Mechanical Properties And Failure Characteristics Of Flexible Fiber Concrete Box Girder

Short cut flexible fibers with random dispersion have a crack-resistance effect on concrete,and it could improve mechanical properties and failure characteristics.Many researches have been done about the effect of flexible fibers on the mechanical properties of concrete,and important achievements have been made^[1-4].However,the application of flexible fiber on railway concrete box girder bridge is still rare.Therefore,it is necessary to study the mechanical properties and failure characteristics of concrete box girder with flexible fibers.The purpose of this study is described as follows:mastering the mechanical properties of flexible fiber concrete;observe the damage characteristics of fiber concrete by high speed video observation test;conducted a miniature box girder model test to reveal the mechanical properties of flexible fiber reinforced concrete box girder and quantify the reinforcing effect of fiber on the concrete box girder.Based on typical railway simply supported box girders,the research contents and results are as follows:（1）The existing theories of fiber reinforced concrete are analyzed.And the finite element model of a finite width plate type I crack of 400mm×200mm×1mm was simulated by ABAQUS.Under the same stress condition,the stress intensity factor and strain energy release rate in the presence of fiber and no fiber were analyzed to verify whether it was consistent with the theoretical value.The conclusions are as follows:When crack passes through flexible fiber,the flexible fiber can effectively reduce the total stress intensity factor at the front of the crack,thus inhibiting the crack growth.The numerical results showed that the strain energy release rate is only 79%when the fiber is without fiber,which indicates that the fiber can effectively reduce the energy for the increment of the new crack surface,thus effectively reducing the generation and expansion of the crack.（2）The change of mechanical properties of C55 concrete mixed with different kinds of flexible fibers at the same volume and different amounts of the same fiber was studied,and a high-speed camera was used to record the failure process.The conclusions are as follows:the compressive strength of concrete was limited by flexible fiber,but it keeps the original shape rather than completely fragmented when the concrete was subjected to compression,and the flexural fiber can effectively improve the flexural and splitting strength of concrete,and polypropylene fiber was more conducive to the improvement of flexural strength of concrete than the polyvinyl alcohol and basalt fiber.（3）Based on the similarity principle,4 pieces of C55 polypropylene fiber reinforced concrete box girder scale model was made.One box girder was the plain concrete used as a contrast and the other three girders are blended with polypropylene fiber according to the volume of concrete volume 0.1%,0.2%and 0.3%.After one month of maintenance,the box girder was subjected to force loading to collect data such as span strain and deflection of the box girder.The conclusions are as follows:The incorporation of polypropylene fiber can improve the overall stiffness of reinforced concrete box girder,especially in the high load condition,which can greatly improve the ability of the box girder to resist deflection.Under the same loading,the transverse strain value across the midsole increased with the increase of fiber content,and it was particularly evident during the elasto-plastic stage,especially near the destruction.It showed that the flexible fiber can effectively improve the plastic deformation of the concrete box girder.（4）Based on the scale model test of box girder,the finite element model of reinforced concrete scaled box girder was established by Midas/FEA,and the nonlinear analysis was done according to the total strain principle.The conclusions are as follows:（1）Under the same loading conditions,the deflection at the midspan is reduced after the flexible fiber are incorporated.（2）The incorporation of flexible fibers can effectively increase the load-bearing capacity of the reinforced concrete box girder;（3）The incorporation of fibers can increase the longitudinal stiffness of the box girder and Lateral stiffness;（4）incorporation of fibers can reduce the deflection of reinforced concrete box girders.The research results in this paper are of theoretical significance for the research on the mechanism of improving the durability of concrete bridges with high performance materials,and it is beneficial to apply the flexible fiber to concrete box girder and has application value.