Development of Nondestructive Methods for Measurement of Slab Thickness and Modulus of Rupture in Concrete Pavements

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This report describes work to develop non-destructive testing methods for concrete pavements. Two methods, for pavement thickness and in-place strength estimation, respectively, were developed and evaluated. The thickness estimation method is based on a new hybrid approach that combines frequency domain (impact-echo) and time domain (seismic) data. This new method makes use of a fuller understanding of the dynamic wave phenomenon, which was developed during the course of the work. The effects of material property gradients (due to aggregate segregation and moisture variation) through the slab thickness are compensated for in the method. A field-testing method is proposed, described, and experimentally verified. Verification tests carried out on full-scale concrete slabs cast on granular base show that the new method provides more accurate thickness estimates than those obtained by the standard impact-echo procedure. On average, the error between predicted thickness and actual thickness determined by cores is less than 6 mm, although some individual estimates exceed this error value. However, the new method does not work on concrete over asphalt or cement-treated base (which accounts for most concrete pavements) or on full-depth asphalt concrete pavements. The in-place strength estimation method is based on ultrasonic surface wave measurements. A field test method is proposed, described, and experimentally verified. Verification tests carried out on a range of concrete mixtures with varying aggregate type and cementitious material, all of which satisfy the requirements of "A3" concrete as specified by the Virginia Department of Transportation. Two data analysis procedures are proposed. One procedure predicts flexural strength within 50 psi of actual strength determined by direct strength measurement of beams, although the procedure requires 1-day strength and ultrasonic values to be known. The second procedure is more flexible but provides strength estimates with lower accuracy. Field tests, which were carried out at two pavement sites in Virginia, are reported for both methods. Finally, a detailed description of the required testing equipment and experimental and analytical procedures for both methods are included in the Appendix. Cost savings from implementing the methods are not obvious, since the methods cannot be used to measure the thickness of most concrete pavements for acceptance and payment. The methods can be used to nondestructively evaluate the modulus of rupture of pavements for analysis purposes, but savings would depend on the nature of the analysis.

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