Comportement des eléments de structures renforcés par des fibres

Abstract

This experimental campaign was carried out with the aim of studying the bending behavior of beams designed with a SFSCC. Before proceeding to the main part of the research program, a comparative study was carried out in order to choose the fiber that has the best impact on the characteristics of the SFSCC, and also to analyze the best technique for their design. Three types of fibers were used, corrugated steel fiber MEDAFAC and polypropylene fibers GRANIFIBRES supplied by GRANITEX, and the by-products which obtained from machining waste of mechanical parts, resulting from the operations of turnings of the national company of the ways of iron (municipality of Constantine). The effect of three mixing methods selected from the literature was additionally evaluated. The mixtures were characterized in the both states “fresh and hardened”. From the results of the preliminary tests obtained, we chose to work with the steel fibers 'MEDAFAC', not only because of their availability and their decent price, but also of their characteristics which improve the mechanical performances of the beams in SFSCC, thus respecting dosages that can be easily applied in the structural elements, the fiber content has been varied between 0.3%-1.2%. Regarding the mixing method, the composites showed superior qualities precisely in the hardened state, when they were prepared respecting a rest time after adding part of the mixing water and the super- plasticizer, hence, the mixtures are prepared for the rest of the work by respecting this protocol. Specimens of identical geometry (10x20cm rectangular transverse section; 120cm length) were tested in three-point bending. They were divided into three series, according to the nature of the concrete: raw concrete NC, SCC and SFSCC without reinforcement. The fiber volume was varied from 0.3% to 1.2%, in order to examine the possibility of completely substituting steel bars by steel fibers. To analyze the contribution of the combination of fibers and reinforcement, as well as the possibility of replacing the shear reinforcements by fibers, longitudinal reinforcements were added to the second series specimens and transverse reinforcements and longitudinal to the third ones. All mixtures were characterized in the fresh state and in the hardened state. In the light of the obtained results, the addition of steel fibers to fresh self-consolidating concrete decreases its workability and fluidity, and the amount of 0.6% of steel fibers is the upper limit of the fiber content with respect to its workability. Meanwhile, hardening properties, such as compressive and tensile strengths, are improved. The 0.9% steel fibers addition improves beams bending capacity and their ductility. Consequently replacing shear reinforcements results in changing the failure mode from a brittle shear collapse to a ductile bending failure mechanism. Increasing fiber volume content to 1.2% does not result in further improvement in the performance of the SFSCC.