Investigation On Compressive Stress-Strain Relationship And Application Of Ultra High Performance Concrete Constrained By Stirrups

Ultra-high performance concrete(UHPC)is a category of ultra-high strength,ultra-high energy consumption,ultra-high density and excellent durability of stone-free cement-based composite materials,which can greatly improve the resistance,energy consumption and durability of stressed structural members in the construction practice with UHPC.When UHPC is used for the enlarged section of the existing concrete column,the thickness of it can be reduced,and the impact on the space of the reinforced building is small.In this work,the UHPC is braced using fibers and configured stirrups.Through a combination of experimental research,finite element analysis and theoretical analysis,the compressive stress-strain relationship of UHPC with steel fibers of the end-hook type,the compressive stress-strain relationship of short UHPC columns with fibers and configured composite stirrups,and the eccentric compressive performance of outsourced reinforced UHPC columns with fibers and configured square spiral stirrups are investigated.The main research contents and results of this work are as follows.(1)Investigation of the mechanical compressive properties of UHPC materials with end-hook steel fibers.First,five groups of UHPC cube specimens were tested in compression,and the effects of doping with end-hooked steel fibers and different sizes on the compressive strength of UHPC cube specimens were investigated,a model for predicting the compressive strength of UHPC cubes was established considering the fibre retention factor.Then,five groups of 30 UHPC prisms were tested under axial compressive loading to investigate the effects of doping with end-hook steel fibers on the test phenomena and damage patterns,axial compressive strength,compressive ductility and Poisson’s ratio of UHPC specimens.According to the test results,the relationship between axial compressive strength,Poisson’s ratio,elastic modulus and fiber confinement coefficient was established,and the uniaxial compressive stress-strain relationships for UHPC were obtained.The results show that the stress-strain relationship established in this paper can better predict the compressive behavior of UHPC materials with end-hook steel fibers.(2)Experimental study of uniaxial compression performance of UHPC columns with fibers and stirrup reinforcement.Using 11 sets of axial compression tests on UHPC columns with fibers and stirrup reinforcement,the effects of parameters such as the amount of dosed steel fibers,the volume stirrup ratio,and the shape of the stirrup reinforcement on the damage pattern,peak stress,and strain corresponding to peak stress(peak strain)of the restrained UHPC columns were investigated.The results show that the damage patterns of the specimens are all in the form of crushed UHPC in the core region and bulged due to a sharp increase in circumferential deformation caused by the yielding of the hoop reinforcement,and that the protective layer of the UHPC is in the form of "cracked but not broken",which is due to the anti-stripping effect of the steel fibers of the end-hook type,the damage patterns of UHPC in the core area are improved by increasing the ring matching properties,decreasing the ring spacing and increasing the fibre dosage.The restraint effect of UHPC specimens under compression was improved by increasing the ring characteristic,decreasing the slope of the falling section of the stress-strain curve of UHPC with fibers and rings,and increasing the ductility and deformation capacity of short UHPC columns,compared with the amount of steel fibers,the ring configuration and ring spacing had a greater influence on the restraint effect of UHPC.(3)The compressive stress-strain relationship between fibers and hoop-bonded UHPC columns based on the constrained stress method.Based on the experimental results and theoretical analysis of hoop-bonded UHPC columns,the formulas for calculating the effective constraint index of bond hoop reinforcement were derived with reference to the formulas for the effective constraint coefficient of hoop reinforcement proposed by Mander,and the formulas for calculating the peak stress and peak strain of UHPC considering the constraint index of the hoop reinforcement and the constraint coefficient of the fibre were established,and the compressive stress-strain relationship between fibers and configured hoop-reinforced UHPC columns was given.The full curve equations for the compressive stress-strain relationship of UHPC columns with fibers and ring reinforcement were given.(4)Stress path based stress-strain relationship for fibers and wrap bound UHPC columns under compression.The hoop stresses with uniformly distributed transverse strains were calculated using the transverse longitudinal strain equation of the UHPC column,and the stress path-based UHPC stress-strain relationship was proposed to compensate for the shortcomings of the hoop stress-based method,mainly considering the following three aspects the effects of fibre restraint coefficient and hoop stress coefficient on the axial strain-transverse strain curve of hoop stress UHPC specimen were analyzed,and the calculation equation for transverse longitudinal strain was proposed,the change law of hoop stress curve with stress path was considered,the stress-strain relationship of hoop restrained UHPC based on stress path is established based on a series of stress-strain curves of actively restrained concrete with uniformly distributed transverse strains,using the method of iterative fitting of subdivided segments.The results show that the transverse longitudinal strain model established in this paper can better predict the transverse strain of wrapped UHPC,the stress-strain relationship of wrapped UHPC based on the stress path agrees well with the experimental results.(5)Study on the eccentric compression behaviour of UHPC reinforced columns with fibers and stirrups and finite element analysis.An experimental study was conducted on five eccentric columns to observe the experimental phenomena of reinforced UHPC columns with different stirrup spacing and analyse their effects on the load-strain curve,load-side deformation curve,and ductility of the eccentric columns.ABAQUS finite element analysis software was used,and a CDP main structure model applicable to fibers and hoop UHPC was established using the proposed compressive stress-strain relationship between fibers and hoop UHPC and the existing tensile main structure model of fibers UHPC.The results show that the strength and ductility of the column with large deflection can be greatly improved by using the reinforcement method with fibers and spiral UHPC,the reinforcement of the concrete column with enlarged cross section using only fibers without hoop UHPC can improve the load-carrying capacity of the specimen,which has brittle damage without obvious signs,the damage pattern and load-side deformation of the specimen from the finite element simulation are in good agreement with the test curve.(6)The load-bearing capacity of fibers and hoop-reinforced UHPC columns was investigated.The effects of the fibre restraint coefficient and hoop restraint index on the peak stress of fibers and hoop UHPC columns were considered,and the formulas for calculating the axial compression load capacity of fibers and hoop UHPC strengthening columns were established,and the results of the axial compression formulas were found to be somewhat more conservative than the experimental results.Based on the assumption of a flat cross-section,a simplified calculation model for the skewed bearing capacity of hoop UHPC columns was established.It was found that the calculation results of the simplified formulas were basically consistent with the test results and simulation results,and the calculation results had a certain safety factor,which can serve as a reference for the engineering calculation of UHPC materials with fibers and ring bonding.