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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

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Publication Number:  FHWA-HRT-16-009    Date:  March 2017
Publication Number: FHWA-HRT-16-009
Date: March 2017




The collection and analysis of pavement deflection data is a relatively quick and easy way to assess the structural condition of an existing pavement. Early work done by Hveem clearly indicated a relationship between the magnitude of pavement deflections and pavement performance, with “weaker” pavements exhibiting larger deflections and “stronger” pavements exhibiting smaller deflections.(1) In later work, Hveem et al. presented safe limits for maximum deflections to preclude cracking for different pavement types subjected to several million repetitions of a 6,800-kg (15,000‑lb) axle load.(2)

With the development of more rapid and sophisticated deflection-measuring equipment and improvements in computer technology, deflection-testing results can now be analyzed to provide a more complete portrayal of pavement behavior. Not only can deflection testing be used to assess the structural condition of existing pavements, but it can also be used to assist in the design of structural overlays, to appraise seasonal variations in pavement response, to assess structural variability along a project, and to characterize paving layer properties and subgrade support conditions. On rigid pavements, deflection testing can also be used to determine load transfer across joints and cracks and to detect underlying voids. Today, pavement deflection testing plays a significant role in the pavement monitoring and evaluation activities of many transportation agencies.

Over the years, a number of different testing devices have been used to obtain pavement deflection measurements, and currently impulse load testing using the falling weight deflectometer (FWD) has become the accepted worldwide standard. The FWD imparts a dynamic load to a pavement structure that is similar in magnitude and duration to that of a moving wheel load, thus producing a representative pavement deflection response. In addition, the FWD provides a relatively rapid (and nondestructive) test procedure that allows a greater sampling frequency than typically possible when using more traditional material sampling methods. The greater sampling frequency allows the engineer to better define both the average and standard deviation of the design inputs while also providing the opportunity to perform the testing under in situ temperature, moisture, and confinement conditions.(3) Although most commonly conducted for project-level testing (and primarily to assist in the development of overlay designs), some State highway agencies have incorporated FWD testing as part of their network-level pavement evaluations (e.g., Texas and Virginia).(4,5)

The need to accurately characterize the structural condition of the pavement has increased with the development of the Mechanistic-Empirical Pavement Design Guide (MEPDG) prepared under National Cooperative Highway Research Program (NCHRP) Project 1-37A and now available in an interim edition from the American Association of State Highway and Transportation Officials (AASHTO).(6,7) In the MEPDG, the performance of the designed pavement is projected by simulating the expected accumulated damage on a monthly or semimonthly basis over the selected design period. The amount of incremental damage occurring during each computation interval (either monthly or semimonthly) varies as the effects of prevailing environmental conditions, changes in material properties, and effects of traffic loading are directly considered. Ultimately, the incremental damage accumulated during each computation interval is converted into physical pavement distresses and projected roughness levels using calibrated models that relate the damage to observable.(6,7)

An integral part of this process is the accurate characterization of material properties of each layer in the pavement structure. Deflection data collected by the FWD can be quickly and easily used to characterize the properties of the paving layers through a methodology called “backcalculation.” This is merely a process whereby the fundamental engineering properties of the paving layers (elastic modulus, E) and underlying soil (resilient modulus MR or modulus of subgrade reaction k) are estimated based on the measured surface deflections, the magnitude of the load, and information on the pavement layer thicknesses. In essence, the set of characteristics for the paving layers and subgrade material is determined such that it produces a pavement response that best matches the measured deflections under the known loading.

Backcalculation has come a long way since the pioneering work performed by Scrivner, Michalak, and Moore, which produced a graphical solution for a simple two-layer system.(8) Since that time, numerous methods have been developed to determine the material properties in each layer of a pavement structure. Flexible pavement systems are typically modeled using a static, linear (or quasi-nonlinear) elastic layered analysis. The material properties of each layer can be determined using either forward calculation or backcalculation. Sometimes both methods are employed. The forward calculation is used first to determine the seed moduli for the backcalculation analysis or to check the “reasonableness” of the backcalculated moduli. However, it is not uncommon to get very different results when using the programs available for analyzing FWD data collected for a specific pavement even though a similar analytical approach is applied by each of these programs. Discrepancies between actual and backcalculated models arise as the result of a departure of the true pavement behavior from the idealized theoretical models. For example, a static analysis is typically performed even though the FWD testing typifies a dynamic loading condition.

Rigid and composite (hot-mix asphalt (HMA) over portland cement concrete (PCC)) pavements are typically modeled as semirigid plates on top of either a dense liquid or elastic solid foundation. The two approaches used for evaluating the support conditions are the Best-Fit and AREA methods. The Best-Fit method is used to define the support layer conditions in the Long-Term Pavement Performance (LTPP) database, but the cumbersome nature of the calculations required for the Best-Fit method has led many researchers and practitioners to use the AREA method. Fortunately, the two methods appear to produce very similar results for specific sensor configurations. Regardless of the method used, the accuracy of the results is limited by the inability to accurately account for variables such as the effective slab size (which is influenced by the level of load transfer efficiency (LTE) present at the longitudinal and transverse joints) and inherent temperature gradients.

Problem Statement

As noted in the previous discussion, FWD testing is a routine pavement evaluation method, and testing results play an integral role in the critical determination of in-place structural characteristics. With the release of the new MEPDG, there was a pressing need for a comprehensive review of the current state of the art/state of the practice of FWD testing, backcalculation, and interpretation. Moreover, there was a need to identify how FWD testing was integrated into the new MEPDG and to provide best practices guidance on how to effectively test existing pavement structures and interpret those results as part of a mechanistic-empirical pavement evaluation and rehabilitation process.

Project Objectives

This project was initiated to address many of the issues noted above. Specifically, the overall objectives for this project can be summarized as follows:

  1. Review the current state of the practice/state of the art of FWD testing and backcalculation, including its use with the new MEPDG.(7)

  2. Demonstrate the use of the FWD testing and analysis as it pertains to the MEPDG.

  3. Provide recommendations for improvements in FWD testing and interpretation, particularly ones relevant to the rehabilitation procedures in the MEPDG.

  4. Develop best practices guidelines for testing with the FWD and for analyzing/ interpreting testing results, particularly as they pertain to the MEPDG or other mechanistic-empirical design processes.

This project addressed FWD data analysis and interpretation of flexible, rigid, and composite pavement systems.

Report Organization

The final report is presented in three s: volume I (Final Report), volume II (Case Studies), and volume III (Guidelines for Deflection Testing, Interpretation, and Analysis). This report (volumeI), which documents the entire research effort that was conducted under the project, contains five chapters in addition to this introduction:



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