The effects of hooked end steel and polypropylene (PP) fibers on the behavior of large-scale doubly reinforced concrete beams under flexure were investigated using experimental and numeric methods. For this purpose, a total of eight beam specimens consisting in two groups were produced in the laboratory and three-point bending tests were conducted under monotonically increasing load. The beams in the groups were designed to have 0.86 and 1.30% tensile reinforcement ratios leading to either flexural or shear critical sections. Three out of eight were produced to be control samples and did not have any fiber additive while remaining five had 0, 0.5 and 1.0% steel or PP fibers by volume. Experimental results showed that the existence of 0.5% either type of fiber in densely reinforced specimens contributed to shear strength and allowed flexural capacities to be fully used instead of an improvement in the capacity. However, when the steel fiber ratio increased to 1.0% flexural capacity was enhanced by 10% for both type of beams. After the experimental study, the beams numerically modeled using nonlinear finite element method and flexural stiffness before yielding as well as yield strength with load carrying capacities were found to be consistent with that of experiments specifically for the beams having stirrup and steel fibers.

The main objective of this study was to inspect the effect of steel fiber ratio into the flexural behavior of large-scale doubly reinforced concrete beams using an experimental method. For this purpose, four RC beams were constructed at the Structural Mechanics Laboratory of İzmir Katip Çelebi University and three-point bending tests were carried out. Two out of four were selected to be control specimens and did not have any fiber additive. To investigate the behavior free from shear reinforcement effect, one of the control specimens did not have stirrups while the remaining one had a minimum amount of stirrup according to TS500 [1]. Last two beams had either 0.5 or 1% volume fractions (Vf) of hooked end fibers, respectively. All the beams were designed to have 150x200x2450 mm prismatic geometry with a 1.30% tensile reinforcement ratio. The used materials were commercially available S420B grade steel for reinforcement and in-house cast concrete having a mean cylindrical compressive strength of 25 MPa. Based on the test results it can be stated that having a minimum amount of stirrup according to TS500 or 0.5% steel fiber enabled the beams to fully use their flexural capacity instead of an enhancement in the capacity. In other words, steel fibers contributed to the shear strength similar to that of beam with minimum amount of stirrup. However, increasing the volumetric ratio of steel fibers to 1.0% did not only contributed on the behavior but also slightly enhanced the flexural capacity (10%) of beam specimen, basically depending on the increase in the moment capacity of the cross-section.