Probing Portland Cement Concrete Pavement for Thickness Determination in Wisconsin

The Wisconsin Department of Transportation (DOT) has used probing of fresh concrete to determine the thickness of portland cement concrete pavements since 1998. Two earlier studies that compared the results from probing and coring in Wisconsin DOT projects concluded that probing did provide an acceptable measure of pavement thickness, but it was recommended that an annual coring program be instituted to verify the probing results. This paper compares core depths with probe measurements for all the projects conducted by the Wisconsin DOT from 2006 to 2011 where both core and probe data were available. A total of 32 projects were analyzed, representing more than 6,600 probes and 1,200 cores. A statistical analysis was done to determine if the core and probe measurements gave the same results. A simulation model was developed to analyze the power of the t-test to predict whether the two measurements are the same and to develop sample sizes needed to ensure that the two measurements differ only by a preselected amount. Results indicate that there are small, yet somewhat negligible in practice, differences between the core and probe methods of measurement. Recommendations are given for sample sizes needed on the basis of the size of the acceptable difference between the two methods.