Improvement of the compressive strength of recycled aggregate concretes and relative effects on durability properties

The present paper deals with the problem of the compensation of the mechanical properties of recycled aggregate concretes (RAC) by adjusting the effective water-to-cement ratio. For this purpose, two substitution rates (60% and 100%) of natural aggregates by recycled ones are first considered and the loss rates of the compressive strength and the elastic modulus of the relative RAC are quantified. In the second part, the improvement of the 28 days compressive strength thanks to the water-to-cement ratio adjustment is proceeded and the effect of this process on the mechanical (elastic modulus), physical and durability properties (water accessible porosity, total shrinkage, chloride diffusion and carbonation) is quantified and presented for discussion. Among many other results, the compensation of the compressive strength (by adjusting the effective water-to-cement ratio from 0.42 to 0.35 for instance) is reported to be insufficient to compensate simultaneously the physical, mechanical and durability properties of the resulting recycled aggregate concrete. Finally, a micromechanics-based model is developed and fed with experimental data collected on the microstructure at different scales. Compared to the macroscopic mechanical properties, the predictions of the established model are shown to express with a good accuracy the effective elastic properties of RAC undergoing the compensation process.