Investigating Issues in Backcalculating Moduli for Thin Pavements

Abstract: Backcalculation is employed to estimate the elastic modulus of the individual layers of a pavement structure utilizing falling weight deflectometer (FWD) data. While static backcalculation is generally successful at estimating elastic moduli for conventional pavements, structures with relatively thin asphalt layers have historically presented difficulties. Conventional back analysis procedures typically involve several simplifying assumptions that may impact the resultant moduli, including the applied contact pressure distribution of the FWD load plate and the interface bonding condition between the asphalt and base courses. The influence of these parameters on the backcalculated moduli was examined for several thin, flexible pavement sections subjected to simulated aircraft traffic. For each test section, four cases were evaluated for different interface bonds and contact pressure distributions by analyzing FWD data collected during trafficking. The results showed that the asphalt and base course moduli were highly dependent on these factors, while the subgrade modulus remained relatively consistent. While non-uniform loading and partial interlayer bonds helped reduce some of the error in the backcalculation results for thin pavements, the inclusion of analytically generated points in the FWD deflection basin, known as the Synthetic Point Method, produced the lowest errors and most reasonable moduli. Results from this study show that the spacing of data points near the FWD load plate in the deflection basin plays a key role in the accurate approximation of backcalculated moduli of thin pavement sections.