The deflection of reinforced concrete beams containing recycled steel fibers

A 3-stage model is used to evaluate the results of an experimental campaign, previously carried out with the aim of studying the deflection of reinforced concrete structures made with reinforced concrete (RC) and fiber-reinforced concrete (R/FRC). In particular, 12 four-point bending tests were performed on beams, whose cross-sections were obtained by combining four effective ratios of the tensile reinforcement (i.e., 0.73%, 1.7%, 3%, and 6.3%), two concrete covers (i.e., 32 mm and 82 mm), and three amounts of steel fibers (0, 0.5%, and 1.0% in volume) recycled from end-of-life tyres. In reinforced beams with low reinforcement ratios, there is a reduction in deflections and the load at yielding of steel increases with the content of fibers. On the contrary, the mechanical behavior of R/FRC and RC beams does not substantially differ in presence of a high effective reinforcement ratio. These experimental observations are in accordance with the results of the three-stage model, introduced herein to predict the possible variation of the load-deflection curves. For a given deflection, the model provides a possible range of applied loads, whose upper bound is related to the situation of incipient cracking (i.e., the maximum tension stiffening effects), whereas the lower bound concerns a fully cracked beam (i.e., the minimum tension stiffening effects). Regardless of the content of fibers, the difference between the upper and lower bounds vanishes in highly reinforced concrete beams.