Assessment of mechanical attributes and microstructural densification of self-healing recycled coarse aggregate concrete using various bacterial immobilizers

Abstract

In recycled coarse aggregate (RCA) based concrete, the strength and durability are influenced by the characteristics of RCA. Micro-level cracks and poor interfacial transition zone (ITZ) lead to macro-level cracks, compromising the integrity of concrete structures. Controlling the formation and propagation of smaller cracks in RCA concrete and improving RCA quality is crucial to have good structural performance. Therefore, microbially induced calcium carbonate precipitation can help to improve the RCA quality, microstructure, cracks propagation, and ITZ. This research compares the effect of different immobilizers and their size on RCA concrete nested with Bacillus Pumilus. Bacteria were incorporated into the concrete by direct introduction and through nano to macro carriers including RCA, sand, and iron oxide nanoparticles (IONPs). The crack healing potential of the concrete samples, mechanical properties, microstructure, and phase configuration were studied. The compressive strength and split tensile strength of IONPs immobilized bacterial mix was 28% and 45% more than the controlled mix, respectively. The results indicated that maximum of 1.3 mm wide cracks could be sealed by the Bacillus pumilus bacteria when immobilized with IONPs and were more efficient upon pre-cracking at 3 and 7 days. In contrast, bacteria immobilized in RCA were more efficient in healing cracks when samples pre-cracked at 28 days. The crack healing precipitate was identified as calcite using field-emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), and x-ray diffraction (XRD).