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.

The feasibility is investigated of various acoustic (sonic), nuclear, and electrical techniques for nondestructive measurement of Portland cement and bituminous concrete pavement thickness both during and after construction. Recommendations are made for equipment development and field testing of three specific methods that could potentially result in such measurements with the desired degree of speed and accuracy.

The proceedings of June 1993 international symposium held in Atlanta, Georgia, called specifically to develop and standardized evaluation procedures for non-destructive methods of testing pavements. The 29 papers discuss analytical models and techniques, measurement and calculation techniques in the field and laboratory, problems and errors associated with backcalculation methods and design parameters, and testing for other pavement uses. Also includes a history of the quest for a standard and the status of that effort. Reproduced from typescripts. Annotation copyright by Book News, Inc., Portland, OR

The nondestructive impact-echo (IE) method offers a simple means for introducing compressional stress waves into a concrete element or slab and measuring the resonance frequencies associated with the reflections of the waves from any internal voids and the bottom of the slab. It is relatively effective for detecting internal voids or delaminations in concrete, which is the application for which it was developed. It may also be possible to use the method for indirect measurement of the thickness of a slab if the wave propagation velocity in the concrete is known. This study was conducted to determine whether the IE method, by itself, could replace the use of coring for quality-assurance measurements of the thickness of concrete slabs in newly built pavements. The results from tests conducted on three pavemems indicated that the wave velocity varied so much, not only between pavements but also within a pavement, that unacceptable errors can result when an average velocity is determined (through limited coring) for a pavemem and subsequently assumed for the entire pavement. To reduce the error to an acceptable level, the wave velocity at any test location must be measured to within an acceptable accuracy by another independent method. In pursuit of this approach, an indirect-transmission procedure based on ultrasonic (UT) measurement was incorporated and tested. This combined IE/UT procedure was able to measure thickness with absolute errors of 5 mm in one pavement and 7 mm in another, at a 90% probability. These results can be considered encouraging since the current procedure requires that the length of a core reported to the nearest 3 mm be the average of several measurements around the core and, in some cores, these measurements can have a spread of as much as 13 mm. In addition, it is expected that these errors can be reduced easily with the use of a transducer with a smaller contact face that would be less sensitive to roughness on the surface of grooved concrete pavements.

The purpose of this research was to summarize existing nondestructive test methods that have the potential to be used to detect materials-related distress (MRD) in concrete pavements. The various nondestructive test methods were then subjected to selection criteria that helped to reduce the size of the list so that specific techniques could be investigated in more detail. The main test methods that were determined to be applicable to this study included two stress-wave propagation techniques (impact-echo and spectral analysis of surface waves techniques), infrared thermography, ground penetrating radar (GPR), and visual inspection. The GPR technique was selected for a preliminary round of "proof of concept" trials. GPR surveys were carried out over a variety of portland cement concrete pavements for this study using two different systems. One of the systems was a state-of-the-art GPR system that allowed data to be collected at highway speeds. The other system was a less sophisticated system that was commercially available. Surveys conducted with both sets of equipment have produced test results capable of identifying subsurface distress in two of the three sites that exhibited internal cracking due to MRD. Both systems failed to detect distress in a single pavement that exhibited extensive cracking. Both systems correctly indicated that the control pavement exhibited negligible evidence of distress. The initial positive results presented here indicate that a more thorough study (incorporating refinements to the system, data collection, and analysis) is needed. Improvements in the results will be dependent upon defining the optimum number and arrangement of GPR antennas to detect the most common problems in Iowa pavements. In addition, refining high frequency antenna response characteristics will be a crucial step toward providing an optimum GPR system for detecting materials related distress.