Reduction of Impulse-Response Data on Rigid Pavement: Comparison of Single Degree of Freedom, Cone, and Lumped Parameter Dynamic Models

The Impulse Response (IR) method is a nondestructive method that has been successfully used to detect voids or loss of support underneath rigid pavement and to evaluate the pavement subgrade. Voids or loss of support can be detected due to changes in the pavement’s mobility spectra, while the subgrade is evaluated based on the static stiffness. The fundamental assumption of the IR method is that the response of rigid pavements can be represented by a single degree of freedom (SDOF) system, with the controlling stiffness being that of the subgrade. Although simple to analyze, this oversimplification of a complex dynamic problem introduces deficiencies that are primarily attributed to a deviation of the shape of the impedance function for rigid pavement systems from that of an SDOF system for frequencies beyond the fundamental frequency. Other dynamic models [i.e., cone and lumped parameter (LP)] are investigated as alternatives to the SDOF model and are implemented in the reduction of field data collected with the Seismic Pavement Analyzer (SPA). These alternative models with additional degrees of freedom provide a significantly better representation of the effect of individual pavement layers and the subgrade on the overall pavement response, and a better fit to the pavement dynamic response curve. In general, the SDOF model results in a higher modulus compared to the cone and lumped parameter models. However, no consistent trend was established between the moduli from the different models, especially when there is a loss of pavement support or void underneath the pavement. Part of the field data used in the analysis came from a grouting program to repair a rigid pavement with voids underneath the slabs. The IR method was successfully used to evaluate support conditions before and after grouting and to determine the effectiveness of the program.