Postpeak behavior of fiber-reinforced concrete under cyclic tensile loads

Although much research has been done on the fatigue behavior of concrete subjected to compressive stresses, the problem of fracture behavior in concrete undergoing cyclic loads is open to further investigation. In this paper, the role of steel and carbon fibers on fatigue behavior of both normal (NSC) and high-strength (HSC) concrete is investigated. Experimental results from uniaxial tensile tests on cylindrical specimens and four-point bending tests on beam specimens allowed a comparison between material and structural behavior. Geometrically similar beams with different sizes were adopted Experiments concerned precracked specimens in which a fracture process zone was present. To study the material behavior both inner loops and cycles on the envelope curve were applied. It is found that the envelope curves obtained from cyclic tests match the curves obtained from the static tests on cylinders of high-strength concrete and of fiber-reinforced concrete (FRC) quite well. This, however, does not occur in beam specimens as the fatigue damage strongly depends on the fracture process zone development which is influenced by the specimen size and loading history. Furthermore, the effectiveness Of steel fibers in improving fatigue life is greater in NSC than in NSC cylindrical specimens. More specifically the presence of steel fibers in HSC resulted in a smaller crack opening increment per cycle. The number of cycles to failure depends on the beam size because of different fracture process zone development. Finally, a comparison between experimental results and an analytical model for crack increment during inner loops is shown.