Nanoindentation testing suggests that creep of hydration products is the microscopic reason for macroscopic creep of cementitious materials. This is supported by a multiscale creep model which explains aging creep of young concretes as the consequence of universal creep of hydration products (Scheiner and Hellmich, 2009), whereby the latter is described with a rheological model consisting of linear springs and dashpots. We here extend the investigation of the origin of creep of cementitious materials further down to the nanoscale of hydration products, where we envision solid matter sliding (upon loading) along interfaces which are filled with lubricating thin layers of adsorbed water, i.e. water in a "glassy", "liquid crystal" state. As for the viscous behavior of the interfaces, we follow (Shahidi et al., 2014) and consider that the shear traction acting on an adsorbed water layer is proportional to the shear dislocation rate of the interface, with an interface viscosity as the proportionality constant. Our analysis starts from corresponding anisotropic creep and relaxation tensors of matrix-interface composites containing parallel interfaces (Shahidi et al., 2014). Considering that hydration products contain interfaces oriented isotropically in all space directions, we here compute complete spatial averages of parallel interface-related anisotropic creep and relaxation tensors, in order to derive isotropic creep and relaxation tensor bounds. Comparing them with creep and relaxation functions of the aforementioned rheological model for universal creep of hydration products allows for identification (ⅰ) of the interface density and (ⅱ) of the product of interface size and viscosity. Based on the Reuss-type creep tensor bound, we obtain, interesting quantitative insight into microstructural features of hydration products.
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