Study On Composite Restraint Performance Of Stirrup And UHTCC Under Axial Compression

Ultra-high toughness cementitious composites(UHTCC)is a new material with cement(as the matrix)and PVA fiber(as the toughening material).When the fiber volume content is less than 2.5%,it can show the strain hardening characteristics stably.It has good tensile toughness,crack control ability and energy absorption capacity,which can greatly reduce the stirrup ratio and reduce the difficulty of construction.Therefore,in order to give full play to the respective advantages of UHTCC materials and stirrups,this paper combines the two to study the composite constraint effect of stirrups and UHTCC.The specific work and main conclusions are as follows:Firstly,the axial compression test of UHTCC stub columns confined by stirrups is carried out.The influence of stirrup spacing,stirrup strength,longitudinal reinforcement ratio and section size on the compressive mechanical behavior have been analyzed.The results show that according to the constraint level and specimen size under axial compression,the test specimen will have shear failure of oblique section,wedge compression failure and vertical bar compression failure.The smaller the stirrup spacing,the higher the stirrup strength,the better the confinement effect of concrete in the core area,and effectively improve the toughness,ductility and energy absorption capacity of the specimen after cracking.Based on the existing research results,the prediction formulas of peak stress,peak strain and stress-strain curves of UHTCC confined by stirrups are given.The predicted curves are in good agreement with the experimental curves.Secondly,the axial compression test of RUHTCC-concrete composite column is carried out.The main variables of the test are stirrup spacing,thickness of UHTCC layer and the type of fiber.The effects of stirrup spacing,outsourcing UHTCC layer thickness and fiber type on the compressive mechanical properties are studied and analyzed.The test results show that the smaller the stirrup spacing is,the greater the thickness of the outsourcing UHTCC layer is,and the ductility,bearing capacity and circumferential deformation capacity of the composite column are improved.The incorporation of PVA fibers improves the ductility and energy dissipation capacity of the composite columns;copper-plated steel fiber can greatly improve the compressive bearing capacity of composite columns.During the compression process of composite columns,the outer layer UHTCC is in biaxial tension-compression state,and the concrete in the core area will weaken the constraint of the outer layer when it is damaged.In order to ensure the constraint effect and energy absorption capacity,the outsourcing layer needs to have a certain thickness.