tesi A. Caggiano.pdf - EleA@UniSA - Università degli Studi di Salerno

Chapter 2. Experimental characterization of steel fiber notched concrete beamsrange [C T ODm 0 ; C T ODm 0 + 0.6mm] and [C T ODm 0 + 0.6 ; C T ODm 0 + 3.0mm] for U 1and U 2 , respectively.Figure 2.12 shows the energy absorption values of each sample, calculated by meansof the Eq. (2.4), while Figure 2.11 reports the values of strength parameters accordingto Eqs. (2.2) and (2.3) and measured for the various tested specimens. Keepingin mind the mechanical meaning of these parameters, Figure 2.11 shows that specimensreinforced with 1.0% of steel fibers exhibit a significant hardening behavior inthe flexural post-cracking regime, whereas a softening behavior has generally beenobserved for specimens with 0.5% of fibers. It is demonstrated that a higher presenceof short fibers increases the peak and post-peak strengths for small crack openings incases with 1.0% of fiber contents. On the other hand, SFRC specimens with only presenceof long fibers (L100-10) deals with a composite characterized by an immediatelypost-peak softening behavior in the first crack range [C T ODm 0 ;C T ODm 0 + 0.6mm]:i.e., f eq (0−0.6)< f l f ; then, a flexural re-hardening response takes place in the secondcrack range [C T ODm 0 + 0.6;C T ODm 0 + 3.0mm]: f eq (0.6−3.0)> f eq (0−0.6). The resultsalso clarify that the composite with the best performance has been represented by theS75-10 mixture.Work capacity [kN mm]70.0060.0050.0040.0030.0020.0010.000.00L100-10 L75-10 LS50-10 S75-10 S100-10 L100-05 L75-05 LS50-05 S75-05 S100-05U1 8.24 8.92 10.17 11.88 11.15 7.90 5.74 6.69 6.73 7.21U2 35.51 38.97 44.82 55.61 49.98 26.60 17.65 27.55 17.90 24.55Figure 2.12: Energy absorption measures U 1 and U 2 according to UNI-11039-2 [2003]. Thevertical segments quantify the min-max range observed for each concrete mixture.Experimental results on SFRC with low fiber contents (0.5%) have been characterizedby high sensitivity to the number of fibers crossing the cracked section. It follows thata lower stability of the experimental results can be recognized. Furthermore, as forthe 1.0% types, the presence of short fibers enhances the post-peak strength for smallcracks (as shown in Figure 2.11 for the cases of LS50-05, S75-05 and S100-05), but theseresidual strengths rapidly decrease, as fibers debond from the cementitious matrix.Further ductility indices can be considered as objective measurements of the fiberbridging effect. Based on the UNI-11039-2 [2003] code, the two following ductility40