Coring is normally done to monitor the thickness and quality of portland cement concrete (PCC) slabs during construction. Because this procedure requires a considerable amount of time, it is done at widely spaced intervals. As a result, the most critical points, in terms of strength or thickness, are sometimes not tested. Their repeatability and extreme sensitivity to the properties of surface layer enable wave propagation techniques to be used for quality control. The main advantage of these techniques is that they are nondestructive. Fortunately, these techniques have been automated in the last few years. Two seismic devices (seismic pavement analyzer and a portable version of it called the Lunch Box) have been used extensively for quality control. With them, slabs can be tested at closely spaced points and at a fraction of the cost and time of coring. The main tests used are the impact echo for determining the thickness of the slab, the ultrasonic body wave for determining the modulus, and the ultrasonic surface wave (an offshoot of the spectral analysis of surface waves method) also for determining the modulus. On the basis of extensive field testing on many types of base and subgrade, the techniques in general—and the two devices in particular—are suitable for many quality-control projects. It was found that the most robust method for determining the modulus is the ultrasonic surface wave. The impact echo also works well, as long as enough contrast exists between the properties of the PCC and the underlying materials.
Long-term use of modern Portland cement concrete: The impact of Al-tobermorite formation
