NONDESTRUCTIVE TESTING WITH FALLING WEIGHT DEFLECTOMETER ON WHOLE AND BROKEN ASPHALT CONCRETE PAVEMENTS

Extensive testing of flexible pavements with the falling weight deflectometer on various test sites in Ontario has aided the development of a rational method of deflection basin interpretation. The goal is a fast computer program (PROBE) that calculates important mechanistic response parameters and determines the quality of data and the degree of structural integrity of the pavement layers. Using the theory of Boussinesq and Odemark's method of equivalent layer thickness, two quantities are defined to help interpret deflection bowl data: (a) the effective modulus of measured surface deflection and (b) the effective modulus of subgrade deflection. Both moduli change with their radial distance from the test load. When plotted they may briefly be referred to as the surface modulus profile and the subgrade modulus profile, respectively. Both moduli provide apparent values of elastic stiffnesses, using uniform elastic half-space solutions. They are parameters for a systematic study of the difference between the theoretically expected and the observed behavior of asphalt concrete pavements. The surface modulus profile evaluates the quality and integrity of pavement layers. Both the surface and subgrade modulus profile are used to estimate the subgrade modulus near the test load, which is the base for further calculation of primary response parameters by the Odemark method. Examples are presented, ranging from very good to poor and broken conditions. Computer simulations with various programs suggest two major points: (a) dynamic effects have only a comparatively minor influence, so that elastostatic modeling appears to be feasible, and (b) deflections die away faster than expected with radial distance, probably because of an increase in the subgrade modulus with depth, an unrecorded presence of a bedrock face, or--more likely--discontinuities of unbound or cracked layer materials. In short the new approach tries to obtain information and interpretations on system features of field cases by systematically studying the deviation from simple elastostatic modeling.