# Fatigue Behaviour of Concrete Using Siderurgical Aggregates
## 2 Methodology
### Fatigue Test  
This methodology allows the determination of the fatigue limit using very few specimens in record time, taking measurements of the resonance frequency and strain of the specimens, in addition to the surface temperature of the specimens. The limitation is that the maximum capacity of the press is almost reached, having to use smaller specimens when the characteristic resistances of the concrete are higher than those of the SLC mix. Fatigue characterisation was done by means of Locati fatigue tests at resonance frequency, in this case \(\sim\)90 hz. The Locati test consists of applying increasing load steps in a concatenated manner until specimen failure. Each of these load steps has a duration of 2 \(\times\) 10\({}^{5}\) cycles. At the end of these cycles, the next step is taken, increasing the maximum load value, while keeping the minimum value constant. In Figure 2, a sketch of the loads applied to the specimen during each test can be seen. Figure 1: Grading distribution of the designed mixes. The load to be applied in each step depends on the test concrete characteristics, specifically the compressive strength; therefore, each step applies a percentage of the _f_c, setting the minimum load applying a stress ratio of 10% of the maximum load. The loads to be applied in each step for each concrete mix (LLC and SLC) appear in Table 4.  
The tests were carried out on two cylindrical specimens of 100 \(\times\) 200 mm per mix (LLC and SLC) at the age of approximately 8 months. A Zick servohydraulic press with a capacity of 400 kN that allows dynamic loading was used. This press provides dynamic loads at the resonance frequency of the specimen-machine set. The upper face of the specimens was cut and polished, while the specimens were fitted with two strain gauges placed diametrically opposite each other. Two-millimeter long gauges were used (Tokyo Sokki Kenkyujo Co. Ltd. model PL-60-11-1 LJC-F (120 \(\pm\) 5 \(\Omega\))). The gauges were affixed to the specimens by means of a two-component epoxy resin. An IRISYS model IRI 1011 thermographic camera was used to record the evolution of the temperature on the surface of the specimens; as a result of internal friction, the camera was positioned outside the resonance chamber to avoid vibrations. Likewise, a stress-strain record was kept by means of digital data collection, and the resonance frequency was recorded during each load cycle using the software of the test equipment. Figure 3 shows the experimental setup used for these tests. It is worth mentioning that in order to carry out the test, the traditional setup of the servo-hydraulic press had to be modified. This machine \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & & \multicolumn{3}{c}{**LLC**} & \multicolumn{3}{c}{**SLC**} \\ \hline
**Step** & **\% f\({}_{\text{c}}\)** & **Min [kN]** & **Max [kN]** & **Mean [kN]** & **Range [kN]** & **Min [kN]** & **Max [kN]** & **Mean [kN]** & **Range [kN]** \\ \hline
1 & 30\% & 14.8 & 148.2 & 81.5 & 66.7 & 20.0 & 199.8 & 109.9 & 89.9 \\
2 & 35\% & 17.3 & 172.9 & 95.1 & 77.8 & 23.3 & 233.1 & 128.2 & 104.9 \\
3 & 40\% & 19.8 & 197.6 & 108.7 & 88.9 & 26.6 & 266.4 & 146.5 & 119.9 \\
4 & 45\% & 22.2 & 222.3 & 122.3 & 100.0 & 30.0 & 299.7 & 164.8 & 134.9 \\
5 & 50\% & 24.7 & 247.0 & 135.9 & 111.2 & 33.3 & 333.0 & 183.2 & 149.9 \\
6 & 55\% & 27.2 & 271.7 & 149.4 & 122.3 & 36.6 & 366.3 & 201.5 & 164.8 \\
7 & 60\% & 29.6 & 296.4 & 163.0 & 133.4 & 40.0 & 399.6 & 219.8 & 179.8 \\
8 & 65\% & 32.1 & 321.1 & 176.6 & 144.5 & 43.3 & 432.9 & 238.1 & 194.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Applied loads in each step of the Locati fatigue test. Figure 2: Locati test scheme. usually works in tension (e.g., tests on steel cords and bars) so two rigid mechanized steel bases have been bolted to the upper and lower jaws and a ball joint has been placed between the upper plate and the specimen.