Backcalculation of composite pavement layer moduli

Abstract : A backcalculation program called COMDEF has been written that utilizes a new database technique to speed program execution. Like most backcalculation algorithms, COMDEF compares measured deflections with theoretical solutions. However, the COMDEF approach is to apply interpolating functions to a database of precalculated solutions, so that the comparison basins are calculated quickly with reasonable accuracy. The state of the art of nondestructive testing of pavements is reviewed and problems in evaluating composite pavements are discussed. A sensitivity study is presented which includes solutions using COMDEF and a general purpose backcalculation program. COMDEF consistently outperformed the general purpose program in both speed and accuracy. Agreement between laboratory modulus tests on field specimens and backcalculated moduli from COMDEF was reasonable. A version of COMDEF using a quasistatic response model is presented and the limitations of this method are discussed. Dynamic theory is presented as a more realistic way to predict structural responses. A version of COMDEF which uses a dynamic response model is presented that clearly shows the importance of dynamic theory in the analysis of nondestructive test data.