## 5 Conclusions  
In this study, the mechanical properties of fibre-reinforced recycled concrete were tested. The effect of steel fibre and polyvinyl alcohol fibre on the mechanical properties of recycled concrete was investigated, and the influence of different volume contents of steel fibre or polyvinyl alcohol fibre on the compressive strength, the flexural strength, the elastic modulus, the Poisson's ratio, and the uniaxial compression toughness of the recycled concrete were discussed. Then, a comparison was made between the experimental results and predictions obtained from the mechanical strength model proposed in this study. Finally, a multiple regression analysis of the uniaxial compression stress-strain curve of fibre-reinforced recycled concrete was carried out to establish the equation of the stress-strain curve associated with the fibre characteristic value. The conclusions of this study are as follows:  
1. After polyvinyl alcohol fibre was added, the cube compressive strength of recycled concrete decreased by 6.5-14.1%, while when the polyvinyl alcohol fibre volume content was 0.2%, the flexural strength increased by 47.9%. The brittleness of recycled concrete was improved obviously after mixing polyvinyl alcohol fibre. A single admixture of steel fibre enhanced the recycled concrete cube compressive strength and the flexural strength. 2. After two types of fibre were added, the cube compressive strength of recycled concrete was reduced compared with the non-fibre, but the flexural strength was improved significantly compared with single admixture fibre, especially when steel fibre and polyvinyl alcohol fibre were mixed at 1.0% and 0.2% by volume percentage, respectively. 3. The uniaxial compressive strength and the deformation capacity of the recycled concrete were enhanced by incorporating steel fibre or polyvinyl alcohol fibre. The  
Figure 18: The comparison of stress–strain measuring curve and calculation curve. deformation performance of the recycled concrete was significantly improved by mixing hybrid fibre compared with single admixture fibre. 4. The incorporation of steel fibre or polyvinyl alcohol fibre reduced the elastic modulus and the Poisson's ratio of recycled concrete but improved the uniaxial compressive toughness. In order to obtain better reinforcement and a better toughening effect, 0.2% by volume of polyvinyl alcohol fibre and 1.0% by volume of steel fibre were recommend to be mixed into recycled concrete. In this study, the formula for calculating the elastic modulus, the Poisson's ratio, and the uniaxial compressive toughness of recycled concrete with a calculation accuracy of more than 80% was proposed. 5. The uniaxial compressive stress-strain curve for each test group was analysed and fitted, then the equation for the uniaxial compressive stress-strain curve associated with the steel-polyvinyl alcohol fibre characteristic value was proposed, which was more complex. However, the calculated curve achieved a high match with the measured curve. Conceptualization, H.N., J.L. and L.W. ; performed the experiments, L.W. and J.J.; writing--original draft preparation, H.N. and L.W. ; editing, J.L. and L.W. All authors have read and agreed to the published version of the manuscript. The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Project No. U1904188), the Natural Science Foundation of Henan Province (182300410247), the Zhejiang basic public welfare research program (LGF19E080014), and the Huzhou Key Laboratory of Green Building Technology, Huzhou, Zhejiang, China. The data are presented in the text. The authors have declared that no competing interest exist. * [PERSON] and [PERSON] (2018) [PERSON]; [PERSON] Flexural performance and evaluation method of steel fiber reinforced recycled coarse aggregate concrete. _Constr. Build. Mater._**2018**, _159_, 126-136. [CrossRef]
* [PERSON] et al.