Experimental evaluation of the influence of fibers content on the tensile behaviour of concrete

The interest about the fiber reinforced concrete, to be adopted for structural issues, is witnessed by the recent bibliography and by the growth of specific conference on this theme, for instance the BEFIB 2004 and the International Workshop on HPFRCC held in Varenna Italy and Honolulu, Hawaii, respectively. The addition of steel, glass or polymeric fibers to the concrete matrix can give rise to a new material working in compression, with reduced value of brittleness. As a matter of fact, when concrete cracks the fibers reduce the microcracking mechanism and limit the cracks propagation, improving both the tensile strength and ductility, providing a post-peak residual tensile stress and improving the toughness of the material by increasing the energy absorption capacity. The mechanical behaviour of the composite depends on the concrete matrix characteristics, on the fiber geometry and material and on the interface properties. Besides the conventional Fiber Reinforced Concrete (FRC), usually characterised by a low amounts of fibers (less than 2% in volume), a great attention is nowadays devoted to the so-called High Performance Fiber-Reinforced Cementitious Composites (HPFRCC), generally related to the addition of high amount of fibers and high performance matrices. According to the definition of Naaman and Reinhardt [12], the high performance is related to the post-cracking stage of tensile elements, during which strain hardening, appears in the tensile stress-strain relationship. By increasing tensile loads, multiple cracking develops in HPFRCCs with reduced width and without any strain localization. Furthermore the possibility of substituting classical reinforcement (longitudinal and transversal) with fibers is a very attractive topic, and its feasibility and application to structures is nowadays the object of different discussions in technical framework. Due to the increasing interest on this topic a number of significant guidelines on FRC is available in Europe in the U.S., authored by the ACI Committee 544 and also by the Precast/Prestressed Concrete Institute PCI and in Japan even if very often devoted to classical composite materials with softening behaviour, and with classical steel fibers. For this reason there still exists an immediate need of material characterizations and design guidelines for other kinds of fibers and above all for HPFRCC materials. Aim of the paper is the analysis of the tensile behaviour of FRC materials, and in the particular the evaluation of the parameters that affect the transition between the brittle and ductile behaviour. Direct tensile tests are carried out at the Laboratory of University of Rome “Tor Vergata” on FRC specimens with different amount of polymeric fibers. The expected values of residual stress are estimated on the basis of classical formulations suitably modified and calibrated with the experimental results.