A best-fit rigid pavement back-calculation method based on site-specific finite element simulations

Traditional best-fit back-calculation methods for rigid pavements seek to determine the slab modulus E and the modulus of the subgrade reaction k by minimizing the squared errors between estimated and measured pavement deflections. The estimated deflections are obtained using theoretical relationships (i.e., Kelvin-Bessel functions). This method is used for back-calculating the rigid layer moduli stored in the LTPP database. The measured deflections are obtained using a Falling Weight Deflectometer (FWD).

This paper describes an innovative best-fit back-calculation approach for improving this traditional method. It allows estimation of the modulus of subgrade reaction supporting the slab using best-fit techniques. It introduces two main innovations: (1) It utilizes the finite element method (FEM) for obtaining the estimated surface deflections by simulating the site-specific field conditions, (slab geometry, dowel/reinforcement configuration and environmental conditions). (2) It assumes that the modulus of the Portland concrete slab is known (i.e., it can be readily obtained through non-destructive wave-propagation techniques).

The new approach is tested using the LTPP database. It utilizes the FEM model EverFE for obtaining the site-specific deflection estimates. Comparisons are made between the subgrade back-calculated moduli using the traditional best-fit method and the proposed best-fit method. These comparisons are focused on LTPP sites that have elastic moduli data for the Portland cement concrete slab (i.e., obtained from core measurements using ASTM C 469), as well as traditional best-fit back-calculated moduli. The new approach shows a marked improvement in predicting moduli of subgrade reaction compared to the traditional best-fit method.