Experimental Study On The Mechanical Behavior Of Fiber Reinforcement Calcareous Sand Cemented By MICP Technology

Calcareous sand have the characteristics of high compressibility,high void ratio and tend to be fragile,which lead to poor mechanical properties and often have adverse effects in the construction of reefs island.In order to improve the mechanical properties of calcareous sand,a new method of strengthening calcareous sand based on microbial induced calcium carbonate precipitation(MICP)technology is proposed in this paper.The main research contents include:by studying the influence of different environmental factors on mineralization efficiency of MICP,mastering the suitable conditions of microbial induced calcium carbonate precipitation;designing a set of curing device for MICP with controllable temperature,and monitoring the process of MICP solidfying calcareous sand to obtain bacterial growth,urease activity and pH in environmental solution.The curing steps of calcareous sand cured by MICP were determined by the change trend of calcium carbonate production with time.The effect of the concentration of cementation solution on the mechanical strength of calcareous sand was studied,and the explanation was put forward according to the morphology,size and distribution characteristics of calcium carbonate crystal.In order to improve the brittle failure of calcareous sand treated by MICP,modification method of calcareous sand cured by polypropylene fiber reinforcement combined with MICP was explored.The influence of fiber content on the mechanical properties and failure modes of calcareous sand was studied through unconfined compression test and direct tension test.The effect of fiber content on the mechanical properties of calcareous sand cured by MICP was also studied by means of scanning electron microscopy and energy spectrum analysis.The coupling mechanism of reinforcement was discussed.On the basis of experimental study,the tensile failure model of MICP cured calcareous sand with different fiber content was also studied.The constitutive equation of tensile failure stress and strain was deduced by using damage theory,Weibull distribution function and linear equation,which made a breakthrough in theory.The thesis was financed by the project of National Natural Science Foundation of China(41772280)and the project of Jiangsu Natural Science Foundation(BK20170394),the tutor,Professor Tang Chaosheng.The main achievements of this paper are as follows:(1)The optimum culture time of Sporosarcina Pasteurii was 24 hours,and the optimum ambient temperature for MICP of Sporosacina Pasteurii is 30-34℃.The yield of calcium carbonate was higher in the range of pH 9.5-10.5,but lower in the acid condition of pH 6.5.CaCl2 can inhibit the activity of bacterial,and over 0.5 mol/L can inhibit the yield of calcium carbonate.(2)The surface of polypropylene fibers is conducive to bacterial colonization.Appropriate amount of fibers can promote the formation of calcium carbonate,but excessive fibers and generated calcium carbonate will fill the interstitial pore,hinder the continuous infiltration of cementation solution and the re-proliferation of bacteria in the pore,so as to reach the "bottleneck stage" of calcium carbonate rising and limit the mechanical properties of the samples.(3)The content of calcium carbonate in the sample of unconfined compressive test is relatively low(21.18%-27.26%)under two-round curing conditions.The content of calcium carbonate,unconfined compressive strength,residual strength,toughness and ductility of the samples increased with the increase of fiber content in the range of 0.00%-0.40%,but the compressive modulus decreased with the increase of fiber content.The cementation of calcium carbonate enhances the pull-out resistance of the fibers,while the fibers can inhibit the crack propagation.(4)The calcium carbonate content of direct tension specimen is higher(25.36%-32.83%)under two-round curing.The optimum content of calcium carbonate,tensile strength,tensile modulus and toughness is about 0.15%.The amount of fibre added has a nearly linear positive correlation with the residual strength of the sample.The failure of specimens with low fiber content(0.05%-0.15%)presents a two-stage failure model,and the residual strength decreases linearly with strain.The failure of high fiber content(0.20%-0.40%)presents a three-stage failure model.The residual strength increases linearly with strain at first and then decreases linearly.(5)The constitutive equation based on Kachanov damage theory,Lemaitr strain equivalent theory,Weibull distribution formula and the MICP-Fiber reinforcement tensile failure model can describe the tensile failure of specimens more accurately,and it has a high fitting degree for specimens less than 0.40%content.