Steel Fiber Reinforced High-strength Concrete Beam Fatigue Test Research

In practical engineering,reinforced concrete structures subjected to alternating load are becoming more and more universal,and the fatigue performance has been attracted more attention.With the development of engineering components toward to high strength and light-weight,it is necessary to make greater demands on the strength of steel and concrete.Nowadays,the high strength steel and concrete have been widely used in engineering.However,due to the poor ductility and significant brittle characteristics of high strength concrete structure,the fatigue problem is becoming more prominent under long-term cyclic loading.In order to improve the overall stiffness and toughness,the method of mixing steel fibers into high strength concrete structures was proposed by many researchers.Meanwhile,the fatigue properties of steel fiber reinforced high-strength concrete(SFRHSC)structure were seldomly studied.Also,the calculation method and design rules of the fatigue performance were not provided by current standards.Therefore,the total of five CF60 SFRHSC beams with HRB500 steel bars were made for this study.The beams with a longitudinal reinforcement ratio of 0.94% have the size of 120 mm× 200 mm×1200 mm.Among them,One beam was used for static load test,and the rest of the four beams were used for constant amplitude fatigue test with the stress level of 0.5,0.6,0.7,0.8.Then,the effect of stress level on the cracking,deflection,strain,stiffness and fatigue life of the beams was studied.The main conclusions are as follows:(1)The test process and failure mode of SFRHSC beams under static load and fatigue load were significantly different.The ductility of beam under static load was higher than that under fatigue load,which showed obvious signs before failure.However,the beams under fatigue load showed brittle failure.The steel fiber used in beams could effectively inhibit the development of cracks as well as improved the flexural rigidity and failure mode of the beams.(2)With the increase of stress level,the fatigue life and fatigue strength of the SFRHSC beams decreased significantly,but the failure process and mode failure were similar.The beams experienced the obvious failure process of three stages.In the first stage of early loading,the failure developed rapidly and exhibited a large extent.After 10 thousand cycles loading,the failure kept a stable state.In the second stage,the characteristic values were stable and uniform,and exhibited a long failure time and a slow failure process which accounted for about 80% of the whole fatigue life..In the last stage,all characteristic values reached to the critical levels,and the suddenly instable failure occurred.(3)Based on the theory of bond slip,the calculation formula of crack width of the SFRHSC beams under static and fatigue loads was presented.According to the linear correlation between the cumulative residual strain and the number of cycles,a method was proposed to calculate the cumulative residual strain of the SFRHSC beams.(4)By fitting the experimental data,the fatigue life curve of the SFRHSC beams between S with lgN was obtained.Moreover,the fatigue life of the SFRHSC beams was predicted by using the numerical value of D,which can provide the basis for the fatigue design of the SFRHSC beams.