DIRECT CALCULATION OF MAXIMUM CURVATURE AND STRAIN IN ASPHALT CONCRETE LAYERS OF PAVEMENTS FROM LOAD DEFLECTION BASIN MEASUREMENTS

Deflection measurements under impulse load carried out with the Falling Weight Deflectometer (FWD) are usually processed by using elastic layer analysis programs, back-calculating layer moduli from measured deflections, and then forward-computing critical performance parameters such as stresses and strains at locations under the load. Presented here is a direct, and more reliable way, of computing the important value of horizontal strain at the bottom of the asphaltic layer. The measured deflections from an FWD test, the radius of load distribution, and the thickness under the load are the only data needed to calculate, first, the curvature, and then, the strains in the top (asphaltic) layer of a pavement structure, in the center under the load. Corresponding stresses can also be calculated but additional information is needed, namely, the elastic stiffness of the first layer, Poisson's ratio, and the load-induced vertical stress. It is shown that strains and stresses in the immediate vicinity of the load position as computed with elastic layer analysis methods via back-calculated moduli are not as reliable as this new proposed strain criterion. At this time, the proposed new method of calculating strain directly is derived, presented, and discussed as a theory, without experimental field verification.