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Recycling construction wastes for their utilisation as aggregates will overcome the dumping and scarcity problems. However, their higher porosity affects the properties of concrete. Hence, this study examines the potential use of microbial-induced calcium carbonate precipitation techniques to improve the quality of recycled coarse aggregates (RCA). The natural coarse aggregate (NCA) was replaced with 50, 100% of RCA and bio-deposited recycled coarse aggregate (BRCA). The aggregate properties were tested and observed that the water absorption and crushing index of RCA were increased by 85.10% and 19.10%, whereas for BRCA, the water absorption and crushing index were increased by only 5.20% and 5.80%. The concrete mixture prepared with optimised and complete percentages of RCA and BRCA was tested for their fresh, hardened, and durable properties. It is observed that the strength of the RAC was 36% lesser than NAC, whereas the strength of the BRAC was 33.40% more compared to the RAC at 28 days. Also, the water absorption and sorptivity of BRAC were reduced by 33.68% and 15.50% and the resistance to carbonation and chloride ingression of BRAC was enhanced by 15.50% and 29.73% when compared to RAC. The microstructural investigations performed through SEM and XRD evident the CaCO3 precipitation that improves the quality of RCA. [ABSTRACT FROM AUTHOR]

A novel technique of self-healing cracks by bacterial species through microbial-induced calcite precipitation (MICP) was developed to counteract the effect of the cracks in concrete structures. This article investigates the influence of two bacterial species, Bacillus subtilis, and Escherichia coli, on the mechanical, durability, and self-healing properties of concrete. The bacterial solutions were cultured in Luria bertani medium to different cell concentrations of 101, 103 and 105 cells/ml. Tests were performed to evaluate the workability, hardened properties such as compressive strength, split tensile strength, flexural strength and durability properties, such as water absorption, acid attack and marine water attack at respective ages. The study concluded that concrete containing bacterial species improves mechanical and durability properties compared to control concrete, and the concrete mixtures with B. subtilis performed better than Escherichia coli. A maximum of 12.8% increase in the strength of the concrete and a 62.1% decrease in water absorption was observed with B. subtilis at 105 cells/ml concentration. SEM analysis performed shows the precipitation of calcite that deposits on the micro-pores and improves the properties of the concrete. Self-healing studies show the deposition of calcite that seals the cracks on the surface of the concrete specimens. [ABSTRACT FROM AUTHOR]

Several research studies have been conducted to investigate the hardened and durability characteristics of the normal aggregate concrete (NAC) and recycled aggregate concrete (RAC). Similarly, few studies have been done to examine the thermal behaviour of the NAC and RAC. However, the influence of elevated temperature on the RAC with treated recycled coarse aggregate (RCA) in the concrete still needs to be investigated. This study presents the comparative discussion on the thermal performance of NAC, RAC, carbonated recycled aggregate concrete (CRAC) and bio-deposited recycled aggregate concrete (BRAC) at various elevated temperatures of 25, 200, 500 and 800°C. The investigation involves the determination of residual compressive strength, tensile strength, elastic modulus and thermal conductivity with a series of concrete mixtures under elevated temperatures. The optimal residual strength was observed at 500°C, beyond exposure of it causes a reduction in the strength. The residual strength of NAC, RAC, CRAC and BRAC at optimal temperature was reduced by 30.10%, 7.06%, and 31.02%. The equivalence of the thermal expansion coefficient of RCA with cement paste improves the residual response of RAC, and higher porosity of RCA decreases the thermal conductivity of the RAC by 43.06% compared to control concrete. Furthermore, no concrete samples crumbled at 800°C, but the residual properties tend to decrease for all mixes. [ABSTRACT FROM AUTHOR]