Heterogeneity of recycled concrete aggregates, an intrinsic variability

Recycled concrete aggregates (RCA) are composed of two different materials: natural aggregate and attached cement paste. It is generally admitted that finer the RCA greater is the quantity of adhered cement paste. In this study it is shown that for a given granular class, very large disparities may be present in the adhered cement paste content, which could generate dispersion of the results of characterization tests of RCA. The heterogeneity of a narrow granular class of coarse RCA (6.3/10 mm) was investigated by sorting samples according to their densities using a water jig. The water jig sorting was essentially a separation by density and not by size. It was shown that the average water absorption after an immersion in water during 24 h was about 5% for a representative sample of the feed RCA, before sorting tests, while for the homogenous specimens separated by density, the water absorption ranged from 2% to 9%. The distribution of the water absorption in a sample of 120 kg of RCA was well estimated by a log-normal distribution. This intrinsic heterogeneity of RCA implies being able to have representative samples with sufficient accuracy, not only concerning the size but also the mineral composition of grains in the stockpiles of RCA. So, after discussing the results of this study, water jig appeared to be an efficient tool to separate RCA and obtain representative samples for their characterization.