Travi in FRC senza staffatura comportamento fessurativo nella regione a momento costante

In the research work presented herein a study of the cracking behavior of fiber reinforced concrete (FRC) beams is illustrated. The beams were tested in bending and were designed according to the guidelines of the National Research Council DT-204/2006, without stirrups in the constant moment region. This choice is due to the fact that consolidated experiences, reported in the literature, show how the presence of transverse steel reinforcement generates a localization of cracks. In the present study FRC with short steel and polyester fibers were investigated. The study was divided into three phases: 1) design of concrete-mix, 2) destructive tests on beams, and 3) analysis of results and analytical comparisons. Scientific investigations in the field of FRC provide information in order to develop an affordable design approach for FRC beams in bending, therefore this research work aims to provide an in-depth knowledge in the field. A comparison with the recent analytical models provided by the National Research Council and 2010 Model Code of CEB-FIB is shown and commented. The case study presented here shows the great importance of the beneficial role exercised by short fibers dispersed in the cement matrix in reducing cracking of reinforced concrete beams. In particular results obtained in absence of transverse reinforcement provide new original data. Stirrups are known to act as a region of discontinuity in the cement hardened paste, creating a cracking opening zone at the interface. Bending tests were performed on 10 beams (2 per concrete mix), deflection at quarter and mid-span were measured, the deformations in the compressed concrete and in the tensioned steel rebars were also measured in correspondence of the increasing applied load. In correspondence of five load levels the distance, the height and the width of the cracks was also measured over the whole length of the beams, with particular attention to the constant moment region. The steel fibers demonstrated to be more effective than respect to polyester fibers in terms of crack arrestors. The experimental data were also compared to the theoretical prediction obtained by applying the analytical models of CNR DT-204/2006 and Model Code 2010.