Experiment And Analysis On Mechanical Behavior Of Columns With High Strength Concrete Core

Compared with ordinary concrete,high strength concrete is dense in micro-structure and is prone to burst under high temperature.This makes the fire-resistance performance of high strength concrete columns relatively poor. In addition,as concrete strength increases,the concrete stress-strain curve under axial compression will turn steeper and concrete will be more brittle,therefore making a relatively poor seismic capacity for the high strength concrete columns.In order to improve the fire-resistance and seismic behavior of high strength concrete columns and to remain their high bearing capacity, a new concept of columns with high strength concrete core (HSCC columns) is proposed in this paper. HSCC columns are ordinary reinforced concrete columns with high strength concrete core,which has higher bearing capacity, finer fire-resistant performance and seismic behavior. This kind of columns has already been patented in the State Intellectual Property Office of China.A certain thickness ordinary concrete can slow down the bursting of high strength concrete and thus improve the fire-resistance performance of columns.Compared with high strength concrete,the stress-strain curve of ordinary concrete drops slower, which makes HSCC columns conceptually have better seismic behavior. This thesis presents a series work carried out to investigate the mechanical behavior of HSCC columns and to propound the corresponding design method, as shown in the following:(1)Due to the different concrete material property, the peak compressive strain in the core high strength concrete and that in the external ordinary concrete are different.In this context,16 plain HSCC columns,3 HSCC columns and 1 ordinary reinforced concrete column are tested to collapse to investigate the strain of the columns corresponding to the peak loading and how to calculate the axial compression bearing capacity. Test results indicate that when axial force of HSCC columns reach to the maximum value,external ordinary concrete stress-strain curve has turned into the drop part, while the core high strength concrete is still in the climbing part of the curve.Considering neither the core high strength concrete nor the external ordinary concrete is in the peak compressive stress state,the coefficient of nc and ne whose formula are regressed from tested and analysis results are introduce into the axial compression bearing capacity predicting equation to adjust the resisting force of core high strength concrete and external ordinary concrete.The results of the bearing capacity calculated by the formula brought up in this paper was in good agreement with the tested results.(2)Twelve HSCC columns which has different area of high strength concrete core are tested to failure under eccentric loading with different eccentricity to investigate the failure characteristic of these specimens,the strain distribution in the section of the half-height of columns,and the development of lateral deflections versus the height of column.Test results indicate that failure characteristic of HSCC columns is similar to the ordinary reinforced concrete columns under eccentric loading,the strain distribution along the cross-section is accordance to the plane section assumption,and the lateral deflection curve follows the sinusoidal half-wave curve.Due to the difficulty in giving the coefficient of equivalent rectangular stress block to calculate eccentrically compression bearing capacity of HSCC columns based on the plane section assumption,the relatively simple strength superposition mehod is adopted in this paper. According to the four different reduced height of compressive zone in the HSCC columns, the calculation method is provided to predict their bearing capacity under eccentric loading.It is shown that the predictive results obtained by this method match well with the experimental ones.(3)Nine low cyclic loading tests are conducted on HSCC columns with different axial compression ratio and area of high strength concrete core to investigate their failure procedures.The loading-displacement hysteretic curves are measured,and the behavior of energy dissipation,strength degradation,stiffness degradation,skeleton curves and ductility of these specimens are analyzed to evaluate the seismic behavior of HSCC columns.Test results show that when the shear-span ratio is 3 and the axial compression ratio no=0.33～0.42,HSCC columns would undergo compression-flexure failure under low cyclic loading,its hysteretic curve is plump and no pinch phenomenon occurs,excellent ductility is observed with the ductility coefficient greater than 3.A computer program is developed using Matlab to investigate the nonlinear behavior of HSCC columns subjected to the combination of axial compression and bending loads,which has been verified by the test results on the 9 HSCC columns mentioned above.Parametric analysis is then conducted to discuss the influences of axial compression ratio,longitudinal reinforcement ratio,core concrete strength,core concrete area ratio,lateral confinement eigenvalue,outer concrete strength and shear-span ratio on the seismic behavior of HSCC columns.The results indicate that the axial compression ratio, core concrete area ratio and shear-span ratio have significant effect on the seismic behavior of HSCC columns.The seismic behavior of HSCC columns is better than high strength concrete columns, and can be as good as ordinary columns by controlling the core concrete area ratio without losing much lateral bearing capacity. By analysing the load-displacement curves of 112 HSCC columns,the expression for characteristic parameters are given with 3 variables of axial compression ratio, core concrete area ratio and shear-span ratio to determine their skeleton curve.The restoring-force model for HSCC columns is further established.The numerical results based on the restoring-force model are in good agreement with the experimental results,which proves that the established restoring-force model performs well in simulating the seismic behavior of HSCC columns.(4)The concept of ultimate axial compression ratio is introduced to ensure the HSCC columns collapse in tension failure when subjected to axial compression and bending loads.The ultimate axial compression ratio is defined to specify the ultimate of tension failure and compression failure for columns subjected to axial compression and bending loads,which is the precondition to determine the limit value of axial compression ratio for the columns in the structures under different classes of seismic measure.With the computer program,768 HSCC columns are analyzed to determine the value of the ultimate axial compression ratio for the designing of this kind of columns.