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Fracture toughness of hybrid fiber reinforced self-compacting concrete Majdzadeh, Fariborz

Abstract

There has been much enthusiasm for hybrid fiber reinforced concrete systems, in which two or more types of fibers are combined in the same concrete matrix. On the other hand, the use of self-compacting concrete (SCC), due to its unique fresh properties and social and environmental benefits, is gaining popularity worldwide. Hence, knowledge on the use of hybrid fibers in SCC deserves attention. The scope of this study encompasses two major research focuses. The first involves the production and evaluation of the fresh properties of high strength fiber reinforced self-compacting concrete and a discussion of its key attributes in the fresh state. The second involves the assessment of the mechanical properties and potential synergistic effects of various fibers in SCC. In this study, 23 mixes containing mono, double, and triple blended fibers were made. The hybrid systems were different combinations of micro fibers (carbon, polypropylene, and steel) and macro fibers (steel and polypropylene). For each mix, six 100x100x350 mm beam specimens for flexural toughness tests and six 100x200 mm cylindrical specimens for compression tests were made. Fresh properties of concrete were measured by a slump flow test. On the flowability aspects, it was found that carbon and micro-polypropylene fibers, even at low volume fractions, caused significant reduction in the flowability of SCC. However, introduction of macro fibers of steel and polypropylene in SCC did not reduce the flowability of concrete as much as micro-fibers did. The maximum amount of microfibers in the hybrid system to retain the self-compacting properties of the fresh mix was 0.25% for carbon fibers and 0.15% for micro-polypropylene fibers provided that they were used individually in the hybrid system. When hardened properties of hybrid fiber SCC where considered the study focused on flexural toughness improvements due to the presence of hybrid fibers in the SCC. Micropolypropylene fibers in combination with all types of steel fibers showed a synergistic effect, while carbon fibers were efficient only in one combination with the steel fiber. According to the results of this study, in a hybrid fiber composite, micro fibers improve the ductility of the matrix. This enhanced the performance of the macro fibers in the pullout process, and the hybrid fiber SCC showed higher flexural toughness as compared to SCC with only one fiber.

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