Research On The Post-cracking Performance Of UHPC With Steel Fiber

Ultra_High Performance Concrete(UHPC)is a new type of cement-based composite material with ultra-high strength and toughness.It is mainly obtained by mixing ultra-fine cementitious material and superplasticizer.It has low water-cement ratio and dense pore structure,its tensile strength and toughness is much higher than the ordinary concrete,it is always used for the high requirements of the tensile bearing capacity of the structure.It can enhance the structural stiffness and the durability of the structure,and the fatigue life of the structure will be improved.As the UHPC matrix still has high brittleness,it is rely on the incorporation of steel fibers for the tensile strength and toughness of UHPC materials,so steel fibers has important influence on the toughness and tensile strength of UHPC materials.In this paper,two different types of steel fibers were selected,and the influence of different length to diameter ratio and fiber content on the mechanical properties and crack performance of UHPC was investigated.The influence of different steel fibers on the flexural toughness of UHPC was analyzed by the load-deflection curve.The stress-crack width constitutive relationship of UHPC was analyzed by back analysis of load-crack mouth opening displacement curves.The effects of different steel fibers on the influence of stress-crack width relationship of UHPC was investigated.Based on the test results,the following conclusions are drawn:(1)The type of steel fiber has some influence on the flowability and mechanical properties of UHPC.The results show that the flowability of the hooked fiber reinforced concrete is lower than the straight fiber reinforced concrete,but the compressive strength and bending toughness of the hooked fiber reinforced concrete is better than the straight fiber reinforced concrete.And the improvement of the tensile strength of the hooked fiber is better than that of the straight fiber,and the stress drop curve of the hooked fiber reinforced concrete is relatively gentle during the stress softening stage,while the straight fiber is steep.(2)The fiber content has a significant effect on the flowability and mechanical properties and post-cracking performance of UHPC.With the increase of fiber content,the flowability of UHPC decreases significantly,but the compressive strength and flexural strength and bending toughness of the UHPC is also improved,but the increase of the strength is slowly reduced,considering the material properties and economic factors,There is an optimum content of steel fiber for improving the post-cracking stress of UHPC.(3)The length to diameter ratio of fiber has little effect on the flowability and strength of UHPC at low content of fiber.However,in the case of higher fiber content,the flowability of the longer fiber is worse,and its compressive strength and flexural strength is also higher,but the effect of different length to diameter ratio of fiber on the improvement of compressive strength of UHPC is not obvious.The ratio of length to diameter has a direct effect on the post-cracking performance of UHPC.With the increase of the length to diameter ratio of fiber,the post-cracking stress of UHPC is also increased,and the toughness of UHPC is improved obviously.However,in the case of higher fiber content,the post-cracking stress of the longer fiber reinforced concrete is decreased,it should be appropriate to choose the appropriate length of steel fiber.