Avaliação do mecanismo de transferência de tensões e formação de fissuras no concreto autoadensável com reforço híbrido

Abstract

This research aimed to analyze the stress transfer mechanism in specimens under tensile stress, through the self-compacting concrete cracking process with hybrid reinforcement, which were casting horizontally and vertically. Aiming to achieve the purpose cylindrical specimens were tested in order to evaluate axial compression behavior and prismatic direct tension test specimens with the view to analyze their tensile behavior. The direct tension test specimens have dimensions of 15x15x75 cm and were casted in the vertical and horizontal position with steel bars of 20 and 25 mm, passing through the center of the rectangular section (15x15 cm). The process of cracking of the tie rod was verified through the use of digital image correlation. Computed tomography was used with a focus on check out the dispersion of fibers and void ratio, in the both direction of casting the specimens. Cylinders and direct tension test samples was carried out using two different mixed of self-compacting concretes, being one without fiber and the other with steel fibers, which have aspect ratio of 65. To characterize the flow, plastic viscosity and flowability of self-compacting concretes in the fresh state, rheological tests were performed using the Abrams cone, with and without the “J” ring, the “V” funnel and the “L" box. The results found from the analysis of self-compacting concrete in the fresh state were consistent with the properties for this type of concrete. The spreading values are greater than 550 mm, and the use of fibers provided an increase in plastic viscosity, since the flow time measured through the “V” funnel, for concrete with fibers was 168% higher than concrete without fibers. The self-compacting ability of fiber-reinforced concrete, in the “L” box and in the “J” ring, demonstrate that the spacing between bars for fiber-reinforced concrete must be wider than fiber-free concrete. In relation to the direct tension test specimens, it was observed that the direction of casting presents differences in the direct tension test specimens fracture mechanism. These were casted horizontally, due to the failure in adhesion, showed asynchronous cracks in the faces, that is, they are not transfer through the cross section. On the other hand, the behavior for the horizontal group, cracks appear simultaneously, that is, there is transfer of cracks through the cross section. Besides, with the addition of fibers to the matrix, the behavior for the horizontal group is modified by the increase in tensile stiffness, caused by the orientation of the fibers during the flow of the mixture in the fresh state into the form, resulting in a parallel direction of the fibers regarding the direction of the request. In the vertical casted group adding fiber, the behavior remains simultaneous in cracking, with higher tenacity, when compared to the group without fiber. Keywords: Mechanical behavior; cementitious composites; anisotropy; casting directions of concrete; concrete tomography.