Indirect evaluation of the compressive strength of recycled aggregate concrete with high fly ash ratios

In this study, the compressive strengths of concrete mixes made with high incorporation of fly ash (FA) and recycled concrete aggregates (RCA), individually and jointly, with and without superplasticisers (SP), were evaluated by non-destructive ultrasonic pulse velocity (UPV) test. Three mix families (0, 30 and 60% FA) were produced and, for each of these three families, three incorporation levels (0, 30 and 60%) of fine RCA were used with 0% and 100% of coarse RCA, with and without SP. In this study, the effects of each of these materials on the UPV of concrete mixes were determined. Different equations are proposed to predict the compressive strength of concrete mixes by means of the non-destructive UPV, according to their mix composition. Moreover, in cases where the FA content is known, as the oven-dried density of concrete can be easily determined, a simplified expression is proposed to estimate the compressive strength of concrete mixes according to their density and FA ratio (in terms of cement mass), regardless of the type and content of aggregates and content of the admixture, within the limits tested in this research.