Determination of layer moduli from falling weight deflectometer measurements

Backcalculation programs are used for evaluation of pavement layer moduli from falling weight deflectometer (FWD) measurements. In this study, a comparison of two programs with different calculation methods, Modulus and Elmod, is made using deflection data derived from 43 SHRP-LTPP (Strategic Highway Research Program, Long-Term Pavement Performance) test sections. First, a brief look is made into the material models, which describe the behaviour of pavement layer materials. Basic concepts of the two backcalculation programs used in this study, Modulus and Elmod, are discussed, and collection of input data for the programs is described. From a comparison of backcalculated layer moduli from the two programs at Finnish standard wheel load of 50 kN, it is concluded that higher subgrade modulus values are obtained with the Elmod-program than with the Modulus-program. Consequently, the Modulus-program yields higher values for asphalt layer modulus than the Elmod-program. Variation in backcalculated layer moduli is somewhat greater within the Modulus-program than the Elmod-program. Stress-sensitivity of paving materials was studied by using the FWD at four different loading levels. The base course modulus was found to be the most stress-sensitive of pavement layers, since the stress level is also highest in the base course of unbound pavement layers. Critical strains were calculated using the linear program (BISAR) with layer moduli from the Modulus-program as input and with the Elmod-program. It was found, that calculated strains from the two programs agree very well, even though the calculated moduli are quite different. Calculated strains from FWD loading were compared with measured strains under moving wheel load from the field at Virttaa test site. The differences between the two are most likely due to differences in the loading conditions and in the method of analysis