###### Abstract  
Ultrasonic pulse velocity (UPV) is a non-destructive measurement technique with which the quality of any concrete element can be evaluated. It provides information on concrete health and for assessing the need for repair in a straightforward manner. In this paper, the relationship is studied between UPV readings and the mechanical behavior of self-compacting concrete (SCC) containing coarse, fine, and/or powdery RA. To do so, correlations and simple- and multiple-regression relationships between compressive strength, modulus of elasticity, splitting tensile strength, flexural strength, and UPV readings of nine SCC mixes were assessed. The correlations showed that the relationship of UPV with any mechanical property was fundamentally monotonic. The inverse square-root model was therefore the best-fitting simple-regression model for all the mechanical properties, although for bending-tensile-behavior-related properties (splitting tensile strength and flexural strength) the estimation accuracy was much lower than for compressive-behavior-related properties (compressive strength and modulus of elasticity). Linear-combination multiple-regression models showed that the properties related to bending-tensile behavior had a minimal influence on the UPV value, and that their introduction resulted in a decreased estimation accuracy. Thus, the multiple-regression models with the best fits were those that linked the compressive-behavior-related properties to the UPV readings. This therefore enables the estimation of the modulus of elasticity when the UPV and compressive strength are known with a deviation of less than \(\pm 20\%\) in 87% of the SCC mixes reported in other studies available in the literature. os 2023 13 874 [supplementary]supplementary_sup  
UPV has been one of the most widely used NDT techniques to evaluate the quality of in situ concrete since the mid-20 th century [14]. This non-destructive measurement consists of placing two transducers on the concrete surface, between which an ultrasonic pulse is transmitted through the concrete [16]. The test result is the time that the ultrasonic pulse takes to travel from the transmitting transducer to the receiving transducer. The quotient between the separation distance of the transducers and the measured time lapse is the UPV reading [17]. The measurement of this time lapse can be done by placing the transducers in three different ways. First, there is the direct method, in which the transducers are placed on opposite sides of the concrete test sample. Second, there is the semi-direct method, in which the transducers are placed on faces of the concrete test sample, usually at an angle of \(90^{\circ}\) to each other. Finally, there is the indirect method, in which the transducers are placed on the same face of the concrete test sample. The most commonly used method is the direct method, as it provides a travel time over a known distance [18]. The value of the UPV, generally measured with the direct method, can be related to different properties of concrete [19; 20]. Thus, the higher the UPV, the higher the density and the lower the porosity of the concrete, and, therefore, the better its mechanical behavior [21; 22]. Although UPV has traditionally been used to estimate the modulus of elasticity of concrete, its close relationship with concrete porosity has led to studies showing its usefulness at estimating other concrete properties such as compressive strength mainly by linear simple-regression models [23; 24]. In this study, the aim is to go still further, by evaluating whether an overall picture of all the mechanical properties of concrete can be obtained only through the UPV index. Currently, various types of concrete are promoted as sustainable alternatives. One of them is recycled aggregate concrete, made with recycled aggregate (RA), widely used in the production of different sorts of concrete elements [25; 26; 27]. This type of aggregate is mainly produced from demolition waste and other concrete elements that have been crushed.