## 1 Introduction  
Polymer-modified mortars and concretes (PCC) have gained wide attention in the fields of repair, restoration and construction [1; 2; 3]. The polymers are added, either in the form of redispersible powders or aqueous dispersions, in order to improve workability, impermeability, adhesive strength or ductility [4; 5; 6]. However, adding polymers to cementitious materials reduces their elastic stiffness and increases their creep activity compared to unmodified pastes [5; 6; 7; 8; 9]. The pronounced creep activity of sprayable polymer cement concretes (SPCC) [10; 11] is beneficial for tunnel linings produced by means of the New Austrian Tunneling Method [12; 13], because creep decreases compressive stresses in the lining, which result from the inward moving ground mass. For many other applications, the increased creep activity is not necessarily an advantage. Therefore, quantitative knowledge regarding the early-age creep behavior of polymer-modified cementitious materials is of significant importance. Macroscopic early-age experiments provide valuable insight into the creep behavior of cementitious materials. As for conventional (unmodified) materials, creep testing enjoys a tradition of already more than 100 years [14; 15] and typically consists of long-term creep tests with durationsof several days to several years; see, e.g., the comprehensive creep database of [PERSON] and [PERSON] [16]. Rather recently, short-term early-age creep testing protocols were developed [17; 18; 19; 20; 21; 22]. They include regularly-repeated creep tests with durations of a few minutes only, during which the chemical reaction between the binder and the water does not progress significantly. Therefore, regularly-repeated ultra-short creep tests provide insight into the hydration-driven evolution of the elastic stiffness and the non-aging creep properties of cementitious materials. The creep testing protocol of [PERSON] et al. [19] was already applied to polymer-modified cement pastes [9]. This showed that polymer-modified cement pastes exhibit a smaller elastic stiffness, as well as a larger creep compliance compared to unmodified reference pastes. In the present contribution, the described short-term creep tests are further evaluated. Thereby, the focus rests on exploiting the macroscopic creep testing data, in the context of top-down analysis, such as to get quantitative insight into the creep properties of the microscopic polymer particles. Multiscale modeling is used as the vehicle for the required macro-to-micro-scale-transition. Multiscale models, developed in the framework of continuum micromechanics, have been successfully applied to cementitious materials, in order to predict the hydration-induced evolutions of their elastic stiffness [9; 23; 24; 25; 26; 27; 28], their compressive strength [29; 30; 31] and their viscoelastic behavior [32; 22; 33]. The hierarchically-organized microstructure of cement pastes was resolved down to the scales of the observation of unhydrated cement grains, capillary pores and hydration products (called \"hydrates\"). Universal, i.e., age- and composition-independent (visco-)elastic and strength constants of these microstructural constituents have been (i) identified and (ii) shown to be the origin of the age- and composition-dependent macroscopic material behavior of cementitious materials. The significance of the universal creep properties of the hydrates, identified by [PERSON] et al. [33], was corroborated by comparing model-predicted creep functions with results from the independent creep tests of [PERSON] and [PERSON] [34], who performed 30-day-long creep tests on 30-year-old Portland cement pastes. The present paper aims at understanding and quantifying the micromechanical origin of the more pronounced creep activity of polymer-modified cement pastes.