Rheology and strength of concrete made with recycled concrete aggregates as replacement of natural aggregates

This paper presents the results of an experimental study carried out to evaluate the influence of the nature and the content of the recycled fine and coarse aggregates from concrete construction and demolition wastes on the rheological and strength properties of concrete. For this purpose, concretes mixtures were made with different percentages (15, 30, 50, 70 and 100%) of recycled sand and recycled gravel. Three different cement / admixture ratios were used. At fresh state, tests applied include slump, yield stress, plastic viscosity, air content and fresh density. Moreover, in order to follow the evolution of these properties over time, the tests were carried out at different times: directly after mixing, 30 minutes, 60 minutes and 90 minutes after completion of the mixing). At hardened state, compressive strength was determined at age of 7 and 28 days. The results show that, the properties of the concretes made with recycled aggregates depend up on the cement / admixture ratio and the substitution level of recycled aggregates. This means that there is indeed an influence of the recycled aggregates on the compatibility (physicochemical equilibrium) of the cement / admixture ratio. This influence is more noticeable on the sand than on the gravel. The results also show that, increasing the percentage of substitution of recycled aggregates in concrete mixture increases the yield stress and the plastic viscosity of fresh concrete. However, a decrease in the compressive strength was found with increasing the content of recycled aggregate.