EVALUATION OF A DUAL-LOAD NONDESTRUCTIVE TESTING SYSTEM TO BETTER DISCRIMINATE NEAR-SURFACE LAYER MODULI

Theoretical analyses were conducted to illustrate the inability of the existing single-load falling weight deflectometer (FWD) to discriminate among near-surface layer moduli of flexible pavement systems. Comprehensive analyses were also conducted to show that this deficiency can be overcome by using a dual-load FWD when the loads are spaced sufficiently apart to induce a concave downward curvature in the pavement surface between the loads (transverse deflection basin). The analyses showed that the asphalt concrete modulus is strongly, and almost uniquely, related to the curvature of this trnasverse deflection basin, whereas the base course modulus is strongly, and almost uniquely, related to the shape of the longitudinal deflection basin. This strong correspondence was shown to hold true for a broad range of pavement geometries and layer moduli. Stress and deformation analyses were conducted to show that the dual-load system works because the shape of the transverse deflection basin is most strongly influenced by the bending moments induced within the asphalt concrete layer between the loads, and by the relatively large changes in vertical compression that are induced in the asphalt concrete layer within this zone. Neither of these effects is observed in the base course, which explains the lack of influence of the base course on the shape of the transverse deflection basin. Finally, an analysis was conducted to select load radii and spacing, and deflection sensor positions that optimize the capabilities of a dual-load system to discriminate among near-surface layer moduli. It was shown that a set of relatively simple equations can be developed to determine (backcalculate) pavement layer moduli obtained from surface deflection measurements using the dual-load system proposed.