## 3 Results and Discussion
### Investigations on TRC Coupons
Figure 8: The evolution of the modulus and the dissipated energy of representative TRC coupons during the loading–unloading cycles: (**a**) an uncracked sample and (**b**) a cracked sample. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & **TRC CYCL** & **TRC CYCL** & **TRC CYCL** \\  & **0.5 MPa** & **1.0 MPa** & **2.0 MPa** \\ \hline A & 0 & 22 & 7 \\ B & 0 & 8 & 12 \\ C & 0 & 0 & 10 \\ D & 0 & 0 & 0 \\ E & 0 & 5 & 16 \\ F & 0 & 1 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Number of cracks formed during cyclic preloading of the TRC coupons. Once a crack was formed, its width tended to stabilize. Both for TRC CYCL. 1.0 MPa as for TRC CYCL. 2.0 MPa, the crack width increased during the first hundred cycles but stabilized afterwards. As an example, the evolution of the crack width was shown for a sample subjected to a maximum cycle stress of 1.0 MPa (Figure 9). After passing all loading cycles, a static test as described in Section 2.3.1 was performed to determine the residual capacity of the TRC coupons. The stresses were calculated using the nominal dimensions of the coupons, the strains were measured using DIC over a length of 320 mm. The obtained stress-strain curves were grouped per series. In Figure 10 one can see that the residual behavior was only slightly affected for series TRC CYCL 0.5 MPa. As shown in Table 5 no cracks were induced during the cyclic loading. The formation of cracks was found as the only degradation fatigue mechanism, leading directly to the explanation of the observed residual behavior: both the modulus, as the strength were comparable to the reference behavior (Table 6). The only difference between the residual curves and the reference one was found in the multiple cracking stage. As can be seen in Table 6, more cracks were formed for TRC CYCL 0.5 MPa samples compared to REF, which could explain the limited stress increase during the multiple cracking stage, and, thus, the shifted stress-strain curves. Figure 10: Residual behavior of TRC CYCL. 0.5 MPa. Figure 9: Evolution of the crack width of some representative cracks formed during cyclic loading of TRC coupons. As the maximum stress to which the TRC coupons were cyclically loaded was increased for series TRC CYCL 1.0 MPa and TRC CYCL 2.0 MPa, so did the probability to the formation of cracks. Once a crack was formed, this directly resulted in a decrease of E\({}_{1}\) measured in the residual stress-strain behavior. The more cracks are formed, the lower E\({}_{1}\). Since this initial modulus was very dependent on the occurrence of cracks, a large scatter exists on the results and the average value of E\({}_{1}\) was not representative and, therefore, not displayed in Tables 7 and 8. A specimen fully saturated with cracks showed a linear behavior in the static tests, for example, specimen A in Figure 11 and specimen E in Figure 12. For these specimens, few extra cracks were formed during the static tests and the residual behavior was characterized with a modulus equal to E\({}_{3}\) (Tables 7 and 8). The cyclic preloading did not affect the modulus in the last branch, E\({}_{3}\) (Tables 7 and 8). Overall, the ultimate strength was lowered after subjection to loading cycles up to 1.0 MPa (Table 7) and 2.0 MPa (Table 8). However, no clear link was found between the number of cracks and the ultimate strength.