## 3 Results and Discussion
### Compressive Strength  
The 7 day and 28 day compressive strengths of the HPSCC mixes containing PP fibre were determined. The cited values are the mean of six companion cube specimens, and there were tested per data point. The effect of PP fibre content on the development of the compressive strength is illustrated in Figure 6.  
As could be seen, the reference mix experienced less compressive strengths than the other mixes containing PP fibres at all ages. The exception was the 28 day strength of  
Figure 5: Variation of blocking ratio with different PP fibre volume content. Figure 6: Compressive strength of PPHPSCC for different polypropylene fibre volume content. the PPHPSCC-0.25 blend containing the largest volume fraction of PP fibres, i.e., 0.25%. This strength was about 3% lower compared to the strength of the reference mix. The mix PPHPSCC-0.05 with 0.05% PP fibres possessed the highest values of strength. The results of the mix with 0.075% PP fibres (PPHPSCC-0.075) are the second-highest values. At 28 day, the addition of 0.125% PP fibres caused a marginal increase in PPHPSCC-0.125 strength over that of the reference cube specimens. The compressive strength of the PP-fibre-reinforced mixes was higher than that of the reference mix by about 9%, 31%, 13.5%, 11.5%, and 5% for 0.025%, 0.05%, 0.075%, 0.125%, and 0.25% PP volume fractions, respectively, at 7 day. On the other hand, at 28 day, the above-mentioned strengths were respectively higher by approximately 5.5%, 8.5%, 7%, and 1% for the first four volume fractions of PP fibres. In general, the increase in the compressive strength of fibre-reinforced concrete is attributed to the retardation of matrix fracturing by limiting and retarding crack propagation through the fibres. In this study, the author found that high volume fractions of polypropylene fibres adversely affected the workability of fresh HPSCC and therefore, holes were created in the PPHPSCC mixes. In fact, it is the formation of these voids that results in a decrease in compressive strength of high-performance self-compacting concrete. The tendency of changes in compressive strength with the increase in the volume content of polypropylene fibres is not unambiguous. For example, [PERSON] et al. [49] reported that the compressive strength increases from 64 MPa for concrete without fibres to 107 MPa for the mixture with the 1.5% content of fibres when using polypropylene fibres with a length of 50 mm and fractions of 0%, 0.5%, 1% and 1.5%. Similar results were obtained by [PERSON] et al. [50] based on tests of polypropylene fibres with a length of 48 mm. It was found that the fibre content at the levels of 0%, 2% and 4% has a positive effect on the compressive strength. Different results were obtained by [PERSON] et al. [51] studying the effect of 0.6 and 0.9 kg/m\({}^{3}\) of 19 mm, 38 mm and 50 mm fibre content on the concrete strength. They found that the lower density of polypropylene fibres compared to the matrix and breaks in the C-S-H bond caused by PP fibre inclusions resulted in lower compressive strength values. A decrease in compressive strength was also noted by [PERSON] and [PERSON] [52] using fibres with lengths of 19 mm and 38 mm and their volume in a mixture of 0.3-0.9%. The increasing percentage decreased the strength of the material compared to the reference mixture; the strength decreased by 6% for 19 mm fibres and 8% for 38 mm fibres. In turn, [PERSON] [53] reported that in the case of using fibres of length 5 mm, 10 mm and 15 mm with a percentage of volume in the mixture of 0.25%, 0.75% and 1%, a slight impact on the compressive strength was obtained. The polynomial correlations between the compressive strength and volume fraction of polypropylene fibre after 7 and 28 days of moist curing are shown in Figure 7. Compressive strengths showed increased trends firstly, and then decreased trends with the PP fibre content from 0% to 0.25% due to PP fibres having a different impact on the strength of HPSCC.