainly at a nominal surface stress of 750 kPa. The PaSE applies a 10 tonne (98 kN) axle load, whereas the other Australian DEFs apply the standard axle load of 8.2 tonne (80 kN). The beam lengths and wheelbase of the PaSE are longer than the other Australian DEFs.

Falling Weight Deflectometer and Heavy Weight Deflectometer (FWD and HWD)

A Falling Weight Deflectometer (FWD) measures surface deflection at offsets ranging from 0 mm to a user-defined maximum offset (normally 1,500 mm, but up to 2,400 mm) from the centre of an impulse test load. A series of geophones is used to measure the deflection. The number of geophones varies, but generally seven are used in normal applications. The magnitude of the load, which is applied through a circular loading plate 300 mm is diameter, is varied by selecting from a range of drop heights. The FWD uses a load cell to measure the actual applied load.

A Heavy Weight Deflectometer (HWD) can also be used in applications such as pavements on heavy duty roads, at airports, container terminals, and other industrial areas. HWDs are available with capacities up to 250 kN, whereas the capacity of a FWD is typically up to 150 kN.

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Table 3: Summary of the main features of Benkelman Beams, Deflectographs, and Falling Weight Deflectometers

Features Benkelman Beam (BB) Deflectograph (DEF) Falling Weight Deflectometer (FWD)

Applied test load A moving wheel load, applied by the dual wheels of a slow moving truck, with standard wheel configuration, tyre pressure and axle load (variable and usually 40kN).

A moving wheel load, applied by the dual wheels of a slow moving truck, with standard wheel configuration, tyre pressure and axle load (variable, usually 40kN, but 49kN for the VicRoads PaSE).

An impact load (up to 150kN for a FWD and up to 250kN for a HWD) in the form of a falling weight with a variable drop height, applied through a standard circular loading plate normally 300mm in diameter.

Outline of test

method A BB comprises a pivot beam and a fixed beam, enabling rebound deflection to be measured at each test point (which can be at any desired spacing) at a series of user-defined ‘offsets’ (or continuously with an automated recording system) along a wheel path behind the applied standard test load. Measures a half deflection bowl while the applied load moves away at creep speed. A BB can be used to measure full bowl deflections if required, however this is not relevant to network surveys.

A DEF is similar to a short BB mounted on a sled under the chassis of a host truck. As the truck moves forward, the DEF is placed for testing, raised and re-placed in a continuous cycle. DEF records downwards deflections in a half deflection bowl in both wheel paths at intervals of between 3m and 7m and at a series of ‘offsets’ 50mm apart up to at least 900mm in front of and a short distance beyond the applied standard test load.

A FWD measures deflections (in a half deflection bowl at a series of user-defined radial ‘offsets’ normally up to 1,500 mm from the applied load) and measures the actual applied load at each test point, which can be at any desired spacing, and at any point on a pavement, whether in a wheel path or not. Deflections are measured by deflection sensors (usually geophones) located at the user-defined radial ‘offset’ positions. Seven geophones are usual, and more are possible.

Transporting the

testing device A BB is a lightweight instrument that can be carried manually, or on a small trolley. The DEF is permanently mounted under the chassis of a dedicated host truck, in front of the rear wheels. FWDs are usually mounted on a small trailer towed behind a light vehicle eg, large car or commercial van. FWDs are relatively small and more readily transportable than DEFs.

Progression along the road during testing

Tests at discrete test points. A brief stop is necessary to put the BB in position before each test. Production depends on traffic conditions, the complexity of traffic control, longitudinal spacing between test points, method of measuring and recording deflections (manual or automated), number of wheel paths being tested (1 or 2), and the mode of transport for operators (walk or ride). Typical rates of progression are of the order of 0.6km/h (test points at 10m spacing) to 1.3km/h (100m spacing). Typical production for a shift of approximately 7 hours ranges from 4 to 7 lane-km.

Continuous movement at 3 to 4 km/h. Typical production for a shift of approximately 7 hours is of the order of 20 to 30 lane-km (viz up to about 15,000 deflection bowls), depending on traffic conditions and the complexity of traffic control.

Tests at discrete test points. The device is stationary during a test. Typical network survey production for a shift of approximately 7 hours is 150 to 200 test points, depending on traffic conditions, the complexity of traffic control, and the longitudinal spacing between test points. Deflections and actual applied load are usually recorded only for the third in a series of three drops at each test point.

Availability of deflection testing equipment in Australia and NZ

A BB is a simple, low-cost and lightweight instrument. Many road agency depots, local government bodies and testing organisations own at least one BB. Suitable trucks are readily available in Australia and NZ.

As at December 2004, there are five DEFs based in Australia (two owned by RTA NSW, and one each by VicRoads (known as PaSE), QDMR and DIER Tas), and none in NZ. TSA expects to commission a new DEF during 2005.

As at December 2004, there are approximately 14 FWDs based in Australia and NZ (3 owned by ARRB TR, 2 by QDMR, 1 by MRWA, 1 by TSA, 3 by PMS Pty Ltd, and a small number owned by various LGAs and private Consulting firms). FWDs based in Australia and UK have been used in NZ.

Suitability for network level deflection surveys

Because of the low number of tests per shift, a BB is suitable only for small

network surveys. Particularly suitable for project level deflection measurement. Suitable for network level surveys, though the rate of coverage is limited. Also suitable for project level deflection measurement. Suitable for network level surveys, particularly for large networks where relatively homogeneous pavements enable long spacings between test points. Also suitable for project level deflection measurement. Personnel

required A truck driver, a BB operator and a data recorder (plus personnel specifically allocated to traffic control duties). A truck driver and a DEF operator (plus personnel specifically allocated to traffic control duties). A driver / FWD operator, or a driver and a FWD operator (plus personnel specifically allocated to traffic control). Advantages Local availability, low establishment costs, and simple technology. No limit on

maximum ‘offset’ for deflection readings, enabling absolute deflection values to be reported.

Close spacing of test points and deflection measurements at each

test point, large number of test points per day. Quickest device for covering a large network, provided sample spacings are long. Applied load can be readily varied. Accurate measurements.

Practical

limitations Personnel are on the road pavement. Relatively slow overall rate of progress with a network deflection survey, and is therefore suitable only for deflection surveys of small networks. Loading from the other rear wheels may affect deflection measurements.

Relatively slow overall rate of progress (distance) with a network deflection survey. Loading from other wheels (front and rear) and vibrations associated with the moving truck may affect deflection measurements. Maximum practical ‘offset’ is 900 mm.

Can be difficult to test the left wheel path where seal or shoulder is narrow and shoulder or edge is not in sound condition. Can be difficult to test the right wheel path, because FWD trailer may encroach on the adjacent traffic lane.

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C2.2 Deflection measuring equipment – general

To reduce costs and speed up the analysis process, surface deflection data from test loads is currently the most commonly used method for assessing pavement strength. Estimation of network strength parameters using deflections, as discussed in Section C1.2.1.2 and Appendix 1, does not require any prior knowledge of the pavement layer structure. However, knowledge of the pavement structure would permit back-analysis of deflection data to estimate moduli, etc for use at a project level.

Three main device types are used for collecting deflection data at network level:

♦ Benkelman Beam (BB) – tests at discrete points, suitable only for small networks due to its relatively slow rate of testing;

♦ Deflectograph (DEF) – conducts almost continuous testing at a constant speed of 3 to 4 km/h; and

♦ Falling Weight Deflectometer (FWD) or Heavy Weight Deflectometer (HWD) – discrete test points.

Table 3 contains a summary of the main features of Benkelman Beams, Deflectographs, and Falling Weight Deflectometers.

All of these deflection measuring devices can be fitted with a Differentially Corrected Global Positioning System (DGPS or GPS) receiver that locates the survey test point to within specified horizontal and vertical tolerances (eg, 2.5 m horizontal and 5 m vertical (TNZ 2002)) of its actual location.

In addition to these three deflection measuring device types, there are other options, such as geophysical techniques combined with cored samples for later laboratory testing, but these methods are slower and often have sampling and interpretation problems greater than those of deflection testing.

In many instances the availability of deflection measuring devices may govern the choice of the device used. The frequency of network sampling (years) may also be influenced by the choice of deflection measurement device - this is discussed in Section C3.

C2.3 Benkelman Beam (BB)

A Benkelman Beam (BB) measures relative deflection of a pivot beam to a base beam at a point on the pavement while the test wheel load (normally the rear axle of a rigid truck) rolls slowly (nominally at 4 to 5 km/h) along the pavement surface. The testing process is relatively slow compared with a Deflectograph. During BB testing, the operator walks behind the truck, which stops at each test point while the BB is set in position for the test.

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Figure 2: General view of Benkelman Beam (BB) with load truck and trolley, and sketch of BB arrangement

A Benkelman Beam can be used to measure full bowl deflections if required. This ability is unique to Benkelman Beams, but collecting full bowl deflection data is not necessary in network surveys.

For a BB, the standard test load consists of an 80 kN dual wheel single axle applying a surface stress of 550 kPa (the tyre pressure) under the test loading. Some road agencies, such as DIPE NT, apply a 750 kPa surface stress with the Benkelman Beam to approximate current tyre pressures used in heavy vehicles. Because of practical difficulties in maintaining tyre pressures on an operational vehicle, variations of up to 5% during a shift are likely. The Benkelman Beam measures the relative deflection up to any desired distance from the moving test wheel load, although this distance is often limited, for example to 2,700 mm (TNZ 1977, RTA NSW 1982) or 1,200 mm (VicRoads 1986), depending on the time available for recording.

Improvements such as electronic measurement and recording of deflections improve accuracy of the data and the efficiency of the survey operation. Two Benkelman Beams can be used simultaneously, one in each wheel path. The Benkelman Beam provides relatively reliable deflection data at a point.

Benkelman Beam with automated deflection recording Benkelman Beam with manual deflection recording

Figure 3: Benkelman Beams (BB) with automated and manual deflection recording

Benkelman Beams are used widely for project level deflection testing on arterial roads. Benkelman Beams may be useful for small scale network level assessments of pavement strength, but due to the slow rate of testing, a Benkelman Beam network survey is unlikely to be

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Sampling

Deflection testing should be initially conducted every 50 to 100 m along the heaviest loaded pavement lane in the left wheel path as a minimum requirement (Roberts and Martin 1996). The longitudinal sample spacing can be increased to an ‘optimal’ value once the investigation processes outlined in Sections C4.2.1 to C4.2.4 are carried out. As noted in Sections C4.2.3 and C4.2.4, this process includes the issue of how many trial sampling lengths are needed along each road link and what is an acceptable level of variation from the representative structural condition in order to establish the ‘optimal’ sample spacing.

Measurement

For sealed granular pavements relative deflections should be measured and reported, as a minimum, at distances of 0, 200, 300, 600 and 900 mm from the centre of the moving test load. These deflections are referred to as D0, D200, D300, D600, and D900, respectively (see Figure 1). As

noted above, deflections can be measured more intensively and also at much greater distances from the test load. Deflections are desirable as far as possible from the centre of the applied load and preferably beyond the 900 mm offset, because deflections at large offsets increase the likelihood of recording the full extent of the bowl. For stiffer pavements, such as asphalt and stabilised materials, a measurement at 1500 mm (D1500) is usually required.

The D0 deflection represents the point of maximum deflection directly under the centre-line of the

applied load (normally the rear wheel of a rigid truck). The D200 deflection is used to indicate bowl

‘shape’ in calculating curvature. The D250 deflection, if not measured, is estimated as the mean of

D200 and D300 (see Figure 1), and can be used in filtering out of the sampled deflections any

sections of pavements with cemented bases (see Section C1.2.1.1).

C2.4 Deflectograph

(DEF)

A Deflectogragh (DEF) is similar to a pair of short Benkelman Beams mounted on a sled under the chassis of a host truck. As the truck moves forward at a fairly constant speed of about 3 to 4 km/h, the DEF is placed for testing, raised and re-placed in a continuous cycle. The test load is applied through the moving dual wheels on the rear axle of the host truck. DEF records downwards deflections in half deflection bowls in both wheel paths with almost continuous sampling at longitudinal spacings of between 3 m and 7 m and at a series of offsets 50 mm apart up to at least 900 mm in front of the applied standard test load.

For network level deflection surveys, a DEF achieves a greater rate of progress along the road and records a larger number of half deflection bowls (ie, shorter longitudinal sample spacings) than a Benkelman Beam.

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Figure 4: Host truck with loaded rear axle and Deflectograph sled in front of rear axle

Figure 5:

DEF with RWP beam shortly after starting a deflection bowl measurement

Two versions of this truck mounted device are in common use in Australia. Both are based on the Lacroix Deflectograph normally referred to as the Deflectograph. RTA NSW designed and built one version, and three State road agencies own and operate four of these Deflectographs, viz RTA NSW (2), QDMR (1) and DIER Tas (1)). QDMR refers to its Deflectograph as ‘PAVDEF’. VicRoads designed, built, owns and operates the other Australian Deflectograph, called the ‘Pavement Strength Evaluator’ (PaSE). TSA is expected to take delivery of a Deflectograph constructed by WDM Ltd (UK) in June 2005.

The main differences in the two Australian versions are that the PaSE has a longer wheel base and beam lengths (PaSE beams are 2.4 m, cf others 1.2 m). Also, the PaSE typically applies a 10 tonne (98 kN) axle load whereas the other Australian Deflectographs typically apply the standard axle load of 8.2 tonne (80 kN). However, axle loadings can be varied on each device.

When they were first introduced, the Deflectographs applied the axle load to the moving wheels in each wheel path at a nominal surface stress of 550 kPa (tyre pressure), as for the Benkelman Beam. Deflectographs are now mainly operated at a nominal surface stress of 760 kPa, which is the usual tyre pressure of most heavy vehicles (Austroads 2004c). However, these guidelines use 750 kPa as the preferred Deflectograph tyre pressure, for consistency with the Austroads Pavement Design Guide (Austroads 2004a, Section 7.2), to eliminate any inference of a need to maintain tyre pressures to such a high degree of accuracy, and in view of the very small difference in practical terms between 760 kPa and 750 kPa in the context of a Deflectograph survey.

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Figure 6: Deflectograph with LWP beam positioned about the middle of a deflection bowl measurement

Figure 7: Deflectograph with RWP beam positioned near the finish of a deflection bowl measurement

Sampling

A Deflectograph measures and records half deflection bowls at longitudinal spacings of between 3 m and 7 m. For example, RTA NSW and QDMR use 4.0 m and 4.5 m respectively as standard longitudinal spacings in Deflectograph surveys.

The characteristic maximum deflection measurements (85th percentile) in both wheel paths are reported every 100 m (Ferne 1997) because of the nature of the sampling from this device.

With a Deflectograph, a network survey can also be conducted using sampling based on surveying a portion (eg, 10% by length) of a defined road link. A useful guide to ‘optimal’ longitudinal sampling (described in Section C4.2.2) under these conditions could be ‘100 m per 1 km’, or ‘500 m per 5 km’. However, such a sampling approach would need to be validated before the survey by more intensive testing over long lengths of the road link. In addition, the time lost in stopping, securing the DEF equipment for travel, travelling to the start of the next sample length, and re-establishing the DEF equipment may reduce if not eliminate any benefit, depending on circumstances such as traffic conditions.

Deflectographs were routinely used in England for comprehensive assessment of network level pavement strength by measuring and recording half deflection bowls at 3.5 m spacings over 100% of the network length, at intervals of 3 to 5 years. However, in recent years there has been a move in England away from comprehensive network deflection testing towards targeted monitoring at selected locations (see Section C4.2.5.2).

C2.5 Falling Weight Deflectometer (FWD and HWD)

Falling Weight Deflectometers (FWD) and Heavy Weight Deflectometers (HWD) are trailer mounted devices (Figure 7) that record half deflection bowls at discrete test points on the pavement surface by measuring surface deflection at distances ranging from 0 mm to a user-defined maximum (normally 1,500 mm, but up to 2,400 mm) from the centre of an impulse test load, which is applied to the pavement surface through a standard loading plate normally 300 mm in diameter (Figure 6) by a falling weight with a variable drop height while the FWD or HWD device is at rest. The FWD produces an essentially half-sine single impact load 25-30 ms in duration, which corresponds to a moving wheel load.

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FWDs and HWDs are essentially the same and are distinguished mainly by their load capacity. A typical FWD has a load capacity of the order of 120 or 150 kN, and a HWD has a load capacity up to 250 kN.

Falling Mass Rubber Buffer

Load Cell Deflection Sensor

Deflection Bowl (not to scale)

Figure 8: Schematic diagram of a FWD

Source: COST 336 Source: COST 336 Figure 9: FWD loading plate

Figure 10: General view of a FWD Figure 11: Rear view of a FWD showing loading plate and geophones

With FWD and HWD testing, various test loads can be applied to the pavement, the most common target loads for granular pavements being 40 kN and 50 kN. However, TNZ uses 35 kN for flexible pavements with chip seal surfacings. As a result of local conditions (eg, longitudinal grade and crossfall at the test site), the actual load may not be exactly the target load. The FWD uses a load cell to measure the actual load. Deflections from FWD testing are ‘normalised’ to the relevant target load by multiplying the measured deflections by the ratio of the target load to the actual load.

With a standard 300 mm diameter loading plate, each target load corresponds to a specific surface stress, as shown in Table 4. For practical purposes, these guidelines adopt rounded values for surface stresses, as also shown in Table 4. The reasons for rounding surface stresses include removal of any implication of practical benefits from more precise figures, and the very small effect of the differences between the precise and rounded values on the resulting reported deflections.

Australean Guidelines for Road Network Condition Monitoring Part 3 – Pavement Strength

AUSTROADS INTERNAL REPORT

Guidelines for Road Network Condition

Monitoring: Part 3 – Pavement Strength

GUIDELINES FOR ROAD NETWORK CONDITION

MONITORING: PART 3 — PAVEMENT STRENGTH

GUIDELINES FOR ROAD NETWORK CONDITION

MONITORING: PART 3 — PAVEMENT STRENGTH

Austroads membership

Acknowledgement

EXECUTIVE SUMMARY

TABLE OF CONTENTS

TABLES

FIGURES

ABBREVIATIONS AND ACRONYMS

SECTION A — ROAD PAVEMENT STRENGTH

A.1 INTRODUCTION

A2. THE NEED AND APPLICATION OF NETWORK PAVEMENT

STRENGTH

A3. OBJECTIVE

SECTION B — GLOSSARY OF TERMS

USED IN THE NETWORK-LEVEL ASSESSMENT OF

SEALED GRANULAR ROAD PAVEMENT STRENGTH

SECTION C – GUIDELINES FOR NETWORK ASSESSMENT

OF ROAD PAVEMENT STRENGTH

C.1 WHAT IS PAVEMENT STRENGTH?

C1.1 Guidelines

C1.2 BACKGROUND NOTES

C1.2.1 Derivation of pavement strength parameters from deflection data

C1.2.2 Assessment of structural capacity from the pavement strength parameter

C2. EQUIPMENT FOR MEASURING PAVEMENT DEFLECTION

C2.1 Guidelines

C2.2 Deflection measuring equipment – general

C2.3 Benkelman Beam (BB)

C2.4 Deflectograph

(DEF)

C2.5 Falling Weight Deflectometer (FWD and HWD)