# High-Frequency Fatigue Testing of Recycled Aggregate Concrete
## 3 Results and Discussions
### Influence of the Number of Cycles Per Step during a Locati Test  
In order to determine the influence of increasing the number of cycles in each step of the Locati test, identical tests were carried out to those described above, but the number of cycles per step was set at \(5\times 10^{5}\) instead of \(2\times 10^{5}\). Figure 9 shows the evolution of the maximum deformation as a function of the number of cycles throughout the Locati test for \(5\times 10^{5}\) cycles per step. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Method-1**} & \multicolumn{2}{c}{**Method-2**} \\ \cline{2-6}
**Material** & \(\Delta\sigma_{\text{max}}\) & \(\mathbf{f_{L}}\) & \(\mathbf{IC}\) & \(\mathbf{f_{L}}\) & \(\mathbf{IC}\) \\ \cline{2-6}  & **(MPa)** & **(MPa)** & **(\%)** & **(MPa)** & **(\%)** \\ \hline RC-B & 32.09 & 25.67 & 43.19 & 26.74 & 44.93 \\ RC-S & 31.51 & 25.21 & 32.40 & 28.01 & 35.99 \\ RC-M & 31.10 & 24.88 & 39.60 & 28.27 & 45.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: High-frequency fatigue limit obtained using the Locati method with \(2\times 10^{5}\) cycles per step. Figure 8: Maximum strain and resonance frequency comparison during high-frequency Locati fatigue tests with \(2\times 10^{5}\) cycles per step. Figure 9 shows that RC-S was the material that had the lowest IC. RC-M was between RC-B and RC-S. In this case, as in the previous case, the two analysis criteria were used to determine the stress range corresponding to the fatigue limit. These results are shown in Table 7.  
In order to compare the evolution of the maximum strain throughout the test, it was decided to divide the number of cycles performed by the number of cycles per step, in this way it was be possible to compare the results of the three variants of the test. An example of the comparison between the Locati test types for each material is given in Figure 10; Figure 12.  
Figure 10 shows the influence of increasing the frequency on the fatigue behaviour of the RC-B by comparing the RC-B-HF-\(2\times 10^{5}\) with the RC-B-MF-\(2\times 10^{5}\). It is possible to conclude that, in the first phase, increasing the test frequency had no influence on the deformation suffered by the specimens. After step 4, the two curves separated, increasing more rapidly in the case of RC-B-HF-\(2\times 10^{5}\). This behaviour can be justified by arguing that, in the first phase, a phase in which the concrete was not damaged, the effect of the frequency seemed irrelevant, whereas when the cracks reached a critical size, an increase in frequency produced a reduction in the number of cycles that RC-B could withstand. It was observed that the specimens tested at very high frequencies showed a markedly higher temperature increase than in the case of tests performed at moderate frequency. This fact is believed to be related to the reduction in fatigue life of the material. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multicolumn{3}{c}{**Method-1**} & \multicolumn{2}{c}{**Method-2**} \\ \cline{2-6}  & \(\Delta\sigma_{\text{max}}\) & \(\mathbf{f_{L}}\) & \(\mathbf{f_{L}/f_{c}}\) & \(\mathbf{f_{L}}\) & \(\mathbf{f_{L}/f_{c}}\) \\ \cline{2-6}  & **(MPa)** & **(MPa)** & **(\%)** & **(MPa)** & **(\%)** \\ \hline RC-B & 32.09 & 24.6 & 41.39 & 26.74 & 44.93 \\ RC-S & 29.76 & 23.81 & 30.60 & 24.51 & 31.49 \\ RC-M & 29.685 & 23.75 & 37.80 & 25.45 & 40.59 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: High-frequency fatigue limit obtained using the Locati method with \(5\times 10^{5}\) cycles per step. Figure 9: Maximum strain of high-frequency Locati fatigue tests with \(5\times 10^{5}\) cycles per step. From this same Figure 10, it is also possible to analyse the influence of increasing the number of cycles per step from \(2\times 10^{5}\) to \(5\times 10^{5}\). Comparing the two curves, it is observed that the effect of each step was similar until the specimen was close to breaking.