# Resonance Fatigue Behaviour of Concretes with Recycled Cement and Aggregate
## 3 Results and Discussion  
### Fatigue Limit  
Table 7 shows the values obtained from the Locati resonant fatigue tests for all mixes. In addition, to complement this parameter, the values of _fcd,180_ (Table 6) and _IC_ were added, which enabled the evaluation of the effect of fatigue on each type of concrete.  
In all cases, fatigue reduced the strength of the specimens to between 30 and 45% of the _fcd,180_. Regarding the effect of the presence of both MRA and OPC-25 CDW on the _IC_, there seemed to be no clear effect. This can be explained mainly by the high dispersion  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multicolumn{6}{c}{**Type**} \\ \cline{2-7}  & **HP** & **HR-25** & **HR-50** & **HP-R** & **HRR-25** & **HRR-50** \\ \hline NA-S & 732.36 & 720.79 & 705.38 & 732.36 & 720.79 & 705.38 \\ NA-M & 382.96 & 282.69 & 184.43 & 382.96 & 282.69 & 184.43 \\ NA-G & 766.69 & 565.94 & 369.22 & 766.69 & 565.94 & 369.22 \\ MRA-M & - & 90.75 & 177.62 & - & 90.75 & 177.62 \\ MRA-G & - & 182.80 & 357.77 & - & 182.80 & 357.77 \\ OPC & 400 & 400 & 400 & - & - & - \\ OPC-25 CDW & - & - & - & 400 & 400 & 400 \\ W & 193.03 & 202.08 & 210.63 & 193.03 & 202.08 & 210.63 \\ SP & 6.20 & 6.20 & 6.20 & 6.20 & 6.20 & 6.20 \\ (w/c)\({}_{\text{effective}}\) & 0.48 & 0.50 & 0.53 & 0.48 & 0.50 & 0.53 \\ \hline \hline \multicolumn{7}{c}{Physical and mechanical properties} \\ \hline \(\rho\) (kg/m\({}^{3}\)) & 2340 & 2310 & 2290 & 2350 & 2310 & 2240 \\ \(f_{cd,180}\) (MPa) & 53.5 & 47.0 & 48.8 & 50.2 & 47.5 & 47.5 \\ \hline \hline \end{tabular} Note: \(\rho\), hardened density and \(f_{cm}\), compressive strength.  
\end{table}
Table 6: Concrete mix proportions.  
Figure 4: Modulus of elasticity of the different mixes.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material** & _fcd,180_ [MPa] & \(\Delta\sigma_{L}\) [MPa] & \(L_{F}\) [MPa] & _IC_ [\%] \\ \hline HP & 53.5 & 27.5 & 22.0 & 41.12 \\ HR25 & 47.0 & 18.4 & 14.8 & 31.49 \\ HR50 & 48.8 & 27.5 & 22.0 & 45.08 \\ HRR & 50.2 & 25.3 & 20.3 & 40.44 \\ HRR25 & 47.5 & 27.5 & 22.0 & 46.32 \\ HRR50 & 47.5 & 20.6 & 16.5 & 34.74 \\ \hline \hline \end{tabular} Note: \(f_{cd,180}\): uniaxial compressive strength at 180 days; \(\Delta\sigma_{L}\): stress range of the Locati test rupture step; \(L_{F}\): fatigue limit; _IC_: Influence coefficient (_Lfcd,180_ 100).  
\end{table}
Table 7: Fatigue limit obtained from Locati Method.  
associated with fatigue failure, as this parameter is highly influenced by local phenomena. Furthermore, analysing the strength properties of concrete [15], neither the presence of MRA nor OPC-25 CDW produced a significant change in the strength of the material. The little influence of MRA and OPC-25 CDW on the \(IC\) is because, as pointed out by [PERSON] et al. [58], the fatigue strength of a recycled concrete does not depend only on the presence of RA, but also depends to a large extent on water/cement ratio, cement content, concrete type, rest periods, curing conditions and age during loading, and in this case, it was concrete with a high cement content per cubic metre and with high-quality MRA [16]. Furthermore, because the method used for the determination of the fatigue limit was based on a test that applies load steps, the resolution of this method will be similar to the size of the steps used. Therefore, if the differences between mixtures were smaller than the step size, they will be masked by the resolution of the test.