Study And Numerical Simulation Of Axial Compression Performance Of 500MPa Grade Steel-reinforced High-strength Concrete Column

The ultra-high strength concrete has been used widely in the construction industry,which can not only reduce the structural weight and speed up the construction progress,but also bear more loads on the unit area,comparing with normal strength concrete.It is possible that the ultrahigh strength concrete members can be with a smaller cross-section area under the same load and it can provide larger space and flexible spatial arrangement.Ultra-high strength concrete has the good character of denser micro structure,frost resistance,impermeability,corrosion resistance and better durability.However,with increase of concrete strength,its brittle characteristics becomes more obvious.How to avoid the brittleness of high strength and improve its ductility become research focus,especially the descending branch of the stressstrain curve of core concrete(the columns confined by reinforcement).The slope of the descending branch is steeper with the increase of concrete strength.More reinforcement was needed to reach enough ductility when the high strength concrete was used.In order to enhance the ductility of the RC columns,fibers was added in the concrete matrix such as Polyvinyl Alcohol(PVA)fiber,steel fiber,carbon fiber etc.PVA fibers with the characteristic of high elastic modulus and tensile strength and when the PVA fibers was added in the concrete,it has a good bond with concrete matrix.The combination of PVA fiber and concrete can effectively enhance the ductility of high strength concrete depending on the fiber pull out from the concrete matrix or fracture with the energy absorption when process of concrete member fracture.Many researcher investigated the behavior of steel fiber reinforced concrete members.The steel fibers which was added in the concrete matrix can be equally to the ties to enhance the ductility of RC columns.This dissertation presents an experimental study of behavior of fiber reinforced high strength concrete confined by reinforcement subjected to monotonically increasing concentric compression.Test program included a total 13 concrete(the concrete strength were over 80 Mpa [100mm*100mm*100mm cube])columns with square cross section(220mm*220mm*600mm).And three plain concrete columns with the same size in them were used to test the in-situ concrete strength.Four hoop configuration were investigated under the same concrete strength.Many researcher have investigated the behavior of RC columns confined by different yield strength reinforcement,some of the yield strength was over 1000 MPa.Because in some model of stress-strain of confined concrete,when the concrete reach the peak strength,the yield strength of reinforcement was used to calculate the confined concrete peak stress in the model.The real stress in the reinforcement should be used when calculating the peak strength of confined concrete.In Chinese construction industry,HRB400 E grade was commonly used,and the higher yield strength reinforcement were only used in some landmark buildings or some high-way bridges.In this dissertation,the BS 500B([British] Hong Kong code)reinforcement was adapted,which was equal to the HRB500 in mainland building code.In his dissertation,except for that the behavior of RC columns with four different hoop configuration was investigated,two different kinds of fibers were added in the concrete under three concrete strength with same hoop configuration to investigate the different function of this two fibers on the behavior of core concrete stress-strain curve especially the descending branch of the curve,and the time of concrete cover spalling.And three concrete strength RC columns without fibers was as the control group.Scanning Electron Microscope(SEM)was used to investigate the PVA fibers state when the concrete fracture to explain the addition of PVA fibers result in the different behavior of the specimens.According to the picture from SEM,the state of fibers was obvious(bridge,pull-out,fracture).The ductility of RC columns was enhanced due to the PVA fibers under different conditions with the energy consumption.And in the specimens with steel fibers,the condition of steel fibers which were pull out from the concrete matrix can be seen.In this dissertation,5 mathematical model proposed by other researchers to represent stressstrain relationship of confined concrete were compared based on the experimental results of 13 high-strength concrete columns(square cross section)confined by high strength reinforcement.Most of the confined concrete stress-strain models proposed by the researchers were based on a certain number of experiments,and these models can give a more accurate prediction of the limit experimental results.The researchers proposed a large number of equations and method to calculate the peak strength of confined concrete.But the universal solution has not been proposed yet.Recent years,various Artificial Neural Networks(ANN)were trained and used in civil engineering and proved to be feasible.ANN is a method which is a simulation of learning process of human beings.In this dissertation,an artificial neural networks(14-10-5-1)including one input layer,two hidden layers and one output layer was trained by collecting the 241 existing experimental data of confined concrete columns from literature.The input parameters include the variables(concrete strength,yield strength and spacing of transverse reinforcement,etc.)influencing the peak stress of confined concrete.And the only one output is peak strength of confined concrete.Based on the ANN structures,the univariate variables(concrete strength and spacing of reinforcement)was analyzed and the concrete strength(xaxial),spacing of reinforcement(y-axial)and strength of confined concrete(z-axial)can be shown in 3-D coordinate system.The ANN can give a more accurate prediction of peak strength of confined concrete than models proposed by other researches,and has a wide range of applicability.Finally,The finite element software ABAQUS is used to simulate the nonlinear behavior of RC columns under uniaxial monotonic loading.The stress of concrete and steel bar are obtained at different loading stage.When using the finite element models,the uniaxial stress-strain model(compression and tension)proposed by Yu and Ding was used in ABAQUS as the constitutive law of the concrete.The damage parameters was used to reflect the damage state of concrete.The axial load capacity obtained from FEM software was compared with the experimental results and the FEM software results are in good agreement with the experimental results.