Micromechanical multiscale modeling of ITZ-driven failure of recycled concrete: Effects of composition and maturity on the material strength

Abstract

Recycled concrete, i.e., concrete which contains aggregates that are obtained from crushing waste concrete, typically exhibits a smaller strength than conventional concretes. We herein decipher the origin and quantify the extent of the strength reduction by means of multiscale micromechanics-based modeling. Therefore, the microstructure of recycled concrete is represented across four observation scales, spanning from the micrometer-sized scale of cement hydration products to the centimeter-sized scale of concrete. Recycled aggregates are divided into three classes with distinct morphological features: plain aggregates which are clean of old cement paste, mortar aggregates, and aggregates covered by old cement paste. Macroscopic loading is concentrated via interfacial transition zones (ITZs)-which occur mutually between aggregates, old, and new cement paste-to the micrometer-sized hydrates resolved at the smallest observation scale. Hydrate failure within the most unfavorably loaded ITZ is considered to trigger concrete failure. Modeling results show that failure in either of the ITZs might be critical, and that the failure mode is governed by the mutual stiffness contrast between aggregates, old, and new paste, which depend, in turn, on the concrete composition and on the material's maturity. The model predicts that the strength difference between recycled concrete and conventional concrete is less pronounced (i) at an early age compared to mature ages, (ii) when the old cement paste content is small, and (iii) when recycling a high-quality parent concrete.