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Mixed-mode fracture toughness of high strength FRC: a realistic experimental approach

Hussien M. A., Moawad M., Seleem M. H., Sallam H. E. M., El-Emam H. M.

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 4

art. no. e168, 2022

Unfortunately, fibrous composite materials' mixed-mode fracture toughness (Keff) was measured using inappropriate through-thickness cracked (TTC) specimens. The problem with such specimens is the ignorance of the fibers in the pre-notch surfaces, i.e., no fiber bridging behind the crack tip. In the present paper, a real Keff of fiber-reinforced concrete (FRC) was experimentally determined using matrix cracked (MC) specimens. Traditional (TTC) specimens were also adopted for comparison. The effect of fiber length (35 mm, 50 mm, and hybrid fibers, 50% from each length) and mode of mixity (Me), Me = 0, 1/4, and 1/2 were studied. Hooked end steel fibers of a volume fraction equal to 1% were used. All cracked beams with a crack-length-to-beam-depth ratio equal to 0.3 were tested under three-point bending in mode I and mixed-mode. The span/depth ratio was equal to two for all specimens. Since there is no equation to predict the Keff of MC specimens and the inapplicability of Griffith's theory to predict the Keff due to the difference in crack paths, new realistic procedures were suggested to overcome this dilemma. The results indicated that MC specimens recorded a lower crack initiation load than the peak load. In contrast, the crack initiation load coincides with the peak load in the case of TTC specimens. This reflected the role of steel fibers behind the crack tip in retarding the specimens to reach their ultimate capacity after crack initiation. Keff increased with increasing Me. Although long fibers recorded higher peak load and energy, their effect on Keff of MC FRC specimens was marginal due to the minor effect of fiber length on the crack initiation loads. The MC specimen is a realistic approach for estimating the Keff of FRC.

Flexural behavior of functionally graded concrete beams with different patterns

Othman M. A., El-Emam H. M., Seleem M. H., Sallam H. E. M., Moawad M.

Archives of Civil and Mechanical Engineering

2021

Vol. 21, no. 4

568--583

Flexural behavior of functionally graded concrete (FGC) beams was experimentally investigated. Fourteen sets of beams, including full depth (FD) fiber-reinforced concrete (FRC) and FGC with different patterns and fiber volume fractions (Vf%), were investigated under three-point bending. These patterns consisted of three layers with a constant middle part of lightweight concrete. The upper layer in the compression zone was either normal strength concrete or FRC having different Vf%. The lower layer in the tension zone was made from either FD FRC has the same Vf% or functionally graded FRC. The fibers used were hooked end steel fibers with Vf% of 0.5, 1.0, and 1.5%. The experimental results were also analyzed numerically and analytically. The experimental results showed that the flexural strength of FGC patterns ranged between 94 and 100% from that of FD FRC beams. However, their toughness indices ranged between 49 and 93% of the corresponding value of FD FRC beams. These ratios depend on Vf% and the presence of fibers in the compression zone. The effect of Vf% is more obvious in the descending part of the load–deflection curve than the ascending part due to the presence of fibers bridging phenomenon following the maximum load. Vf% is more pronounced in the descending portion in all FGC patterns than in the FD FRC beams. There is a good agreement between the experimental results and those predicted by analytical and numerical models.