Issue 10

B. Chiaia et alii, Frattura ed Integrità Strutturale, 10 (2009) 29-37 ; DOI: 10.3221/IGF-ESIS.10.04 30 Short steel fibers randomly dispersed in a cement-based matrix can generate confining pressures comparable with that of stirrups. The experimental campaign of Ganesan and Ramana Murthy [5], performed on short confined columns with and without fibers (Fig.2a), investigates on this aspect. As shown in Fig.2b, the applied load- average strain ( P -  cm ) diagram of RC columns, made with ordinary concrete and a transversal reinforcement percentage equal to  s =1.6%, is more or less similar (in terms of strength and ductility) to that of fiber-reinforced (FRC) columns, made with a reduced quantity of stirrups (  s =0.6%) and FRC (volume fraction V f = 1.5%, aspect ratio L /  = 70). Figure 1 : The stress-strain relationship of compressed concrete with and without confinement [4]. Figure 2 : The columns tested by Ganesan and Ramana Murthy [5]. Although fiber-reinforcements have been introduced in order to increase the ductility of cement-based composites in tension, they can also provide a sort of confinement, and therefore higher ductility in compression. For this reason, when a better fiber matrix bond can be achieved, like the Fiber-Reinforced Self-compacting Composites [6], higher compressive fracture toughness should be expected. To confirm such a conjecture, the post-peak responses of different cementitious composites under uniaxial and multi-axial compression are here investigated. P OST - PEAK RESPONSE OF CONCRETE UNDER COMPRESSION he stress-strain relationships of concrete and quasi-brittle materials in compression (Fig.3a) can be divided into two part s (Fig.3b). In the first part, when the stress is lower than the strength f c (and  c <  c1 ), the specimen can be considered undamaged. In the case of plain concrete, the ascending branch of  c -  c can be defined by the Sargin’s relationship proposed by CEB-FIP Model Code [7]. As soon as the peak stress is reached, localized damage develops and strain softening begins. In this stage, the progressive sliding of two blocks of the cement-based material is evident. In Fig.3a, the angle between the vertical axis of the specimen and the sliding surfaces is assumed to be  =18°. This value, as measured in many tests, can be also obtained through the Mohr-Coulomb failure criterion, if the tensile strength is T