Identification of viscoelastic C-S-H behavior in mature cement paste by FFT-based homogenization method

a r t i c l e i n f o A powerful and robust numerical homogenization method based on fast Fourier transform (FFT) is formulated to identify the viscoelastic behavior of calcium silicate hydrates (C-S-H) in hardened cement paste from its heterogeneous composition. The identification is contingent upon the linearity of the creep law. To characterize cement paste microstructure, the model developed by Bentz at the National Institute of Standards and Technology, which has the resolution of 1 μm, is adopted. Model B3 for concrete creep is adapted to characterize the creep of C-S-H in cement paste. It is found that the adaptation requires increasing the exponent of power law asymptote of creep compliance. This modification means that the rate of attenuation of creep with time is lower in C-S-H than in cement paste, and is explained by differences in stress redistribution. In cement paste, the stress is gradually transferred from the creeping C-S-H to the non-creeping components. The viscoelastic properties of C-S-H at the resolution of 1 μm were identified from creep experiments on cement pastes 2 and 30 years old, having the water–cement ratio of 0.5. The irreversible part of C-S-H creep, obtained from these old specimens at almost saturated state, is found to be negligible unless the specimens undergo drying and resaturation prior to the creep test. The existing models for predicting concrete creep are either entirely empirical or are based only on macroscopic mathematical arguments [1]. They do not take into account the random microstruc-ture and its evolution. Thanks to the recent progress in micromecha-nics, material modeling and numerical simulation, it has now become possible to relate creep to the material properties on the micrometer and nanometer scales. Here we aim at mathematical formulation of this relationship. It is beyond doubt that the mechanics of composite materials can bring new insight into the creep behavior of concrete. The recent work in linear elastic homogenization of concrete [2] or cement paste [3] demonstrated the potential of analytical or numerical homogenization methods (although, for very young hydrating concrete, these methods might not be sufficiently realistic, due to the simplifying assumption of perfect bonding among all phases, which is poor for early ages [2]). Extension of these methods to basic creep of aging concrete or hardened cement paste is made possible by linearity of the constitutive law, which permits reducing the creep problem to a quasi-static elasticity problem. …