Issue 10

B. Chiaia et alii, Frattura ed Integrità Strutturale, 10 (2009) 29-37; DOI: 10.3221/IGF-ESIS.10.04 35 However, a direct comparison between the analyzed concretes is not possible in terms of nominal stress and strain, because specimens have different nominal strengths. Post-peak comparison in terms of F(w) A more accurate comparison between the post-peak responses of Sismabeton, NC and SC under compression can be conducted in terms of F ( w ) (Fig.7). In particular, for a given  c   c1 , the decrease of compressive stress  c = f c -  c (and F =  c / f c ) can be obtained through the  c -  c diagrams experimentally evaluated (Fig.6), whereas the corresponding w (Fig.3a) can be obtained from Eq.(3) ( f c ,  c1 , E c and H are known from the tests). The F ( w ) curves reported in Fig.7 are limited to w = 2mm, when compressive strains  c are relatively high although, in some cases, stresses are higher than zero. However, in all the tests the relative stress F =  c / f c decreases with w . The dashed curves reported in Fig.7 r epresent the behaviour of NC and SC as predicted by Eq.(4) in the case of zero confinement. As in the case of  3 = 0 the post-peak responses of the specimens NC0, SC0, SC0b are correctly predicted by Eq.(4), and all the tests can be considered consistent [10]. Both for NC and SC, Fig.7a and Fig7b, respectively show the increase of the compressive fracture toughness (within the range w  0-2 mm) with the confining pressure  3 . However, this phenomenon is also evident in the case of Sismabeton, which can show, in absence of confinement, more or less the same F ( w ) obtained for NC and SC when  3 = 1MPa. Figure 7 : The post peak behaviour in terms of F ( w ). Figure 8 : The active confinement of Sismabeton. Fig.8a shows the post-peak responses of the specimens HC0 and HC0b, which are closer to those of confined SC and NC (i.e., the range defined by NC1, SC1, SC1b), than to the theoretical F ( w ) obtained in absence of confinement [10] (the dashed line i n Fig.8a) . Within the observed range ( w  0-2 mm), compressive facture toughness of different concretes can be objectively measured by the area A F under the function F ( w ):   dwwF A F   2 0 (6)