ILLI-SLAB and FWD Deflection Basins for Characterization of Rigid Pavements

Deflection-based nondestructive evaluation methods for highway and airfield pavements rely wholly on mechanistic models of pavement behavior under load to characterize certain fundamental properties of individual pavement features (pavement section of similar thickness, construction history, and traffic). Once these key pavement parameters are quantified, areas of immediate concern can be identified for maintenance or rehabilitation, and an evaluation of the future performance of the entire feature can be made. The procedure sounds simple enough, until one tries to accomplish the task for an entire airfield which might contain well in excess of 200 distinct features. This paper details how the deflection basin created at the center of a rigid pavement slab under loads produced by the falling weight deflectometer (FWD) can be used in conjunction with the ILLI-SLAB finite-element model to backcalculate the two key parameters needed to characterize a classical Westergaard rigid pavement, a dynamic Young's Modulis (E) of the concrete surface, and a composite dynamic modulus of subgrade reaction (k) for the supporting layers of the system. The deflection basin is described in terms of two independent variables, the maximum deflection under the center of the FWD loading plate (D0) and the cross sectional “area” of the basin. The independent nature of these two variables is critical to the uniqueness of the backcalculated parameters. Using the ILLI-SLAB model, ranges of dynamic E and k that bound the actual field values are input to the computer, along with the actual FWD load, to produce a graphical solution. An iterative computer solution is then outlined that makes the task of backcalculating dynamic E and k for several hundred features more manageable. A correlation is presented that relates dynamic k values to traditional static k values determined from plate-bearing tests. Finally, comparisons between measured deflections in the field and predicted deflections using backcalculated parameters on the computer are made at center slab to verify the accuracy and repeatability of the technique for a wide variety of temperatures and thicknesses. It is only after the validity of the technique is established that confidence can be placed in the calculated stresses due to actual loads and, therefore, the evaluation itself.