Modellazione analitico-numerica e comportamento sperimentale di elementi in calcestruzzo fibro-rinforzato

The aim of the paper is the characterization of the mechanic properties and the constitutive law, for the usual utilization in the design practice, of the composite Fiber reinforced concrete material (FRC). Nowadays in this field a great interest is represented for this material, both new structural element construction and external application of a thin layer in old or damaged conventional concrete elements. The addition of polymeric or steel fibers to a cementitious matrix increases the tensile strength and provides a residual strength in tension: this condition particularly proves, for usual content of fiber (1% - 2%), that the FRC material presents softening branches and residual strength; on the contrary for larger content of fiber (almost 4%) the global response of the FRC material is ductile and the dangerous phenomena of strain localization generated near the cracks is more reduced. This condition produces the possibility of optimal use of the new material in the structural design (also for strengthening intervention) because it assures adequate ductility in order to dissipate the energy related expecialy to seismic phenomena. It’s required a deep evaluation of the constitutive material response; for the constitutive law identification the international and Italian guidelines suggest a “prestactional” approach. In Italy the CNR DT204/2006 document proposes the mechanical characterization with the direct traction test or the undirected 4 points flections test. The equivalent constitutive response obtained with this method is also simplified. In this paper the constitutive law of FRC and the influence of fibers content is evaluated with experimental analysis for the three points flections - beams and for special traction specimens in accordance with CNR DT 204/2006. There are also proposed a numerical model and an analytical model that reproduce the experimental tests; the analytical and numerical models are based on the study of a cracked element and these models account for cracking, material non-linearity, tension-stiffening effects and presence of slip at the interfaces (the last one assumption only for analytical model). The numerical model allows catching bi - dimensional and three – dimensional aspects. It’s shown a good agreement in the graphic results in the paper. Finally, in order to define the optimum material for construction or strengthening intervention in seismic area, the ductility of r.c. real beams in FRC or r.c. real beams damaged for vertical and seismic actions (and then reinforced with FRC material with high performance) is evaluated with analytical and numerical examples.