Use of recycled concrete aggregates as carriers for self-healing of concrete cracks by bacteria with high urease activity

Microbial induced carbonate precipitation (MICP) has become one of the most attractive strategies to achieve self-healing of concrete cracks in recent decades. In order to provide a moderate space for bacteria in the harsh environment of concrete, the incorporation of protective carriers is a necessity. In this study, the microbial urease activity at different generation of propagation in the selected culture medium was first investigated, followed by exploring the potential of using recycled concrete aggregates (RCAs) as microbial carriers, considering the porous nature of RCAs. The specific urease activity of bacteria at 48 h kept as high as 2.4 x 10(-8) mM.min(-1).cells(-1) even after 5 generations of propagation in the selected culturing medium. RCAs would not result in a loss of ureolytic activity after immobilization and were capable of retaining bacterial viability during concrete mixing. Compared with concrete without any healing agents or with only substrates and calcium source, the one with bio-agents loaded in RCAs completed most of the crack healing in the first week. When the initial crack widths were less than 0.6 mm, the average crack healing ratio and the crack area healing ratio were 71% and 84%, respectively, for microbial self-healing. The depth of crack healing was 17.8 mm as indicated by the microbial precipitates on the crack wall. After microbial self-healing, the compressive strength regain ratio achieved 99.7% while the water tightness regain ratio attained 99.2%, and the cracks were filled with densely packed irregular rhombohedral-shaped crystals consisting of calcite and a small amount of vaterite.