Indirect estimation of compressive strength through non-destructive testing is key to monitoring the strength of structural concretes used in construction and rehabilitation works. However, no models are available to perform this estimation in highly Self-Compacting Concrete (SCC) with Recycled Concrete Aggregate (RCA). To fill this gap, two indirect measures were tested in this paper, the hammer rebound index and Ultrasonic Pulse Velocity (UPV), to predict the compressive strength of highly SCC. To do so, 24 SCC mixes were developed with different aggregate powders, binders, such as Ground Granulated Blast Furnace Slag (GGBFS), and contents of fine RCA. Compressive strength, and both indirect measures of all mixtures were determined at 1, 7, 28, and 90 days. The development of specific models for highly SCC responded to the inappropriateness of conventional models that are not adapted to its high fines content. Modelling as a function of either UPV or the hammer rebound index yielded accurate predictions, although the UPV model proved more sensitive to compositional changes and presented higher uncertainty. The best predictions were modelled by combining both indirect measures. The models provided safe and accurate indirect estimations of the compressive strength of high flowability SCC in real structures.
