# Resonance Fatigue Behaviour of Concretes with Recycled Cement and Aggregate
## 3 Results and Discussion
### Frequency Evolution  
Figure 7 shows an example of the evolution of the resonant frequency of the set during the fatigue test as a function of the number of cycles. The fatigue testing machine used is characterised by always testing at the resonance frequency of the test machine and specimen assembly, which depends mainly on the stiffness of the assembly; the stiffer it is, the higher the resonance frequency. As shown in this figure, the resonance frequency was not constant throughout the test, which implied that the specimen underwent a series of changes in its mechanical properties during the test. First, it can be seen that when each time a step change occurred, there was also an increase in the resonance frequency. This is because a step-change implies an increase in the stress range applied to the specimen, which generally results in a higher stiffness. Second, it can be seen that, after the first few cycles, there was a reduction in the resonance frequency, corresponding to a stabilisation phase. In the fourth step, this effect was not seen because it is a phenomenon of short duration, which only affects a few cycles, and the data acquisition did not record any of them. However, it is a phenomenon observed by other authors [29; 31]. In the first step, it can be seen that, once this first phase was overcome, the resonance frequency remained constant, which means that the specimen was in a stationary state, i.e., the specimen was suffering little damage, and, therefore, it could withstand a large number of cycles without breaking. It was only in the last step that the resonance frequency decreased. This drop in frequency was the consequence of an increase in the specimen deformation, which can be explained by the growth of micro-cracks in the concrete specimen that will lead to specimen failure [60]. For this reason, observing that the resonance frequency decreased (after the initial stabilisation phase) is considered as an indicator of being close to failure of the specimen and that the endurance of the material has been exceeded. Finally, this methodology may be a new way to measure the fatigue limit through resonance frequency monitoring.  
Figures 8 and 9 show an example of the comparison between the test frequencies obtained for each type of concrete as a function of the percentage of NA replaced by MRA for each type of cement.  
Figure 8 shows the results for the OPC concretes. It can be seen that the behavioural phases described above were satisfied. As the percentage of replacement increased, the  
Figure 8: Resonance fatigue frequency evolution during a Locati test for OPC mixes.  
Figure 7: Resonance fatigue frequency evolution during a Locati test.  
Figure 9: Resonance fatigue frequency evolution during a Locati test for OPC-25 CDW mixes.  
resonance frequency decreased, decreasing the stiffness of the specimen, which was in agreement with the results obtained from the elastic modulus tests and with the strain values previously shown. Figure 8 shows the results of the mixes with OPC-25 CDW. As in the case of Figure 7, the phases described above can be seen. In these mixes, it was also observed that in those concretes with MRA, the resonance frequencies were lower than in the case of concretes without MRA. Likewise, these values agreed with the results obtained in the elastic modulus and strain tests.