The use of ultrasonic pulse-echo tomography to assess UPV in soft calcarenitic stones

Measuring the ultrasonic pulse velocity of stone ashlars belonging to masonry structures provides useful indications for assessing the materials' strength and state of conservation, the need for their possible substitution, and the effectiveness of consolidating treatments. Ultrasound techniques as nondestructive tests preserve the integrity of masonry buildings that especially in Italy often have a cultural and historical-artistic value.
Several correlations linking ultrasonic velocity with the compressive strength of stone materials are available in the literature. These correlations are often developed in the laboratory using small-size samples, high measurement frequencies, and direct transmission measurement modes. In situ, it is often difficult to make measurements under such conditions due to the inaccessibility of both the surfaces of materials and the higher dimension of the elements to investigate. In addition, material surfaces are often affected by deterioration which causes problems in the transducer coupling with measurement surfaces. Furthermore, the use of coupling agents is usually forbidden in the case of artistic artifacts. The ultrasonic tomograph with pulse-echo technology (PE UT) overcomes such difficulties. Pulse-echo method introduces a stress pulse by a transmitter into an object at an accessible surface. The pulse propagates into the test object and is reflected by flaws or interfaces. The surface response caused by the arrival of reflected waves, or echoes, is monitored by receivers. Tomography gives visualization, either by cross-section or 3D images, of the interior structure of the object to find anomalies and determine the material physical properties. PE UT has an array of dry-point contact transducers (DPCT) that enable ultrasonic measurements on rough surfaces without coupling gel, reducing the measuring time and test invasiveness. PE UT employs transverse S-waves instead of longitudinal P-waves because they give more reliable results for degraded materials and in the presence of defects. In the literature, there are several studies in which tomographic images were used to detect the presence of defects in concrete and rock members. In these cases, a reference value of UPV is assumed. On the contrary, few studies use PE UT images to determine UPV on concrete or stone members. Thus, the appropriateness of this technique to assess UPV is still under research due to the lack of a sufficient number of published experimental studies.
In the present paper, the reliability of PE UT in determining the ultrasonic velocity of soft calcarenitic stone elements, known as Lecce stone, is investigated. The measurements are compared with those conducted with conventional transducers at different frequencies in direct and indirect modes. The measurements are also carried out considering the presence of water in the material, which strongly influences not only the ultrasonic measurements but also the strength of the material itself. Four levels of saturation from dry to fully saturated states are considered. The results of the experimental campaign evidence that PE UT is a reliable technique for UPV assessment, allowing rapid one-side measurements on surfaces in different conditions.