Issue 10

B. Chiaia et alii, Frattura ed Integrità Strutturale, 10 (2009) 29-37; DOI: 10.3221/IGF-ESIS.10.04 29 Sismabeton: a new frontier for ductile concrete Bernardino Chiaia, Alessandro P. Fantilli, Paolo Vallini Politecnico di Torino, Dep. of Structural and Geotechnical Engineering Corso Duca degli Abruzzi, 24 -10129 Torino, Italy fantilli@polito.it, vallini@polito.it, chiaia@polito.it R IASSUNTO . I calcestruzzi fibrorinforzati ed autocompattanti (definiti Sismabeton) manifestano una elevata duttilità non solo in trazione ma anche in presenza di sforzi compressione. Ciò e messo in evidenza nel presente lavoro attraverso la misura della risposta meccanica, in regime di compressione triassiale, di calcestruzzi ordinari (NC) ed autocompattanti (SC) con e senza fibre. In strutture semplicemente compresse, la presenza del Sismabeton è da sola sufficiente a garantire un confinamento attivo uniforme. A BSTRACT . The high ductility of Fiber Reinforced Self-consolidating concrete (called Sismabeton) can be developed not only in tension but also in compression. This aspect is evidenced in the present paper by measuring the mechanical response of normal concrete (NC), plain self-compacting concrete (SC) and Sismabeton cylindrical specimens under uniaxial and triaxial compression. The post-peak behaviour of these specimens is defined by a non-dimensional function that relates the inelastic displacement and the relative stress during softening. Both for NC and SC, the increase of the fracture toughness with the confinement stress is observed. Conversely, Sismabeton shows, even in absence of confinement, practically the same ductility measured in normal and self-compacting concretes with a confining pressure. Thus, the presence of Sismabeton in compressed columns is itself sufficient to create a sort of active distributed confinement. K EYWORDS . Fiber-reinforced concrete, self-compacting concrete, confining pressure, triaxial tests, fracture toughness. I NTRODUCTION everal reinforced concrete (RC) structures fail via concrete crushing in compressed zones. This is the case, for instance, of over-reinforced concrete beams, like those in four point bending tested by Mansur et al. [1] . When fiber-reinforced, the post-peak behaviour of such members is remarkably more ductile than that observed in beams having the same geometry, the same steel rebars, and the same bearing capacities, but made of normal concrete (NC) without fiber. Thus, when crushing occurs, the type of concrete rules both the mechanical response and the ductility of RC structures. The experimental campaign conducted by Khayat et al. [2] on highly confined RC columns, subject to concentric compression, also confirms the influence of the cement-based composites on the structural performances. More precisely, for a given cross-section, the load vs. average axial strain diagrams appear more ductile in the case of columns made of self-compacting concrete (SC) than in NC columns. These experimental observations can be usefully applied to designing RC compressed columns in seismic regions. According to Eurocode 8 [3], if a required ductility cannot be attained because concrete strains are larger than 0.35% , a compensation for the loss of resistance due to crushing can be achieved by means of an adequate confinement. Such a confinement, usually provided by transversal steel reinforcement (i.e., stirrups), and indicated by the confining pressure  3 (Fig.1), allows designers to consider a more ductile stress strain (  c -  c ) relationship in compression. For instance, Fig.1 shows the so-called parabola-rectangle diagrams proposed by Eurocode 2 [4] for confined and unconfined concretes. S