Mechanical properties, microstructure, and environmental assessment of recycled concrete from aggregate after fire

Abstract

In terms of ever-increasing fire accidents in cities together with upgrades in the amount of construction and demolition (C&D) waste, the rapid modernization and urbanization process have caused an adverse effect on environmental protection. The lack of natural resources in concrete provides a driving force for the reuse of C&D waste caused by fire accidents. Aiming to conserve natural resources and reduce environmental pollution, the feasibility of using aggregate after fire as an alternative material to produce recycled aggregate concrete (RAC) is investigated experimentally. The materials and environmental properties of the RAC are evaluated with the change of four different replacement ratios (25%, 50%, 100%, including a control group of 0%), and five different heating temperatures (200 °C, 400 °C, 600 °C, and 800 °C, including a control group of 20 °C). The results showed that with the increase of the substitution ratio, the strength, elastic modulus and ascending-branch slope for the stress–strain curves decrease gradually, which the brittle failure phenomenon becomes more obvious, which may have a bearing on the presence of old adhered mortar (AM) on recycled coarse aggregate (RCA). Nevertheless, the utilization of waste concrete treated by heating in RAC can mitigate the adverse effect of increasing the replacement ratio, which is attributed to the decreasing content of old AM in RCA. Numerous calculation formulas related to the material properties and constitutive model are proposed by considering different factors. Based on SEM tests and X-ray diffraction techniques, it was identified that the strengthening mechanism in the mechanical properties and uniaxial compressive behavior for RAC prepared with waste concrete after heating is mainly ascribed to the decreasing possibility of penetrating cracks in concrete under compressive loading. The cost and EI assessment shows that RAC with a 25% replacement ratio produced by waste concrete after 400 °C can probably be a suitable selection in concrete materials from an economic and environmental perspective.