When backcalculating layer moduli from Falling Weight Deflectometer (FWD) tests and when forward calculating critical stresses and strains, a number of assumptions have to be made. Some of the important assumptions normally are that 1) the system is in equilibrium, 2) the materials are continuous and remain continuous under deformation, and 3) the materials are elastic, isotropic and homogeneous.
None of these assumptions are valid for FWD testing of real pavement structures. A number of mathematical models exist for describing the pavement response, but they are all based on idealization of real pavement structures and materials. The only way of determining whether a given mathematical model is acceptable for pavement design or evaluation, is by comparing the response predicted by the model to that measured in real pavements.
Many attempts at doing this have been done over the years, but none of them have let to the conclusive verification of a specific mathematical model. This paper presents the results of measurements carried out on four instrumented test sections, one in the Danish Road Testing Machine and three on experimental sections in Sweden. On all sections the deflections were measured with a FWD, the layer moduli were backcalculated using different methods, and the stresses and strains in different materials, caused by FWD loading or by traffic loading, were calculated and compared to the stresses and strains measured at the same locations with pressure and strain gauges.