## 4 Discussion  
Herein, key model assumptions and corresponding limitations and potentials are discussed in more detail. This includes the potential discontinuity of strength predictions for replacement ratios approaching zero, initial cracks in recycled aggregates, the ITZ porosity, the use of elastic microstresses for strength upscaling, and model predictions in case of multiaxial or tensile macroscopic loading. The model predictions are discontinuous at the limit case of vanishing recycled aggregate volume fraction, if macroscopic failure is triggered by failure in ITZs related to class II or class III recycled aggregates (see the composition with \((w/c)_{ncp}=0.35\) and \(\xi_{ncp}=0.75\) in Figure 10a). As for \(f_{ra}^{a}=0\), the modeled strength is 12% higher than for the case of infinitely small, but non-zero recycled aggregate content \(f_{ra}^{a}\to 0\). This originates from the concept of brittle failure upscaling, since, as soon as class III recycled aggregates are present, macroscopic failure is triggered by ITZ failure around them. Practically, however, ITZ failure of a small sub-volumes of ITZs would not result in macroscopic failure. This shows that the concept of brittle failure upscaling is not suited to study the strength reduction for small aggregate replacement ratios for the aforementioned compositions. In order to overcome this difficulty and to determine the related threshold, one would need to consider damage laws to characterize the stress concentrations after failure of small sub-volumes, which is beyond the scope of the paper, but provides lots of motivation for future research. Next, we discuss the effect of crushing on the recycled aggregate microstructure. One might think that the mechanical loads during the crushing process might lead to damage, most likely in the old ITZs [44]. Available experimental investigations, however, do not corroborate such a presumption. Nanoindentation studies showed that the indentation moduli of old and new ITZs around aggregates are very comparable and mainly driven by the \(w/c\)-ratio of the adjacent paste [8; 16]. By means of optical microscopy coupled with image analyses, [PERSON] et al. [45] could detect only a very minor amount of (micro-)cracks in the old cement paste. Only after more quantitative experimental results on damage characteristics of recycled aggregates are available, modeling existing cracks in recycled aggregates seems, in the authors' opinion, beneficial. Another interesting discussion point concerns the microstructural homogeneity of cement paste close to the aggregate surface. The ITZs of conventional concrete are (slightly) more porous than the bulk paste due to segregation effects [46; 47], in particular at early material ages. Herein, the ITZs between aggregates and cement paste (both old and new) are considered as a \"geometrical\" concept rather than a \"mechanical\" one, i.e., their microstructures are considered to be identical to the ones of the adjacent cement paste. Critical stress concentrations only occur in the ITZs' hydrates due to their location around aggregates. This choice is motivated based on the fact that blind model predictions for conventional concrete (\(f_{ra}^{a}=0\)) agree exceptional well with independent experimental results, and weakening the ITZs between plain aggregates and new cement paste does not improve the predictive capabilities significantly (see [PERSON] et al. [21] for the related validation and sensitivity study). As for the ITZ between old and new cement paste, however, many experiments showed that water migration effects (adsorption of mixing water and internal curing, respectively) or chemical reactions can lead to considerably weaker ITZs than those encountered around aggregates(see Section 2.5). This provided the motivation to incorporate \"mechanical\" ITZ weaknesses by means of a phenomenological porosity factor. Future modeling work should focus on quantifying the related physical phenomena, and incorporating them into a multiscale micromechanics model.