Experimental Study of Concrete by Using Ultrasonic Surface Waves at Hundreds of Kilohertz Frequency Range

In this paper, a characterization study of concrete samples with ultrasonic surface waves at hundreds of kilohertz frequency range is presented. These waves are generated and received by using 0.5 MHz-nominal frequency transducers. This investigation therefore concerns the first centimeters in the nearby of the material surface. The study was applied to concrete specimens for which a compositional parameter which is the water-to-cement ratio (W/C) has been varied. The latter affects the density and porosity of the material and therefore its mechanical properties. In addition, the evolution of acoustic and mechanical parameters of the concrete during its curing period has been investigated. The acoustic velocity and attenuation parameters are determined by exploiting the time of flight and the amplitude of the received ultrasonic signals. This study shows that the variation of the water-to-cement ratio affects the velocity of propagation of the surface waves and also leads to a variation of the mechanical strength of the concrete. It concludes that there is a strong correlation between the strength of the concrete, the ultrasonic velocity, and the W/C ratio. The results obtained by the destructive evaluation, which provides a measurement of the compressive strength by mechanical crushing test, confirm those obtained by the non-destructive evaluation of concrete. The study shows that this type of non-destructive testing using ultrasonic surface waves is beneficial particularly when the concrete structure is only accessible from the surface or when the propagation of the bulk waves is perturbed by the presence of reinforcements.