Who Says Backcalculation Is Only about Layer Moduli?

Abstract

In this study, an existing finite layer algorithm for dynamic analysis of pavement structure was enhanced to incorporate the nonlinear behavior of unbound pavement materials. The nonlinear (stress-dependent) modulus was approximated in the vertical direction, which is similar to the approach used with multi-layered elastic and viscoelastic analysis methods for incorporating material nonlinearity. First, the enhanced finite layer algorithm was used to backcalculate the layer thickness, unit weight, Poisson’s ratio, and damping ratio in addition to the linear (viscoelastic and elastic) modulus of all layers. Then, the parameters backcalculated from the linear analysis were used to estimate the seed values for the subsequent nonlinear analysis in which the stress-dependent moduli of the unbound layers were backcalculated. Deflection data from two field sections (with thick and thin asphalt concrete layer) were used for demonstration. The results showed excellent agreement between the measured and backcalculated deflection time histories. In addition, it was found that the use of backcalculated parameters for the thickness, unit weight, Poisson’s ratio, and damping resulted in lower errors for both the linear and nonlinear analyses. Furthermore, the results of the backcalculation indicated that the material nonlinearity was more pronounced for the thin pavement, in which case the backcalculation error may be reduced further by incorporating the stress-dependent modulus.