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The main objective of this study was to assess the performance of concrete containing recycled springs with various volume fractions of 0·2, 0·4 and 0·6% at temperatures of 25, 100, 250, 500, 700 and 900°C. In addition, a comparison between concrete mixes containing steel and polypropylene fibres was performed using tensile and compressive strength tests to attain an optimum mix design. The results showed that the compressive and tensile strengths of concrete specimens were improved by adding the percentages of 0·2, 0·4 and 0·6% springs. However, these strengths decreased by increasing the volume fraction of springs. Furthermore, the optimal compressive strength of concrete containing springs was 2–3 times greater than that of concrete containing steel and polypropylene fibres at various temperatures, but there was no significant difference between the tensile strengths of concrete containing springs and steel fibres. In addition, the use of fibre in concrete caused a decrease in the width of the cracks created after the splitting tensile test, by up to three times.

The use of fibers in concrete improves strength, ductility, and durability of concrete. Concrete has fireproofing properties, but rebars are the most important concern of reinforced concrete structures in the event of a fire outbreak. Therefore, one of the recommendations to reduce these risks is the use of alternative materials like fibers. In this article, the effects of different temperatures on the mechanical properties of concretes with different cement contents containing steel and polypropylene fibers were studied. The specimens were placed under temperatures of 25°C, 100°C, 250°C, 500°C, and 700°C, and the results revealed that the effects of fire on concrete containing steel fibers are more damaging. Also, the compressive strength and modulus of rupture at 25°C and tensile strength at 250°C have the maximum values. Compressive and tensile strengths of concrete containing steel fibers were 40 % and 50 %, respectively, more than the concrete containing polypropylene fibers. However, their flexural strength was almost equal. By comparing the flexural strength of specimens with grade 400 and grade 700, it can be concluded that the specimens with grade 400 have more resistance by about 10 to 40 %.

Recycled fibers can enhance the compressive, tensile, and flexural strength of the concrete exposed to high temperatures. The main effect of these fibers on concrete is to reduce the generation of wide cracks developed by heat and pressure. Their behavioral mechanism is such that by bridging between cement paste and aggregates, the concrete structure is strengthened and as a result, cracks with great widths will not be developed. In this paper, the effect of elevated temperatures on the mechanical properties of concretes containing steel and polypropylene fibers as well as recycled metal springs with bore diameters of 8, 12, 16, and 20 mm, is experimentally studied. The results approve that using metal springs increases the concrete's strength for up to 80%. Among different types of fibers added to the concrete, the springs with a bore diameter of 12 mm and steel fibers could most effectively improve the compressive and tensile strengths, respectively. Furthermore, the threshold temperature at which the strengths will severely decline is measured at 500°C.

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