The production and use of durable materials in construction are considered as one of the most challenging things for the professional engineers. Therefore, this research was conducted to investigate the mechanical properties and the durability by using of different percentages of steel fiber with high-strength flowable mortar (HSFM) and also the use of the hybridization of steel fibers, palm fibers, and synthetic fiber (Barchip). Different experimental tests (compressive strength, splitting tensile strength, flexural strength, and static modulus of elasticity among others) were determined after 90 days of normal water curing and 180 days of seawater exposure. The results indicate that hybrid fibers of 1.5% steel fibers？+？0.25% palm fibers？+？0.25% Barchip fibers provide significant improvement in the different mechanical properties of HSFM. Besides, the hybridization of fibers was found to be effective in the terms of durability (exposure to seawater). Therefore, the minimum reduction in static modulus of elasticity, compressive, splitting and flexural strength was obtained for the HSFM mixes of hybrid fibers using steel fibers with palm fibers and also for the use of steel, palm, and Barchip fibers. 1. Introduction High-strength concrete or mortar subjected to axial compression is known to be a brittle material with almost no strain-softening behavior. Adding fibers to plain matrix has little or no effect on its precracking behavior but does substantially enhance its postcracking response, which leads to a greatly improved toughness and impact behavior [1]. Besides that, ductility in fiber reinforced cementitious composites is enhanced because fibers bridge crack surface and delay the onset of the extension of cracks [2]. Shah and Naaman [3] investigated tensile strength, flexural strength, and compressive strength tests on mortar specimens reinforced with steel fibers. It was observed that the tensile or flexural strength of steel fiber-reinforced mortar was at least two to three times higher than that of plain mortar. However, the addition of steel fibers at higher dosages has some disadvantages in terms of poor workability and higher cost. In addition, the high stiffness of steel fibers in the matrix means that voids and honeycombs could be formed during placing as a result of improper compaction at low workability. In order for good flow ability of mortar, the addition of two or three different fiber types can yield the optimum performance [4, 5]. Sustersic et al. [6] stated that the effect of corrosion of surface steel fibers is not detrimental to
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