Previous microwave reflection measurements made on simple, unreinforced concrete blocks have shown that the transit time of a microwave through concrete is linearly related to its thickness. In this study measurements were conducted on concrete slabs that were built to simulate pavements and on an actual continuously reinforced concrete pavement to determine whether this type of rapid nondestructive measurement could be used in lieu of coring in the inspection of newly built concrete pavements for compliance with slab thickness specifications. It was found that reflections from the bottom of a concrete pavement slab may often be too weak and difficult to identify in a radar profile; therefore, the transit time of a microwave through reinforced concrete slab would often be difficult to measure with reasonable accuracy. The results obtained with radar measurements made on simulated pavement slabs showed that radar was only able to indicate whether a slab is too thin (i.e., less than 8 in); but those measurements made on an actual pavement yielded reasonably good agreement between the thickness determined by coring and those determined by reflection measurements in conjunction with a calibration procedure. Based on these mixed results, it would be difficult to predict how reliable radar would be for precise quantitative measurement of slab thickness of a particular pavement until an actual radar scan of the pavement has been conducted. If the reflection from the bottom of the slab appeared to be identifiable, then the calibration procedure could be used to determine the slab thickness for the entire pavement. Otherwise, radar can be used at least as a screening tool to spot areas in the pavement that appear to be to thin. These areas would then be cored to verify the radar readings.

The use of microwave reflection measurements to continuously and nondestructively monitor the hydration of concrete is described. The method relies upon the influence of the free-water content on the dielectric properties of the concrete. Use of the method, on concrete blocks showed linear relations between the compressive strength, water-cement ratio, and block thickness with the dielectric properties. Some possible applications of the method are discussed.

Civil engineers will value this resource that examines the tools and techniques used to estimate the in-place strength on concrete, permeation properties that relate to potential durability, and the methods used to assess the internal condition of concrete and the corrosion activity of steel reinforcement.

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.

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.