Resistance to fragmentation of recycled concrete aggregates

This work deals with the characterization of fragmentation of recycled concrete aggregate ( RCA) by the Los Angeles test. The standard testing procedure for the Los Angeles index requires the measurement of the mass passing 1.6 mm after 500 revolutions of the drum. Questions arise on the significance of this measure for RCA as the resistances of the mortar, of the original aggregate and of their interface simultaneously affect the measurement result. In order to clarify the interpretation of such a measure, three aspects were investigated: the influence of the number of revolutions, the evolution of the full particle size distribution and the distribution of mortar in the fractions after the test. The results suggested that the fracture mechanisms are similar between recycled and natural aggregates with some quantitative differences. RCA was less resistant to fragmentation and displayed a non linear evolution of mass passing 1.6 mm with the number of revolutions. During fracture of RCA, mortar progressively accumulates in smaller fractions, with coarse fractions asymptotically behaving like natural aggregates. The analysis of the rate of breakage of the coarsest fraction appeared to be a richer indicator of crushability as a function of time and composition than the amount of the mass passing 1.6 mm: its typical evolution allows estimating a rate of breakage which correlates well with indirect measures of composition ( water absorption and density). The possible existence of an asymptote in the rate of breakage of the coarsest fraction suggests that it can be used to estimate at the same time the resistance of the recycled aggregate and of its original natural component, and probably give an estimate of the amount of mortar.