| Nowadays, reinforced concrete deep beams are more and more widely used in civil engineering because of their tremendous bearing capacity. The shear capacity calculation of deep beam is of especial significance in the structural design because the span-depth ratio of deep beam is relatively small, which leads to a small moment but fairly high shear stress under loads. In order to meet the needs of shear resistance, deep beams are usually high with lots of steels inside, which on the other hand means more materials needed and more difficulties met in construction. High performance concrete (HPC) is featured in high strength and good durability. The application of HPC to deep beam structure will improve the bearing capacity of deep beam, reducing the component dimension and self-weight, to save the construction materials. However, HPC still fails to get rid of the brittle nature of concrete, and the higher strength of HPC, the brittler the concrete will be. Due to the bad ductility in shear failure, HPC deep beams are rarely favored to seismic resistance. Meanwhile, hybrid fiber reinforced concrete has been paid more and more attention to by researchers because of its eminent mechanical performance. So is it practical to reinforce HPC with steel and polypropylene hybrid fiber in order to obtain better-performance HPC: On the one hand, the advantage of hybrid fiber can increase the toughness of HPC and the ductility of deep beam in shear failure; on the other, making use of the reinforcement of hybrid fiber to improve the shear capacity of deep beam, which can reduce the amount of reinforcing steel so as to solve the construction problems caused by over-dense steel. Considering the reasons above, experimental and theoretical researches are carried out to study the shear performance of hybrid fiber reinforced HPC deep beams in this paper, with the following contents mainly concerned:(1) Orthogonal experimental method is adopted to conduct the cubic compression tests and tensile splitting tests of hybrid fiber reinforce HPC. The effects of several parameters (including the shape, length-diameter ratio, volume ratio of steel fiber, and the volume ratio of polypropylene fiber) on the cubic compression strength and tensile splitting strength of hybrid fiber reinforce HPC are analyzed respectively, and comparison is made to list these factors in order according to the magnitude of their effects. The test results show that the cubic compression strength, splitting tensile strength and the tensile-compression strength ratio of the HPC increased generally after being reinforced by the steel- polypropylene hybrid fiber, and among all the influencing factors, the shape of steel fiber is the most influential one, while the length-diameter ratio of steel fiber has the least effect.(2) Based on the shear tests of 18 HPC deep beams reinforced by steel- polypropylene hybrid fiber and 2 ordinary HPC deep beams, direct-viewing method is applied in comparing and quantitatively analyzing the effects of different factors on the shear capacity of deep beams. The test results indicate that the hybrid fiber can significantly increase the shear capacity of HPC deep beams. Among all the factors influencing the shear capacity of HPC deep beam reinforced by hybrid fiber, the reinforcement ratio of horizontal distribution bars is ranked as the top, the length-diameter ratio of steel fiber takes second place, and the volume ratio of polypropylene fiber is the least influential one.(3) Through combining the shear capacity calculation formulas for deep beams in current codes "Code for design of concrete structures" (GB50010-2002) with "Technical specification for fiber reinforced concrete structures" (CECS 38:2004), and referring to related research findings from scholars both at home and abroad, comparison is made to analyze the applicability of current shear capacity calculation formulas for HPC deep beams. Based on this analysis, a shear capacity calculation formula for HPC deep beams reinforced by steel- polypropylene hybrid fiber is proposed, which can be taken as a reference in engineering design.(4) Based on the shear tests of deep beams, direct-viewing method is adopted to quantitatively analyze and compare the effects of different factors on the initial cracking strength of deep beams. The test results show that the hybrid fiber can notably increase the initial cracking strength of HPC deep beams. Among all the factors influencing the initial cracking strength of HPC deep beam reinforced by hybrid fiber, the length-diameter ratio of steel fiber is the most influential one, the volume ratio of polypropylene fiber being less influential, and the reinforcement ratio of vertical distribution bars the least.(5) By combining the crack resistance capacity calculation formulas for steel fiber reinforced deep beams in current codes "Code for design of concrete structures" (GB50010-2002) with "Technical specification for fiber reinforced concrete structures" (CECS 38:2004), and basing on relevant research findings of scholars both at home and abroad, a crack resistance capacity calculation formula for HPC deep beams reinforced by steel- polypropylene hybrid fiber is come out with, which can provide references to engineering design.(6) Built upon the shear tests of HPC deep beams reinforced by steel- polypropylene hybrid fiber, orthogonal analysis is conducted to investigate the effect of different parameters, including the shape, length-diameter ratio, volume ratio of steel fiber, volume ratio of polypropylene, and horizontal and vertical reinforcement ratio, on the shear failure modes and ductility of the deep beams. Comparison is made to find out the magnitude of effect of different factors on the deformation and ductility of the deep beams. Hence, the shear failure mechanism of hybrid fiber reinforced HPC deep beams is revealed. The analysis results reflect that the hybrid fiber can greatly increase the ductility of HPC deep beam in shear failure. Among all the factors influencing the ductility of HPC deep beam reinforced by hybrid fiber in shear failure, the volume ratio of steel fiber is the most influential one, even more influential than the reinforcement ratio of vertical and horizontal distribution bars, and the shape of steel fiber being the least influential.(7) By using the large-scale finite element analysis software ABAQUS, numerical simulations of the shear performance of both ordinary HPC deep beams and HPC deep beams reinforced by hybrid fiber are made, and the simulation results and test results are compared. Meanwhile, expanding analysis is carried out to study those factors which have not been considered in experiments, for their effects on the shear performance of hybrid fiber reinforced HPC deep beams. The results of finite element analysis indicate that with the increment of reinforcement ratio, the shear capacity of HPC deep beams reinforced by hybrid fiber increases, but quite limited. When the shear span ratio is no more than 1(λ≤1), the variation of shear capacity of HPC deep beams reinforced by hybrid fiber is not that obvious as the shear span ratio changes. The increment of span-depth ratio can improve the shear capacity of HPC deep beams reinforced by hybrid fiber, but only with small amplitude. All these regularities are similar to those of ordinary reinforced concrete deep beams. |
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