Study On Stress-strain Relation Of Ultra-high Performance Concrete With Coarse Aggregate Under Uniaxial Compression

Ultra-high Performance Concrete(UHPC),as a new cement-based composite material with super-high strength,high toughness and excellent durability,has attracted great attention from civil engineering circles at home and abroad since its advent,and is the development direction of new concrete materials in the 21st century.Previous studies have shown that incorporating appropriate types and proportions of coarse aggregates into UHPC can effectively reduce the amount of cementitious materials used in the preparation of UHPC,reduce the self-shrinkage rate of UHPC,improve the mechanical properties of UHPC,improve its working efficiency and save production costs,thus promoting the engineering application of UHPC.At present,the working mechanism and mechanical properties of coarse Aggregate Ultra-High Performance Concrete(CA-UHPC)under axial compression are not clear,and the stress-strain relationship under axial compression is not mature enough.In view of this,this paper studies the stress-strain relationship of CA-UHPC under axial compression based on the key project of National Natural Science Foundation of China "Research on mechanical properties and design calculation theory of super-high performance concrete filled steel tube under complex action"(51738011).The main work and results are as follows:(1)Considering the three factors of coarse aggregate content,coarse aggregate type and steel fiber volume,63 cubic and 51 prism specimens were designed and manufactured.The fluidity of CA-UHPC was analyzed during the preparation process.The whole process of force and failure mode of CA-UHPC were investigated through the axial compression test,and the actual force was measured according to the test results.In the whole process,the stress state of CA-UHPC in each stage is analyzed,and the influence of coarse aggregate and steel fiber on the axial compression performance of CA-UHPC is expounded in depth.The results show that the compressive strength and toughness of CA-UHPC increase with the increase of the content of coarse aggregate and steel fiber,and are positively correlated with the strength and toughness of coarse aggregate itself.(2)Based on the measured compressive strength,peak strain,modulus of elasticity and stress-strain curves,the experimental results are compared and analyzed.Through data analysis and calculation,the effects of coarse aggregate content,types of coarse aggregate and volume fraction of steel fiber on the basic mechanical properties of CA-UHPC are studied.The calculation method of basic mechanical properties of high strength concrete under compression is also presented.Based on the fitting and analysis of the test data,the formulas for calculating the peak strain and modulus of elasticity of CA-UHPC are established,and the formulas for calculating the peak strain and modulus of elasticity of CA-UHPC are proposed.(3)Based on the analysis of the basic mechanical properties of CA-UHPC,the basic mathematical characteristics of the stress-strain complete curve of CA-UHPC under axial compression are studied.The overall shape and mathematical characteristics of the curve are similar to those of ordinary UHPC.The slope of the ascending section decreases monotonously,but the change is small and increases linearly;the descending section has a turning point and a maximum curvature point,and the descent is steep.Based on the mathematical characteristics of curves,the mathematical expressions of stress-strain curves of CA-UHPC under axial compression are established by fitting and analyzing the existing expressions of stress-strain curves at home and abroad.The results are in good agreement with the experimental data and can be used for reference in engineering design calculation and analysis.Finally,on the basis of summarizing the analysis results of the full text,the prospect of the follow-up research on this topic is put forward.