## 5 Conclusions  
A micromechanics multiscale model for predicting the macroscopic strength of recycled concrete was developed. Therefore, the complex and heterogeneous nature of recycled aggregates is modeled by means of three recycled aggregate classes, (I) old plain aggregates without attached old mortar or paste, (II) old mortar aggregates, resolved at a smaller observation scale, as a cement paste matrix hosting small old aggregates, and (III) old aggregates covered by a layer of old cement paste. This representation entails interfacial transition zones (ITZs) between aggregates and old paste, aggregates and new paste, and also between old and new paste. Micromechanics-based scaling relations allow for quantifying the micro-stresses of all ITZs based on the macroscopically applied load. The ITZ stresses are further \"downscaled\" to micron-sized hydration products and failure of the most unfavorably loaded sub-portion of the hydrates in any of the ITZs results, in the framework of a brittle failure concept, in macroscopic failure at the concrete scale. The mechanical model is combined with [PERSON] hydration model to obtain strength prediction for any composition and maturation states of the new and the old cement paste. By means of extensive sensitivity analyses regarding changes in composition and maturity parameters, the model is exploited to decipher which ITZs are most prone to failure and to quantify the corresponding macroscopic uniaxial compressive strength. This way, the model presents an interesting alternative or complement to time-consuming and expensive experimental campaigns. The model qualifies very well to study composition and maturity states, which cannot be studied experimentally, given the large amount of possible mix design parameters and maturity states. Explicit model validation against experimental data is, unfortunately,still out of reach, given that experimental databases typically do not contain information on all required composition parameters and maturity states. A comprehensive experimental campaign which combines strength testing with microstructural characterization of the ITZs at different ages, the quantification of the hydration degree of the new cement paste, and the knowledge on the content and quality of old cement paste, on saturation state, and on stiffness of recycled and natural aggregates, would be a prerequisite for such validation attempts. Clearly, only after extensive experimental validation, quantitative conclusions and model predictions are credible. Moreover, we note that quantitative conclusions might depend on the underlying model assumptions such as the recycled aggregate morphology. Qualitatively, the model suggests that:  
* The extent of the strength reduction for recycled concrete compared to conventional concrete is determined by the ITZ, where failure is induced, and thus most importantly by the mutual stiffness contrasts between old cement paste, new cement paste, and aggregates. * Old concretes with high \(w/c\) ratios do not qualify as source of recycled aggregates if high-strength recycled concretes are targeted, since the aggregate-old cement paste ITZ will trigger macroscopic failure and the potential of a high-quality new cement paste cannot be exploited. Ideally, the \(w/c\)-ratios of old and new cement paste match, resulting in less pronounced stress concentration and consequently an optimized use of cement. This calls for careful selection of the parent concrete. * Modeling the commonly observed weakness of ITZs between old and new cement paste, resulting from water migration or chemical reactions, by means of increasing the porosity of the ITZ shows that the early-age strength significantly reduces with increasing ITZ porosity. For mature pastes, however, weak ITZs between old and new paste do not significantly alter the uniaxial compressive strength. * The strength of recycled concrete does generally decrease with increasing content of old cement paste in the recycled aggregates, whereby the decrease is less pronounced at early ages.