## 3 Results and Discussion
### Splitting Tensile Strength  
To further study the contribution of PP fibres to the tensile strength of PPHPSCC materials, splitting tensile tests were carried out on cube specimens with varying volume fractions of PP fibres, i.e., 0.025%, 0.05%, 0.075%, 0.125%, and 0.25%. Six cube specimens  
Figure 8: Correlation between 28 day and 7 day compressive strength for HPSCC and PPHPSCCs.  
Figure 7: Correlations between 7 day or 28 day compressive strength and volume content of PP fibre.  
were prepared and tested for each mix and curing age. The mean strengths of different mixes when cured for 7 days and 28 days were compared, as shown in Figure 9.  
There were improvements in the splitting tensile strength effectiveness with increasing PP fibre content, with values of 11.5% and 15.5% at 0.025% PP fibre content, 25% and 30.5% at 0.05% content, 29% and 35% at 0.075% content, 38% and 44.5% at 0.125% content, and 5% and 21% at 0.25% content after 7 days and 28 days of curing, respectively. It can be noted that the 7 day splitting tensile strength improvements of PPHPSCC ranged from 11.5% to 38% at the volume contents of 0.025% to 0.25% in comparison with the reference HPSCC without PP fibres. On the other hand, the 28 day splitting tensile strength of PPHPSCC reached the improvements of 15.5% to 44.5% at the same PP fibre fractions. As expected, even a low content of polypropylene fibres increased the splitting tensile strength. Theoretically, the greater the number of fibres bridging the splitting crack, the greater the tensile strength should be. However, in this study, it was possible for self-compacting concrete to achieve the desired rheological parameters with a relatively low content of polypropylene fibres, i.e., 0.125%. [PERSON] et al. [33] reported that the splitting tensile strength of HPC with 0.1-0.2% polypropylene fibres increased by 32.86-44.52% compared to HPC without fibre. [PERSON] and [PERSON] [54] noted an increase in splitting tensile strength from 3.75 MPa to 5.11 MPa with an increase in the content of polypropylene fibres 50 mm long and volume fractions from 0% to 1.5%.  
Figure 10 shows the strong relationships between 7 day or 28 day splitting tensile strength and volume content of PP fibres regarding high-performance self-compacting concrete. Splitting tensile strengths displayed an increased trend initially, and then decreased tendencies with the PP fibre content decreasing from 0.125% to 0.25%, due to fibres having a different influence on the tensile strength of HPSCC. Direct relationships are completely clear according to high correlation coefficients equal to 0.96 and 0.91, respectively, with their equations in the form of a polynomial-quadratic function as seen below:  
\[f_{ct,spl7}=-68.95V_{PP}^{2}+17.97V_{PP}+3.05 \tag{4}\]  
\[f_{ct,spl28}=-103.5V_{PP}^{2}+29.51V_{PP}+4.68 \tag{5}\]  
where \(f_{ct,spl7}\) and \(f_{ct,spl28}\) are the 7 day and 28 day splitting tensile strength of cubic specimens (MPa).  
Figure 9: Splitting tensile strength of PPHPSCC for variable content of polypropylene fibres.  
The greatest dispersion of the 7 day and 28 day splitting tensile strength results was noted with the highest content of PP fibres. This may be due to its agglomeration caused by a decrease in workability. Figure 11 exhibits the linear correlation between the 28 day and 7 day splitting tensile strength values of polypropylene-fibre-reinforced HPSCC. This correlation demonstrated a strong value of R\({}^{2}\) = 0.94. The predicted equation for HPSCC after 28 days of water curing is specified by:  
\[f_{ct,spl28}=1.64f_{ct,spl7}-0.23 \tag{6}\]  
The derived equation may successfully be used to predict the relationship between the 7 day and 28 day splitting tensile strength for HPSCC.  
Figure 11: Correlation between 28 day and 7 day splitting tensile strength for HPSCC and PPHPSCCs.  
Figure 10: Correlations between 7 day or 28 day splitting tensile strength and volume content of PP fibre.