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

1113

3

can enhance concrete mechanical properties. PVA microfibers reduced concrete shrinkage, although a clear effect on concrete tensile strength and toughness was not identified. Hence, it can be concluded that PVA macrofibers had a major role in increasing the concrete post-cracking tensile residual strength. 2. Materials and methods Ten concrete notched beams were tested under a three-point bending test set-up. Nine specimens were made of FRC with different dosages of PVA fibers, while one plain concrete specimen served as control. To evaluate the effect of micro and macrofibers on concrete tensile and residual strength, two types of PVA fiber were considered. PVA microfibers (Fig. 1a) had nominal length and diameter of 6 mm and 26  m, respectively, and tensile strength and elastic modulus of 1600 MPa and 39 GPa, respectively (MAHAC srl (2023a)). PVA macrofibers (Fig. 1b) had nominal length and diameter of 30 mm and 600  m, respectively, and tensile strength and elastic modulus of 900 MPa and 23 GPa, respectively (MAHAC srl (2023b)). Both fiber types had a specific weight of 1.3 g/m 3 .

Fig. 1. Shape and dimensions of PVA fibers employed: a) microfibers and b) macrofibers.

Specimens were named NBT-S-L-N, where NBT = notched beam tests, S (if present) = content of PVA microfibers (in kg/m 3 ), L (if present) = content of PVA macrofibers (in kg/m 3 ), and N is the specimen number. Specimens tested and corresponding amount of fibers used are summarized in Table 1. A single concrete batch was used to cast one plain concrete beam and nine FRC beams with different amounts of PVA microfibers (2.6, 3.9, and 5.2 kg/m 3 ) and macrofibers (12.0 and 15.0 kg/m 3 ). Being a preliminary study, two specimens for each fiber combination were tested (except for the plain concrete specimen and group NBT_2.6, in which only one specimen was tested). Considering that PVA fibers have lower stiffness than steel fibers and that stiffer fibers are less likely to spread homogeneously during casting (Buratti et al. (2011)), PVA fibers were progressively added to the mix and the exact amount of concrete needed for each beam was poured subsequently to control the fibers amount and their homogeneous distribution. This was possible thanks to the small amount of concrete needed to cast the specimens presented in this work. A concrete mixer with 170-liter capacity was used and the casting time was approximately 20 minutes. The same concrete batch was used to cast three 102 mm-diameter×203 mm-height plain concrete cylinders. The concrete mix design used for the specimen manufacturing was comprised of cement (Portland cement type II), water, fine aggregate, coarse aggregate, and fly ash (maximum aggregate size 16 mm) with a mixture proportion by weight of 2.0:1.0:4.7:4.7:0.1 (w/c=0.5). A liquid superplasticizer (Master Builders Solutions (2022)) was added in 0.3%wt. of the cement. Compressive tests performed after 28 days from casting on three concrete cylindrical specimens provided an average cylindrical compressive strength, f ’ c , of 40.4 MPa (CoV=2.21%) (European Committee for Standardization (2004)). All specimens were subjected to a three-point bending test according to the draft of the report developed by the Joint ACI/ASCE 446 Technical Committee (Carloni et al. (2019), Zhao et al. (2022)). The test set-up is reported in Fig. 2. The specimen depth and height were D = 152 mm and H = 152 mm, respectively, and they had a length L of 550 mm. A V-shaped tip notch with a nominal width of 3 mm and nominal depth of one-third of D ( a 0 =50.8 mm) was cut at the beam midspan using a diamond saw with a V-tip. Two steel plates were epoxy-glued in correspondence of the support points on the FRC notched beams, which were positioned on two steel support cylinders with a loading span S equal to three times the depth D of the beam (456 mm). The load was applied with an additional steel cylinder placed at the mid-span of the beam. A clip-on-gauge was mounted at the bottom edges of the notch to measure the