Mechanical Properties Of Cement-Based Materials With Surface Modified Steel Fiber

Steel fiber has high elastic modulus,high strength and good ductility.Adding steel fiber to UHPC can effectively improve its toughness.However,the bonding strength between steel fiber and matrix is weak,which is not enough to make full use of the strengthening and toughening of steel fiber.Therefore,this paper studies the methods to improve the interface performance and the influence of interface bonding performance on the macroscopic mechanical properties of UHPC.The main contents are as follows:Firstly,the influence of surface modification method,content of surface modified steel fiber and silica fume on the flexural performance of UHPC was studied.A prediction model of UHPC flexural strength for steel fiber content was established based on composite material theory.The mathematical model of UHPC bending load displacement curve is proposed,and the model of UHPC load-deflection curve in mathematical form was fitted well with the test curve.Based on the theory of composite materials,the prediction model of UHPC flexural strength was established.The relationship between the length-diameter ratio and the content of steel fiber is studied under uniaxial tension and bending of UHPC.The results show that the critical content of steel fiber to achieve strain hardening is inversely proportional to the fiber length-diameter ratioSecondly,the calculation formulas of bridging stress and theoretical crack spacing of steel fibers are derived considering the snubbing effect and matrix spalling effect.The formula for calculating theoretical crack spacing shows that when the fiber aspect ratio is determined,the theoretical crack spacing decreases with the increase of fiber content,while the influence of ?_f/?_mu on theoretical crack spacing increases with the decrease of steel fiber content,and when ?_f/?_mu?3.5,the influence on theoretical crack spacing decreases.The smaller the length-diameter ratio is,the faster the theoretical crack spacing decreases with the increase of ?_f/?_mu .When the length-diameter ratio is greater than 80,the influence of length-diameter ratio on crack spacing decreases.When the ratio of interfacial bond strength to matrix tensile strength is less than 3.5,increasing the length-diameter ratio can significantly reduce fracture spacing.When the ratio of interfacial bond strength to matrix tensile strength is greater than 4.5,interfacial strength and fiber length-diameter ratio have almost no influence on fracture spacing.At last,based on the single fiber pull out model,the stress field of steel fiber bridged and the matrix near the crack surface was simulated numerically,and the stress field distribution near the crack surface and the stress-displacement curve of UHPC bridged fiber pulled out were calculated.The pull-out strength,energy dissipation and stress transfer distance between the fiber and the matrix are also analyzed.