Seismic Performance Of Steel-Polypropylene Fiber Reinforced Concrete Columns

Many seismic disaster surveys show that the reinforced concrete columns under loads imposed by earthquakes are easy to appear shear failure characteristic due to lack of adequate ductility, and failure to effectively resist seismic action by large deformation. To solve this problem, adding fiber in concrete is one of the effective solutions. Currently, the seismic performance of steel fiber reinforced concrete columns and polypropylene fiber reinforced concrete columns have some research findings, but the seismic performance of steel-polypropylene hybrid fiber concrete column is rarely reported. It has been shown that by using the way of mixing different kinds and types of fibers into concrete, the hybrid fiber composite materials can offer more attractive engineering properties, because the presence of different fibers can synthetically enhance the concrete performance at different stages and levels. In addition, the hybrid mix of metal fiber with cheap synthetic fiber can reduce cost and improve cost performance of the composite materials. For these reasons, with the support of national natural science foundation of China (NO.51278388), the seismic performance of steel-polypropylene hybrid fiber (hereinafter, denoted as hybrid fiber) reinforced concrete column was studied by quasi-static test, the main work and results are as follows:(1) The seismic performance of hybrid fiber reinforced concrete column was investigated by pseudo-static test. Twenty-two specimens were tested, the parameters of these specimens included fiber type, axial-load ratio, reinforcement ratio and shear span ratio. Test results indicated that:?Compared with normal concrete column, when the axial-load ratio was bigger (nt?0.308), hybrid fiber avoided the shape of hysteretic curve displaying "reverse S", increased the energy dissipation index of hysteretic curve peak and limit point by6.2%?23.6%and35.9%?65.0%respectively, displacement ductility ratio by35.9%?65.0%, and significantly improved the seismic performance of concrete columns.?When the axial-load ratio was small (nt?0.186), hybrid fiber could not significantly improved the seismic performance of concrete columns.?The steel fiber also improved the seismic performance of concrete columns, but the effects were not as good as hybrid fiber, and the effects of polypropylene fiber were small. A formula that can be used to calculate displacement ductility factor of steel-polypropylene hybrid fiber concrete columns is proposed.(2) Based on experimental studies and theoretical analysis, the failure pattern of specimens, influence factor of bearing capacity and working mechanism were investigated. The calculation formulas that can be used to estimate flexural capacity and shear capacity were proposed respectively. Analysis results indicated that:?failure patterns of hybrid fiber columns could be divided into3kinds:flexural failure, bending-shear failure and shear failure.?The hybrid fiber avoided protective layer peeling off completely, made the failure pattern of concrete columns transfer from bond-cracking failure to bending failure, and the crack patterns transfer from single crack pattern to the smeared crack pattern.?The hybrid fiber increased the seismic flexural capacity of column by5.1%?19.9%, and the seismic shear capacity by8.4%-15.6%, and its beneficial effect mainly came from steel fiber, the influence of polypropylene fiber could be ignored. The formulas that can be used to calculate the bending bearing capacity of hybrid fiber reinforced concrete column and shear bearing capacity of hybrid fiber reinforced concrete column were proposed respectively.(3) Using ABAQUS software, the fiber model of steel-polypropylene hybrid fiber reinforced concrete columns was established, and the influence of axial compression ratio, longitudinal steel ratio and shear span ratio on energy dissipation capacity of the columns was analyzed. Results indicated that:?When the axial compression ratio nt was between0.24and0.47, hybrid fiber could obviously improve the energy dissipation capacity of concrete columns, with the increase rate of20%?65%. When the axial compression ratio nt>0.47or nt<0.24, the energy dissipation capacity of hybrid fiber reinforced concrete columns increased by less than15%.?When steel ratio ??O.04, the energy dissipation capacity increased by32.3%?35.7%. When steel ratio p>0.04, the energy dissipation capacity increased by less than10%.?The energy dissipation capacity went up and then down with the increase of shear span ratio:When3???5, the hybrid fiber could obviously improve the energy dissipation capacity with the increase rate of18%-32.3%, and the energy dissipation capacity went up with the increase of shear span ratio. When?>5, the improvement of capacity was not obvious, and it decreased with the increase of shear span ratio.(4) Based on measured hysteresis curve, A three-linear restoring force model with considering stiffness degradation that can be used to predict the hysteretic behavior of hybrid fibers reinforced concrete columns was proposed, including the calculation formulas of load, placement, stiffness degradation, strength degradation, shear deformation, bond slip deformation and so on.Finally, based on the summaries and conclusions of all research works, the proposal and forecast for further study are put forward.