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Real fracture toughness of FRC and FGC: size and boundary effects

Elakhras A. A., Saleem M. H., Sallam H. E. M.

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 2

art. no. e99, 1--17

The present dilemma is how to simulate the real crack in full depth (FD) fiber-reinforced concrete (FRC), FD FRC, to get the actual fracture toughness of such fibrous composites, i.e., through-thickness pre-cracks are inappropriate for such materials. To overcome this dilemma, a new technique was adopted to create a pre-matrix crack (MC) without cutting the fibers bridging the two surfaces of the pre-crack. The main objective of the present work is to study the size and boundary effects on the real fracture toughness of MC-FD FRC and functionally graded concrete (FGC). Forty-eight MC-FD FRC and MC-FGC beams with three different span to depth ratios L/d equal 4, 5, and 6, and three different beam depths of the same beam span have been tested under three-point bending. All beams have the same pre-MC length to beam depth ratio (ao /d) of 1/3. Hooked end steel fibers of 1% fiber volume fraction produced FRC. FGC beams consist of three equal layers, FRC layer at the tension side, normal strength concrete layer at the middle of the beam, and high strength concrete layer at the compression side. The applied load versus all beams' crack mouth opening displacement (CMOD) curves have been analyzed. The present load/ CMOD results showed that beams having constant L/d ratios are recommended to capture independent size effect parameters. The size effect law (SEL) and boundary effect model (BEM) are good candidates to predict the size effect. According to the maximum non-damaged defect concept, the SEL is more reliable in predicting MC FD FRC fracture toughness than BEM.

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.