Flexure-Shear Behavior Of Hybrid Fiber Reinforced Self-consolidating Concrete Beam Elements

Currently, the poor workability of new fresh concrete including low flowability and segregation affects the casting quality of construction engineering. Specailly, the effective bond behavior between rebar and concrete matrix, the thickness of concrete cover would be reduced in seriously reinforcement region or high dense space. In site, the inaccuracy arrangement of rebar and shear reinforcement induces crack of concrete member. The mentioned above affects safety, serviceabilty and durability of structures directly. Fiber reinforced high performance self-consolidating concrete (FRSCC) and replacement of rebar and shear reinforcement by macro-fiber (steel fiber and plastic fiber) is one of the new developed research trends. FRSCC has good and high workability. Macro-fiber can improve the tensile strength, flexural strength and flexural toughness of concrete, which is the prerequisite of shear reinforcement replaced by macro-fiber. However, the studies about the workability of FRSCC, replacement of shear reinforcement by macro-fiber still are immature. There are no specifications and formulas about the ultimate shear load of hybrid fiber reinforced self-consolidting concrete memebers and relative research report about the shear behavior of hybrid fiber reinforced self-consolidting concrete beams.In this work, the influence of macro-steel fiber, macro-plastic fiber and hybrid fiber on workability, different strength and toughness of self-consolidating concrete (SCC) is studied. Based on many laboratory trials, several mix compositiones which have good and high workability, high toughness are found. Then, the flexural behavior and shear behavior of steel fiber and hybrid fiber reinforced SCC rectangular beams are tested and analyzed. The comprehensive comparison about the shear behavior between the rectangular beams and T-beams is studied. The following aspects are carried out:(1) With reference to the widely used methods about workability of FRSCC and latest studies, the workability of steel fiber, plastic fiber and hybrid fiber reinforced SCC with different fiber dosages, types and fiber aspect is tested using the methods of flowability test, J-ring test, L-box test and U-box test. The compressive strength, flexural strength and flexural toughness, splitting strength and direct shear strength of steel fiber, plastic fiber and hybrid fiber reinforced SCC with different fiber dosages, types and fiber aspect are done. The mix compositions including steel fiber dosages of 20kg/m3,25kg/m3,40kg/m3,50kg/m3 and hybrid fiber dosages of 20+6kg/m3,40+4kg/m3 are selected to the placement of members. The investigation shows the steel fiber dosage of 50kg/m3 and hybrid fiber dosage of 40+4kg/m3 are the maximum fiber dosage in condition that the reinforcement ratio is 3.3%.(2) The problem on the mimimum reinforcement ratio of fiber reinforced RC beams is studied. The influence of hybrid fiber and steel fiber on yield load, ultimate load, mid-span reinforcement strain, concrete strain in compressive region and flexural ductility of RC beams with miminum reinforcement ratio is analyzed. Whether the plane sections remain plain or not is discussed. The results show that the flexural ductility of RC beam with mimimum reinforcement ratio is reduced by fibers. Based on the mentioned above, the mimimum reinforcement ratio should be increased when design fiber reinforced RC beams.(3) The shear behavior of hybrid fiber and steel fiber reinforced RC rectangular beams and T-beams are studied. The influence of hybrid fiber and steel fiber on load-deflection curve (shear load-displacement at loading point), failure mode, crack pattern, load bearing capacity and deformability (steel strain, concrete strain, crack width) are analyzed. The differences of shear behavior between rectangular beams and T-beams are compared comprehensively. The results present that steel fiber and hybrid fiber can enhance ultimate load and ultimate deflection, improve crack pattern, transfer failure mode, reduce crack width in shear span and crack spacing, limit the development of stirrup strain and steel strain, decrease longitudinal concrete strain. The longitudinal concrete strains in flange of hybrid fiber reinforced RC T-beams have phenomenon of shear lag. The plane sections in flange under different load did not remain plane. Hybrid fiber reduces longitudinal concrete tensile strain in flange of shear span. Flange of T-beams can transfer the failure mode of beams with stirrups or fibers, limit development of crack. The contribution of flange to shear behavior can not be ignored.(4) A novel method is proposed in order to evaluate the post-peak behavior of RC beams made of various fiber composites. The post-peak nominal average equivalent shear strength and shear toughness indexes can quantitatively give the describtion of the residual load-bearing capacity after the peak load of the specimen. For beams with various reinforcements failing in shear collapse, the pre-peak energy is compared by the area under load-deflection curves or shear load-displacement curves. The investigation indicates that hybrid fiber and steel fiber can increase pre-peak energy largely. The pre-peak energy increases with increasing of fiber dosages. The improving performance of hybrid fiber on pre-peak energy is better than that of steel fiber. For beams with stirrups and steel fibers, the post-peak load decrease clearly, the decreasing extent is larger than that of beams with only stirrups. However, for beams with hybrid fibers and stirrups, the decreasing extent is slower than that of beams with only stirrups. For beams without stirrups, a combination of steel fibres and plastic fibres indicates the positive hybrid effect on shear toughness. The effect of hybrid fibers or steel fibers and stirrups on shear toughness is based on fiber dosages, stirrup ratios and beam section forms.(5) Based on the analysis of load-deflection curves (shear load-displacement at loading point), ultimate load (shear load), ultimate deflection (ultimate displacement), pre-peak energy, failure mode, maximum crack width in shear span, shear toughness etc. between 23 group comparative beams, the possibility of totally or partially replacing of stirrups by steel fibres and hybrid fibers is studied. The results present that the stirrups can be replaced by hybrid fibers and steel fibers in rectangular beams and T-beams. The replacement ratio is based on fiber dosages, stirrup ratios and beam section forms. The combination of steel fibers or hybrid fibers and stirrups behave positively hybrid effect. The shear behavior of beams with the combination of steel fibers or hybrid fibers and stirrups is better than that of beams with only stirrups or fibers. In this paper, hybrid fibers or steel fibers can not totally replace constructive reinforced stirrups according to requirements in specification. The flange is beneficial to replacing of stirrups by steel fibres and hybrid fibers.(6) Based on shear mechanism of fiber reinforced RC beams and latest theory, the formulas of shear strength of steel fiber reinforced SCC rectangular beams and T-beams are founded. The ultimate shear load of steel fiber or hybrid fiber reinforced SCC rectangular beams and T-beams are predicted by using stress-strain method. Based on load-deflection curves of simply supported norched beam, theσ-w relations of steel fiber and hybrid fiber reinforced SCC beams are derived by using inverse analysis method. Then, the ultimate shear load of steel fiber or hybrid fiber reinforced SCC rectangular beams and T-beams are predicted based onσ-w relations. The formula on replacing of stirrups by hybrid fibers is proposed. The shear strength or ultimate shear load recorded experimentally is compared with the predicted values, and the correlation is satisfactory.In a word, the combination of steel fiber and plastic fiber as well as stirrups can improve flexural strength, flexural toughness, shear strength, shear toughness, load-deflection curves (shear load-displacement curves), failure mode, crack pattern, load bearing capacity and deformability of beams at different levels, indicate positive hybrid effect, can represent a stronger composite reinforcement for RC-SCC beam.