Influence of urea and Ca²⁺ concentration on microbial based repairing efficiency in a visible crack

During concrete crack repair using microbially induced carbonate precipitation (MICP), both the quantity and distribution of bio-precipitates in cracks directly determine repair efficiency. The concentrations of precipitation precursors (urea and Ca²⁺) strongly affect the precipitation behavior. So far, most studies were conducted under static submersion condition, which differ from practical crack repair situations involving dynamic grouting. This study developed a visual model of mortar cracks to investigate the influence of precipitation precursors on the distribution and repair effectiveness of cracks during dynamic grouting. The results showed that precursor concentration significantly affects both the amount of bio-precipitates and its spatial distribution. At low concentrations of precipitation precursors (0.25 M/0.25 M), precipitates slowly formed and concentrated at the bottom, resulting in insufficient filling and weak bonding. While at high precursor concentrations (≥0.75 M/0.75 M), a dense precipitate layer formed quickly near the injection point due to rapid instantaneous precipitation rate, hindering the transport of the repair solution into the deeper part of the crack, resulting in uneven distribution and low Ca²⁺ utilization (40%). A more uniform spatial distribution of bio-precipitates and highest repair efficiency were observed when the concentration of precipitation precursors was 0.5 M/0.5 M, with a crack sidewall coverage of 85% and a crack filling rate of 81.8%, along with higher Ca²⁺ conversion efficiency (79%) and excellent bonding. These findings indicated that the repair efficiency under dynamic grouting conditions was influenced by the synergistic interaction among the precursors’ concentration, instantaneous precipitation rate, injection flow rate, and transport capacity of repair solution.