Abstract
The durability of reinforced concrete structures is closely related to moisture state in cement-based materials. Therefore, it is crucial to develop moisture models that can accurately predict moisture state in the materials. However, many studies reported anomalous moisture transport in cement-based materials that cannot be well simulated by the conventional models. Several reasons have been investigated in the literature, such as the complex pore structure, chemical reactions with water, dimensional changes of the tested specimen, etc. Nevertheless, only a few models are able to capture the anomaly of moisture transport. This study viewed the main moisture transport coefficient—permeability—as a kinetic variable that depends on both the degree of moisture saturation and the contact time. The time-dependence was formulated by the decay (for drying) or growth (for wetting) functions. The saturation-dependence was calculated by the van Genuchten–Mualem (VGM) model. These functions were then implemented into a moisture transport model that was developed in previous studies. The proposed model was validated by experimental data and showed a good agreement for cement pastes that were dried or wetted in the hygroscopic range. Numerical simulation results were also compared with the simplified solutions to a fractional derivative model (FDM) of anomalous diffusion and the empirical Weibull function. We found that the solutions to the FDM cannot provide appropriate results. Weibull function performs as well as the proposed model, but the empirical function lacks physical meanings.
Highlights
Concrete is the most widely used man-made material in the world
The empirical solution to the Fractional Derivative Model (FDM) is involved by the apparent diffusivity
Since the size of the specimen is known, the apparent diffusivity D f can be calculated by rewriting Equation (22)
Summary
Concrete is the most widely used man-made material in the world. In 2018, the global cement production was about 4.1 billion tons [1], equivalent to more than a half ton per person. When cement is mixed with water, the chemical reaction (so-called hydration) starts immediately, and eventually, the mixture becomes a dense solid, which contains a large number of pores whose sizes range from nm to mm. The presence of these pores can lead to the deterioration of concrete and corrosion of reinforcing steel bars (rebars). Saturated concrete favors carbonation as there are enough empty pores for CO2 diffusion and sufficient water for the reaction with the alkali ions Another type of commonly mentioned corrosion is the chloride-induced corrosion. The process is accelerated in the drying-wetting condition [7]
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