Local bond performance of rebar embedded in steel-polypropylene hybrid fiber reinforced concrete under monotonic and cyclic loading

• The effects of hybrid fibers on the local bond performance were investigated.
• Improvements in bond strength, slip and energy dissipation capacity were observed.
• Empirical laws for monotonic and cyclic bond stress–slip relations were developed.

The performance of the bond between reinforcing bars and fiber reinforced concrete (FRC) plays an important role in determining the mechanical behavior of FRC structures when they are subjected to static or dynamic loadings. This paper presents an experimental study on the local bond performance of rebar embedded in steel-polypropylene hybrid fiber reinforced concrete (HFRC). A total of 102 specimens under monotonic and cyclic loading are investigated by means of pull-out tests. The main variables include fiber volume fraction and aspect ratio, concrete strength and stirrup confinement. The results show that the introduction of hybrid fibers had a synergetic effect on improving the bond performance in terms of peak bond strength and corresponding slip, resulting in a more ductile bond behavior. The improvement becomes more pronounced as the fibers content and concrete strength increase. With respect to the energy dissipation capacity, the hybrid fibers also exhibit a great influence. Specimens with higher fibers content always demonstrate a better energy dissipation capacity, while the opposite is true for increasing the aspect ratio of both fibers. Furthermore, two phenomenological models were proposed to predict the monotonic and cyclic bond behavior of well-confined HFRC specimens, in which the benefits of hybrid fibers were taken into account. The models, as well as involved equations, were verified by independent experimental results.