<b>ABSTRACT </b></br>

One of the main operations in any civil work is to verify that the concrete supplied and placed on site reaches the required minimum compressive strength. This verification is usually performed statistically through the preparation and testing of a large number of specimens. However, its indirect control is also useful due to its simplicity, ease of execution, and low cost. One of the most common methods to perform this operation is the measurement of the Ultrasonic Pulse Velocity (UPV) of the cast concrete, checking that its value corresponds to the necessary concrete’s strength class. This study aims to analyze whether this indirect measurement, widely used in vibrated concrete produced with natural aggregate, is also valid when large quantities of Recycled Concrete Aggregate (RCA) are added to Self-Compacting Concrete (SCC). For this purpose, six SCC mixes were produced with 100% coarse RCA and variable fine RCA contents (0%, 50%, and 100%). In addition, two different types of powder were used: limestone filler <0.063 mm and limestone fines 0/0.5 mm. The determination of the compressive strength and the UPV at 7 and 28 days in all mixes allowed demonstrating the existence of a close relationship between these two magnitudes in this type of concrete. It was even possible to develop highly accurate simple-regression models to interrelate both variables. These findings show that the use of SCC with RCA in engineering works would still allow the control of compressive strength using one of the most common traditional techniques, the ultrasonic pulse.</br>

<b>ACKNOWLEDGEMENTS</b></br>

The authors acknowledge the funding of this research work to the Spanish Ministry of Universities,

MICINN, AEI, EU and ERDF [FPU17/03374; PID2020-113837RB-I00; PID2021-124203OB-I00;

RTI2018-097079-B-C31; 10.13039/501100011033]; JCyL and ERDF [UIC-231, BU119P17]; Basque Government [SAREN group, IT1619-22]; and the University of Burgos [SUCONS, Y135.GI].