Evaluation of backcalculation methods to predict pavement layer moduli

The objective of this study is to evaluate five backcalculation procedures (BKCHEV, BOUSDEF 2.0, EVERCALC 3.3, MODULUS 4.0 and WESDEF) to determine the moduli of asphalt concrete pavement layers and the subgrade. The sensitivity of each procedure is evaluated using ranges of input variables: seed modulus, range of modulus, Poisson's ratio, layer thickness, rigid bottom, convergence criteria and others. The surface deflection data at each site are obtained from the SHRP LTPP study database. Evaluation of each procedure is based on the backcalculated moduli and the sum of absolute error between measured and calculated deflection basins. Also, normalized surface deflections are used to evaluate the stress sensitivity of each pavement structure. Comparison of backcalculated moduli to laboratory tests is not made because test equipment was not available during the project. The most influential variables for backcalculated moduli are location of the rigid bottom and AC layer thickness. Establishing a statewide database on the characteristics of pavement materials in California highways is strongly recommended for successful future operation.