The Tensile Properties Of Ultra-high Performance Concrete Under Monotonic And Cyclic Loading

Ultra high performance concrete(UHPC)is a new type of concrete material,which has attracted the attention of the engineering field because of its excellent durability and mechanical deformation.An appropriate amount of steel fiber in UHPC can change the material from brittle failure to ductile failure.Therefore,steel fiber is the main factor affecting the tensile performance of UHPC.Based on this background,this article live in National Natural Science Foundation of China for project\ "Research on seismic performance and design method of active powder concrete compression members\" as the background with studing the axial tensile,splitting tensile and flexural tensile properties of UHPC by taking the content and shape of steel fiber contained in UHPC as experimental parameters,expecting to provide basis for performance evaluation and analysis of UHPC materials and structures.The main research contents completed are as follows:(1)Study on the influence of steel fiber content and shape on compressive performance of UHPC.UHPC materials with different steel fibers and strength classes of 150 MPa were prepared.Based on the compression performance test results of cube and prism specimens,the influence of steel fiber content and shape on the compressive strength and elastic modulus of UHPC was determined.The results show that: the compressive strength of UHPC material increases with the increase of steel fiber content.Compared with the matrix strength without steel fiber content,when the content of straight steel fiber is 1%,2% and 3%,the cube strength increases by 5.3%,14.8% and 23%,and the prism strength increases by 20%,22% and 37%,respectively.When the fiber volume content is 2%,the cube strength and prism strength of the end-hooked fiber specimens increase by 8.6% and 1.7% respectively compared with that of the straight fiber specimens.The fiber content and shape had little effect on the elastic modulus of UHPC.(2)Study on the influence of steel fiber content and shape on axial tensile performance of UHPC under monotonic loading.Based on axial tensile performance test results of dog bone specimens under monotonic loading,the influence of steel fiber content and shape on failure pattern,stress-strain curve characteristics,ductility,energy dissipation capacity and tension cracks of UHPC was determined.The stress-strain curve equation and stress-crack width curve equation of UHPC undermonotonic loading were established.The results show that: when the steel fiber volume content is 1%,the axial tension of UHPC showed the strain softening characteristic of single-crack failure,and when the steel fiber volume content is no less than 2%,the strain strengthening characteristic of multiple-crack failure was shown.The whole tensile stress-strain curve of UHPC under monotonic loading,including the unloading curve,presents linear evolution between the characteristic points such as the initial crack point,the peak point,the limit point and the unloading point.The elastic tension modulus is close to the elastic compression modulus.The plastic energy dissipation capacity of the material is basically provided by the plastic deformation of the fiber in the matrix.End-hooked fiber only has significant influence on the peak point strain,plastic energy dissipation capacity and crack spacing,but has little influence on other properties.(3)Study on the influence of steel fiber content and shape on axial tensile performance of UHPC under cyclic loading.Based on axial tensile performance test results of dog bone specimens under cyclic loading,the influence of steel fiber content and shape on failure pattern,stress-strain curve characteristics,unloading modulus and reloading modulus.The stress-strain envelope curve equation,unloading curve equation and reloading curve equation of UHPC under cyclic loading were established.The results show that: there is no significant difference in the failure pattern of UHPC under monotonic and repeated loading.With the increase of fiber content,the stress-strain envelope curve and the hysteresis loop of loading and unloading gradually become full under cyclic loading.With the same fiber content,the envelope curve and the hysteresis loop of the specimens containing end-hooked fiber were significantly more plump than that of the specimens containing straight fiber.The stress-strain curve of ultra-high performance concrete under cyclic loading is basically the same as that under monotonic loading.The unloading modulus and reloading modulus decrease gradually with the increase of loading and unloading times,and the decline rate decreases gradually with the increase of strain,and finally tends to be stable.The values of unloading modulus and reloading modulus were determined based on the test results.(4)Study on the influence of steel fiber content and shape on splitting and flexural tensile performance of UHPC.Based on the splitting tensile performance of cube specimens and flexural tensile performance of prism specimens,the influence of steel fiber content and shape on splitting tensile strength,load-displacement curve,flexural energy dissipation capacity and flexural toughness as well as the relationshipbetween axial tensile strength,splitting tensile strength and flexural tensile strength was clarified.The results show that: axial,splitting and flexural tensile strength increased linearly with the increase of steel fiber content,and flexural tensile strength is the most sensitive to it.The ratio of axial tensile strength to flexural and splitting tensile strength decreases gradually.The energy dissipation capacity and flexural toughness of flexural specimens increase with the increase of the steel fiber content,and end-hooked fiber has better effect on UHPC energy dissipation and flexural toughness than straight fiber.