Mechanical properties of recycled aggregate concrete prepared from waste concrete treated at high temperature

Abstract

The ever-increasing generation of construction and demolition (C&D) waste caused by the frequent occurrence of building fires has posed a threat to environmental protection and waste management over the last few decades. The shortage of natural resources provides an impetus for the utilization of C&D waste after fire. Aiming to provide a cleaner option for waste concrete after fire, the viability of using waste concrete after fire as a substitute for coarse aggregate in the production of recycled aggregate concrete (RAC) was investigated and analyzed in detail. The failure mode, stress–strain behavior, mechanical properties and microstructure of the RAC was evaluated with the variation of four different replacement ratios (0%, 25%, 50%, and 100%) and five different treatment temperatures (20 °C, 200 °C, 400 °C, 600 °C, and 800 °C). The experimental results showed despite the failure mode of RAC being similar to that of natural concrete (NC), an increase in the recycled coarse aggregate (RCA) replacement ratio would decrease the slope of curves and strength, and the fcu and fc with 25% replacement ratio for RAC decrease by about 15% and 10% respectively as compared to those for NC. Nevertheless, treatment of elevated temperature for waste concrete can eliminate the damage of replacement ratio to mechanical properties, and the fcu and fc with 25% replacement ratio after 400 °C increased by approximately 12% and 8% respectively as compared with untreated RAC. Against the test result, a series of calculation formulas is proposed to predict the mechanical properties of the RAC whilst a constitutive model is modified with the existing standard to describe the stress–strain behavior of that with the consideration of replacement ratio and treatment temperature. The microstructure analysis indicated that the increase in the RCA replacement ratio would result in the increasing content of adhered mortar characterized by numerous poles and less bond on RCA, whereas treatment of high temperature for waste concrete can decrease the content of adhered mortar on RCA, which makes the microstructure of the RAC more densified.