A Data Base Method for Backcalculation of Composite Pavement Layer Moduli

A program called COMDEF has been written for use in nondestructive evaluation of composite pavements. The program uses a new data base technique to speed program execution. COMDEF, like most backcalculation algorithms, compares measured deflections with theoretical solutions. However, the COMDEF approach applies interpolating functions to a data base of precalculated solutions, so that the comparison basins are calculated quickly with reasonable accuracy. A description and flowchart of the method used by COMDEF is included. This paper discusses the role of nondestructive testing in pavement evaluation and the particular problems in evaluating composite pavements. Lack of knowledge about the degree of cracking in the rigid layer is a major problem, and backcalculated modulus is presented as a way to assess the degree of cracking in the rigid layer. The use of a representative basin is discussed, and data are presented which indicate that this approach should not be used for composite pavements. 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. Laboratory modulus tests were performed on samples taken from three verification sites. Agreement between laboratory modulus tests on field samples and backcalculated moduli from COMDEF was excellent.