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Conventional construction cementitious composites typically contains cement, fine and gravel aggregate and additive in a specific ratio along with water. It is extensively used to bind the structural elements together like the bricks, stones, and concrete blocks, or end connection of the column and beam, and to develop a sufficient bond with the substrate as a repair cementitious composite, due to its several advantages, such as low cost, appropriate compressive strength, and easy access. However, some weaknesses of the cementitious mortar influence its performance under different conditions. For example, low tensile strength, brittle behavior, unacceptable performance against shrinkage cracks, and lack of resistance against stress concentration are some of these critical properties of the mortar, if not modified, the structures will be deteriorated in a short time. These deficiencies emerge from extravagance water, bleeding, plastic settlement, shrinkage stress and strain concentrations due to external limitations. When loads are applied and further increased, type of cracks grow and reach a critical condition, and catastrophic failure is precipitated. In this situation, the mortar will be exposed to severe damaging factors such as premature saturation, disadvantage of freeze-thaw, scaling, and corrosion of steel. In recent years, researchers in the field of concrete technology have focused on the using of a variety of fibers such as carbon, steel, glass, polypropylene and basalt fiber into the cementitious composites to improve their mechanical properties (especially their ductility behavior) and to some extent their durability. Accordingly, in the present study, the hybrid effect of different percentages of basalt and polypropylene fibers on the workability, mechanical behavior and durability properties of cementitious mortar was investigated. Polypropylene fibers are known in the field of reinforced concrete, but basalt fibers are a new potential additive in this field. In recent decades, researchers have studied more about basalt fibers because of their potential reinforcement properties in composite materials. The basalt fibers are an appropriate replacement for another fibers, including glass, steel, and carbon fibers in plenty of applications due to their excellent properties such as high mechanical properties specially tensile strength and flexural strength, good resistance to low and high temperature, low cost, durability, vibration resistance, high elasticity modules, great failure strain, acceptable persistence to chemical assault, impact load and fire with less toxic materials. Samples containing a hybrid composition of 0.05 and 0.125 percent (weight percent of total cement and aggregate) of the basalt and polypropylene fibers have been used in order to evaluate the effect of fibers so that a total of 4 types of mixed designs containing hybrids of basalt and polypropylene fibers were made and its results have been compared with a control sample. As expected, after analyzing the results, the fibers had no significant effect on the compressive strength of the cementitious composite, while the results of this study reported a favorable and remarkable performance of these fibers in increasing flexural and splitting tensile strength, as well as the water absorption of cementitious mortar is favorably decreased by the fiber. The sample containing 0.125% basalt and polypropylene fibers increased flexural and splitting tensile strengths by 28 and 23%, respectively. Also, the sample containing 0.125% basalt fiber and 0.05% polypropylene fibers resulted in 9.3% increase in compressive strength, 24% decrease in sorptivity and 15% water absorption. The results of the current study have shown that the simultaneous use of basalt and polypropylene fibers can improve significantly the mechanical behavior and durability properties of cementitious mortar, whereas according to the previous studies if each of these fibers is used separately, such a desirable result will not be obtained.

Keywords: Cementitious mortar, basalt fiber, polypropylene fiber, mechanical properties, durability

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