## 3 Experimental Results
### Compressive Strength  
Figures 4-6 show the range of compressive strengths by number of cycles for the three mixtures and for both series.  
Figure 4: Variation of compressive strength by number of cycles. HPC (**a**) L-Series. Raw values. (**b**) H-Series. Raw values. (**c**) L-Series. Maximum, minimum, and average relative values, and tendency lines. (**d**) H-Series. Maximum, minimum, and average relative values, and tendency lines.  
Figure 5: _Cont._  
Figures 4-6 show some interesting results. First, it should be highlighted that although the expected fatigue life was 1.9 \(\times\) 10\({}^{8}\) cycles for the L-Series and 1.5 \(\times\) 10\({}^{3}\) cycles for the H-Series, no collapse occurred during the cyclic tests. It was especially noticeable for the H-Series, where the number of cycles was, in all cases, greater than the expected fatigue life. When the expected fatigue life corresponding to the real concrete quality at the beginning of the cyclic tests is considered, the results are more consistent.  
The results also showed no significant variation in the average compressive strength of the concrete by the number of cycles. In all cases, the variation was less than 7%. This behavior was observed for  
Figure 5: Variation of compressive strength by number of cycles. SFHPC (**a**) L-Series. Raw values. (**b**) H-Series. Raw values. (**c**) L-Series. Maximum, minimum, and average relative values, and tendency lines. (**d**) H-Series. Maximum, minimum, and average relative values, and tendency lines.  
Figure 6: Variation of compressive strength by number of cycles. PPFHPC (**a**) L-Series. Raw values. (**b**) H-Series. Raw values. (**c**) L-Series. Maximum, minimum, and average relative values, and tendency lines. (**d**) H-Series. Maximum, minimum, and average relative values, and tendency lines.  
all the mixtures and for both series. It means that, in this case, any damage caused by cyclic loading was very small, and no appreciable mechanical consequences were observed.  
On the contrary, a relevant increase of the scatter of the results was observed. Specimens not previously subjected to cyclic loads showed very low scatter, while the rest of the specimens, in general, showed a higher scatter. In general, the scatter increased more than two-fold, and this variation is quite similar in all cases.  
This phenomenon can be explained as follows. Concrete specimens have internal stress caused by the curing process, shrinkage, etc. Cyclic loads release the internal stresses, resulting in microcracking. This phenomenon has been observed in previous research [35; 36; 37]. Depending on the real internal stress field, the releasing process is more or less intense and more or less microcracking occurs, resulting in different macroscopic responses.  
In some cases, this internal stress relaxation is not harmful and a greater compressive strength is observed.  
This scatter was higher in the case of the mixtures including fiber, which reveals that the fibers introduced additional restrictions to the concrete mixture during the curing process, resulting in more internal stress. Cyclic loads released this stress, and the macroscopic result was a higher scatter.  
The mixture including steel fibers showed, in general, higher compressive strength, which denotes that steel fibers help to increase the mechanical capacity of concrete. In contrast, the mixtures without fibers and with polypropylene fibers showed similar results.