Research On Restoration Of Early Age Cracks In Cement-based Materials By Microbe

Concrete is one of the most widely used building materials in the world. Cracks occur easily because of the low tensile stress during the service life, which will degrade the performances of concrete. The reduction of durability will cause serious damage to buildings. Therefore, lots of attentions were put upon to the repair techniques of concrete cracks. This paper centers on the self-healing of early age cracks by microbiologically precipitating calcium carbonate in cement-based materials. A new type of self-healing agent and mechanism was applied to repair the cracks and the mechanism was revealed.The microbes in this paper are able to survive in the high alkali environment of concrete for a short period of time, and maintain the enzyme activity in concrete cracks. Increasing the dosage of self-healing agent would affect the performance of cement-based materials, cause liquidity reducing, setting time delaying and strength losing. But proper dosage range of self-healing agent had no significant impacts on the performance. Microbes could induce the deposition of CaCO3 efficiently in the calcium salt solution. Small cracks below 0.3mm at early age almost closed completely, proving the self-healing agent was viable.Crack repair effect reduced when crack width increasing. The cracks below 0.3mm could be repaired using lower healing agent, but the large cracks （1.0mm） couldn’t obtain a better repair effect even if raising the dosage of healing agent. Self-healing agent worked best at about 30℃, and temperature below 10℃ is not benefit to cracks repair. Cracks couldn’t be repaired when cracks covered by wet geotextile. But experimental result showed an excellent repair effect under dry-wet cycle curing condition. The self-healing effect could be improved for the cracks formed at late ages when the bacteria were immobilized by sound ceramsite.Finally, the mechanism of microbiologically repairing cracks was revealed based on numerical simulation and experimental results. By calculating the concentration distribution of CO₂ in cracks and simulating the absorption of CO₂ by microbes, the process of precipitation of CaCO3 could be demonstrated. The maximum dissolution depth and dissolution rate of Ca²⁺ were studied to learn the migration of Ca²⁺. The concept of valid Ca²⁺was put forward. The depth of precipitated CaCO3 could be predicted base on valid Ca²⁺ and then verified by practical experiments. Moreover, it also explained why cracks closing only occurred on the surface of specimen.