Study On Service Performance Of Hybrid Fiber Concrete In Metro Shield Tunnel

Fiber concrete is a new type of engineering material developed in recent years.The concrete can effectively prevent the occurrence of concrete cracks,give the concrete better durability,and effectively improve the mechanical properties of the concrete.The application of hybrid fiber concrete to urban subway projects requires research on its service performance.Service performance includes both safety performance and durability.In this paper,the characteristics of three kinds of fibers,cellulose fiber(CTF-960),polyvinyl alcohol fiber(PF-2000)and micro-rib fiber(VS-3000),and the demand for concrete performance of subway shield tunnel environment are as follows.Aspect research:1.According to the"Fiber Concrete Test Method Standard"for axial compression test and residual bending strength test,study different fiber types,different fiber content and different fiber mixing methods,concrete axial compressive strength and residual bending strength Impact.The test results show that for the concrete axial compression test,the CTF fiber has a significant positive influence,and the PF fiber significantly reduces the axial compressive strength.The effect of the VS fiber on the axial compressive strength is not obvious.For the residual bending strength test,the incorporation of CTF fiber and PF fiber can not improve the bending toughness of concrete,but CTF fiber can increase the initial cracking strength,and PF fiber can reduce the initial cracking strength.VS fiber can significantly improve the toughness of concrete,so that the concrete still has a certain bearing capacity after cracking.2.The experimental study on the carbonation resistance and chloride ion penetration resistance of hybrid fiber concrete with different fiber types,different fiber content and different fiber mixing modes.The test results show that for carbonization test,CTF and PF fiber can effectively reduce the carbonation depth of concrete at different ages.The effect of VS fiber is related to the amount of VS fiber.When the VS fiber content is 2.0 kg/m~3,there is no significant effect on the carbonization depth.When the VS fiber content is 3.5,4.5kg/m~3,the concrete carbonation depth is significantly reduced.When the CTF and PF fibers are double-doped,the fiber positive mixing effect occurs,and the carbonization depth is further reduced.For the electric flux test,the CTF fiber can effectively reduce the electric flux,and as the CTF fiber content increases,the electric flux decreases.With the increase of PF fiber content,the concrete flux increases first,then decreases and then increases.VS fibers generally reduce concrete flux.When mixed with CTF fibers and PF fibers,a positive fiber mixing effect occurs and the electric flux is further reduced.3.Based on the results of axial compression test and residual bending strength test,the safety of reinforced fiber concrete used in subway shield tunnel is studied.Referring to Model Code 2010,considering the contribution of VS fiber to the post-crack bearing capacity,using the residual bending strength obtained from the test,the tensile strength of the mixed fiber concrete with different mixing amount in the calculation of the normal section is obtained,and the reinforced fiber concrete is established.A calculation diagram of the limit state of the load carrying capacity of the segment and a corresponding bearing capacity formula.Combined with the engineering example,the maximum bearing capacity of the mixed fiber concrete segments with different bearing capacity is obtained.Refer to the Technical Specification for Fiber Reinforced Concrete Structure,introduce the influence coefficient of hybrid fiber on the crack width,and obtain the corresponding coefficient according to the residual bending strength test result.Finally,calculate the maximum crack width of the hybrid fiber concrete segment under normal service limit state..Based on the calculation of the combined maximum bearing capacity and maximum crack width,the safest blend of hybrid fiber concrete in the subway shield tunnel is VS4.5.4.Based on the results of concrete carbonation test and chloride ion penetration test,combined with reliability theory,the service life of hybrid fiber concrete segments under carbonization conditions and chloride salt environment was predicted.The carbonization test results were substituted into the carbonization depth model to calculate the carbon dioxide diffusion coefficient and the carbon dioxide absorption per unit volume of the mixed fiber concrete with different amounts.Based on the engineering practice and reliability theory,the service life under carbonization conditions is predicted.Using the results of the electric flux test,the chloride diffusion coefficient of the mixed fiber concrete with different contents was determined and substituted into the chloride ion diffusion model.Combined with the engineering practice and reliability theory,the service life in the chloride salt environment was predicted.According to the durability evaluation results of the integrated carbonization environment and the chloride salt environment,the service life of the reinforced fiber concrete segment is controlled by the chlorine salt environment,and the corresponding durability is VS4.5.