Issue 42

A. Jadidi et alii, Frattura ed Integrità Strutturale, 42 (2017) 249-262; DOI: 10.3221/IGF-ESIS.42.27 261 C ONCLUSION Concrete tensile strength . Reinforcing samples with steel fibers, results in considerable increase in tensile strength. The more fibers used in concrete, the more its tensile strength increases 2. The results show that with the use of steel fibers after 3 days, 7 days and then 28 days, the concrete tensile strength increases in all cases. The tensile strength is higher in concrete containing hybrid steel fibers 3. By Reinforcing samples with steel fibers, the behavior of the concrete under tensile strength takes on more plasticity than samples with no steel fibers. Based on these results, the fracture of samples without steel fibers has been brittle, whereas those of the samples reinforced with steel fibers have been gradual and soft. Therefore, after samples crack, no fractures occur and the fibers join aggregates to each other. 4. The highest 28-day tensile strength exists in samples reinforced with 2.5% hybrid steel fibers (hooked steel fibers L/D=30 and crimped steel fibers L/D=50) with 7.11 MPa and 80% tensile strength increase. 5. The highest 7-day tensile strength exists in samples reinforced with 2.5% hybrid steel fibers (hooked steel fibers L/D=30 and crimped steel fibers L/D=50) with 5.48 MPa and 83.27% tensile strength increase 6. The highest 3-day tensile strength exists in samples reinforced with 2.5% hybrid steel fibers (hooked steel fibers L/D=30 and crimped steel fibers L/D=50) with 3.98 MPa and 73.79% tensile strength increase 7. The lowest tensile strength increase occurred in samples containing 1.5% hooked steel fibers (L/D=30) with 18.34%, 37.79% and 38% in the ages of 3, 7 and 28 day respectively. 8. The lowest tensile strength increase occurred in samples containing 1.5% hooked steel fibers (L/D=50) with 17.03%, 38.79% and 41.77% in the ages of 3, 7 and 28 day respectively. Concrete bending strength 1. The results acquired from the study show that after 28 days, there is a significant rise in bending strength, and the 28- day bending strength of all samples has increased. 2. The use of fibers considerably increases bending strength which is noticeably related to fiber type and content. Fiber increase with higher percentages had greater effect. 3. The lowest bending strength increase occurred in samples containing 1.5% steel fibers (L/D=50) with 5.16% MPa and 70.86% bending strength increase 4. The highest 28-day bending strength exists in samples reinforced with 2.5% hybrid steel fibers (hooked steel fibers L/D=30 and crimped steel fibers L/D=50) with 12.96 MPa and 329% bending strength increase. 5., Another important impact of adding steel fibers to concrete in bending test, as in indirect tensile test, is increased toughness and energy absorption which play an important role in the type of concrete fracture. The use of fibers causes a considerable increase in concrete tensile toughness and softer behavior. Compressive strength The results of compressive strength test of samples reinforced with 1.5%, 2% and 2.5% steel fibers are as follows: 1. Using steel fibers in this study decreased the efficiency of concrete. Based on these results, in cases where steel fibers are employed the compressive strength of some samples decreased. The use of pozzolanic materials caused a significant increase in the compressive strength of fiber reinforced concrete. Based on these results, the highest 28-day compressive strength with 22.84 compressive strength increase exists in samples reinforced with 2.5% hybrid steel fibers (hooked steel fibers L/D=50 and crimped steel fibers L/D=50). Further, the highest amount of compressive strength decrease with 6.39% exists in crimped steel fibers (L/D=50). 2. One of the reasons for compressive strength increase of steel fiber-reinforced concrete is the application of micro silica with its filling and pozzolanic properties. The pozzolanic materials react to calcium hydroxide in the concrete and by producing more cement gel make the concrete denser which in turn increases the compressive strength. 3. The increase in compressive strength of concrete by adding fibers is not significant; however, adding fibers increases concrete compressive toughness and makes its behavior softer. Moreover, the increase in the weight percentage of fibers in hybrid samples increases their compressive strength and combining fibers has positive impact on the degree of compressive strength. 1