Fatigue Performance And Calculation Method Of Steel Fiber Reinforced High-strength Concrete Beams

In order to meet the requirements for establishment of architectural industry standard entiled "Technical specification for steel fiber reinforced concrete structures" in China, one high-strength concrete (HC) beam and fourteen steel fiber reinforced high-strength concrete (SFRHC) beams were designed and tested under the static load and fatigue load in this thesis. Through the measurement of steel fiber distribution in beam sections, strains of the reinforced bars and concrete, cracking load and width, deformation, fatigue life and residual strain of concrete at compressive region, the effects of fatigue load characteristics (equivalent stress ratio), volume fraction of steel fibers, steel fibers types, concrete strength, distribution altitude of steel fibers, and grade of reinforced bars on fatigue performance of SFRHC beams were discussed. The calculation models and methods of the residual strain of concrete at compression zone, the maximum crack width, rigidity and fatigue life, etc. for SFRHC beams, were put forward to complete the computational theory in the field of fiber reinforced concrete. The relevant conclusions in this thesis can provide evidences for the edition of "Technical specification for steel fiber reinforced concrete structures". The main research and conclusions completed in this thesis as follows:(1) The cumulative residual strain of concrete at compressive region increases with the increasing of the maximum stress level and fatigue cycles, and it's development has significant stage. With the addition of steel fiber into the high strength concrete beams, the increasing rate of cumulative residual strain of concrete at compressive region decreases with the addition of steel fibers. Based on the test results, the method to evaluate the cumulative residual strain of concrete at compressive region of SFRHC beams under fatigue load was discussed, the formulas which considers the impacts of steel fiber characteristic value, distribution altitude of steel fiber, fatigue cycles and strength of concrete, were proposed.(2) Adding steel fiber into high-strength concrete beams can prevent the development of the fatigue crack and reduce crack width and crack spacing effectively. Maximum crack widths of SFRHC beams decrease 44%?83% under static load, fatigue crack width 15%?38% and crack spacing 28%-67%. Finally, the calculation method of maximum crack width under static and fatigue load was discussed, and the formulas for maximum crack width of SFRHC beams under fatigue load was put forward, which considers the impacts of fatigue cycles, steel fiber characteristic value and concrete strength.(3) The addition of steel fiber into high-strength concrete beams can restrain rigidity attenuation and control the deformation effectively. The mid-span deflection of SFRHC beams with reinforcements decreases 4%?43%% under static load and 7%?21% under fatigue load respectively. Based on the test results and theoretical analysis, the calculation method and formulas of rigidity of SFRHC beams were put forward, which consider the influences of fatigue cycles and steel fiber characteristic value.(4) On the basis of the analysis for influence factors and development law of SFRHC beams under the static and fatigue load, the effective moment of inertia was discussed by considering the nonlinear features of geometric and physical relations of cracking cross section. Meanwhile, the rigidity reduction factor was introduced to consider the influences of craking on rigidity. Then the calculation method and formulas of rigidity of for SFRHC beams were put forward based on the effective moment of inertia method.(5) Based on the results of fatigue tests for fourteen SFRHC beams, the distribution of fatigue life of SFRHC beams with different parameters was statistically analyzed; the main influential factors on the fatigue life and the cumulative damage development process of SFRHC beams were studied. The results show that adding steel fiber into high-strength concrete beams can increase fatigue life and control the cumulative damage development effectively. Based on the deterioration law of the rigidity and the second phase of rigidity injury linear attenuation, the fatigue life prediction method for SFRHC beams were presented by using the theory of damage mechanics, which consider the influences of fatigue cycles, steel fiber characteristic value and concrete strength.