Resistência ao cisalhamento de vigas de concreto com adição de fibras de polietileno de alta densidade

Abstract

The usage of fibers to increase the mechanical properties of concrete has been studied for several decades. It is known that the fibers act by absorbing the tensions inside the concrete, containing the cracking advance. However, the understanding of the stress transfer mechanisms is still insufficient to quantify the contribution of fibers to the shear strength of concrete. In addition, the use of fibers reduces the workability of concrete, which can cause other structural problems. Therefore, the present study aims to determine the shear strength of concrete beams reinforced with high-density polyethylene (HDPE) fibers, in order to verify their post-cracking behavior. Thus, concrete beams will be molded without transverse reinforcement and fiber contents corresponding to 0.0%, 1.5% and 3.0% of the concrete volume. The shear strength will be evaluated using the three-point bending test, also the digital image correlation methodology was used to evaluate the cracking process. The presence of fibers affected the workability of the concrete, presenting the slump of 80 mm for the concrete with 1.5% and 0 mm for the concrete with 3.0%, both with 0,1% of water reducer additive, requiring mechanical consolidation of the beams, while the slump for plain concrete was 90 mm without water reducer addition. It was verified that the concrete with 1.5% presented an increase in the compressive strength of 2.91% when compared to the plain concrete, while the concrete with 3.0% exhibited a decrease of strength of 16.58%. Analogously, in the modulus of elasticity there was an increase of 3.25% in the concrete with 1.5% and a reduction of 2.41% in the concrete with 3.0%. In terms of tensile strength, in both cases the FRC showed lower strength than plain concrete, with a reduction of 1.37% in concrete with 1.5% and 33.34% in concrete with 3.0%. Regarding the shear strength, the concrete with 1.5% showed a decrease of 9.07% in relation to the plain concrete while there was an increase of 16.67% for the concrete with 3.0%. It was found that the fibers acted to control cracking during failure, showing openings of 3.785 mm in plain concrete, 1.768 mm in concrete with 1.5% and 4.966 mm in concrete with 3.0%. Moreover, in both FRC mixes, the openings occurred less abruptly than in the reference concrete, guaranteeing a longer time until the element failure. Only three of the eight equations used to estimate the ultimate shear force approximate to the experimental results, which indicate the need for further studies in order to estimate the shear strength of polymeric-fiber reinforced concrete. Thereby, it can be concluded that the addition of HDPE fibers does not contribute to the increase in tensile strength, but guarantees a post-cracking behavior, while for the compressive strength, it only contributes with the addition of moderate levels and for the shear strength only contributes to high grades. However, when comparing the three resistances and loss of workability, it becomes impracticable to use this type of fiber as structural reinforcement.