Shear behavior of deep and wide-shallow beams in fiber reinforced concrete

The behavior and design of reinforced concrete members subjected to shear remains an area of much concern in spite of the vast number of experiments that have been carried out to assess the shear capacity of structural members. Consequently, design codes are frequently changed and are generally becoming more stringent, so that structures designed several decades ago typically do not comply with the requirements of current codes. In fact, there are many parameters which affect the shear strength of a reinforced concrete element without transverse reinforcement: shear-span-to-effective-depth ratio, size of the element (better known as size effect), presence or absence of axial load (tensile or compression), longitudinal reinforcement ratio, concrete compressive strength, loading conditions, coarse aggregate size, shape of the cross section and the distribution of longitudinal reinforcement along the beam depth. Therefore, incorporating the influence of all these parameters in a formulation that can be applied in the design is rather complicated. The International Shear Workshop held in Salò, Italy, in 2010 provided an opportunity to discuss the new approaches and verify how they meet or diverge from one another. Moreover, it is of essential importance to recognize the effects that new models may determine in the field and for structural design and applications. In the workshop, both the shear behavior of reinforced concrete (RC) and fiber reinforced concrete (FRC) elements has been discussed. In fact, fiber reinforced concrete is now a material widely recognized by the international codes, such as in the Model Code 2010. Several reports published over the past twenty-five years confirm the effectiveness of steel fibers as shear reinforcement. Fibers are used to enhance the shear capacity of concrete or to partially or totally replace stirrups in RC structural members. This relieves reinforcement congestion at critical sections such as beam-column junctions in seismic applications. Fiber reinforcement may also significantly reduce construction time and costs, especially in areas with high labor costs, and possibly even labor shortages, since stirrups involve relatively high labor input to bend and fix in place. Fiber concrete can also be easily deployed in thin or irregularly shaped sections, such as architectural panels, where it may be very difficult to place stirrups. This is of essential significance for many secondary structural elements in which a minimum conventional reinforcement is not required for equilibrium. The present PhD thesis intends to be a further contribution to the knowledge of shear behavior of structural elements made of concrete and fiber reinforced concrete. Firstly, an extensive literature survey on the structural typology discussed in this PhD thesis, RC beams and RC wide-shallow beams will be presented. This literature review will also address the material fiber reinforced concrete and its possible structural applications, including its use as shear reinforcement. Afterwards, an experimental campaign on RC deep beams will be presented, where nine full-scale structural elements have been made and tested with the aim of evaluating the influence of the element size on the shear strength of fiber reinforced concrete beams. Particular attention will be placed on two specific experimental campaigns regarding the shear behavior of wide-shallow beams (WSBs) in both reinforced concrete and fiber reinforced concrete (with steel fibers or synthetic fibers), in which thirty wide-shallow beams have been tested. The main purpose of this research has been the study of the shear behavior of wide-shallow beams, as well as the role of the width on the shear strength. The experiments have been also studied numerically by means of the finite element program VecTor2 based on the Modified Compression Field Theory (MCFT) and the Disturbed Stress Field Model (DSFM). An interesting numerical study on the size effect influence on the shear strength of fiber reinforced concrete elements will be presented, discussed and compared with the experimental results obtained from the experimental campaign on deep beams. Finally, in order to summarize the main experimental tests done at the University of Brescia on reinforced concrete elements critical in shear, the last chapter will present the overall University of Brescia’s shear database. This shear database consists of ninety-one elements, in which sixty-two are beams and twenty-nine wide-shallow beams, both in RC and FRC.