Study On Axial Compression Behavior And Compressive Constitutive Relation Of ECC Cylinder Restrained By Stirrups

Ultra-high toughness concrete(also known as engineering cement-based composite material,ECC for short)as a kind of ultra-high performance cement-based composite material,has excellent toughness and crack control ability.Based on the axial compression test of stirrup-restrained ECC cylinder,this paper analyses the stress characteristics and failure modes of components,studies the mechanism of internal crack development and transmission,and restraint mechanism,establishes the stress-strain curve formula of stirrup-restrained ECC cylinder,and provides theoretical basis for improving the research system of ECC.The main research contents and results are as follows:(1)ECC material which satisfies both compressive strength and tensile toughness is prepared by optimizing the composition of ultra-high toughness concrete under the condition of structural performance requirements.(2)The stress-strain curves of different specimens are obtained by the axial compression test of the confined ECC column with stirrups.The failure characteristics and failure modes of the confined ECC short column are summarized.The influence of stirrup spacing,stirrup strength and ECC strength on the confined effect of the column is analyzed.The results show that there is no ECC spalling when the specimens are destroyed,and the integrity of the specimens is good.When a certain confinement force is satisfied,the descending section of the curve is very gentle,showing a longer platform section,and the compressive strain of the specimen can reach more than 7%.It is noteworthy that when the first stirrup is pulled off,a small section of the curve falls suddenly and then remains in a horizontal state and continues to bear.(3)The stress state of confined concrete and the restraint mechanism of stirrups are analyzed.The linear relationship between peak strength,peak strain and effective confinement index of stirrup confined ECC cylinder is obtained by regression analysis.Based on the analysis of restraint mechanism,the formula of bearingcapacity of stirrup restrained ECC cylinder under axial compression is deduced.(4)Based on the analysis of the existing constitutive model of confined concrete and the experimental results,a complete stress-strain curve model suitable for stirrup confined ECC cylinder is proposed by means of mathematical statistics and theoretical research.The model can be divided into two cases: platform segment and non-platform segment.The form of the model is rational fraction with continuous ascending and descending segments.