| Sumario: | Fiber-reinforced concrete (FRC), which has become quite popular in recent years, improves many of concrete’s mechanical properties. It uses fibers discretely and is utilized in different structures. This paper proposes, between steel fibers and concrete, a fiber interfacial transition zone (FITZ) which is the most vulnerable part of steel FRC (SFRC) because it has a high cracking and microcracking potential due to fiber-concrete separation. In the prepared specimens, steel fibers were added to concrete in hooked and twisted forms, the SFRC microstructure was studied in both cases under a scanning electron microscope (SEM), and the related images were compared as secondary electron (SE) images. The SEM analysis showed highly precise images of the cracks and their microstructures in the FITZ and lab results show that the newly defined FITZ illustrates the cracking patterns well for both fiber types. Because twisted fibers have cracking angles and larger contact surfaces, the concrete-fiber bond is increased and the related crack widths decrease considerably. A comparison of the crack widths showed that those in the FITZ of specimens with twisted fibers decreased by a factor of approximately seven compared to those with hooked fibers.
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