The construction sector is among the most resource- and energy- consuming industrial activities. Particularly, according to recent surveys, the emissions of greenhouse gases due to concrete industry correspond to about 10% of the total, almost half of which is due to the cement production. Thus, to enhance the sustainability of the construction sector, the design of integrated recycling processes aimed at reducing wastes to produce new materials is of particular interest. The compressive strength of concrete made of recycled constituents (i.e. aggregates, fibers, alternative binders, and other) is an issue that is still lacking robustness and the effect of aggregate replacement to the ultimate capacity is unclear. This paper proposes a conceptual compressive strength model which is based on both the development of the microstructure, i.e. representing the strengthening of the paste, and the different characteristics of the aggregates using a parallel-series model. The combined concept relates the development of the compressive strength to the evolution of the degree of hydration while taking into account the ultimate strength capacities of the recycled and natural aggregates. The model is able to assess the effect of blended aggregates systems, with different strength properties (low and high stiffness, low and high intrinsic strength) on the compressive strength development. The actual predicting capacity of the model is assessed through an experimental validation. Both the experimental results and the theoretical formulations proposed in this paper stem out from the inter-university collaboration developed within the framework of the European Project EnCoRe (www.encore-fp7.unisa.it).
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