Book Description

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.