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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1. Introduction Thanks to the availability of its components and their low cost, concrete is one of the most important building materials. Concrete is a quasi-brittle material with high compressive strength and limited tensile strength. To improve its tensile mechanical properties, short fibers can be added as reinforcement to create fiber-reinforced concrete (FRC). FRC is a composite material in which short dispersed fibers are added to concrete to enhance its tensile and post cracking tensile residual strength and can be effective in delaying the formation of cracks both at the micro and macro level. Indeed, fibers can inhibit the formation and growth of microcracks and avoid unstable propagation of macrocracks through the so called crack bridging effect (Banthia and Gupta (2004), Naaman (2018)). The FRC residual strength depends significantly on the number of fibers crossing cracks and on their orientation (Barros et al. (2005)). Studies on FRC focused on both fresh and hardened concrete (Zollo (1997), Di Prisco et al. (2009)), since the addition of fibers to concrete reduces its workability. Depending on shape, dimension, and mechanical properties, specific types of fiber can enhance different concrete properties. Usually, high-modulus fibers are used to substitute conventional steel transverse reinforcement, whereas small fibers with low modulus can be used to reduce shrinkage cracking and improve fire resistance (Brandt (2008), Di Prisco et al. (2009), Signorini et al. (2020)). Short dispersed alkali resistant (AR) glass fibers are often used in non structural concrete applications (e.g., precast panels (Bartos (2017)) thanks to their resistance to the highly alkaline concrete environment. Synthetic fibers such as polypropylene (PP) fibers are used in concrete in a volume percentage of 0.5-2%vol. to increase concrete toughness, to avoid the formation of shrinkage cracks, and to increase concrete fire resistance - they relieve internal pressure of concrete thanks to their sublimation (Naaman and Reinhardt (1996), Rossi (1997), Nobili et al. (2013)). Their main applications are road and airport pavements (Conforti et al. (2019)). Although steel fibers nowadays remain the most used type of fiber in concrete (Di Prisco et al. (2009), Plizzari (2018)), synthetic fibers represent a promising alternative due to their resistance to aggressive environments and lightweight. However, due to their hydrophobic nature, workability of the concrete mix and mix procedures should be carefully designed. Steel FRC (SFRC) is used in structural elements when crack propagation control is of primary importance, such as in precast tunnel segments, or in beams without internal transverse shear reinforcement (Plizzari (2018), Facconi et al. (2021)). In the correct dosage and for specific applications, the addition of steel fibers can replace the use of mild steel reinforcement (Visintin and Oehlers (2018)). Cracking phenomenon of SFRC is characterized by narrower and more closely spaced cracks (Tiberti et al. (2015)). Steel fibers usually have a diameter equal to 0.4 mm, length of 30 mm, can have different shapes, e.g., can be crimped, hooked or twisted (Xu and Zhang (2009), Plizzari (2018), Martinelli et al. (2021), Facconi et al. (2021)), and are added to concrete in 1-12%vol. (Di Prisco et al. (2009)). Polyvinyl alcohol (PVA) fibers were recently introduced in concrete mix designs (Mostofinejad et al. (2022)). PVA microfibers with diameter of 0.1-0.6 mm and length of 5-10 mm are mainly used to reduce the plastic shrinkage cracking of concrete, whereas PVA macrofibers (with diameter of 0.6-1.5 mm and length of 30-60 mm) are used to enhance concrete toughness and control crack propagation (Dopko et al. (2018)). The latest development in FRC is hybrid-FRC (HyFRC), in which two or more types of fiber can be suitably combined to improve concrete physical and mechanical properties, taking advantage of a possible synergistic effect. Fibers made of different materials and/or different sizes can be used, and different effects seem to be triggered by varying the fiber dosage. Usually, steel or synthetic (PP or PVA) macrofibers are combined with synthetic (PP or PVA) microfibers (Banthia and Gupta (2004), Banthia and Sappakittipakorn (2007)). Considering the difficulties faced when performing uniaxial tensile tests, constitutive laws for FRC are determined by means of three-point bending tests on small notched beams following the prescription of EN 14651 (European Committee for Standardization (2007)) or ASTM C1609 (ASTM International (2017)), which allow to obtain the nominal stress (  N ) – crack mouth opening displacement (CMOD) curves. FRC design follows a performance-based approach. Depending on a large number of parameters (such as fiber type, dosage, and shape), FRC can exhibit a hardening, softening, or plastic behavior in bending after cracking. In this paper, a preliminary experimental campaign was carried out to investigate the effect of PVA fibers with different dimensions (namely microfibers and macrofibers) on concrete tensile strength and toughness, with particular emphasis on their possible synergy. Three-point bending tests were performed on notched PVA-FRC and plain concrete beams following the European standard EN 14651 (European Committee for Standardization (2007)) or the draft of ACI/ASCE 446 Technical Committee report (Carloni et al. (2019), Zhao et al. (2022)). Different dosages of PVA fibers were considered. Results showed that in the correct dosage the addition of micro and macro PVA fibers