## 1 Introduction  
The uniaxial compressive strength of recycled concrete, i.e., concrete which contains aggregates produced by crushing waste concrete, is typically smaller than the strength of conventional concrete [(1; 2; 3; 4; 5; 6)]. The extent of the strength reduction depends on several factors including the replacement ratio of natural by recycled aggregates [(1; 6)], the strength of the parent concrete [(7; 8; 9)], the water-to-cement ratio of the new mix [(1; 8)], the maturity [(9; 10)], the saturation state of the recycled aggregates [(9)], the mixing method [(11; 12)], and the curing conditions [(13)]. In order to design recycled concrete compositions that can compete with conventional concretes, and this way to aim towards a more sustainable construction process, knowledge on the micromechanical origin of the experimentally observed strength loss is crucial. The failure mechanism of conventional concrete under uniaxial compressive loading is well understood: concrete failure is governed by cracking in the \"weakest link\" of its microstructure, the so-called interfacial transition zones (ITZs) between aggregates and cement pastes [(14; 15)]. As for recycled concrete, however, ITZs do not only occur between aggregates and the (new) cement paste, but also between the old and the new cement paste and even\"within\" the recycled aggregate, i.e., between the old aggregate and the old cement paste. Experimental identification of the failure-governing ITZs is thus significantly more challenging and mostly limited to visual inspection of crushed concrete samples. This way, cracking in plate-type concrete specimens containing cylindrical recycled aggregates (natural aggregate core with old cement paste cover) is observed to occur around both the ITZ between aggregates and old mortar and between old and new mortar [16]. By analyzing the fracture surface after crushing a recycled concrete specimen, [PERSON] et al. [17], however, showed that cracks through the old mortar-new mortar interfaces rarely occur; instead, cracking through aggregate-mortar (old and new) ITZs is likely. [PERSON] et al. [8] coupled strength measurements with microhardness tests of the ITZs between aggregates and the old and new mortar, respectively. They observed that, when preparing recycled concrete with high-strength new cement paste associated with high-strength ITZs between aggregates and new cement paste, cracks concentrate within the ITZs between aggregates and old mortar. In case of a low-strength new cement paste, the cracks, however, concentrate within the (low-strength) ITZ between aggregates and new cement paste. Then, the strength of the old mortar does not affect the macroscopic strength of concrete. These experimental observations underline that either of the ITZs might represent the \"weakest link\" of the microstructure of recycled concrete. The identification of failure-governing ITZs calls for micromechanics-based modeling approaches, since they allow for quantification of the ITZ-related microstresses as a function of the stiffness contrasts between old cement paste, new cement paste, and aggregates. Such modeling techniques have resulted in interesting insights on ITZ failure modes of conventional concrete [18; 19] and eventually led to very successful micromechanics-based strength predictions [20; 21]. As for recycled concrete, neither critical microstress states in the different ITZs nor micromechanics-based strength predictions are, to the authors' knowledge, available in the open literature. We herein aim at closing this gap by applying 3D multiscale modeling in the framework of continuum micromechanics [22]. Continuum micromechanics modeling approaches were applied very successfully to decipher the mix- and age-dependent material behavior of cement paste, mortar, and concrete in the context of homogenization of elastic [23; 24], creep [25; 26], and thermal material properties [27], as well as of autogenous shrinkage strains [28]. [PERSON] [29] and [PERSON] et al.