PSI - Issue 64

Veronica Bertolli et al. / Procedia Structural Integrity 64 (2024) 1111–1117 Veronica Bertolli , Tommaso D’Antino , Christian Carloni/ Structural Integrity Procedia 00 (2019) 000 – 000

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enhancement of concrete tensile strength. Thus, the amount of microfibers to be added in FRC could be limited to that needed to avoid plastic shrinkage cracking of concrete (e.g., 1.0 kg/m 3 (Brandt (2008)). A post-cracking residual tensile strength was observed for specimens with both micro and macrofibers (Fig. 3b), with average values of f R 1 = 2.54 and 2.52 MPa for specimens NBT-5.2-12 and NBT-5.2-15, respectively, and f R 2 = 2.52 and 2.31 MPa for specimens NBT-5.2-12 and NBT-5.2-15, respectively. The amount of microfibers used in this work did not allow for determining a clear effect on the concrete mechanical properties, whereas the amount of macrofibers used entailed for a clear enhancement of concrete toughness. The addition of microfibers tended to limit the mixture workability. Fig. 4a shows specimen NBT-5.2-15-2 at failure, whereas Fig. 4b shows its crack surfaces, in which a homogeneous distribution of macrofibers along the crack can be observed. The results of the three-point bending tests reported in this work were compared with those reported in Bertolli et al. (2023), which were performed on the same type of PVA-FRC, which comprised specimens with 6.0, 26.0, and 39.0 kg/m 3 of macrofibers and specimens with both microfibers (2.6, 5.2, 7.8, and 13.0 kg/m 3 ) and macrofibers (12.0 and 26.0 kg/m 3 ). From this comparison, it can be noticed that the amount of macrofibers up to 12.0 and 15.0 kg/m 3 entailed for a softening behavior after cracking, whereas higher amount of macrofibers (within the range 26.0-39.0 kg/m 3 ) entailed for a hardening behavior after cracking (Bertolli et al. (2023)). However, amount higher than 15.0 kg/m 3 may cause an uneven distribution of fibers within the concrete mix and reduces concrete workability. A clear effect of the addition of PVA microfibers up to 5.2 kg/m 3 was not identified.

Fig. 4 Specimen NBT-5.2-15-2 a) at failure and b) crack surfaces.

5. Conclusions In this paper, the effect of micro and macro PVA fibers on the performance of fiber-reinforced concrete notched beams was investigated. Three-point bending tests were performed on plain concrete and fiber-reinforced concrete (FRC) specimens. Results showed that the amount of microfibers considered in this work did not provide a significant change in the concrete tensile and post-cracking residual tensile strength, whereas when both micro and macrofibers were considered an enhancement of concrete post-cracking tensile residual strength was observed. Comparison with results obtained in a previous work on the same PVA-FRC highlighted a possible synergy between micro and macro PVA fibers, but further investigations are needed to better identify their role and to select the minimum amount of micro and macrofibers required to enhance concrete toughness. Acknowledgements The support of the RES515729 from National Center for Transportation (Washington State University) is gratefully acknowledged.