## 3 Model Predictions
### Sensitivity Study Regarding the ITZ Porosity  
After examining the sensitivity of the concrete strength with respect to typical stiffness contrasts occurring during the maturation of the new cement paste, we herein discuss the model-predicted strength variations with respect to varying ITZ porosities, by means of changing the ITZ porosity factor within the interval \(F_{por}\in[1,1.5]\). We recall that only the porosity of the old paste-new paste ITZs, i.e., of \(\mathcal{I}_{onia}^{ncp}\) and \(\mathcal{I}_{ocpc}^{ncp}\), are increased this way (according to Equation (22)), while the aggregate-paste ITZs remain unmodified. Given that \(\mathcal{I}_{oma}^{ncp}\) and \(\mathcal{I}_{ocpc}^{ncp}\) are not weakened (\(F_{por}\!=\!1\)), the ITZ around old plain aggregates triggers macroscopic failure during early and medium ages for the benchmark composition exhibiting \((w/c)_{ncp}\!=\!0.5\) and also for the corresponding composition exhibiting \((w/c)_{ncp}\!=\!0.35\), see Figure 7. Increasing the ITZ porosity of \(\mathcal{I}_{oma}^{ncp}\) and \(\mathcal{I}_{ocpc}^{ncp}\), respectively, beyond a certain threshold, leads to a change in failure mode and consequently to a kink in the strength-ITZ porosity plot. Then, the ITZ around the old mortar aggregate (\(\mathcal{I}_{oma}^{ncp}\)) becomes critical and an increase of the ITZ porosity decreases the macroscopic strength. The strength loss of recycled concrete, in case of an ITZ which is 25% more porous then the bulk new cement paste matrix, amounts to 25%. Notably, the earlier the change in failure mode occurs, the younger the concrete is. This implies that young recycled concrete is very sensitive regarding additional porosity in the weak ITZs.  
Figure 6: Uniaxial compressive strength evolution with respect to the hydration degree of the new cement paste matrix \(\xi_{ncp}\): (**a**) for an old cement paste-related \(w/c\)-ratio amounting to \((w/c)_{ocp}\!=\!0.5\) and different new cement paste-related \(w/c\)-ratios, whereby solid lines refer to concrete containing recycled aggregates only (\(f_{ra}^{a}\!=\!1\)) and dashed lines refer to conventional concretes without recycled aggregates (\(f_{ra}^{a}\!=\!0\)); (**b**) for different old and new cement paste-related \(w/c\)-ratios \((w/c)_{ocp}\in\{0.2,0.35,0.5,0.7\}\) and \((w/c)_{ncp}\in\{0.2,0.35,0.5,0.7\}\).  
Mature recycled concrete, in turn, is less sensitive. As for \((w/c)_{ncp}=0.35\) and mature ages (\(\xi_{ncp}=0.75\)), the macroscopic failure is driven by failure around old covered aggregates (\(Z_{ocd}^{ncp}\)-failure), as discussed in the previous section. Interestingly, then the ITZ around old mortar aggregates does not become critical, not even for \(F_{por}=1.5\). Consequently, the concrete strength is constant with respect to the porosity of the \"weak\" ITZs, see the top solid line in Figure 7a. Full hydration of the benchmark composition which exhibits \((w/c)_{ncp}=0.5\) results in critical failure around the old embedded aggregates at the mortar scale (\(Z_{ocd}^{ncp}\)-failure), as indicated by the top solid line in Figure 7b. Increasing the ITZ porosity beyond \(F_{por}>1.4\) finally results in failure of the weakened ITZ around mortar aggregates.