## 3 Results and Discussion
### Flexural Strength  
Flexural strength tests were performed at 7 and 28 days. Figure 12 displays the differences in the flexural strength of HPSCC with the various PP fibre contents. The flexural strength effectiveness at 7 and 28 days indicates that the flexural strength values were higher by 12.5% and 9.5%, 29% and 18%, 33.5% and 28.5%, 45% and 42%, 16.5% and 12% at the volume contents of 0.025%, 0.05%, 0.075%, 0.125%, and 0.25% after 7 and 28 days, respectively, in comparison with HPSCC reference mix. During bending, the ends of the polypropylene fibres are pulled out of the cement matrix. Initially, the deformation of the interfacial transition zone occurs and the adhesion of fibres with different Young's moduli and deformability effects to the cement paste is disturbed. A further increase in load causes one end of the fibre to pull out while the other end is firmly anchored in the matrix. [PERSON] et al. [33] reported that the flexural strength increased by 19.64% to 24.46% when the volume fraction of polypropylene fibre is in the range of 0.025% to 0.042%. [PERSON] and [PERSON] [55] noted an increase in flexural strength with increasing the fibre volume fraction for PP fibres with lengths of 48 mm and 60 mm at volume contents of 0.33-1%. Similar results were obtained using fibres with a length of 19 mm and 38 mm for a volume content of 0.3-0.9%. The increase in flexural strength was 25-59% and was greater for 38 mm-long fibres [52]. Based on the above-mentioned literature [33; 52; 55], it can be concluded that the use of polypropylene fibres with a length of 12-60 mm and a content of 0.3-1% is beneficial and increases the flexural strength. The optimally fitted curves on the basis of the flexural strength and fibre content are presented in Figure 13, and it is pronounced that there are strong relationships between 7 day and 28 day flexural strength and volume fraction of PP fibres concerning HPSCC. Based on the results, the following quadratic functions between 7 day and 28 day flexural strength as well as PP fibre content with favourable correlation coefficients R\({}^{2}\) = 0.93 and R\({}^{2}\) = 0.84, respectively are obtained:  
\[f_{ct,f17}=-69.19V_{PP}^{2}+19.32V_{PP}+3.01 \tag{7}\]  
\[f_{ct,f28}=-110.2V_{PP}^{2}+30.69V_{PP}+6.37 \tag{8}\]  
where \(f_{ct,f27}\) and \(f_{ct,f28}\) are the 7 day and 28 day flexural strength of prismatic specimens (MPa). Figure 12: Flexural strength of PPHPSCC for different polypropylene fibre volume content. The obtained curves are analogous to those for HPSCC splitting tensile strength. [PERSON] [56] found that the model can be considered as a reasonable when the R\({}^{2}\) coefficient is at least 0.7. For that reason, the obtained equations can be successfully used to predict the relations between the 7 day and 28 day compressive strength and the fibre content. According to the test data, the relation between the 28 day and 7 day flexural strengths PPHPSCC can be expressed as below, with Figure 14 presenting a comparison between the fitting linear function and test results. This relation established a quite strong value of R\({}^{2}\) = 0.86 for HPSCC with PP fibre content. \[f_{ct,f128}=1.34f_{ct,f77}+2.51 \tag{9}\]  
The linear relation between splitting tensile strength and flexural strength was obtained based on the regression analysis of the tensile strength results. Figure 15 and the following formula indicate the correlation between flexural strength and splitting tensile strength for HPSCC with PP fibres, which has a linear relationship with a strong coefficient of determination R\({}^{2}\) = 0.93. \[f_{ct,fl}=1.56f_{ct,spl}-1.76 \tag{10}\]  
Figure 14: Correlation between 28 day and 7 day flexural strength for HPSCC and PPHPSCCs. Figure 13: Correlations between 7 day or 28 day flexural strength and volume content of PP fibres. where \(f_{ct,spl}\) is the splitting tensile strength of cubic specimens (MPa), and \(f_{ct,fl}\) is the flexural strength of prismatic specimens (MPa).