Experimental Study On Concrete Crack Self-healing Performance Based On Reclaimed Aggregate Impregnated Microorganism

Concrete is a kind of building material with strong comprehensive performance and is widely used in construction engineering industry.However,cracks tend to occur in the service engineering after concrete forming,and small cracks may further develop into wider macroscopic cracks.Although most is to work with cracks in the process of concrete in service,but for concrete service process will be in contact with the water,the generation of cracks can provide channels for some aggressive ion infiltration,corrosive ion deeply affect concrete internal environment,durability and mechanical properties of concrete have a significant impact.Therefore,scholars began to study concrete with self-healing properties of cracks.Cracks self-healing concrete based on microbial mineralized deposition technology is to add microorganisms and corresponding nutrients in the process of concrete mixing.Once cracks occur in concrete and the internal environment of concrete changes,microorganisms begin to self-repair cracks.However,the complex environment inside concrete reduces the activity of microorganisms to a large extent,so scholars begin to study the use of porous lightweight materials as the carrier of microorganisms mixed into concrete to ensure the activity of microorganisms.At present,the carrier used by scholars is not compatible with concrete,costs are high,mechanical properties are poor,and the selected microbial type is single,and the mineralization robustness against environment is not strong,which limits the application of cracks self-healing concrete.Based on the research group’s previous research on microorganisms and reclaimed aggregate,this paper screened the mixed bacteria with mineralizing ability,and used reclaimed aggregate as microbial carrier to prepare concrete with self-healing performance of cracks,and conducted in-depth analysis of its self-healing mechanism of cracks.The main work and conclusions are as follows:(1)Screening(mixed bacteria)and culture(Bacillus coli,Bacillus pasteurii,mixed bacteria)of microorganisms,and the mineralized deposition efficiency of different microorganisms were simply tested to verify the feasibility of these three microorganisms used in self-healing concrete;Using reclaimed aggregate as carrier,the basic performance indexes of reclaimed aggregate as carrier were studied:water absorption,crushing index and apparent density.Microbial mineralized deposition technology was used to enhance the treatment of recycled aggregate,and the basic properties of the enhanced treatment were studied.The results showed that the mineralized deposition quality of B.barneri and b.coli was 1.1 times that of B.coli after 7 days.The water absorption rate of reclaimed coarse aggregate is 6.75%,the crushing index is 16.5%,and the water absorption rate of reclaimed fine aggregate is 10.85%,which can provide enough living space for microorganisms.After 14d mineralization enhancement,the apparent density of reclaimed aggregate reached 2642kg/m~3,increased by 1.73%,and the crushing index decreased by 29.6%,which can provide better protection for microorganisms and improve the mechanical and working performance of concrete.(2)The self-repair carrier was prepared by microorganisms embedded in unreinforced recycled aggregate,and the microscopic observation of concrete crack surface and continuous monitoring and quantification of crack width were used as characterization methods to investigate the effects of microbial types,carrier types and nutrient fixation methods on the self-repair performance of concrete cracks.It is verified that recycled aggregate as microbial carrier can make concrete have self-healing property of crack.The results show that the recycled aggregate,as a microbial carrier,can provide concrete with good crack self-healing ability.In addition to maintaining a high rate of complete fracture repair,the average fracture repair width of the mixed bacteria reached 0.35mm at 56d,while the average fracture repair width of Bacillus barneri and Bacillus coli was 0.32mm and 0.31mm.Reclaimed aggregate is more suitable as microbial carrier than ceramsite.When 50%regenerated fine aggregate was added,the average fracture repair width was increased from 0.35mm to 0.41mm by 17.14%.The average repair width of 56d cracks in the recycled concrete with fine aggregate and nutrients was 0.47mm,which was 34.28%higher than that of the recycled concrete with nutrients mixed in 0.35mm.(3)The compressive strength and slump were used to characterize the mechanical properties and working performance of reinforced concrete.The crack width was quantified and the water permeability at the crack was tested to characterize the self-healing performance of concrete.It is proved that the recycled aggregate can improve the performance of concrete.The test results show that the compressive strength of recycled concrete increases by 15%when the mixed bacteria mineralization is enhanced for 14 days.The slump increased by 20%after 7days of mineralization enhancement.The average repair widths of cracks of recycled concrete after 28d repair and curing were 0.28mm and 0.30mm,respectively,which were larger than that of Bacillus coli(0.25mm).The impermeability of recycled concrete specimens was significantly improved.After 28d and 56d of repair and curing,water seepage coefficient of all specimens was zero,that is,water seepage stopped.(4)Based on the research and characterization of the reinforcement of reclaimed aggregate and self-healing performance of cracks from a macro perspective,SEM was used to characterize the surface crystals of reclaimed aggregate after enhanced mineralization and the mineralized crystals generated by mineralization at cracks at the micro level.X-ray diffraction,energy spectrum analysis and thermogravimetric analysis were used to analyze the material composition level of the mineralized crystals on the surface of the reclaimed aggregate and the mineralized crystals generated in the cracks after mineralization enhancement.The experimental results show that microbial mineralization-enhanced reclaimed aggregate can encapsulate aggregate and improve its physical and mechanical properties.As microbial carrier,recycled aggregate can realize self-healing of concrete cracks.In addition,microbial mineralization enhanced the crystal on the surface of the reclaimed aggregate and the mineral crystal at the crack after repair and curing.Although the morphology was different,their material composition both contained calcite calcium carbonate with good compatibility and stable properties with concrete.