Comparative Analysis of Using AASHTO and WESDEF Approaches in Back-calculation of Pavement Layer Moduli

This paper presents an evaluation of the falling weight deflectometer testing data using the AASHTO and WESDEF back-calculation procedures. General concepts of each procedure are highlighted, together with data utilized in the analysis. The back-calculated layer moduli and effective structural number $\left({\mathrm{SN}}_{\mathrm{eff}}\right)$ of an existing flexible pavement were determined from both procedures and compared to see the difference between the procedures. The average subgrade modulus of resilience $\left(M_R\right)$ obtained from WESDEF was found to be about 0.7–0.9 of the value obtained from AASHTO with the factor (C) of 1.0. Modulus values obtained from California bearing ratio tests were in close agreement with AASHTO values only when the factor (C) was set to 0.33, the AASHTO recommended value. The analysis also suggests that a similar factor, about 0.5, for WESDEF should be used. WESDEF, without adjustment, produced higher ${\mathrm{SN}}_{\mathrm{eff}}$ values compared to AASHTO, with the difference being significant only at higher levels of reliability. This difference could reach 0.44 and 0.66 for reliability levels of 95 and 99%, respectively, and could result in a difference in the overlay thickness by up to 25 and 38 mm, respectively.