Ultrasonic study of alteration processes in granites caused by freezing and thawing

Abstract

Ornamental rocks, and particularly granites, represent one of the most important industrial activities in the region of Extremadura (SW Spain). Therefore, knowledge about the intrinsic properties of this natural material and its evolution in the environment is required in order to ensure the quality of construction activities based on these types of rocks. In this work, we describe a detailed experimental study on the use of different types of granites which were repeatedly frozen and thawed in a controlled environment. Specifically, our experiments were based on 56 freezing-thawing cycles, where the temperature was varied from -12°C to 20°C at time intervals of 4, 8, or 12 hours, depending on the experiment. The ultrasonic wave velocity was measured at the beginning and the end of each cycle. In addition, several measurements were made during the process to characterize the evolution of the granites. A low-frequency transceiver set-up, based on a high-voltage BPV Steinkamp instrument and two 50 kHz probes, were used to measure pulse travel times by ultrasonic through-transmission testing. There was a reduction in the ultrasound propagation speed through the samples which reached, depending on the species considered and the cycle- time, a maximum of about 15%. In addition, we found strong positive linear correlations in the plots of the reduction in speed against the differences in propagation speeds in water- saturated and dry granites, before and after being tested. Although the porosity of the tested granites was below 2% before the experiments, this velocity reduction could be explained by bearing in mind that freezing and thawing can significantly increase the size and number of pores, fractures, and microfractures, thus decreasing the propagation speed of ultrasound in the material after several cycles.