Experimental Study On Axial Compressive Performance Of UHPC-NSC Circular Section Composite Short Column

Ultra-high performance concrete（UHPC）is a high-performance cement-based material with ultra-high strength and excellent durability,which can effectively prevent the piers column from various physical and chemical corrosion and early cracking and other durability problems.At the same time,the combined structure of UHPC and NSC can reduce the cost and balance the mechanical properties.However,there are few researches on UHPC-NSC composite short columns constrained by stirrups.Therefore,12 short columns were produced in this paper,including 1 UHPC,10 UHPC-NSC composite short columns and 1 RC short column.Mainly analyzed the axial compression performance of UHPC-NSC composite short columns,and proposed a UHPC-NSC composite short column stress-strain theoretical model according to its failure mode and restraint mechanism,which provides a basis for the application of UHPC and its composite structure.The main research content includes:（1）Axial compression test of UHPC-NSC composite short columns with circular section is carried out.The failure modes and stress-strain curves of the short columns under different UHPC thickness,stirrup spacing,and pouring methods were measured,and the effects of each parameter on the mechanical properties of composite short columns were studied;（2）The restraint mechanism of the UHPC-NSC circular cross-section combined short column is theoretically deduced,and the restraint effect of stirrups and UHPC on the combined short column is considered,and the constraint stress calculation formula of the UHPC-NSC combined short column is obtained;（3）By introducing effective constraint indicators of dimensionless coefficients,the theoretical formulas for peak stress,peak strain and axial bearing capacity of UHPC-NSC circular cross-section composite short columns are established;（4）The existing stress-strain curve models of confined concrete are compared and studied,and a calculation model suitable for the stress-strain relationship of UHPC-NSC circular cross-section composite short columns is proposed.The test results show that the UHPC shell of the UHPC-NSC composite short column is beneficial to improve the ultimate bearing capacity,ductility and crack resistance of the short column.Among them,the peak strength of the composite short column is almost proportional to the thickness of the UHPC;when the UHPC thickness and the stirrup ratio are the same,the composite short column with the NSC core first and then the UHPC shell has a higher bearing capacity.Different form of traditional RC columns,UHPC-NSC composite short columns will produce a unique stress drop after reaching the peak stress.The new trilinear stress-strain model proposed based on previous research and the characteristics of the UHPC-NSC combined short columns stress-strain curve has a certain accuracy,and its calculation value is in good agreement with the experimental value.