Experimental Reaserch Of Uniaxial Behavior And Constitutive Model Of Stirrup Confined Ultra-High Performance Concrete

Ultra-high Performance Concrete(UHPC)is a new type of cement matrix composite material with High compressive strength,High elastic modulus,High toughness,High durability,tensile strain hardening and other mechanical properties that are better than ordinary Concrete under normal temperature or High temperature curing conditions.At present,there are few studies on the stress-strain curve relationship of stirrup-confined UHPC short columns at home and abroad.Many constitutive models of confined concrete proposed are difficult to be applied to UHPC components,and the constitutive relationship of components is the basis for further study and discussion.In this paper,the mechanical behavior of stirrup-confined coarse aggregate UHPC short column under axial compression was studied.The main work and conclusions are as follows:The mixture ratio test of coarse aggregate UHPC material and various mechanical properties test were completed,which provided data support for the research on the relationship between stirrup confined UHPC short column axial compressive stress-strain curve.The results show that the better performance of fresh concrete can be obtained when the water-binder ratio is controlled at 0.143.Due to the small effect of hoop effect of coarse aggregate UHPC,the ratio of compressive strength of prism to compressive strength of cube is obviously higher than that of ordinary concrete,and various mechanical properties are much better than that of ordinary concrete.The uniaxial tensile test of UHPC shows that the initial crack strength,post-crack strength and energy consumption of UHPC can be effectively increased with the increase of steel fiber content.The introduction of coarse aggregate can improve the compressive strength and elastic modulus,but it will weaken the dispersion ability of steel fiber and reduce the flexural strength and tensile strength of UHPC.Through the whole process test of 16 stirrup confined coarse aggregate UHPC short column axial compression loading,it was found that the deformation of the specimen was stable and slow at the initial stage of loading.When the critical peak load was reached,the steel fiber was pulled out and the internal circumferential constraint stress was released,the stirrup was broken and the specimen was brittle failure.The elastic section length exceeds 0.95 fc,and its stress-strain curve presents the form of a bifold line.Different from ordinary reinforced concrete,the stress-strain curve of the stirrup confined UHPC short column does not decrease significantly at low stirrup ratio,and the ultimate strain is equal to the peak strain.When the stirrup ratio is greater than or equal to 4%,an obvious yield platform section appears in the short column,and the ductility coefficient reaches 1.16.The rise section of reinforced concrete constitutive model widely used at home and abroad usually adopts the form of quadratic parabola,so the peak stress of coarse aggregate UHPC is generally underestimated,and its peak strain is overestimated,which makes this kind of constitutive model difficult to be applied to the stirrup confined UHPC axial compression short column.Combined with the loading test results of short column,the constraint mechanism of stirrup confined UHPC short column is discussed,which is combined by internal constraint and external constraint.The inner restraint is the circumferential restraint caused by the bridging action of steel fiber.The external constraint is the hoop constraint.When the stirrup ratio is low,the internal constraint is dominant.When the stirrup is close to failure,the internal constraint fails,and the stirrup is unable to bear the hoop constraint released by the internal constraint and thus ruptures,resulting in brittle failure.When the stirrup ratio is high,the stirrup can bear the energy released by the failure of internal constraint,and the external constraint makes the specimen show certain plastic performance.In this thesis,the calculation method of circumferential constraint stress is derived,and the formula for calculating the peak stress and peak strain of stirrup confined UHPC short column is given,and a theoretical constitutive model of uniaxial compression is constructed.By introducing the ratio of internal and external constraining stress and the ratio of internal and external constraining strain energy to compare their constraining effects,the results show that under the condition of satisfying the conventional reinforcement rate in engineering,the stirrup confined UHPC short column is dominated by internal constraints,and the external constraints can be ignored.At the same time,the minimum stirrup ratio under three conditions is proposed for different engineering requirements.Based on the PQ-Fiber fiber model combined with the axial compression constitutive model of the coarse aggregate UHPC short column proposed in this paper,the ABAQUS finite element three-span continuous beam bridge seismic model is established.The NC+UHPC composite pier can effectively raise the pier.The seismic performance of the column can significantly reduce the cross-sectional size and reduce the structural weight when compared to the conventional NC pier.