Experimental investigation of effect of content and length of steel fiber on mechanical properties and impact resistance of high-strength concert

Various fibers have been used by researchers to reinforce concrete and extend the service life of structures, also researchers are using different fibers to improve behavior of different concretes, especially high-strength concrete, against impact and dynamic loads. This study investigate the effect of different contents and lengths of steel fiber on the mechanical properties of high-strength concrete. In this study, wavy hooked-ended fibers of two length (30, 50 mm) with three volume fraction 0.5%, 1% and 1.5% were added to concrete mixes and 150×300 mm cylindrical specimens were made, then different tests were performed for determination of compressive strength, splitting tensile strength and impact resistance at 7 and 28 days, in accordance with standards and procedure proposed. The results indicated that the addition of different contents and lengths of steel fiber caused significant change in the mechanical properties of high-strength concrete. In the best case, using 1.5% of 50 mm long steel fibers, increased compressive strength and splitting tensile strength by 25% and 40%, respectively, compared to non-fibrous high-strength concrete. A remarkable improvement was observed in impact resistance of the fibrous concretes, as compared with the reference materials. By incorporating steel fibers into the mixtures, specially longer fibers, a conclusive increase in the number of blows required for first and final cracking (as compared to reference values) was observed, as well as the number of blows from the first cracks to the final failure in the high percentage of the fiber increased up to  80%. Moreover, it can be concluded that, by adding fiber, the failure crack pattern was changed from a single crack to a group of narrow crack, which demonstrate the beneficial effect of fiber reinforced concrete when subjected to impact loading.