Crack Repair in In-Service Tunnel Linings Using Chitosan-Combined Enzyme-Induced Carbonate Precipitation

Water seepage in tunnel lining cracks considerably influences the structural safety of a tunnel. In this study, chitosan was introduced in the enzyme-induced carbonate precipitation (EICP) to repair the cracks in in-service tunnel linings. The influence of chitosan incorporation on the ${\mathrm{Ca}}^{2+}$ precipitation ratio during EICP was analyzed by aqueous solution experiments, and the optimal content of added chitosan was obtained. Moreover, in terms of specimen scale and field tests, the determination of permeability characteristics, and mass loss, scanning electron microscopy and ground penetrating radar technology were used to analyze the changes in permeability coefficient and mass loss of cracked concrete repaired by chitosan-combined EICP under normal temperature ($25°\mathrm{C}\pm 2°\mathrm{C}$) and freeze–thaw (FT) cycling conditions. The effect, feasibility, and action mechanism of chitosan-combined EICP for tunnel crack repair in extreme environments were explored. The results showed that the incorporation of an appropriate amount of chitosan in traditional EICP could accelerate ${\mathrm{Ca}}^{2+}$ precipitation, provide nucleation sites for the precipitation of ${\mathrm{CaCO}}_3$, promote the existence of the deposited ${\mathrm{CaCO}}_3$ crystals in the form of calcite with higher strength, and reduce mass loss in extreme circumstances. The combination of hydrogel and ${\mathrm{CaCO}}_3$ makes the impermeable layer more compact and reduces the permeability coefficient of repair concrete. The permeability coefficient descends exponentially with the decrease of mass loss under FT conditions. Chitosan-combined EICP represents an environmentally friendly and feasible method for crack repair in in-service tunnel linings.