| Ultra-high performance concrete is a new type of cement-based composite material made of cement,silica fume,sand,water,water reducing agent and fiber.The cement matrix itself is a typical brittle material,and the fiber is introduced to change its failure mode from brittle failure to plastic failure,in which the bridging between the fiber and the matrix plays a great role.Traditional UHPC mainly uses single steel fiber incorporation so that the bridging effect is single.In addition,the incorporation of fiber leads to a new fiber-matrix interface transition zone,fiber-matrix interface bonding performance of traditional UHPC is not ideal.Both factors will have a great influence on the tensile properties of the UHPC.Aimed at the above problems,in this paper,the tensile properties of UHPC were improved by mixing polyvinyl alcohol fiber(PVA fiber)with steel fiber and the surface modification of steel fiber,and its mechanism of action was further studied,and those achieve comprehensive improvement of UHPC mechanical properties.And the tensile properties of UHPC materials were simulated by ABAQUS finite element model.The main results of this paper are as follows:UHPC was prepared by replacing steel fiber with PVA fiber with different content.The flow properties and mechanical strength of UHPC blended with PVA fiber and steel fiber were tested respectively,and the tensile properties were tested by direct tensile test.At the same time,combined with the macro test results,the micro test technology was used to analyze its mechanism.The results show that:with the increase of PVA fiber replacement content,the compressive strength of UHPC decreases gradually;the flexural strength increases firstly and then decreases;the tensile properties increase firstly and then decrease.When the content of PVA fiber is 0.1%,the comprehensive performance of UHPC is the best.The two kinds of fibers cooperate to achieve the balance of strengthening and toughening.The surface of steel fiber was modified with silane coupling agent solution.The flow properties and mechanical strength of the modified steel fiber UHPC were tested,and the tensile properties were tested by direct tensile test.At the same time,combined with the macroscopic test results,the mechanism of silane coupling agent modification was revealed by means of microscopic test.The results show that: the compressive strength of UHPC increases firstly and then decreases with the increase of modification concentration,with the same modification concentration,the compressive strength of the pretreated is higher than that without pretreatment.After modification of steel fiber,the flexural strength of UHPC decreases,and the decrease in group NS3 is negligible.The tensile properties of UHPC show a tendency that low concentration modification is better than high concentration modification,and the pretreatment with sodium hydroxide is better than that without pretreatment.When the modification concentration is 3% after pretreatment with sodium hydroxide,the comprehensive performance of UHPC is the best.It is shown that the modification with silane coupling agent solution can comprehensively improve the performance of UHPC.The whole process of bond-slip between steel fiber and matrix is reproduced by numerical simulation,and the simulation results are in good agreement with the experimental results.On this basis,a 2D finite element model is established to simulate the direct tensile test of UHPC.The simulation results are basically in agreement with the experimental results,which verifies the validity of the model.At the same time,the whole tensile failure process of UHPC was reconstructed.Moreover,the 2D finite element model of UHPC material with arbitrary number,diameter and length of rectangular steel fibers randomly distributed in rectangular matrix of any size can be obtained.Finally,the 3D finite element model of UHPC material with arbitrary number,diameter and length of solid steel fibers randomly distributed in rectangular matrix of any size is realized. |
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