Experimental Study On Uhpc Short Columns Confined By High Strength Stirrups Under Axial Compressive Loadings

This study focused on the blank area of the axial compression performance of the stirrup-confined UHPC short column,and total of 72 steel-confined UHPC specimens were performed axial compression tests,including high strength stirrup s-confined UHPC and ordinary stirrups-confined UHPC.The axial deformation behavior and stirrup strain development were measured in the study.The influence of stirrup spacing,stirrup type and cover thickness on the failure mode,load carrying capacity,ductility and initial stiffness were investigated.A calculation method was proposed to predict the load carrying capacity of the stirrups-confined UHPC columns.The main results can be drawn,as follows:（1）The failure mode of stirrups-confined UHPC columns is different with the conventional reinforced concrete columns.Multiple cracks occurred on the surface of RPC tube,while almost no UHPC peeled off from the specimens even at peak load.Compared with the ordinary steel-confined UHPC specimens,the UHPC columns confined by the high strength stirrups exhibit relatively severe damage,especially for the specimen with high volumetric hoop ratio.（2）Compressive strength and ductility of the UHPC columns significantly increase with the increase of the volumetric hoop ratio.Generally,the high strength stirrups-confined UHPC columns show relatively higher strength and deformability compared with the ordinary stirrups-confined UHPC columns.The test results prove that UHPC cover can carry a considerable ratio of axial load under the peak load.So the load carrying capacity from the portion of unconfined UHPC section could not be ignored.Compared with the CFRP hoops,using steel hoops has a higher confining effect for UHPC.（3）UHPC cover has relatively obvious effect on the initial stiffness of the stirrupsconfined UHPC columns,and increasing thickness of the cover can increase the initial stiffness.However,the stirrup spacing and the stirrup type only exhibit slightly influence on the initial stiffness.（4）Existing models can be used for evaluating the load carrying capacity of the stirrups-confined UHPC columns.Strength models were proposed to predict the axial load carrying capacity of UHPC columns confined by high strength steel hoops and ordinary steel hoops,respectively.The models were also validated with experimental results.（5）Based on the existing constitutive model and test data,a special constitutive model was proposed for steel-confined UHPC column.The calculated curves by the proposed model were basically consistent with the experimental curves.