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Nowadays, the use of recycled concrete has increased significantly for economic and environmental reasons. Increasing the replacement percentages of recycled aggregates change the mechanical properties of concrete. In this research, the mechanical properties of concrete containing recycled aggregates with different percentages of steel fibers and polypropylene fiber has been investigated in the structural laboratory. A total of 36 cylindrical compression specimens, 36 cylindrical tensile specimens with dimensions of 20 * 10 cm and 36 flexural specimens with dimensions of 10 * 10 * 35 cm were tested. Recycled aggregates (coarse aggregates) in ratios of 25 and 50% (weight ratio) replaced with natural materials (coarse aggregates). Also, steel and polypropylene fibers were added to recycled concrete samples in ratios of 0%, 0.5%, 1% and 0%, 0.4%, respectively. Fiber concrete samples containing recycled aggregates under compressive force, indirect tension and three-point bending are tested and factors such as compressive strength, tensile fracture toughness, modulus of elasticity, flexural strength and fracture energy were investigated. The results show that the increase in porosity in recycled concrete is affected by the increase in the percentage of replacement of recycled aggregates and reduces the specific gravity of concrete. Increasing the composition of 1% steel fibers with polypropylene fibers without the presence of recycled aggregate has a greater effect on increasing the specific gravity of concrete than polypropylene fibers (alone). By increasing 25 and 50% replacement of recycled aggregate, 0.8% and 2.5% of specific gravity of concrete were reduced, respectively. Compressive strength decreases with increasing replacement percentage of recycled aggregate. Increasing the percentage of replacement of recycled aggregate due to poor transmission area reduces the amount of energy absorption. So that by replacing 25% and 50% of recycled aggregate, the energy absorption rate decreases by 21.7% and 26%, respectively, compared to the control samples. Polypropylene fibers have a positive effect on increasing compressive strength and combination of polypropylene fibers with 0.5 and 1% steel fibers increases the energy absorption. Also, increasing the replacement percentage of recycled aggregates reduces the hardness. Maximum tensile strength decreases by 3% and 13% with 25 and 50% increase in replacement of recycled aggregate, respectively. The results of flexural strength test show that increasing the replacement percentage of recycled aggregate has a negative effect on reducing the final load and reduces the final load in flexural specimens. Also, polypropylene fibers have a positive effect on increasing the loads and preventing the collapse of concrete, and combining polypropylene fibers with 0.5 and 1% steel fibers, respectively, increases the final load by 20% and 95% in 25%, replacing recycled aggregates and increasing 19% and 21% of rainfall in 50% recycled aggregate replacement. Increasing the percentage of steel fibers, the amount of deformation in the area after cracking and the amount of energy absorption increases, so that by increasing the amount of steel fibers to 0.5% and 1% by volume of concrete, the amount of energy absorption to 2 and it increases 3 times. The use of polypropylene fibers in the area after cracking has little effect and increases the load capacity.

Keywords: Recycled Concrete, Fiber reinforced Concrete, Steel Fiber, Polypropylene Fibers, Compressive Strength

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