# High-Frequency Fatigue Testing of Recycled Aggregate Concrete
## 3 Results and Discussions
### Influence of the Frequency on Fatigue  
Figure 6 shows an example of the maximum deformation envelope recorded during the Locati tests at a frequency of 10 Hz. Figure 6 shows, first of all, that RC-B was able to resist the most steps, which meant that it was the material with the highest IC, with RC-S being able to resist the least and RC-M was in an intermediate situation between the other two materials. These results agree with the results of other authors who state that the presence of adhered mortar in the RA reduces this coefficient [15; 16; 42]. It can also be seen that the deformation values suffered by the specimens was lower in the case of RC-B. It should be noted that, as each of the steps is a fixed percentage compared to the compressive strength of each material, they are not directly comparable to each other as they are different stress values. Table 5 shows a brief summary of the results obtained from the moderate-frequency tests carried out using method-1. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Material**} & \(\Delta\sigma_{\text{max}}\) & \(\mathbf{f_{L}}\) & **IC** \\ \cline{2-4}  & **(MPa)** & **(MPa)** & **(\%)** \\ \hline RC-B & 38.75 & 31.0 & 52.2 \\ RC-S & 45.5 & 36.4 & 46.8 \\ RC-M & 39.6 & 31.7 & 50.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Low-frequency fatigue limit (f\({}_{\text{L}}\)) obtained using the Locati method with \(2\times 10^{5}\) cycles per step. Figure 6: Maximum strain of low-frequency Locati fatigue tests with \(2\times 10^{5}\) cycles per step. Analysis of these results shows that the material with the highest stress range was RC-S, the least was RC-B and the RC-M had an intermediate value. Likewise, it is possible to determine that, although the RC-S had the highest stress range values, it had the lowest IC. This loss of compressive strength was due to the presence of mortar adhered to the aggregate. In any case, these values of IC are within the usual range for concretes and agree with values found in the literature [15; 16]. Figure 7 shows an example of the maximum deformation suffered by a specimen of each material during Locati tests at the resonance frequency of the machine test set. Figure 7 shows that RC-S was the material that had the lowest IC. RC-M was generally between RC-B and RC-S. Figure 8 shows that the resonance frequency evolved throughout the test. For the first steps, there was an increase in the frequency with the cycles throughout each step, as well as a punctual increase when there was a change of step. At the end of each test, a drop in the resonance frequency of the system was seen. This resonance frequency depended on the stiffness of the system; an increase in the stiffness of the system resulted in an increase in the resonance frequency, while a reduction in the stiffness of the system reduced it. For this reason, it was interpreted that both point and distributed frequency increases occurred as a consequence of a stiffening of the system, while the fall that occurred in the final part of the test was a consequence of the damage suffered by the specimen, where the cracks had reached such a size that they produced a flexibilization of the specimen, which indicated that it was close to breaking. Figure 7: Maximum strain of high-frequency Locati fatigue tests with \(2\times 10^{5}\) cycles per step. Table 6 shows the results obtained from the Locati tests with \(2\times 10^{5}\) cycles per step at a very high frequency using the two criteria previously established. Analysing the results of Table 6, it can be determined that, as had been deduced from the evolution of the maximum strain, RC-B was the material with the highest IC, although the compression strength of RC-S was the highest of all. The stress ranges corresponding to the fatigue limit of the three concretes were similar.