Application Of Hybrid Fiber In Metro Segment Concrete

In recent years,the method of adding one or more fibers into concrete has become one of the effective methods to improve the performance disadvantages of ordinary concrete,such as easy cracking and low tensile strength.The manufacturing material of metro shield segment is ordinary reinforced concrete.When hybrid fiber concrete is applied to metro shield segment,its mechanical properties and durability need to be studied.In view of the performance characteristics of steel fiber(SF),polyvinyl alcohol fiber(PVA)and glass fiber(GF)and the performance requirements of subway shield tunnel environment on concrete,the following studies are carried out:(1)According to the Standard for Test Methods of Fiber Reinforced Concrete CECS13:2009 and the Standard for Test Methods of Mechanical Properties of Ordinary Concrete(GB/T50081-2002),compressive,splitting tensile and flexural properties tests were carried out to study the effects of different types and amounts of fiber hybrid on the mechanical properties of hybrid fiber high-strength concrete.(2)In order to study the influence of the fiber content and the proportion of the first admixture on the durability of concrete,the carbonation resistance test and chloride ion permeability test were carried out.(3)The microstructure of hybrid fiber concrete was analyzed by XRD diffraction test and SEM test.The test results show that:(1)Hybrid fibers can improve the mechanical properties of concrete.Among the three mechanical properties tests(compressive test,splitting tensile test,and flexural test),hybrid fibers have the best effect on improving the splitting tensile strength;When the fiber content is 40kg/m3 of steel fiber,2kg/m3 of glass fiber and 1.8kg/m3 of polyvinyl alcohol fiber respectively,the hybrid fiber has the best effect on strengthening the mechanical properties of concrete,while excessive fiber content will produce negative hybrid effect and reduce the strength of concrete.In addition,the influence mechanism of different fiber content and proportion on the mechanical properties of concrete is analyzed.(2)Fiber can improve the durability of concrete.However,if the three fibers are mixed into concrete at an inappropriate amount or proportion,they cannot achieve a good mixing effect.Only by adding them to concrete at an appropriate proportion can the durability of concrete achieve the best.The three types of fibers form a disorderly and disorderly distributed fiber network within the concrete matrix,which jointly inhibits the generation of cracks and pores within the concrete structure,hinders the transmission of CO2 between the cracks and pore channels,thereby weakening the carbonation reaction within the matrix and enhancing the carbonation resistance of the concrete matrix.The amount and proportion of hybrid fibers have a significant impact on the carbonation resistance of concrete,but the impact on the chloride ion permeability of concrete is relatively small.The chloride ion permeability of all single fiber reinforced concrete and hybrid fiber reinforced concrete is evaluated using the NEL method.The low chloride ion diffusion coefficient indicates that the improvement effect of fibers on the chloride ion permeability of concrete is not significant.(3)After obtaining SEM spectra through scanning electron microscope tests,the microscopic details of the bonding interface between the fibers and the cement substrate were observed and analyzed.It was found that the three fibers did not fully adhere to the cement substrate,but there were certain gaps.The bonding interface between the three fibers and the substrate has a concentration of crystals such as Ca(OH)2,calcium sulphoaluminate hydrate,and ettringite.These crystals can also reduce the porosity of the matrix to a certain extent and improve the compactness.Thereby enhancing the mechanical properties and durability of concrete.The peaks of XRD spectra of hybrid fiber reinforced concrete are slightly lower than those of plain concrete.