## 3 Model Predictions
### Sensitivity Study Regarding the w/c-Ratio of New and Old Paste and the Age of New Paste  
Next, we study the strength of recycled concrete for different material ages, as expressed by means of new cement paste-related hydration degrees \(\xi_{ncp}\!\in\! [0,1]\), as well as for different water-to-cement ratios related to the old and the new cement paste, \((w/c)_{ocp}\!\in\!\{0.2,0.35,0.5,0.7\}\) and \((w/c)_{ncp}\!\in\!\{0.2,0.35,0.5,0.7\}\). All other composition parameters remain unchanged as compared to the benchmark composition. Naturally, the recycled concrete strength increases with increasing hydration degree and decreases with increasing \(w/c\)-ratio, see Figure 6a. As for the compositions with \((w/c)_{ncp}\!\in\!\{0.5,0.7\}\), the new cement paste is, for all hydration degrees \(0<\xi_{ncp}<1\), weaker than the old cement paste which exhibits \((w/c)_{ocp}\!=\!0.5\) and \(\xi_{ocp}\!=\!1\). By analogy to the benchmark composition, the macroscopic strength is therefore triggered by ITZ failure around the old plain aggregates (class I recycled aggregates). As for recycled concrete produced with low-\(w/c\) pastes and hydration degrees \(\xi_{ncp}\) which are considerably smaller than the ultimate hydration degrees according to Equation (20), macroscopic failure is again triggered by \(\mathcal{I}_{opa}^{ncp}\) failure. However, as the new cement paste becomes stiffer due to progressive hydration, the failure mode changes. For mature pastes with \((w/c)_{ncp}\!\in\!\{0.2,0.35\}\), the most unfavorably loaded hydrate needles are located within the ITZ around the old covered aggregates (class III aggregates), since the old cement paste covers constitute the weakest links within the microstructure of concrete and concrete failure is consequently initiated at \(\mathcal{I}_{oca}^{ocp}\). Once \(\mathcal{I}_{oca}^{ocp}\)-ITZs become critical, further hydration (of the new cement paste matrix) does not lead to a significant strength increase. Next, the sensitivity of the recycled concrete strength with respect to the interplay between old cement paste stiffness and new cement paste stiffness is further elaborated by studying the strength of recycled concrete with respect to the \(w/c\)-ratio of the old paste. The ITZ around old plain aggregates (\(\mathcal{I}_{opa}^{ncp}\)) is critical for all studied old and new cement paste compositions \((w/c)_{ocp}\!\in\!\{0.2,0.35,0.5,0.7\}\), \((w/c)_{ncp}\!\in\!\{0.2,0.35,0.5,0.7\}\) at early and medium ages (and even at mature ages for \((w/c)_{ncp}=0.7\)), and the corresponding macroscopic strength values increase progressively with increasing hydration degree of the new cement paste, see Figure 6b. The model shows that the strength of young recycled concrete increases only slightly when using a high-quality old cement paste with low \(w/c\)-ratio. This observation originates from the micromechanical contribution of the three recycled aggregate classes to the load transfer, as elaborated next. If the old cement paste exhibits a high \(w/c\)-ratio, the old mortar aggregates as well as the old covered aggregates attract a smaller share of the macroscopic load, thereby leaving a larger part to be overtaken by the old plain aggregates. This results in more pronounced stress concentrations in the \(\mathcal{I}_{opa}^{ncp}\)-ITZ and consequently in a slightly reduced macroscopic strength. At mature ages, however, the \(\mathcal{I}_{opa}^{ncp}\)-failure is replaced by ITZ failure around the old covered aggregate (\(\mathcal{I}_{oca}^{ocp}\)-failure) for the compositions with relatively \"weak\" old cement pastes exhibiting \((w/c)_{ocp}\!\in\!\{0.5,0.7\}\). As an example, we focus on the composition exhibiting \((w/c)_{ncp}\!=\!0.35\) and \((w/c)_{ocp}\!=\!0.7\), where the change from \(\mathcal{I}_{opa}^{ncp}\)-failure to \(\mathcal{I}_{oca}^{ocp}\)-failure occurs at hydration degrees of approximately \(\xi_{ncp}\!\approx\!0.5\), and results in a kink in the strength evolution (see Figure 6b).