Back-calculation of layer parameters for LTPP test sections, volume II: layered elastic analysis for flexible and rigid pavements - Recent Communications Products - Brief Article

Public Roads, March-April, 2003

Publication No. FHWA-RD-01-113

This report documents the procedure and steps used to back-calculate the layered elastic properties (Young's modulus and the coefficient and exponent of the nonlinear constitutive equation) from deflection basin measurements for all of the LTPP test sections with a Level E data status. The back-calculation process was completed with the program MODCOMP4 for both flexible and rigid pavement test sections in the LTPP program. The report summarizes the reasons why MODCOMP4 was selected for the computations and analyses of the deflection data, provides a summary of the results using the linear elastic module (Young's modulus) for selected test sections, and identifies those factors that can have a significant effect on the results.

Results from this study do provide elastic layer properties that are consistent with previous experience and laboratory material studies related to the effect of temperature, stress-state, and season on material load-response behavior. In fact, more than 75 percent of the deflection basins analyzed with the linear elastic module of MODCOMP4 resulted in solutions with a root mean square (RMS) error of less than 2.5 percent. Those pavements exhibiting deflection-softening behavior with Type II deflection basins were the most difficult to analyze and generally were found to have RMS errors greater than 2 percent. In summary, the nonlinear module of MODCOMP did not improve the number of reasonable solutions significantly, and it is recommended that nonlinear constitutive equations not be used in a batch mode basis.