Using Falling Weight Deflectometer Data With Mechanistic-Empirical Design and Analysis, Volume I: Final Report

CHAPTER 8. FWD DATA ANALYSIS AND INTERPRETATION

INTRODUCTION TO THE GUIDELINES

One of the primary objectives of this project was to develop guidelines for conducting FWD testing, analyzing the resultant deflection data, and interpreting the results for pavement rehabilitation design. Volume III of this report is a standalone guide that addresses each of those topics, providing general guidance and direction throughout the entire testing and analysis process. It is organized into the following chapters:

The guidelines are structured to be a “how-to” or “step-by-step” guide for pavement engineering practitioners.

RECOMMENDATIONS FOR NEXT-GENERATION TOOLS AND ANALYSES

Backcalculation techniques and the available tools have evolved significantly since the early days of pavement deflection testing. Although analysis methods and tools continue to improve, a number of shortcomings have yet to be overcome, and several cutting-edge advancements have yet to make it to mainstream use. This section provides commentary on areas requiring improvements in the backcalculation and interpretation process, along with a discussion of impending advancements, as observed during the course of this project.

For flexible pavements, one should ideally be able to determine a curve of HMA layer modulus as a function of frequency using a (dynamic) frequency-based backcalculation algorithm, which would give a more direct estimation of the HMA layer modulus with frequency from actual field conditions rather than relying on a laboratory-derived curve such as the Witczak equation. However, care should be taken in interpreting and using such data with the existing MEPDG performance predictions because they were calibrated using laboratory-derived moduli. Although dynamic backcalculation methods can backcalculate layer moduli and thicknesses accurately from synthetically generated FWD data for pavement systems with three or more layers, they present some serious challenges when using field data.⁽⁴⁶⁾ The frequency-domain method can lead to large errors if the measured FWD records are truncated before the motions fully decay in time. Also, dynamic, time-domain backcalculation algorithms cannot directly determine the HMA modulus as a function of frequency. They either assume a constant HMA modulus (similar to static backcalculation) or a prescribed function of the HMA layer modulus with frequency (e.g., linear relation in the log-log space).

The procedures available for the evaluation of the structural support conditions under a rigid pavement could potentially be improved to enhance the current analytical capabilities. For example, most methodologies assume a flat slab, but temperature and moisture gradients have been shown to affect the interpretation of FWD data when evaluating support conditions as well as monitoring joint performance (particularly for nondoweled joints) and detecting voids.⁽¹⁰²⁾ Improved guidelines are needed to define when FWD testing can be performed so data interpretation is not influenced as much by temperature/moisture/construction gradients. However, a procedure to account for this influence when testing a slab that is not flat would also be helpful.

For rigid pavements, substantial improvements could be made in the backcalculation process to help reduce the variability found between backcalculated moduli calculated at the same location at different times. The development of correction factors that account for dynamic effects would help reduce seasonal variability. These correction factors would be used to account for the effects of changes in the inertia of the pavement system and damping of the subgrade. Curling/warping of the slab, on the other hand, can increase the variability of the backcalculated moduli calculated at the same test location throughout the day, although the magnitude of the variability will fluctuate seasonally. With the large quantity of rehabilitation work that needs to be performed by many State agencies, it is not feasible to limit FWD testing to the morning hours when gradients are not likely to be present. For this reason, the development of correction factors that account for the effects of temperature and moisture gradients in the slab on backcalculated moduli would be very useful. Some steps have already been taken to help address these issues.^(104,52,102)

Alternatively, a dynamic analysis could be performed. As discussed, dynamic backcalculation has been applied successfully for flexible pavement analysis, and advances are being made for rigid pavements as well. However, the use of a static analysis should be evaluated to determine whether performing a dynamic analysis is necessary for rigid pavements. Although a static analysis is being applied to a dynamic load condition, it might not be necessary to backcalculate the k-value based on a dynamic analysis unless it were shown to help decrease unexplained variation. It should be noted that the dynamic k-value is required as input for the MEPDG program.

Another limitation in modeling rigid pavements when evaluating support conditions is that it is based on a slab of infinite length and width. Crovetti developed procedures, which were later modified by Hall et al., to adjust for the effective slab size but this still does not consider the effect the transverse or longitudinal joint load transfer and edge support have in increasing the effective slab size.^(104,51) This is another potential area for which improvements are needed.

For HMA/PCC pavements, although the available analysis tools are lacking in their ability to fully model the behavior, this limitation does not seriously impair the ability to evaluate FWD data for these types of pavements. That is, the limitations of the available tools do not have a significant practical impact on the ability to evaluate the characteristics of HMA/PCC pavements. The subgrade k-value can be determined reliably using the simplest of the available tools, the outer-AREA method. Reasonable estimates can also be made for the PCC elastic modulus using the outer-AREA method. As with the other pavement types, a limited amount of core testing is highly recommended to evaluate the material properties and to validate the pavement layer moduli determined from FWD data. Core testing can also assist with determining appropriate modular ratios for the HMA and PCC used in the analysis to determine individual layer properties.

The typical reasons for rehabilitating HMA/PCC pavements are related to material and functional issues. The continued load-related deterioration of the underlying PCC pavement is rarely the principal mode of deterioration for HMA/PCC pavements. The results that can be obtained using the available analysis tools are adequate for the purposes of conducting what is essentially a design check on rehabilitation design of HMA/PCC pavements. Nevertheless, improved models for both forward analysis and backcalculation would help more accurately model the behavior of composite pavements. The structural models based on plate theory do not correctly model the behavior of HMA/PCC pavements because the through-thickness deformations are ignored. Elastic layer programs do not yield the k-value typically needed for the analysis of PCC pavements. An elastic layer program that allows the subgrade to be modeled as a Winkler foundation has been developed, which may be ideal for the backcalculation of HMA/PCC pavements, although it is currently a proprietary program.⁽¹⁰⁹⁾ Researchers have also demonstrated the advantages of using ANNs for backcalculation. Development of software specifically designed for the backcalculation of HMA/PCC pavements (complete with an automated process for data screening to identify problem data points) would facilitate the process of analyzing the FWD data.