ABSTRACT
The objective of this research is to study the effect of initial moisture of RA and also the cement-admixture combination on the behavior of RAC with different substitution percentages of coarse and/or fine NA by RA. Regarding the rheological properties, the measured parameters are: torque and yield stress, torque/rotation rate ratio at steady state flow and plastic viscosity, workability, air content and density. Furthermore, in order to follow the evolution of these quantities versus time the tests were carried out from the end of mixing to 90 min of age. Water demand and superplasticizer contents are fixed to have the same initial S4 class of workability (a slump of 200 ± 20 mm is targeted). There is no significant adverse effect of the pre-humidification of RA on the rheological properties of fresh concrete at the end of mixing. The mixing time seems sufficient for under saturated RA to absorb a large part of the water necessary to their saturation. The moisture content of RA should be ranged from 0.8 to 1.0 Ab as a compromise between the yield value and the viscosity to minimize the adverse effect of time on the flow properties of concrete. The properties of recycled aggregates concrete depend on the couple cement/admixture and the level of substitution on recycled aggregates. This means that there is indeed an influence of recycled aggregate on the compatibility (physico-chemical equilibrium) of the couple cement/admixture. This influence is more noticeable on the fine aggregate than on coarse aggregate. The results also show that for concrete based on recycled aggregate and in our conditions of test, the higher 110the percentage of substitution, the higher the rheological loss. We can distinguish also concretes made with recycled fine aggregate for which beyond a substitution rate of 30% a degradation is observed and concretes made with recycled coarse aggregates for which an only few degradation is observed for 100% of substitution.
