Ductility of lightly reinforced and fiber-reinforced concrete elements: A unified approach

Lightly Reinforced Concrete (LRC), Fiber-Reinforced Concrete (FRC), and Hybrid Reinforced Concrete (HRC) elements subjected to static bending actions exhibit a similar behavior, which depends on the amount of rebar and/or fibers added to the cementitious matrix. In all the cases, if a suitable reinforcing system is not provided, the brittle failure occurs at the cracking of concrete. Hence, a new and unified approach is introduced in the present thesis to evaluate the minimum reinforcement for static reasons. Such approach is based on the definition of the ductility index ( DI ), which is a function of the difference between the ultimate load and the effective cracking load. Therefore, DI is higher than zero when a lightly reinforced member shows a ductile response, whereas it is negative in case of brittle behavior. To study the brittle/ductile transition (i.e., the minimum reinforcement), the flexural behavior of concrete beams containing low amounts of rebar, fibers, or a combination, is predicted through three general models for LRC, FRC, and HRC members. In addition, test results coming from the available literature and a specific experimental campaign are considered. Both numerical and experimental data reveal the existence of a generally valid linear envelope of DI when the reinforcement varies in a concrete member. Based on these results, a design-by-testing procedure can be established for determining the minimum reinforcement of a LRC and/or FRC element, which corresponds to DI equal to zero. Moreover, the minimum reinforcement of an HRC element is defined by any linear combination of the associated minimum amounts of sole rebar and fibers. The proposed approach is adopted to design the minimum reinforcement of precast concrete segments for a tunnel lining. It is applied to LRC, FRC, and HRC members, not only subjected to pure flexure but also under combined axial force and bending moment. Finally, the ductility index is used as functional unit of a simplified sustainability analysis. In the specific case of lightweight FRC one-way plates, this new parameter allows to measure the performances of the concrete elements in combined ecological and mechanical analyses, with an integrated holistic approach.