Experimental Study On Shear Resistance Properties Of Steel Fiber Reinforced High-Strength Concrete Members

Since "Specification for Design and Construction of Steel Fiber Reinforced Concrete Structures(CECS38:92)" was formally issued for enforcement in 1992, as a kind of high-performance construction material, steel fiber reinforced concrete has been applied to many branches of engineering construction. But with the development of research and application of steel fiber reinforced concrete and revisions of its dependent codes and specifications, some provisions in this specification cannot coordinate with the dependent codes and specifications. Therefore according to the instruction of China Association for Engineering Construction Standardization, the compilation group organized by Dalian University of Technology compiled "Technical Specification for Fiber Reinforced Concrete Structures(CECS38:2004) " to replace "CECS38:92". As a part of this revisal project, a special study on shear resistance of steel fiber reinforced high-strength concrete beams was performed to enlarge the application scope of concrete strength grade of "CECS38:2004" in the first part of this study. Then a large-scale experiment study on steel fiber reinforced high-strength concrete coupling beams with small span-depth ratio(l/h≤2.5) was conducted to overcome the defects in shear capacity, ductility and dissipation of energy of traditionally reinforced concrete coupling beams. The main research work is performed as follows:1 Based on the test results of 20 steel fiber reinforced high strength concrete beams and 2 control beams, the effects of steel fibers and stirrups on diagonal cracking strength and shear strength of beams are investigated and compared quantitatively. Test results obtained so far indicate that, though steel fibers have distinct superiority over stirrups in improving diagonal cracking strength and restraining diagonal cracks of beams, they are less effective than stirrups in improving shear strength. So it is feasible and effective to replace part of stirrups by steel fibers to improve the deformation property and ductility of beams, but it is not reasonable to replace all the stirrups by steel fibers to resist shear. The influence coefficients of different kind of steel fibers on diagonal cracking strength and shear strength of beams are obtained, which provides a direct foundation for the revision of related provisions in "CECS38:2004". And some empirical formulas are given to predict the diagonal cracking strength and shear strength of steel fiber reinforced high strength concrete beams.2 The deformation and ductility properties of steel fiber reinforced high-strength concrete beams failed in shear are investigated in this study. Shear span-effective depth ratio is still the main factor to govern the failure mode of fiber reinforced high-strength concrete beams. Though steel fibers cannot change the brittle property of shear failure of beams with small shear span-effective depth ratio, increasing steel fiber volume fraction ρf can transform the failure section from a flattening shear plane into a narrow fracture zone. Steel fibers canimprove the strength, toughness and deformability of concrete under combined action of shear and compression, restrain the development of diagonal cracks, lower diagonal crack width of beams under average service load effectively. As a result, the ductility and energy dissipation capacity of beams are improved substantially by the addition of steel fibers.3 On the basis of the Modified Compression Field Theory(MCFT), a section analysis model of steel fiber reinforced concrete beams subjected to combined action of shear and moment is presented, and it has been used to predict the complete behavior of beams in this study and other research. The analysis results indicate that this model can give reasonably accurate predictions of shear resistant capacity, diagonal cracking strength, the ultimate compressive strain of concrete in compression & shear region and tension strain of stirrups. Then the efficiencies of steel fibers and stirrups are compared with this model, and the result agrees with this experiment.4 On the basis of a large-scale experiment of 9 steel fiber reinforced high-strength concrete coupling beams and 4 control coupling beams(///*<2.5), the failure mode, load-displacement response, strains of reinforcements, axial elongation and shear distortion of coupling beams subjected to cyclic load or static load are investigated in detail. Test results reveal that steel fiber volume fraction /?f is another governing factor on failure mode of coupling beams besides span-depth ratio llh and stirrup content fraction psv, adding appropriate mount of steel fibers can convert a brittle shear failure of coupling beams to a ductile flexural failure. Steel fibers can prevent the concrete cover, concrete in compression region and failure section from spalling, which results in the delay of stiffness and shear deterioration of coupling beams. Increasing steel fiber volume fraction pi can inhibit the pinching effect of hysteretic response of coupling beams subjected to cyclic load.5 The response skeleton curve, displacement ductility, deterioration of stiffness and shear resistant capacity and energy dissipation capacity of steel fiber reinforced concrete coupling beams are studied systematically. Test results indicate that, except for specimen CCB2-3(/>f=0.5%), all the other coupling beams reinforced with steel fibers replacing the additional stirrups due to seismic design requirement exhibit superior ductility property. The minimum and average displacement ductility ratio of them equal to 3.80 and 4.33 respectively, and when pf=1.0%, specimen CCB2-4 has a comparable displacement ductility ratio with that of coupling beams solely reinforced with close packed stirrups(psv=1.675%). Steel fibers slower the deterioration of stiffness and shear resistant capacity and better the energy dissipation capacity of coupling beams remarkably. The test proves that, adding an appropriate mount of steel fibers instead of part stirrups in coupling beams can obtain better energy dissipation capacity without any lost of displacement ductility.6 The ultimate shear strength of steel fiber reinforced coupling beams is investigated in detail. Test result shows that replacing the portion of stirrups due to seismic design requirement by steel fibers improves the shear strength rather than decreases it when compared with those of coupling beams with close packed stirrups. Some empirical formulas for calculating shear capacity of steel fiber reinforced concrete coupling beams subjected to static load or cyclic load (l/h<2.5) are presented. After a comprehensive analysis on the effectsof steel fibers on failure mode, ductility, energy dissipation capacity and shear resistant capacity of coupling beams, some suggestions for shear design of steel fiber reinforced concrete coupling beams are proposed.