Abstract
This paper deals with the bond strength of deformed bar embedded in steel-polypropylene hybrid fiber reinforced concrete (HFRC) matrix. The benefits of hybrid fibers were evaluated through a series of monotonic/cyclic pull-out testing on 72 specimens. The influences of fiber characteristics, i.e. the volume fraction and aspect ratio, on the failure mode and the complete bond-slip responses were analyzed. Upon a simplified spring model, the effects of fiber reinforcement at different loading stages were further discussed. The results showed that for a well-confined HFRC specimen, the cyclic bond response at the pre-peak stage almost approached its monotonic response with slight reduction in the ultimate bond strength. Compared to the specimen made with plain concrete, the introduction of hybrid fibers could exert obvious positive influences on the bond strength, due to the synergetic effects in inhibiting the propagation of cracks at multi-scale and multi-stages. Finally, an analytical model was proposed to estimate the ultimate bond strength, in which the effects of fiber reinforcement, stirrup confinement and geometrical shape of deformed bar were taken into consideration. The model was subsequently well validated by various independent experimental results.
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