Research On Mechanical Performance Of Ultra-high Performance Concrete

Ultra-High Performance Concrete(UHPC)is one of the most researched new materials in the field of civil engineering nowadays.There are already some practical engineering applications of UHPC in the world,but no unified norms have been formed in China.One of the reasons is that the basic mechanical research on UHPC is still insufficient,and theoretical research is relatively lagging behind.In order to further understand the mechanical properties of UHPC,and provide a reference for engineering application.This paper focuses on the mix design,compressive properties,tensile properties and bond properties of UHPC,both experimental and theoretical analysis were conducted.The main research results are as follows:(1)Preparation technology and mix ratio study of UHPC.Firstly,by comparing the different mix ratio calculation models,considering the characteristics of the local raw materials to determine the most suitable grading calculation model of quartz sand;then the Weatman and Hugill algorithm was used to determine the relative amount of cement and silica fume;finally according to a series single variable trial tests,A group of best mix ratios was seletaed considering workability and cubic compressive strength.(2)Uniaxial compressive performance study of UHPC.In order to investigate the effect of fiber content on the failure mode,axial compressive strength,deformation capacity,elastic modulus and Poisson's ratio of the UHPC specimens,6 groups of UHPC prism specimens were tested.The influence of fiber volume content on the compressive behavior of UHPC specimens was analyzed based on the test results and finite element analysis results;compressive damage constitutive equations of UHPC were established considering the damage threshold.The constitutive equations are in consistent with the experimental curves.(3)Study on tensile mechanical properties of UHPC.Firstly,in order to study the effects of fiber content,fiber length,fiber single-mixing and complex doping on the failure process,tensile strength and deformability of the UHPC tensile specimens,9UHPC uniaxial tensile tests were carried out.Then 5 four-point flexural tests were carried out to explore the influence of fiber content on the flexural properties of UHPC.The theoretical calculation formula between tensile strength and flexural strength was established based on the constitute equations,equilibrium equations and deformation coordination relationships.After this,the theoretical calculation formula of uniaxial tensile strength of UHPC was established based on meso-mechanics.Finally,the uniaxial tension damage constitutive equations were established based on experimental results.The theoretical curves agree well with the experimental curves.(4)Study on the bond-slip performance of deformed steel bars embedded in UHPC.Firstly,the effects of fiber content,steel bar diameter,embeddment length,concrete cover thickness and steel bar grade on the failure mode,bond strength and slip of the specimens were studied through 26 groups of pull-out tests.The expression of the bond strength considering various factors was established based on statistical regression method.Then,according to the thick-walled cylinder theories and elastic mechanics,the theoretical calculation formula of the bond strength between steel bar and UHPC is established.Then the bond stress-slip constitutive equations were founded based on the test results,which in consistent with the experimental curve.Finally,the central point method,the checkpoint method and the canonical method were used to determine the anchorage length of the deformed steel bar embedded in UHPC.The recommended anchorage length values were given based on the anchor reliability theory considering the reliability.