Experimental Study On Effect Of Size Effect On Shear Performance Of High Strength Reinforced UHPC Beams

Ultra-High Performance Concrete(hereinafter referred to as UHPC)is a new type of building material with superior mechanical properties and good durability.Ultra-high performance concrete and HRB500-grade high-strength steel work together to make full use of the excellent performance of both,which greatly promotes the engineering application of UHPC.Based on the National Natural Science Foundation of China(51368013),this paper studies the effect of size effect on the shear performance of high-strength reinforced UHPC beams through shear tests and theoretical analysis of five high-strength reinforced UHPC beams.The effect of size effect on the shear behavior of high-strength reinforced UHPC beams is proposed.The main research results are as follows:(1)The raw materials of UHPC and its mixing ratio are introduced.The relationship between the axial compressive strength and the cubic compressive strength is fitted through multiple sets of test values.The conversion coefficient is in line with the rule of ultra-high performance concrete conversion.The relationship between the compressive strength of the cylinder and the compressive strength of the cube,the relationship between the split tensile strength and the compressive strength of the cube and the relationship between the flexural strength and the compressive strength of the cube are also fitted through multiple sets of test values.The mechanical properties of UHPC are less affected by size effects than ordinary concrete.(2)The high-strength reinforced UHPC beams were subjected to shear tests through asymmetrical concentrated loading.Each test beam suffered shear and compression failure.The deflection change trend of each test beam was consistent with the bending moment diagram.The shear ductility of high-strength reinforced UHPC beams is superior to reinforced steel fiber high-strength concrete beams.The longitudinal reinforcement of each test beam did not yield,and the size effect affected the distribution of longitudinal reinforcement strain along the beam length.The change trend of the main tensile strain of the concrete in the middle of the shear span of each test beam was basically the same.When the steel fiber "bridge effect" exists,the stiffness of the test beam increases as the size increases.As the "bridge effect" of steel fibers subsides,the stiffness loss of large-scale test beams is obvious.(3)The ratio of cracking shear strength to ultimate shear strength is used to describe the occurrence time of oblique cracks.The size effect has little effect on restricting the development of oblique cracks.With the increase of the size of the test beam,the inclination angle of the main oblique cracks generally increases,and the test data can be well applied to the calculation method of the oblique crack dip angle of the Canadian Concrete Design Code.With the increase of the size of the test beam,the projected length and width of the main oblique cracks are increasing.With the increase of the size of the test beam,the cracking load of diagonal cracks increases substantially linearly,and the test data can better match the empirical formula of cracking shear strength.(4)Normative formulas at home and abroad cannot predict the shear capacity of high-strength reinforced UHPC beams.Based on the shear capacity calculation theory such as truss theory,plastic theory and limit equilibrium theory,taking into account the size effect and the role of steel fiber,the three recommended shear bearing capacity calculation formulas have better applicability to the shear strength of high-strength reinforced UHPC beams.Among them,the recommended calculation formula of shear capacity based on the limit equilibrium theory of inclined section can accurately predict the shear capacity of high-strength steel UHPC beams.