Modeling Anomalous Moisture Transport in Cement-Based Materials with Kinetic Permeability (original) (raw)
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Transport in Porous Media, 2020
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.
Investigation of moisture transport properties of cementitious materials
Cement and Concrete Research, 2016
Moisture transport in cementitious materials is directly related to the durability of concrete structures. When the material loses moisture, the drying shrinkage induces cracks which are harmful to the solid body. When liquid moves into the material, it can carry aggressive ions. Hence, moisture transport properties are important to cope with durability issues. The present paper is focused on two ways to enhance our understanding of moisture transport properties of cementitious materials. They are based on the indirect determination of the liquid water permeability and water vapour diffusion coefficient. The first one is known as the " inverse analysis " method. In a moisture transport model, the liquid permeability and diffusion coefficient can be adjusted in order to fit the measured mass loss curves and water content profiles. In the second way, measured apparent diffusion coefficient (also called moisture diffusivity) curves are fitted over a large range of relative humidity (RH) by a general expression which includes both liquid transport and vapour diffusion. Due to different RH ranges of predominance of liquid and vapour transport, the liquid water perme-ability and vapour diffusion coefficient can be determined separately. Input experimental data on cement pastes are collected from the literature. Discussions on relative permeability and Knudsen diffusion show a significant influence on modelling of moisture transport. A further comparison with measured permeability data is able to provide a better understanding of moisture transport properties. Keywords: cement paste (E), moisture transport (C), diffusion coefficient (C), liquid water permeability (A), inverse analysis (B), Knudsen effect (A)
Analysis of moisture transport in cementitious materials and modelling of drying-wetting cycles
Proceedings of the international conference on numerical modeling strategies for sustainable concrete structures (SSCS 2012), 2012
Aiming at studying moisture transport in cementitious materials, the paper describes a multiphase isothermal modelling of moisture transport which incorporates the coupled movements of liquid water, water vapor and dry air. The model is based on mass balance equations, transport laws and material properties, such as water vapor sorption properties and permeability. Numerical simulations show that a gas overpressure can occur owing to the evaporation coupled with the diffusion of dry air while a gas underpressure is observed if the initial saturation is close to saturated state. The moisture transport during drying is either in liquid form or vapor form depending on external relative humidity and material microstructure (porosity). Furthermore, a simplified method for the modelling of moisture transport during wetting is proposed, using adsorption isotherm and different expressions of relative permeability. Moreover, mass loss kinetics data obtained from experiments during the first drying and wetting cycle are used for the validation of the numerical results.
Moisture Diffusion in Cementitious Materials
2000
This article describes an improvement on a previous model proposed by Ba~ant and Najjar, in which moisture diffusivity and moisture capacity are treated as separate parameters. These parameters are evaluated from independent test results, and are shown to depend on the water:cement ratio, curing time, temperature, and cement type. The moisture capacity is obtained as the slope of the adsorption
Cement and Concrete Research, 2007
This paper focuses on behaviors of moisture dispersed in nano-macro scale pores under various temperature and relative humidity conditions. The authors formulated an equilibrium relationship between liquid and vapor phases and a moisture flux driven by pore pressure, vapor pressure and temperature gradients. In addition, liquid and interlayer water were measured separately by ethanol in order to reveal each temperature sensitivity in saturation-humidity paths. Based on the experiments, a modified hysteresis model for moisture isotherm was proposed. Verifications with experimental data showed that the proposed method can simulate moisture behaviors under various temperature conditions.
Moisture diffusion coefficient of impregnated concrete
2005
In order to understand the mechanisms of water repellent agents it is important to have reliable data on how the moisture diffusion coefficient is affected by hydrophobic treatment. The results from the experiment described below will be used as input data in a project aiming at creating a computer model for simulations of moisture and material transport in impregnated concrete structures. Two types of concrete with water/cement-ratio 0.8 and 0.45 have been investigated with the cup-method to determine the moisture diffusion coefficient. Half of the specimens have been completely impregnated with triethoxy(2,4,4-trimethylpentyl)silan by capillary suction and the other half were left untreated. This silan is one of the most common used water repellents on the Swedish market. Four different saturated salt solutions are used to create relative humidity (RH) between 85 % and 97 % inside the cups while the surrounding environment holds 50 % RH. Three cups for each situation and in addition two cups with pure water give a total of 50 cups. The loss of weight has been monitored ones a week until stable results were obtained.
Moisture distribution, diffusion coefficient and shrinkage of cement-based materials
Doboku Gakkai Ronbunshu, 1999
The purpose of this study was to develop a new method for obtaining diffusion, film and shrinkage coefficients of cement-based materials. These coefficients are required for the numerical simulation of the effect of shrinkage strain on the deformation of concrete structures using finite element method. An experimental approach for obtaining the relative humidity distribution in the specimen at arbitrary drying times and a numerical method for determining the material coefficients from the experimental data are proposed in this paper. Results show that this method can provide us with the diffusion coefficient not only as a function of moisture content but also as a function of relative humidity in cement-based materials.
An equation of drying kinetics for cementitious materials
Drying Technology , 2018
Concrete structures are often subjected to drying in the natural environment. To simulate moisture transport in concrete during drying, a continuum model is commonly used, which generally requires the measured water vapor desorption isotherm as input data to address the equilibrium between liquid phase and water vapor at a given temperature. The main problem in measuring desorption isotherms is that the sample needs very long time to reach mass equilibrium at a certain relative humidity. To improve the method of measurement of sorption isotherms, we proposed to use Weibull equation to determine the mass loss at the infinite time. However, Weibull equation does not work well with the anomalous drying kinetics that were recently reported in the literature. In this paper, based on the theory of dual-porosity, a new equation is proposed by separating the moisture transport in the large and small pores to estimate the drying kinetics for cementitious materials. Fitting the drying kinetics measured by the dynamic vapor sorption analyzer shows that the newly developed equation has a very high capability for a large range of relative humidities, especially for the anomalous drying kinetics. This paper also demonstrates that using the proposed equation can reduce the duration of measuring desorption isotherms.
Determination of water permeability for a moisture transport model with minimized batch effect
Construction and Building Materials, 2018
Values of water permeability for cementitious materials reported in the literature show a large scatter. This is partially attributed to the fact that materials used in these studies are different. To eliminate the effects of cements, specimen preparation, curing conditions and other batch effects, this study employs a single cement paste to prepare all specimens for a variety of permeability determination methods , such as beam-bending, sorptivity, Katz–Thompson and Kozeny-Carman equations. Permeabilities determined by these methods are then used in a moisture transport model. Compared with the measured mass loss curves, we found that permeability determined by the beam-bending method is more suitable for the moisture transport model than the other methods. The difference results from the use of a saturated specimen in the beam-bending method, while specimens in the other methods are dried (or rewet-ted). As already shown in the literature, the microstructure of the dried or rewetted specimens is altered and different to the original microstructure of the water saturated specimens. In addition, we found that drying tests for the inverse analysis method must be done at high RHs (63% in this study) to reduce the effect of vapor diffusion on the determination of water permeability.
2011
Early-age cracks in fresh concrete occur mainly due to high rate of surface evaporation and restraint offered by the contracting solid phase. Available test methods that simulate severe drying conditions, however, were not originally designed to focus on evaporation and transport characteristics of the liquid-gas phases in a hydrating cementitious microstructure. Therefore, these tests lack accurate measurement of the drying rate and data interpretation based on the principles of transport properties is limited. A vacuum-based test method capable of simulating early-age cracks in 2-D cement paste is developed which continuously monitors the weight loss and changes to the surface characteristics. 2-D crack evolution is documented using time-lapse photography. Effects of sample size, w/c ratio, initial curing and fiber ACKNOWLEDGMENTS I would like to start by expressing my gratitude to my advisor Professor Barzin Mobasher for giving me the opportunity to work with him and study at Arizona State University. His guidance and support are the reasons I was able to accomplish this work. Thank you to Professor Subramaniam D. Rajan and Dr.