VARIABILITY IN MEASURED DEFLECTIONS AND BACKCALCULATED MODULI FOR THE STRATEGIC HIGHWAY RESEARCH PROGRAM SOUTHERN REGION

It is well known that excessive variability in subgrade characteristics, layer thicknesses, material characteristics, and other such construction variables are major causes of distress and loss of performance in pavements. As the use of deflection measurements to characterize pavement structural capacity and to determine the elastic moduli of the separate layers increases, variability in the deflection measurements and the resulting estimates of the elastic moduli become more important. One study reported was intended as a preliminary evaluation of the amount of variability in deflections that may be expected in relatively short test sections (500 ft for the Strategic Highway Research Program test sections studies), and to learn what characteristics of pavements contribute to this variability. The relative causes of the variability were studied using correlations of variations in deflections with various characteristics of the pavements, such as layer thicknesses, layer stiffnesses, and asphalt viscosity. These studies indicate that the typical coefficients of variation for deflections range between 4 and 18% of the mean deflections, but some coefficients of variation for specific test sections will run over 40% of the mean deflections. A second study identified variations in backcalculated moduli for four test sections. This limited set of backcalculated moduli indicates that variability in backcalculated moduli for base layers is much higher than that for the asphalt concrete (AC) layers and subgrade and that the coefficients of variation for backcalculated AC moduli appear to be proportional to those for measured deflections. Some causes of the variability appear to have been identified, but variability in deflection measurements were found to result from a multitude of pavement characteristics, each resulting in minor effects to create the whole.