| It has been realized that the damage and cracking(cracks or micro-cracks)which are usually present in concrete caused by combined actions with various loadings and environment,can significantly accelerate the ingress of water,oxygen and soluble aggregessive agents into concrete,because they provide the preferential flow channels and allow more aggregessive agents to penetrate.To restrain structural damage and reduce high maintenance cost caused by crack propagation in concrete,the microbial induced calcium carbonate precipitation(MICP)to heal the micro-cracks within concrete,also named as microbial self-healing concrete,becomes a hot recent topic in civil engineering.The definition,classification,mechanism,and influence factors on the microbial self-healing concrete as well as its evaluating indicators for healing ability are summarized based on the literature survey.The latest developments and main conclusions on the technology of microbial self-healing concrete are reviewed.Results show that micro-cracks in concrete are mainly filled with calcium carbonate,a metabolite of microorganisms.The previous results show that besides filling and repairing micro-cracks,mineral precipitation produced by microbe can also enhance the mechanical properties of concrete.In addition,the decrease of porosity,water absorption,permeability and chloride transport capacity of the repaired concrete are beneficial to improving the durability of concrete structures by microbial healing agents.Therefore,microbe as a healing agent to repair concrete damage has a capacity for sustainable repair and can improve both the mechanical property and durability of concrete to achieve the purpose of maintaining the concrete structure.However,microbial self-healing technology is a complex physical and chemical reaction process,and the related researchs on mechanism are still in its infancy.In addition,it is difficult to observe the process of microbial self-healing,especially the sealing of concrete cracks.Based on the above considerations,the experimental and numerical simulation studies on the mechanism of microbial self-healing and the effect of concrete durability improvement are carried out.The specific contents of the research work and main conclusions are summarized as follows:(1)A kind of alkaliphilic microorganisms(Bacillus pseudofirmus)was cultivated,and the microbial mineralization deposition test was carried out to verify the feasibility of repairing concrete cracks by using Bacillus pseudofirmus.The mechanical properties test ofmicrobial self-healing concrete was carried out,results reveal that the mechanical properties of microbial self-healing concrete decreases in varying degrees compared with ordinary concrete.(2)At present,the research mainly focuses on the self-healing performance of early age microbial concrete,which can not reflect the influence of the harsh internal environment of concrete on the self-healing performance of microbial concrete.In this paper,the ability of microbial self-healing technology to repair cracks after curing in water for 150 days was studied.The effect of microbial self-healing technology repairing concrete cracks and the improvement of durability was quantitatively evaluated by multiple indicators such as crack repairing level(width and area),capillary water absorption.The results reveal that microbial self-healing technology can repair the crack width up to 507.5 ?m in a short time.The sealing rate is more than 90% when the crack width is less than 200 ?m.The cumulative water absorption is reduced by 26.3%,and the rate of average water absorption is decreased by about 50%.(3)The urea hydrolysis-diffusion transport model and calcium carbonate precipitation model based on microbe are established.The discrete form of urea hydrolysis-diffusion transport process in cracks was given by Galerkin method.The process of urea transport and crack closure in the crack are simulated by MATLAB.The accuracy and reliability of the model is verified by the prediction of the time to seal the crack width up to 300 ?m with the microbial self-healing concrete model.Through the sensitivity analysis of the prediction model,it's found that the distribution of initial urea,crack length and crack width are the main factors determining the accuracy of the prediction model.(4)Based on the mesoscopic lattice network model,the capillary water absorption and chloride diffusion process in microbial self-healing concrete are simulated.The numerical simulation of capillary water absorption and chloride diffusion in concrete is carried out when the repair rate is 0%,25%,50%,75%,100%.The results show that the repair level has a significant impact on the water and chloride transport in self-healing concrete.According to the improvement level of durability,the self-healing process can be divided into three stages.In the first stage,when the repair rate is in the range of 0~50%,there is no significant relationship between the mass transport process and the repair level,and the microbial self-healing process can not improve the durability of concrete.In the second stage,the repair rate is 50~75%,and the rate of mass transport in the cracks decrease rapidly with the improvement of the repair level,which indicates that the microbial self-healing technology can significantly improve the durability of concrete at the same time.In the third stage,the repair rate is more than 75%,which can be approximately regarded as reaching the limit of microbial self-healing technology.At this time,the repair rate has no significant effect onmass transport process.Through the sensitivity analysis of the mesoscopic microbial self-healing concrete model,the results show that the initial water content,crack length and coarse aggregate shape have no significant effect on the mass transport in self-healing concrete.The aggregate volume fraction and aggregate distribution have certain influence on the mass transport law in the microbe self-healing concrete. |
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