Deflections and Strength

Pavement deflections and strength.

Although road condition data, e.g. visual distress and ride quality, provide a good indication of the overall network-level road condition, it does not give a direct measure of structural integrity and capacity of a road pavement. To address this issue, the road structural condition parameters based on the falling weight deflectometer (FWD) measurements are proposed and implemented in the road deterioration model for Thailand pavement management system (PMS). This paper highlights the practical implication of structural condition parameters suggested in the past publications for road maintenance, rehabilitation, and reconstruction. The preliminary analysis of this study showed that the FWD was the most comprehensive approach for road structural condition assessment and road deterioration model for Thailand pavement performance prediction, which was an integral part of PMS. The Department of Highways, Thailand aims to reduce road deterioration and maintenance cost through using improved PMS which employs the road deterioration model uniquely developed based on Thailand road conditions.

2015 - South Africa - FWD Benchmark Study
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A benchmark analysis method was developed using Falling Weight Deflectometer (FWD) data for comparative evaluation of the structural condition of flexible pavement structures. This is established as a preliminary design and analysis tool, and aspects of it are incorporated in TRH 12. Experiences with benchmark analyses on roads and airports are reviewed and adjusted criteria are recommended with a colour coded three tiered condition assessment method. Exploratory studies on additional deflection bowl parameters are conducted to gauge their potential for inclusion in benchmark analyses. Deflection bowl parameter benchmarking has found application at network level analysis in a number of road authorities worldwide. Modified Structural Number (SNP) and Pavement Number (PN) have recently also been illustrated as being able to accurately calculate from the full deflection bowl and can be used in such enhanced benchmark analyses of flexible pavement structures.

The modified or adjusted structural number (SNC or SNP) is widely used to define the structural
capacities of various flexible pavements. Viable correlations between either SNP or SNC and a variety of falling weight deflectometer (FWD) deflection bowl measuring points or parameters exist. Whilst historical use of maximum deflection continues, a large portion of the inherent structural information in the rest of the deflection bowl goes underutilised. The paper presents and validates a single relationship of parameters, representing the full deflection bowl, and effective adjusted structural number (SNPeff). SNP and the structural condition index (SCI) are widely used on network and preliminary project level investigations, but cannot identify origin of the distresses. The complementary use of a deflection bowl parameter benchmark analysis can greatly enhance such
investigations. In this paper the use of deflection bowl derived SNPeff, SCI complemented with deflection bowl parameter benchmark analysis is demonstrated with a case study.

This report documents a review and comparison of Falling Weight Deflectometer (FWD) and Traffic Speed Deflectometer (TSD) deflections, based on different approaches, at a network level. It
draws on the work undertaken for Austroads by Roads and Maritime Services of New South Wales (RMS) and the Department of Transport and Main Roads Queensland (TMR) in the evaluation and
assessment of the TSD for its application in Australasia. The report complements and is in direct response to the State-ofthe-Art review of current TSD practice completed under Austroads
Project AT1730 (this project which drew on information gained from attending the deflection at road traffic speed workshop held in the United Kingdom in June 2012. It also builds on earlier work under project AT1613 which defined six research goals for this area of work, and reported studies conducted on behalf of RMS and TMR.

This report documents discussions about international traffic speed deflectometer (TSD) data analysis and reporting held at the second Deflection and Road Traffic Speed workshop in Norway in June 2013. It also documents TSD data procurement and analysis processes from Poland that confirm the reliability of the ‘area under the curve’ method using data that was not typical of that encountered on flexible pavements in Australasia. ARRB has implemented interim operational procedures for a second generation TSD acquired for network strength testing in Australasia.
This research will help ensure the data collected for road agencies is reliable.

The objective of this project was to develop a structural index for use in network-level pavement evaluation to facilitate the inclusion of the pavement’s structural condition in pavement management applications. The primary goal of network-level pavement management is to provide the best service to the users for the available, often limited, resources. Pavement condition can be described in terms of functional and structural condition. The current widespread practice of network-level pavement evaluation is to consider only the functional pavement condition. This practice results in suggested treatments that are often under-designed or over-designed when considered in more detail at the project level. The disagreement can be reduced by considering the structural capacity of the pavements as part of a network-level decision process.

This study developed a flexible pavement structural index to use for network-level pavement applications. Available pavement condition data were used to conduct a sensitivity analysis of the index, and example applications were tested. The results indicated that including the structural index developed, named the Modified Structural Index (MSI), into the network-level decision process minimized the discrepancy between network-level predictions and project-level decisions when compared to the current network-level decision-making process. A pilot implementation of the MSI showed that it can be used to support various pavement management decision processes, such as network-level structural screening, deterioration modeling, and development of structural performance measures. The pilot test also indicated that the impact of the structural condition of the pavement on the performance of a maintenance treatment and its impact on life-cycle costs can be quantified.

The objective of this study was to develop an approach for incorporating techniques used to interpret and evaluate deflection data for network-level pavement management system (PMS) applications. The first part of this research focused on identifying and evaluating existing techniques, seeking out those that were simple, reliable, and easy to incorporate into current PMS practices, as well as those that produced consistent results. The second part of the research

detailed the development of guidelines for the application of recommended techniques and procedures for determining optimum falling weight deflectometer (FWD) test spacing and data collection frequency. While there are many viable techniques available for evaluating the structural capacity of pavements that use FWD for project-level analysis, many of these techniques are time consuming and require an experienced analyst. As a result, using pavement deflection testing for network-level analysis has been limited to date. This guide contains step-by-step instructions for applying appropriate evaluation techniques for network-level

(not project-level) measurements and analyses.

The objective of this study was to develop an approach for incorporating techniques used to interpret and evaluate deflection data for network-level pavement management system (PMS) applications. The first part of this research focused on identifying and evaluating existing techniques by seeking out those that were simple, reliable, and easy to incorporate into current PMS practices, as well as those that produced consistent results. The second part of the research detailed the development of guidelines for the application of recommended techniques, along with procedures for determining optimum falling weight deflectometer (FWD) test spacing and data collection frequency. While there are many viable techniques available for evaluating the structural capacity of pavements that use FWD for project-level analysis, many of these techniques are time consuming and require an experienced analyst. As a result, using pavement deflection testing for network-level analysis has been limited to date. The findings presented in this report suggest that it is possible and, in fact, advantageous to define simplified techniques for the evaluation and interpretation of pavement deflections for network-level analysis.

2010 - NZ - Structural Capacity Definition with FWDs
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Pavement performance modelling for New Zealand roading networks, currently relies on an adjusted structural number (SNP) which is a single parameter intended to describe the performance of a multi-layered pavement structure in terms of its rate of deterioration with respect to all structural distress modes, as well as non-structural modes. This parameter had its origin in the AASHO road test in the late 1950s, before the advent of analytical methods. Hence refinement to keep abreast of current practice in pavement engineering is overdue.

This research describes the basis for a new set of structural indices and how these can be used to obtain improved prediction of pavement performance: both at network level and for project level rehabilitation of individual roads. The results are (i) effective use of all the data contained in RAMM, (ii) more reliable assignment of network forward work programmes, (iii) reduced cost through targeting only those sections of each road that require treatment and (iv) more efficient design of pavement rehabilitation through informed appreciation of the relevant distress mechanism that will govern the structural life of each individual treatment length.

Portable Falling Weight Deflectometer (PFWD) that can be considered as simple equipment is mainly used to measure elastic moduli of pavement unbound layers. This paper evaluates the potential use of PFWD to reliably measure the elastic modulus of pavement layers. To achieve this, PFWD tests were conducted on highway sections selected from different projects in Tehran. The California Bearing Ratio (CBR) laboratory tests were also conducted on samples collected during field tests. PFWD testing parameters were varied while performing the field testing. These included drop weight, drop height, plate diameter and position of additional geophones. In addition, PFWD moduli were compared with those obtained from performing FWD testing on the same site. It was found that drop mass and loading plate size affect PFWD modulus significantly. In addition, the results indicated that good correlation exist between PFWD moduli and FWD and CBR results.

2009 - UK - Portable FWD Guide
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Deflectometer devices are dynamic non–destructive testing tools commonly used in the field of pavement systems to measure a layer or surface modulus. Among the various testing devices used for non–destructive insitu assessment of pavement layers the Light Weight Deflectometer (LWD) has become the focus of increasing interest. In particular, the changes introduced in the Design Manual for Roads and Bridges (HD 26/06, and IAN 73/09 which superseded HD 25/94), introduced a requirement for field compliance testing of the surface modulus of constructed pavement foundations. In addition, other applications for deflectometers and the portable versions in particular include compliance testing for general highway investigation, (re)construction, highway utility trench reinstatements, and many other similar instances. The portable Light Weight Deflectometers (LWD) are considered in general to be relatively rapid and cost effective tools, if used appropriately.

This report presents a good practice guide for the LWD.

Compliance Testing Using the Falling Weight Deflectometer for Pavement Construction, Rehabilitation and Area Wide Treatments. The Falling Weight Deflectometer (FWD) which measures pavement deflections was assessed for its ability to predict the life of a newly constructed or rehabilitated pavement. FWD measurements used in the study were from New Zealand Transport Agency’s test track CAPTIF, roads that have failed and from two Performance Specified Maintenance Contracts where the actual life from rutting and roughness measurements could be determined.

 

Three different methods to calculate life from FWD measurements were trialled. The first, a simple Austroads method that uses the central deflection only and was found to either grossly over predict life by a factor of 1000 times more than the actual life or grossly under predict the life. The second two methods trialled were based on Austroads Mechanistic Pavement Design where the life is determined from the vertical compressive strain at the top of the subgrade. For the mechanistic approach the FWD measurements are analysed with specialised software that determines a linear elastic model of the pavement which computes the same surface deflections as those measured by the FWD. From the linear elastic model the subgrade strain is determined and life calculated using the Austroads equation.

 

It was found when using this approach that predictions of life from individual FWD measured points within a project length can range from nearly 0 to over 100 million ESAs (Equivalent Standard Axles). To cater for this large scatter in results the 10th percentile value was used as the predicted life of the pavement. In general the Austroads Mechanistic approach under-predicted the life, sometimes by a factor of 10 or more. The third approach trialled was adjusting the Austroads Mechanistic approach by applying a factor determined from past performance to calibrate the subgrade strain criterion to local conditions. This third approach greatly improved the predictions but it was found that the multiplying factor was not consistent for a geographical area and thus the factor found from one project may not be suitable for the another similar project.

 

2007 – NZ – Software for Predicting Pavement Strength
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Windows application for predicting the strength of a pavement for analysis with HDM-4 using different techniques.

This paper reviews the LWD as a field evaluation tool, discusses the test variables and data quality and concludes both on its usefulness and also its limitations for a variety of earthwork and road assessment scenarios. The paper aims to provide a state of the art reference document for LWD users, consultants, material specifiers, contractors and clients.

Portable falling weight deflectometer has been used to evaluate stiffness of subgrade and/or subbase. The stiffness is estimated using a pair of peak values of load and displacement. However, test data are dependent on specification of measurement apparatus.

Instead of peak values of load and displacement, this study have used historical time data of load and displacement to estimate layer stiffness. The estimated stiffness was comparable to the one obtained from plate bearing tests. The objectives of the study are to improve accuracy on estimated stiffness and to decrease influence of individual apparatus specification.

Paper on study to collect network level FWD data along Viriginia's interstate system. The FWD data collection aims to build a comprehensive database of deflection data and associated structural analysis to be used for future implementation of the Mechanistic-Empirical Pavement Design Guide.
This manual is intended for use in collection of Falling Weight Deflectometer (FWD) data for the Long-Term Pavement Performance (LTPP) program. As such, it contains background information on FWD equipment and the general role of FWD testing within LTPP as well as a set of field operations guidelines for the data collection process.
NHCRP Report 538 describing survey procedures and analysis

2005 - Training Course on Stength Testing
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Lecture notes on how to conduct non-destructive pavement strength tests

2005 - Europe - European Approach to FWD Calibration
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Paper on how FWDs are calibrated in Europe

2005 - Bangladesh Benkelman Beam Survey Manual
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Conducting Benkelman Beam surveys in Bangladesh

Prepared by Austroads, 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.

Report describing calibration use of Benkelman Beam for deflection testing
Report describing calibration use of Benkelman Beam for deflection testing

This guide provides procedural information for measuring pavement surface deflections, directly under, or at locations radially outward (offset) from a known static, steady-state, or impulse load. Guidle is applicable for deflection measurements performed on flexible asphalt concrete, rigid portland cement concrete or composite pavements.

This paper describes how the Greenwood High Speed Deflectograph applies Laser Doppler Technology to provide information about pavement response to traffic load.

2002 - Denmark - Development of a High Speed Deflectograph
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Report on a project to develop a prototype of a testing device, which (at normal traffic speed) is capable of monitoring road pavement bearing capacity using advanced laser technology. The idea behind the project emerged at Greenwood Engineering A/S, who has patented the concept in a number of countries all over the world.
Technical Report No. 4. The objective of this study is to compare the results from Benkelman Beam tests and the existing Department of Highways Overlay Design Method with the results from Falling Weight Deflectometer tests and an Analytical Overlay Design, and to discuss the legitimacy of any deviations.

Paper by John Emery

2000 - USA - LTPP FWD Survey Manual
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FWD survey requirements for LTPP study

2000 - Hong Kong - FWD Survey Manual
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FWD survey requirements for Hong Kong
This paper describes comparative trials of five non-destructive pavement tests (Loadman, FWD, Benkelman Beam, Nuclear Density Meter and Clegg Hammer), conducted at the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) and on sections of in-service pavements. The tests were evaluated to determine their suitability for quantifying the condition and predicting the future performance of unbound granular pavements.
This test method covers the measurement of deflections of paved and unpaved surfaces with a falling-weight-type impulse load device. These devices are commonly referred to as falling-weight deflectometers or FWDs. This test method describes the measurement of vertical deflection response of the surface to an impulse load applied to the pavement surface. Vertical deflections are measured on the load axis and at points spaced radially outward from the load axis.

1994 - India - Loadman Training Guide
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Manual for how to use the LOADMAN portable falling weight deflectometer

Report comparing deflections from two instruments

Report comparing deflections from two instruments

1993 - USA - SHRP FWD Survey Manual
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FWD survey requirements for the US SHRP project

1993 - USA - SHRP Approach to FWD Calibration
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Paper on how FWDs are calibrated for SHRP project

1992 - Canada - Saskatchewan DCP Survey Manual
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DCP testing procedure

Report on results of experimental measurements made on public main roads to determine some features of the fluctuations of the vertical forces applied to the road surface by the wheel of a two-axis semi-trailer travelling over a variety of pavements.