# Fatigue Behaviour of Concrete Using Siderurgical Aggregates
## 3 Results
### Fatigue Test  
Figure 5 shows the evolution of the resonance frequency over time, where each load step can be seen, which in turn corresponds to \(2\times 10^{5}\) load-unload cycles. For the LLC mix (Figure 5a), it can be observed that in step 8 for specimen 1 the resonance frequency begins to decrease, indicating that the stiffness of the specimen begins to decrease, which means that the specimen is highly cracked and close to fracture. For specimen 2, it can be seen that when starting step 8, the specimen has broken just as the test load increases, establishing that step 8 defines the fatigue limit for both specimens. For the SLC mix (Figure 5b), very similar behavior is seen for the two specimens tested. In both cases, the resonant frequency begins to decrease slightly in step 5, and  
Figure 4: Physical properties of the mixes produced; (**a**) densities and (**b**) porosity and absorption. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mix** & **fc [MPa]** & **fct [MPa]** & **E [GPa]** \\ \hline LLC & 62.90 \(\pm\) 6.22 & 3.54 \(\pm\) 0.19 & 41.25 \(\pm\) 0.31 \\ SLC & 89.45 \(\pm\) 3.15 & 4.12 \(\pm\) 0.16 & 53.48 \(\pm\) 1.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Mechanical properties of the mixes produced. very markedly in step 6. The SLC results show that siderurgical concrete specimens can withstand a greater loss of stiffness before breaking. On the other hand, the mix with siderurgical aggregates managed to resist fewer steps. This is because fatigue is a subcritical mechanism, causing cracks to grow slowly until the specimen weakens enough to break. Siderurgical aggregates have irregular surfaces that are in contact with the mortar paste, and these paste surfaces form elements that are more susceptible to breakage, or in other words, the irregular shapes of the siderurgical aggregates behave as stress concentrators (there are sudden changes in section), causing premature failure of the ITZ between paste and aggregate. Another issue is that an aggregate much stiffer than the paste causes the bond at the interfaces to deteriorate much earlier, since aggregate and paste do not deform in the same way at the same stress. Finally, it is necessary to analyse the effect of the apparent shape of the aggregates, which being sharper (flaky) will be able to penetrate the paste more easily. Together, all the causes described favour the appearance of defects in the microstructure of the specimens with passing cycles, which will reduce the fatigue life of the concrete with respect to conventional limestone concrete. Another criterion to establish the step of the fatigue limit consists of measuring the evolution of strain over the cycles, or in other words, over time. For this, a reading of the maximum strain (corresponding to the maximum load within each cycle) and the minimum strain (corresponding to the minimum load within each cycle) for each of the \(2\times 10^{5}\) cycles of each step was made. Figures 6 and 7 show the maximum and minimum strain for the LLC and SLC mixes, respectively. For the LLC mix (Figure 6), the two specimens show a very similar strain in each step, although specimen 2 shows some noise during the first three steps, possibly due to the vibration of the strain gauge cables. It can be observed that the slope of the deformation increases slightly with each step, until the specimens finally break at the beginning of step 8 and 9. This slope increases because the growth rate of the cracks is increasing. The behaviour of the SLC mix (Figure 7) is similar to that of the LLC mix with some differences. The strain is less until the fifth step, and in the sixth step (step of the fatigue limit) it shoots up until the specimen finally breaks. Figure 5: Evolution of the resonance frequency in each step for the LLC (**a**) and SLC (**b**) mix. Figure 8 shows the evolution of the mean strain with the passage of the cycles. As expected, the mean strain increases as the load in each step increases. On the other hand, there is a high correspondence between the two specimens of each mix.