Experimental Study On Mechanical Performance Of Demountable Steel-UHPC Composite Structure

Steel-Ultra high performance concrete(UHPC)composite structure is a novel type of structure which is composed of steel member and UHPC member connected through shear connectors.Due to the excellent mechanical properties and durability,the steelconcrete composite structure could effectively improve the ultimate bearing capacity,crack width control ability,and the durability service life of the composite structures.The demountable steel-UHPC composite structures,in which,steel members and UHPC members are assembled by demountable shear connectors,have advantages of light weight,high strength,high ductility and high fatigue life and so on.Each component can be segmentally prefabricated and assembled on site,which is accommodate for standardized and industrial production.The components can be disassembled to facilitate the later maintenance and recycling of the component.This paper takes the demountable steel-UHPC composite structures as the research object.The cyclic tensile behavior of reinforced UHPC,the flexural behavior of UHPC beam reinforced with negative Poisson’s ratio steel rebar under monotonic/cyclic loading,the shear behavior of a demountable bolted connector in steel-UHPC lightweight composite structure,and the flexural behavior of demountable steel-UHPC composite slab under positive/negative bending moment are investigated by experimental and theoretical analysis.The main work of this thesis is as follows:(1)Cyclic tensile test and acoustic emission location technology are adopted to study the axial tensile behavior and damage evolution mechanism of reinforced high strain-hardening UHPC under cyclic loading.The effects of different reinforcement diameter and different target strain on cyclic tensile behavior of reinforced high strainhardening UHPC are analyzed.The results show that regardless of the level of target strain,the cumulative axial tensile strain and tension stiffening effect both decrease with the reinforcement diameter increases.In addition,the Kaiser effect occurs in reinforced high strain-hardening UHPC under cyclic tensile load obviously.And the stiffness of reinforced UHPC under damage state is controlled by the effective tensile length of steel fiber after cracking.Finally,based on the experimental data,the relationship between the degradation rate of the loading stiffness and the cumulative residual strain of reinforced UHPC under cyclic axial tension is fitted,which shows an exponential relationship.(2)Through the combination of negative Poisson’s ratio steel rebar(termed as NPR steel rebar)and high strain hardening UHPC,a type of reinforced UHPC beam which possesses both high bearing capacity and high ductility is proposed.It breaks the contradiction between high bearing capacity and high ductility of traditional reinforced concrete structural members.The effects of different reinforcement ratios and reinforcement types on the flexural behavior of reinforced UHPC beams are studied.The results show that the failure mode of UHPC beam reinforced with NPR steel rebar is distinctly different from that of the UHPC beam with conventional longitudinal reinforcement,which exhibits multiple(7-8)main cracks approximately uniformly distributed within the whole pure bending region,and the longitudinally horizontal splitting cracks along the whole pure bending region in the compression zone.Additionally,on the premise of the same reinforcement ratio,the flexural performance of UHPC beams reinforced with NPR steel rebar is far superior to that of UHPC beams with conventional steel rebar.Finally,the minimum reinforcement ratio of UHPC beam with conventional steel rebar is proposed and the crack clustering mechanism of UHPC beam reinforced with NPR steel rebar is clarified.(3)The flexural behavior of reinforced UHPC beams under cyclic loading is experimentally studied.The results show that the development of crack width and crack height of reinforced UHPC beam is effectively controlled due to the unique negative Poisson’s ratio effect of NPR steel rebar under plastic strain,which results in that stiffness degradation rate of reinforced UHPC beams with NPR steel rebar is lower than that of conventional reinforced UHPC beams.In addition,the energy dissipation capacity of UHPC beams with conventional steel rebars mainly comes from the plastic deformation of steel rebar after yielding.Whereas,the energy dissipation capacity of UHPC beam reinforced with NPR steel rebar mainly comes from the ultra-high tensile strength and uniform elongation of NPR steel rebar,which endows the beam with more proportion of elastic deformation.Finally,the analytic stiffness method based on the assumption of average curvature and plane section is proposed,and the strain inhomogeneity coefficient of NPR longitudinal reinforcement under monotonic and cyclic loading is both set as 0.2,which can effectively predict the deflection deformation of UHPC beam with NPR reinforcement in normal service stage,even prior to the yielding of NPR reinforcement.(4)Considering that the UHPC layer in the lightweight composite deck system,composed of steel and thin UHPC,is too thin to use conventional shear connectors,and in order to fulfill the rapid assembly and disassembly,replacement of prefabricated UHPC,a novel shear connector consisting of specified designed nut pre-embedded in thin UHPC layer and high strength bolt is proposed.Seven groups of push-out specimens are designed and conducted.The failure pattern and load-slip curve characteristics of novel shear connectors are analyzed.And the effects of different parameters on mechanical properties of the novel shear connector,such as ultimate slip,shear capacity,shear stiffness and so on,are discussed.Based on the test results,the load-slip model of high-strength bolted connectors is proposed,and the shear capacity design formula of demountable shear connectors is established.Based on the different mechanical properties of high strength bolt and negative Poisson’s ratio bolt,design recommendation on shear connectors for steel-UHPC lightweight composite members is put forward.(5)The effects of shear connector spacing and steel deck type on the failure mode,loaddeflection curve,ultimate bearing capacity,interface relative slip,crack development and distribution and other mechanical properties for the demountable steel-UHPC composite slab with the high-strength bolt under positive and negative bending moment loads are experimentally studied.The results show that under the positive bending moment,the failure mode of composite slab specimen using Q355 steel deck is that the high-strength bolt is shear off.However,the failure mode of composite slab specimen using high-strength steel slab is that part of the high-strength bolt is cut off,part of the embedded elongated nut with cushion is pulled out,and the UHPC deck collapses in a large area due to instability.The cooperative effect and flexural failure mechanism of steel-UHPC composite slab are further explored,and the bearing capacity calculation method of demountable steel-UHPC composite slab under negative bending moment load is established.