| Strain Hardening Cementitious Composites(SHCC)is a kind of fiber reinforced composite materials with the typical strain hardening property and the characteristics of multiple cracks.The cracked cement-based materials are easy to suffer from the ingress of aggressive ions and further induce the corrosion of steel bars,resulting in the serious durability problems of reinforced concrete structures.Cracks can be healed by bacteria mineralization,it can heal the cracks by the self-sustained repair process,where the repaired products have good compatibility with cement matrix.However,most of current studies on the crack healing property of SHCC are focused on the physical and chemical methods.The studies on the application of biological technology in the healing of SHCC are less.Particularly,the influential mechanisms of the optimum substrate concentration and its ratio as well as the source of calcium ion in bacteria mineralization reactions are not sufficiently studied.Based on this,the effects of mixing ratio of bacteria and cementing liquid,types of calcium sources and calcium ion concentration on the production and its microscopic characteristics for bacteria induced calcite precipitation(MICP)would be studied.Capillary water absorption and crack observation were used to investigate the effect of surface coating methods(immersion,brushing and spraying)and the mixed bacteria liquid or self-healing particles methods on the crack healing performance of SHCC.The main work and obtained conclusions are shown as follows:(1)The effect of four kinds of mixing ratio of bacteria and cementing liquid(1:0.5,1:1,1:1.5,1:2),three sources of calcium ions(calcium chloride,calcium acetate,calcium nitrate)and four concentrations of calcium ion(0.5 mol/L,0.7 mol/L,1 mol/L,1.5 mol/L)on the production of MICP was studied.The influential mechanism of different factors on MICP was analyzed by the microscopic test of mineralized products.The increase of cementing liquid proportion in the mixing ratio and the concentration of calcium ion improves the production of calcium carbonate generated by the mineralization reaction.However,the overlarge proportion of cementing liquid causes this phenonmen that the bacteria can not fully utilize the mineralized substrate.Similarly,the higher concentration of calcium ion affects the urease activity of bacteria,leading to the decrease of calcium carbonate production.This study could find that the optimum mixing ratio of bacteria liquid and cementing liquid is between 1:1～1:1.5,and the suitable concentration of calcium ion is between 0.7 mol/L～1.5 mol/L,while there is no significant difference in the amount of calcium carbonate produced by the mineralization of the three calcium ion sources and a great difference in the microstructure.The morphology of calcium chloride product is independent loose granular composed of two calcium carbonate mixed crystal types,calcite and vaterite.Calcium acetate and calcium nitrate are clumped particles composed of a single vaterite calcium carbonate crystal,and the effect of bacteria on nucleation of calcium carbonate was confirmed by element distribution analysis.(2)Three surface coating tests of cracked SHCC including immersion,brushing and spraying methods were conducted.Effect of surface coating method on the healing efficiency of SHCC cracks was contrastively analyzed,and the healing products were observed on microscale.According to the results of capillary water absorption and crack observation,it is found that brushing and spraying coating can achieve 100% area healing,while the highest healing rate of immersion coating is less than 80%.Three methods can all achieve different degrees of crack healing and decrease the amount of water absorption.Calcium carbonate sinks in immersion coating due to gravity,which can not realize effective accumulation at the crack location causing poor healing effect.Brushing method creates the superior mineralization reaction conditions on the surface of specimens.The products are deposited evenly in the crack by brushing method and less materials are consumed,resulting in the better healing efficiency of cracks.For spraying method,bacteria and cementing liquid are easy to be lost on the specimen surface,inadequate mixing of bacteria and cementing liquid results in uneven distribution of products,causing the greater discreteness of healing effect.(3)The mechanical and healing performance of SHCC with internally incorporated bacteria solution and self-healing particles were investigated.Based on capillary water absorption performance and area healing rate,the healing effects of SHCC cracks with two kinds of internally incorporated bacterial methods were compared and analyzed.The results show that the addition of bacteria solution has a negative effect on the strength and changes the interface bond performance between the fibers and matrix,improving the ductility and deformation.Self-healing particles can effectively protect bacteria.After 28 days of immersion in simulated pore solution,the self-healing bacteria particles with protected coating is 670% compared to unprotected group.The addition of proper amount of self-healing particles can improve the compressive strength,flexural strength and deformation properties of SHCC.The white healing products accumulated in the cracks caused 100% area healing of some cracks.Based on the results of capillary water absorption and area healing rate,it was found that the healing effect of adding bacteria solution was relatively poor,while the healing ability of internally incorporated bacteria self-healing particles was more than 10 times that of the control group.The complete healing could be achieved and the capillary water absorption decreased by up to 44.09%after the healing of 7 days. |
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