A Dual-Permeability Approach to Study Anomalous Moisture Transport Properties of Cement-Based Materials
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A Dual‑Permeability Approach to Study Anomalous Moisture Transport Properties of Cement‑Based Materials Zhidong Zhang1 · Ueli Angst1 Received: 14 June 2019 / Accepted: 23 August 2020 © The Author(s) 2020
Abstract Anomalous moisture transport in cement-based materials is often reported in the literature, but the conventional single-porosity moisture transport models generally fail to provide accurate simulation results. Previous studies suggested that the anomalous moisture transport could be caused by different moisture transport velocity in large and small pores. Based on this concept, the present study proposes a continuous dual-permeability model for cement-based material. The proposed model includes the transport contribution of both liquid water and water vapor, which are governed by liquid advection and vapor diffusion, respectively. We explicitly consider that moisture transport in the large pore region is faster than the small pore region. The volumetric fraction of each region is determined when fitting the measured sorption isotherms by using a bimodal equation. The validation with experimental data shows that the dual-permeability model can well simulate both the “normal” and the anomalous moisture transport. The applicability of the proposed model implies that the “dual-porosity property” could be one of reasons that cause anomalous moisture transport in cementitious materials. In addition, results show that vapor diffusion can be neglected for moisture transport in both porosities at high relative humidity (RH), while at low RH, vapor diffusion must be considered. Keywords Water vapor sorption isotherm · Permeability · Van Genuchten–Mualem model · Moisture transfer factor · Volumetric fraction
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1124 2-020-01469-y) contains supplementary material, which is available to authorized users. * Zhidong Zhang [email protected] Ueli Angst [email protected] 1
Institute for Building Materials (IfB), ETH Zurich, 8093 Zurich, Switzerland
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1 Introduction The moisture state in cement-based materials is related to their durability (Baroghel-Bouny et al. 2017). Aggressive agents (e.g., chloride) can penetrate into concrete with liquid water (e.g., Nilsson 1985; Liu et al. 2018; Baroghel-Bouny et al. 2012; Mao et al. 2019). When concrete is unsaturated, carbon dioxide (CO2) can rapidly diffuse into the pores that are not filled with liquid water. These processes are able to cause corrosion of the reinforced steel (rebar) and deterioration of concrete structures (Angst 2018). Therefore, appropriate moisture transport models are essential for the predication of structures durability. Conventionally, a moisture transport model based on the Darcy’s or Fick’s law is used to predict the moisture state in cement-based materials (e.g., Bazant and Najjar 1972; Coussy 1995; Mainguy et al. 2001). However, concerning anomalous moisture transport reported in the literature
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