## 3 Results and Discussion
### Stress-Strain Curve  
Figure 12 shows the uniaxial compressive stress-strain curves for recycled concrete and natural concrete. The uniaxial compressive stress-strain curves of the RC specimen were significantly lower in both uniaxial compressive strength and slope compared to the NC specimen, as shown in Figure 12. The main reason for this is that the old mortar adhering to the surface of the recycled coarse aggregate affects the adhesion between the aggregate and the cement matrix, resulting in more weak interfaces within the recycled concrete, and therefore the uniaxial compressive strength and elastic modulus of the recycled concrete were significantly reduced. Figure 11: Flexural strength of fibre-reinforced recycled concrete. The uniaxial stress-strain curve for single admixture fibre recycled concrete is shown in Figure 13. The uniaxial compressive strength of SF recycled concrete increased with the increase in fibre admixture and increased by up to 14.2% compared with that of non-fibre recycled concrete. The softening branch tends to flatten out. The main reason is that the steel fibre volume amount increased. On the one hand, the number of steel fibre involved in restraining concrete cracking increased, while, on the other hand, steel fibre absorbed more of the concrete's compression deformation energy, which was converted into their own tensile deformation energy. So, the softening branch of the uniaxial stress-strain curve for recycled concrete became smoother, as shown in Figure 13a. As can be seen from Figure 13b, the difference between the RP-1 stress-strain curve and the RC was not significant, indicating that a small amount of polyvinyl alcohol fibre admixture had a small effect on the uniaxial compressive stress-strain behaviour of the recycled concrete. Comparing the curves of RP-1, RP-2, and RP-3, it can be seen that as the volume admixture of polyvinyl alcohol fibre increased, the uniaxial compressive strength and the peak strain first increased and then decreased, and the softening branch of the stress-strain curve tended to flatten from a steep drop. This indicates that the deformation capacity and the energy absorption capacity of the specimens increased during the stress-softening stage. The reason for this is that more polyvinyl alcohol fibre was used to link the cracked surfaces of the recycled concrete, improving the bond of the cracked surface of concrete. Thus, more energy was required to deform the specimen by cracking. Figure 12: Uniaxial compression stress-strain curve of recycled concrete and natural concrete. Figure 13: Uniaxial compression stress–strain curve of recycled concrete with single fibre mixture. The uniaxial compressive stress-strain curve for both single and hybrid fibre recycled concrete is shown in Figure 14. The uniaxial compressive stress-strain curve for the RSP-1 specimen has a similar shape to the RP-3 specimen, but the falling section of the RSP-1 specimen stress-strain curve is more smoothness, as shown in Figure 14. The reason for this is that the RSP-1 specimen not only had a larger amount of polyvinyl alcohol fibre, but it also had a small amount of steel fibre compared to RP-3, and these steel fibres further shared the load on the fracture surface of the specimen. Therefore, the RSP-1 specimen had an increased capacity to dissipate energy during the compressive damage period, which is reflected in a smoother falling section of the stress-strain curve. The uniaxial compressive stress-strain curve for the hybrid fibre recycled concrete RSP-2 specimen showed an increase in the slope of the rising section of the curve compared to both RS-2 and RP-2, and a smoother softening branch, as shown in Figure 14. The possible reason for this is that the mixing of polyvinyl alcohol fibre and steel fibre produced a synergistic effect compared to the single-fibre recycled concrete, with the double fibre contributing to each other and together enhancing the compressive strength, the elastic modulus, and the deformation properties of the recycled concrete.