## 4 Results and Discussion  
Since the compression and tension strengths of concrete determine the bending performance of concrete beams and their cracking, the impact of the SF and RCA contents on the concrete compression capability was studied. The results are represented in Figure 11. According to this figure, the cracking occurred only in the concrete surface when SFs were added and the specimen did not collapse, which became more perceptible when 2% SFs were added. The lack of concrete tensile strength led to collapsing of the specimen and the cores of the specimens were broken. Hence, by adding SFs, a resistant core of concrete specimens prevented it from collapse, and only the specimens' surface was cracked and detached. This issue resulted from the bridging role of SFs that kept the particles close together and increased the tensile strength of the concrete. Alternatively, due to RCA's higher and more angled specific surface compared to NCA, they were better impregnated with cement paste, which prevented the complete disintegration of the sample. Hence, the incorporation both SFs and RCA increased the adhesion between the cement paste and the aggregates and fibres. Therefore, more force was needed to separate the particles from each other and break than the unreinforced concrete, which improved the behaviour of the concrete.  
The tensile behaviour of the concrete specimens with RCA and SFs was also studied. In order to investigate the failure of specimens under the Brazilian test (splitting tensile strength), the results are presented in Figure 12. According to this figure, a specimen with no SFs was divided into two parts. However, the addition of SFs prevented the two parts from separating when it failed due to the bridging role of the fibres in the concrete paste. Furthermore, using RCA weakened the tensile strength of the specimen, which could be compensated for by adding SFs. Hence, RCA could be used as a replacement for NCA when SFs were used, with no considerable reduction in the tensile strength.  
Figure 10: Test setup: (**a**) four-point bending test of the beams and the (**b**) compressive stress-strain setup.  
Figure 11: Effects of the SF and RCA contents on the concrete’s compressive behaviour.  
Figure 12: Effects of the SF and RCA contents on the concrete’s tensile behaviour.  
To evaluate the compressive and tensile behaviour of the unreinforced concrete in detail, the stress-strain performance of the specimens produced using different RCA and SF contents were studied. Figure 13 shows the stress-strain curves of the tested specimens.  
Each stress-strain curve was obtained by averaging the stress-strain outcomes of three specimens. These relationships were determined using uniaxial compression tests. Using SFs led to an increased strain, which increased the deformation of the concrete and reduced the crack widths, but it did not have an impact on the maximum stress. Therefore, the strain was significantly improved when 2% SFs  
Figure 13: Stress–strain curves of the specimens (**a**) with no RCA, (**b**) 50% RCA and (**c**) 100% RCA.  
were used and the concrete beams could be deformed more without a sudden collapse with the use of SFs. Contrariwise, raising the RCA content reduced the maximum strain but the maximum stress increased. The maximum stress and strain increased when both RCA and SFs were used. Therefore, NCA could be replaced by RCA when SFs were used with no reduction in the tensile performance of the concrete. Furthermore, trend lines with high R\({}^{2}\) values were determined in order to predict the stress and strain of the RCARC. As per Figure 13, the maximum error between the experimental outcomes and presented formulas was 6%, which indicates a high accuracy. Table 4 presents the obtained formulas for different specimens.