Structural concrete made with recycled aggregates: Hydration process and compressive strength models

This paper reports a study on the mechanical behaviour of recycled aggregate concretes (RACs) and is mainly intended at unveiling how they are fundamentally influenced by the mixture constituents and conditions. To this end, it summarises the results of some experimental tests carried out at the Laboratory of Material testing and Structure (LMS) of the University of Salerno (Italy) on samples of concrete mixtures made with natural aggregates (NAs) and recycled concrete aggregates (RCAs), characterised by two different initial moisture conditions. The compressive strength determined at different curing ages and the time evolution of temperature developed within the concrete samples during setting and hardening are reported for each concrete mixture. By making use of a theoretical heat flow model, temperature measurements could be employed for identifying the hydration process of cement, which take place in such mixtures. These analyses clearly highlighted that the hydration reaction is significantly influenced, not only by the nominal water-cement ratio, but also by the initial moisture conditions of recycled aggregates, whose significantly higher porosity modifies the water content actually available in the mixture for cement hydration. Moreover, linear correlations emerged between the so-called "degree of hydration" and the corresponding compressive strength. In fact, introducing the degree of hydration as a fundamental parameter connects the micro-structural evolution to the development of the macro-scale mechanical properties of RACs. This multi-scale approach is considered as a key step in simulating the behaviour of structures made of concretes with recycled aggregates.