Zhidong Zhang | Swiss Federal Institute of Technology (ETH) (original) (raw)

Papers by Zhidong Zhang

Construction and Building Materials, 2023

Excessive CO2 curing of concrete may significantly increase the risk of steel corrosion, which li... more Excessive CO2 curing of concrete may significantly increase the risk of steel corrosion, which limits the application of this technology in reinforced concrete. Considering the substantial potential advantages of seawater sea sand concrete (SWSSC) structures reinforced by fiber reinforced polymer (FRP) bars in coastal infrastructure, FRP-SWSSC is proposed to capture CO2 by means of carbonation curing in this study. The effects of long-term CO2 curing on the compressive strength, pore structure, interfacial transition zone, CO2 uptake and pH of SWSSC were examined. It is found that CO2 curing can achieve an increase of approximately 25% in both 28-d and 56-d compressive strengths of SWSSC. Additionally, the porosity experiences a reduction of approximately 3%. The increased carbonation depth and higher CO2 uptake in CO2-cured SWSSC lead to significantly greater CO2 storage. Even after 28 days of additional water curing, the pH of CO2-cured SWSSC remains below 9, thus preventing any damage caused by the high pH environment to the mechanical properties and microstructure of embedded FRP bars. Therefore, CO2 curing of FRP-SWSSC offers three-fold great benefits: (1) improved SWSSC performance, (2) increased CO2 storage amount, and (3) reduced adverse effects of high alkaline concrete pore solution on embedded FRP bars.

Drying Technology, 2021

The accurate simulation of moisture transport in cement-based materials is an important step to p... more The accurate simulation of moisture transport in cement-based materials is an important step to predict many durability processes of concrete structures. This paper studied the effects of boundary ...

The durability of reinforced concrete structures and their service life are closely related to th... more The durability of reinforced concrete structures and their service life are closely related to the simultaneous occurrence of many physical and chemical phenomena. These phenomena are diverse in nature, but in common they are dependent on the moisture properties of the material. Therefore, the prediction of the potential degradation of cementitious materials requires the study of the movement of liquid-water and gas-phase transport in the material which is considered as a porous medium. In a natural environment, structures are always affected by periodic variations of external relative humidity (RH). However, most moisture transport models in the literature only focus on the drying process. There are few studies considering both drying and wetting, although these conditions represent natural RH variations. Much fewer studies take into account hysteresis in moisture transport. Therefore, this work is devoted to a better understanding of how the moisture behaviour within cementitious ...

Corrosion of steel in concrete is a common degradation mechanism. With the growth of corrosion pr... more Corrosion of steel in concrete is a common degradation mechanism. With the growth of corrosion products, expansive pressure is created that may damage the concrete. In conventional studies, corrosion products are assumed to precipitate on the steel surface and from there, exert stresses on the surrounding concrete. However, before precipitating, the released ferrous ions can penetrate into the concrete. This means that corrosion products are not necessarily formed immediately after the dissolution of iron. Instead, the formation of corrosion products can be viewed as kinetic processes, which has been already studied in aqueous solution, depending on oxygen concentration, pH, ferrous ion concentration, etc. In our previous studies, a reactive transport model was proposed, which includes the processes of iron dissolution, ferrous ion diffusion, oxidation, and precipitation of ferrous and ferric compounds. In this paper, the First Order Reliability Method (FORM) was used to analyze the sensitivity of the different model parameters, such as ferrous ion oxidation reaction rate, ferrous hydroxide precipitation rate, tortuosity, and constrictivity. The results show that the tortuosity factor is the most dominant parameter, followed by ferrous ion oxidation reaction rate and constrictivity, which have an equal sensitivity. Ferrous hydroxide precipitation rate is not sensitive regardless of its reference value. In addition, we found that the sensitivity of tortuosity factor increases with its reference value and the corrosion rate does not affect the sensitivity of studied parameters

SPE Journal, 2020

Summary The performance of oil-well cement is altered if contaminated by spacers. Very few studie... more Summary The performance of oil-well cement is altered if contaminated by spacers. Very few studies in the literature are found on this topic, in particular for newly developed microemulsion spacers. Therefore, it is worth investigating the properties of cement contaminated with spacers. In this study, material characterization techniques including chemical shrinkage, ultrasonic pulse velocity (UPV), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and nitrogen adsorption (NAD) were used to study the hydration and microstructure of contaminated cement. Results showed that the studied microemulsion spacer was less compatible with the cement slurry than the conventional one. The microemulsion spacer has complex effects on cement hydration: it slightly enhances hydration for the low dosage but can retard hydration at the early ages and accelerate hydration in the later ages with the increase of dosage. The conventional spacer caused early acceleration of cement hydr...

Cement and Concrete Research, 2021

Abstract Drying cement-based materials is a necessary step to characterize the microstructure by ... more Abstract Drying cement-based materials is a necessary step to characterize the microstructure by microscopy, adsorption, or mercury intrusion. The isopropanol (IPA) exchange method is commonly used to replace the pore solution prior to drying, as it reduces the capillary pressure and thereby helps to preserve the microstructure of the hydrated cementitious materials. However, some physical and chemical effects of IPA on hydration products have been reported. These effects cannot be completely avoided, but can be reduced by shortening the exchange duration, in particular for effects on ettringite. This study carried out experiments with different exchange durations and IPA removal techniques to address research gaps in the literature. For 1-mm cubes of cement paste with IPA diffusion coefficient of 1 × 10−11 m2/s, the compared exchange durations are 5 min, 15 min (value from the literature), 40 min (when the mixture at the sample center achieves 95% of the drop in surface tension from water to IPA), 80 min (when the IPA/water mixture at the sample center reaches an azeotrope), 5 h (when over 99% IPA is replaced at the sample center), and 24 h (prolonged exchange). After IPA exchange, samples were either directly dried at 40 °C with flushing N2 or washed by diethyl ether prior to N2 drying. Even though the exchange duration of 40 and 80 min, in theory, can reduce the damage from capillary pressure, the nitrogen adsorption results do not show any advantage in preventing microstructural alteration. The 5-h exchange provided results similar to the 24-h exchange, but the latter induced slightly more serious chemical effects resulting from the longer contact of IPA with hydration products. In addition, the compared IPA removal techniques do not show significant differences. Therefore, this study suggests using small samples (~1 mm) for microstructural studies following IPA exchange for a period corresponding to the time when 99% of the water (mole fraction) at the sample center is replaced.

Drying Technology, 2017

ABSTRACT Concrete structures are often subjected to drying in the natural environment. To simulat... more ABSTRACT 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.

Cement and Concrete Research, 2020

The steel-concrete interface is fundamental to many aspects related to the performance of reinfor... more The steel-concrete interface is fundamental to many aspects related to the performance of reinforced concrete, particularly for corrosion behavior. A recently published paper by Chen et al. [Effect of design parameters on microstructure of steel-concrete interface in reinforced concrete, Cem. Concr. Res. 119 (2019) 1-10] quantitatively characterized the thickness of the gap observed between the steel and concrete. Considering the low number of available publications, studies documenting the microstructure at the steel-concrete interface are important. However, in our discussion, we think that aspects related to the terminology, the specimen preparation and possible experimental artefacts in the discussed paper can be improved.

Journal of Natural Gas Science and Engineering, 2018

The primary function of natural gas well cementing is to provide downhole zonal isolation. Howeve... more The primary function of natural gas well cementing is to provide downhole zonal isolation. However, the residual non-aqueous drilling fluid in cement and on the surface of the casing can weaken the bond of hardened cement to the casing. In this paper, the amount of remaining drilling fluid both in cement and on the casing is quantified by the fluorescent dye method, which is verified by other two methods, mass difference and laser scanning. The essential factor-surface roughness that can significantly affect the amount of residual drilling fluid-is investigated. Results show good correlations between the thickness of the remaining drilling fluid and the surface roughness. The comparison of mud removal efficiency of two commercial spacers indicates that microemulsion can remove more drilling fluid than the common spacer, as pointed out in the literature; however, microemulsion is also apt to remain on the casing and thus reduce the bonding between cement and the casing.

Cement and Concrete Research, 2019

When cementitious materials are subjected to drying, the microstructure is significantly altered ... more When cementitious materials are subjected to drying, the microstructure is significantly altered by high capillary forces established in the fine pores. To avoid drying stresses when preparing samples for microstructural characterization, our previous studies developed the supercritical drying method (SCD) (Zhang and Scherer, 2017) which showed rather good preservation for the microstructure of hydration products compared with the other conventional methods (Zhang et al., 2018). In this study, various drying methods, including SCD, are evaluated by using the nitrogen adsorption (NAD) technique in terms of BET surface area, sorption isotherms, pore size distribution and pore volume. Results show that the isopropanol (IPA) replacement method can better preserve the microstructure than the other methods with a slight chemical effect of IPA on the calcium silicate hydrate (C-S-H). Contrary to the previous studies, IPA was not found to have a greater influence on calcium hydroxide (CH) than the other drying methods. SCD shows slightly lower microstructural preservation than IPA replacement due to pore blockage. The fluid used for SCD (trifluoromethane) also chemically interacts with both CH and C-S-H, which may result from the pressurization during drying. Freeze-drying is suggested if one wants to avoid chemical effects on the hydration products, at the expense of damage to the microstructure.

Aiming at studying moisture transport in cementitious materials, the paper describes a multiphase... more 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.

Mass transport (moisture and ions) at the steel-concrete interface is closely related to corrosio... more Mass transport (moisture and ions) at the steel-concrete interface is closely related to corrosion of rebar in reinforced concrete structures. Thus, in the model simulating mass transport, the structure of the steel-concrete interface must be well represented. In this study, an image-based local homogenization method is proposed based on images taken in the scanning electron microscope (SEM) under the backscattered electron (BSE) detector. According to the gray level of the image, porosity can be calculated and then proposed equations are used to associate the transport properties (sorption isotherms, diffusion coefficients and permeability) with porosity. Experimental data of chloride concentration taken from the literature are used to validate the proposed method and a good agreement with simulated results them is found.

Http Www Theses Fr, May 13, 2014

with hysteretic eects provided much better results than non-hysteresis modelling. Among different... more with hysteretic eects provided much better results than non-hysteresis modelling. Among different hysteresis models, the use of the conceptual hysteresis model, which presents closed form of scanning loops, showed more accuracy predictions. Further simulations for dierent scenarios were also performed. All comparisons and investigations enhanced the necessity of considering i Abstract hysteresis to model moisture transport for varying RH at the boundary. The investigation of moisture penetration depth could provide a better understanding of how deep moisture as well as ions can move into the material. Furthermore, this research investigated dierent methods to determine the liquid-water intrinsic permeability, including the inverse analysis with dierent boundary conditions and tting measured apparent diusivity values for dierent equations to calculate relative permeabilities.

The corrosion of the reinforcement steel is the most common cause of degradation of reinforced co... more The corrosion of the reinforcement steel is the most common cause of degradation of reinforced concrete. The repair work is always associated with high costs which is mainly due to the lack of fundamental understanding of the processes of corrosion and cracking. In the literature, most degradation models are empirical and based on the assumptions, such as homogeneous concrete structure and uniform corrosion products growth from rebar. According to the steps of the corrosion-related degradation, this paper proposes a framework for modeling corrosion-related degradation in reinforced concrete. Models that can be related to each step in this framework are reviewed. The proposed framework can enhance the understanding of corrosion-related degradation for researchers and engineers.

In this research, a model of the couplings between hydration and moisture transfer within cementi... more In this research, a model of the couplings between hydration and moisture transfer within cementitious materials is proposed. Based on taking into account the microstructure properties and the moisture transfer properties due to hydration, the model describes the couplings between the moisture transfer at early-age owing to external drying and the internal water depletion by hydration. Furthermore, the slowing down of the hydration kinetics caused by the decrease of relative humidity prevailing in the pores is also investigated. Experimental data, such as drying kinetics, as well as porosity and degree of hydration profiles, are used to verify the proposed modelling. The materials used here are three OPC cement pastes with different water-to-cement ratios. The agreement between observed and modelled results is considered satisfactory and confirms the possibilities of using the coupling model to simulate moisture transfer for cementitious materials at early-age.

Water vapour sorption isotherms (WVSIs) are one of main material properties that are important fo... more Water vapour sorption isotherms (WVSIs) are one of main material properties that are important for modelling of moisture transport in porous medium, such as cementitious materials. However, measurements of WVSIs are time-consuming. In this paper, we have proposed a method that uses the fewest measured data to predict both desorption and adsorption isotherms. Experimental data have been collected from the literature to verify the proposed simplified method. We concluded that for OPC materials three points on the desorption isotherm and two points on the adsorption isotherm are enough to calculate both curves. For high performance materials, one or more points are needed to predict the desorption curve. One statistical criterion developed based on and MME were used to quantify the proposed method. Results show that this new method has the same prediction accuracy was using the original experimental data. The proposed method not only can save experimental time but also can reduce the m...

Cement and Concrete Research, 2020

The microstructure of the steel-concrete interface (SCI) in reinforced concrete is closely relate... more The microstructure of the steel-concrete interface (SCI) in reinforced concrete is closely related to corrosion of reinforcing steel bars. Accordingly, characterization of the SCI is receiving increasing research attention. For microscopical observations of the SCI, a cutting process is needed to create a flat cross-section. Cutting carries the risk of damaging the SCI because of the considerable difference of hardness between concrete and steel. However, studies on characterizing the microstructure of the SCI rarely consider the damage induced by the potentially inappropriate cutting process. This study investigated the damage created by three cutting methods, namely, mechanical sawing, laser cutting, and combined laser-water cutting by the Laser MicroJet technology (LMJ). The SCI of the cut sections was imaged by scanning electron microscopy equipped with a backscattered electron detector. Additionally, the specimens were non-invasively studied by X-ray microtomography before and after cutting, to compare the impact of various cutting techniques on inducing damage to the SCI beneath the cutting surface. The results showed that if a bleed water zone (BWZ) is present, the cutting technique and protocol can significantly influence the morphology of this zone and adjacent regions. This study recommends an optimized mechanical sawing protocol with low feed speed as this led to considerably less SCI damage than laser and LMJ cutting. Moreover, it was found that adjusting the cutting direction can further significantly reduce the damage created during cutting. The least damage was found when the saw blade cut through the steel before cutting the BWZ. The main problem with laser cutting was heat generated even for a relatively low laser power; therefore, a heat-affected zone was created which significantly altered the microstructural features of the SCI not only on the cutting surface but also a certain depth below the surface. In LMJ cutting, this thermal effect was significantly reduced, however, the high-pressure water eroded the porous SCI and caused cracks. These effects can penetrate along the BWZ into the interior material. To complete this study, two applications demonstrate that the optimized mechanical sawing protocol is applicable to concrete specimens with rebars of actual size and corroded rebars.

Transport in Porous Media, 2020

Anomalous moisture transport in cement-based materials is often reported in the literature, but t... more 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 ...

International Journal of Molecular Sciences, 2020

The durability of reinforced concrete structures is closely related to moisture state in cement-b... more 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 we...

Construction and Building Materials, 2023

Excessive CO2 curing of concrete may significantly increase the risk of steel corrosion, which li... more Excessive CO2 curing of concrete may significantly increase the risk of steel corrosion, which limits the application of this technology in reinforced concrete. Considering the substantial potential advantages of seawater sea sand concrete (SWSSC) structures reinforced by fiber reinforced polymer (FRP) bars in coastal infrastructure, FRP-SWSSC is proposed to capture CO2 by means of carbonation curing in this study. The effects of long-term CO2 curing on the compressive strength, pore structure, interfacial transition zone, CO2 uptake and pH of SWSSC were examined. It is found that CO2 curing can achieve an increase of approximately 25% in both 28-d and 56-d compressive strengths of SWSSC. Additionally, the porosity experiences a reduction of approximately 3%. The increased carbonation depth and higher CO2 uptake in CO2-cured SWSSC lead to significantly greater CO2 storage. Even after 28 days of additional water curing, the pH of CO2-cured SWSSC remains below 9, thus preventing any damage caused by the high pH environment to the mechanical properties and microstructure of embedded FRP bars. Therefore, CO2 curing of FRP-SWSSC offers three-fold great benefits: (1) improved SWSSC performance, (2) increased CO2 storage amount, and (3) reduced adverse effects of high alkaline concrete pore solution on embedded FRP bars.

Drying Technology, 2021

The accurate simulation of moisture transport in cement-based materials is an important step to p... more The accurate simulation of moisture transport in cement-based materials is an important step to predict many durability processes of concrete structures. This paper studied the effects of boundary ...

The durability of reinforced concrete structures and their service life are closely related to th... more The durability of reinforced concrete structures and their service life are closely related to the simultaneous occurrence of many physical and chemical phenomena. These phenomena are diverse in nature, but in common they are dependent on the moisture properties of the material. Therefore, the prediction of the potential degradation of cementitious materials requires the study of the movement of liquid-water and gas-phase transport in the material which is considered as a porous medium. In a natural environment, structures are always affected by periodic variations of external relative humidity (RH). However, most moisture transport models in the literature only focus on the drying process. There are few studies considering both drying and wetting, although these conditions represent natural RH variations. Much fewer studies take into account hysteresis in moisture transport. Therefore, this work is devoted to a better understanding of how the moisture behaviour within cementitious ...

Corrosion of steel in concrete is a common degradation mechanism. With the growth of corrosion pr... more Corrosion of steel in concrete is a common degradation mechanism. With the growth of corrosion products, expansive pressure is created that may damage the concrete. In conventional studies, corrosion products are assumed to precipitate on the steel surface and from there, exert stresses on the surrounding concrete. However, before precipitating, the released ferrous ions can penetrate into the concrete. This means that corrosion products are not necessarily formed immediately after the dissolution of iron. Instead, the formation of corrosion products can be viewed as kinetic processes, which has been already studied in aqueous solution, depending on oxygen concentration, pH, ferrous ion concentration, etc. In our previous studies, a reactive transport model was proposed, which includes the processes of iron dissolution, ferrous ion diffusion, oxidation, and precipitation of ferrous and ferric compounds. In this paper, the First Order Reliability Method (FORM) was used to analyze the sensitivity of the different model parameters, such as ferrous ion oxidation reaction rate, ferrous hydroxide precipitation rate, tortuosity, and constrictivity. The results show that the tortuosity factor is the most dominant parameter, followed by ferrous ion oxidation reaction rate and constrictivity, which have an equal sensitivity. Ferrous hydroxide precipitation rate is not sensitive regardless of its reference value. In addition, we found that the sensitivity of tortuosity factor increases with its reference value and the corrosion rate does not affect the sensitivity of studied parameters

SPE Journal, 2020

Summary The performance of oil-well cement is altered if contaminated by spacers. Very few studie... more Summary The performance of oil-well cement is altered if contaminated by spacers. Very few studies in the literature are found on this topic, in particular for newly developed microemulsion spacers. Therefore, it is worth investigating the properties of cement contaminated with spacers. In this study, material characterization techniques including chemical shrinkage, ultrasonic pulse velocity (UPV), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and nitrogen adsorption (NAD) were used to study the hydration and microstructure of contaminated cement. Results showed that the studied microemulsion spacer was less compatible with the cement slurry than the conventional one. The microemulsion spacer has complex effects on cement hydration: it slightly enhances hydration for the low dosage but can retard hydration at the early ages and accelerate hydration in the later ages with the increase of dosage. The conventional spacer caused early acceleration of cement hydr...

Cement and Concrete Research, 2021

Abstract Drying cement-based materials is a necessary step to characterize the microstructure by ... more Abstract Drying cement-based materials is a necessary step to characterize the microstructure by microscopy, adsorption, or mercury intrusion. The isopropanol (IPA) exchange method is commonly used to replace the pore solution prior to drying, as it reduces the capillary pressure and thereby helps to preserve the microstructure of the hydrated cementitious materials. However, some physical and chemical effects of IPA on hydration products have been reported. These effects cannot be completely avoided, but can be reduced by shortening the exchange duration, in particular for effects on ettringite. This study carried out experiments with different exchange durations and IPA removal techniques to address research gaps in the literature. For 1-mm cubes of cement paste with IPA diffusion coefficient of 1 × 10−11 m2/s, the compared exchange durations are 5 min, 15 min (value from the literature), 40 min (when the mixture at the sample center achieves 95% of the drop in surface tension from water to IPA), 80 min (when the IPA/water mixture at the sample center reaches an azeotrope), 5 h (when over 99% IPA is replaced at the sample center), and 24 h (prolonged exchange). After IPA exchange, samples were either directly dried at 40 °C with flushing N2 or washed by diethyl ether prior to N2 drying. Even though the exchange duration of 40 and 80 min, in theory, can reduce the damage from capillary pressure, the nitrogen adsorption results do not show any advantage in preventing microstructural alteration. The 5-h exchange provided results similar to the 24-h exchange, but the latter induced slightly more serious chemical effects resulting from the longer contact of IPA with hydration products. In addition, the compared IPA removal techniques do not show significant differences. Therefore, this study suggests using small samples (~1 mm) for microstructural studies following IPA exchange for a period corresponding to the time when 99% of the water (mole fraction) at the sample center is replaced.

Drying Technology, 2017

ABSTRACT Concrete structures are often subjected to drying in the natural environment. To simulat... more ABSTRACT 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.

Cement and Concrete Research, 2020

The steel-concrete interface is fundamental to many aspects related to the performance of reinfor... more The steel-concrete interface is fundamental to many aspects related to the performance of reinforced concrete, particularly for corrosion behavior. A recently published paper by Chen et al. [Effect of design parameters on microstructure of steel-concrete interface in reinforced concrete, Cem. Concr. Res. 119 (2019) 1-10] quantitatively characterized the thickness of the gap observed between the steel and concrete. Considering the low number of available publications, studies documenting the microstructure at the steel-concrete interface are important. However, in our discussion, we think that aspects related to the terminology, the specimen preparation and possible experimental artefacts in the discussed paper can be improved.

Journal of Natural Gas Science and Engineering, 2018

The primary function of natural gas well cementing is to provide downhole zonal isolation. Howeve... more The primary function of natural gas well cementing is to provide downhole zonal isolation. However, the residual non-aqueous drilling fluid in cement and on the surface of the casing can weaken the bond of hardened cement to the casing. In this paper, the amount of remaining drilling fluid both in cement and on the casing is quantified by the fluorescent dye method, which is verified by other two methods, mass difference and laser scanning. The essential factor-surface roughness that can significantly affect the amount of residual drilling fluid-is investigated. Results show good correlations between the thickness of the remaining drilling fluid and the surface roughness. The comparison of mud removal efficiency of two commercial spacers indicates that microemulsion can remove more drilling fluid than the common spacer, as pointed out in the literature; however, microemulsion is also apt to remain on the casing and thus reduce the bonding between cement and the casing.

Cement and Concrete Research, 2019

When cementitious materials are subjected to drying, the microstructure is significantly altered ... more When cementitious materials are subjected to drying, the microstructure is significantly altered by high capillary forces established in the fine pores. To avoid drying stresses when preparing samples for microstructural characterization, our previous studies developed the supercritical drying method (SCD) (Zhang and Scherer, 2017) which showed rather good preservation for the microstructure of hydration products compared with the other conventional methods (Zhang et al., 2018). In this study, various drying methods, including SCD, are evaluated by using the nitrogen adsorption (NAD) technique in terms of BET surface area, sorption isotherms, pore size distribution and pore volume. Results show that the isopropanol (IPA) replacement method can better preserve the microstructure than the other methods with a slight chemical effect of IPA on the calcium silicate hydrate (C-S-H). Contrary to the previous studies, IPA was not found to have a greater influence on calcium hydroxide (CH) than the other drying methods. SCD shows slightly lower microstructural preservation than IPA replacement due to pore blockage. The fluid used for SCD (trifluoromethane) also chemically interacts with both CH and C-S-H, which may result from the pressurization during drying. Freeze-drying is suggested if one wants to avoid chemical effects on the hydration products, at the expense of damage to the microstructure.

Aiming at studying moisture transport in cementitious materials, the paper describes a multiphase... more 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.

Mass transport (moisture and ions) at the steel-concrete interface is closely related to corrosio... more Mass transport (moisture and ions) at the steel-concrete interface is closely related to corrosion of rebar in reinforced concrete structures. Thus, in the model simulating mass transport, the structure of the steel-concrete interface must be well represented. In this study, an image-based local homogenization method is proposed based on images taken in the scanning electron microscope (SEM) under the backscattered electron (BSE) detector. According to the gray level of the image, porosity can be calculated and then proposed equations are used to associate the transport properties (sorption isotherms, diffusion coefficients and permeability) with porosity. Experimental data of chloride concentration taken from the literature are used to validate the proposed method and a good agreement with simulated results them is found.

Http Www Theses Fr, May 13, 2014

with hysteretic eects provided much better results than non-hysteresis modelling. Among different... more with hysteretic eects provided much better results than non-hysteresis modelling. Among different hysteresis models, the use of the conceptual hysteresis model, which presents closed form of scanning loops, showed more accuracy predictions. Further simulations for dierent scenarios were also performed. All comparisons and investigations enhanced the necessity of considering i Abstract hysteresis to model moisture transport for varying RH at the boundary. The investigation of moisture penetration depth could provide a better understanding of how deep moisture as well as ions can move into the material. Furthermore, this research investigated dierent methods to determine the liquid-water intrinsic permeability, including the inverse analysis with dierent boundary conditions and tting measured apparent diusivity values for dierent equations to calculate relative permeabilities.

The corrosion of the reinforcement steel is the most common cause of degradation of reinforced co... more The corrosion of the reinforcement steel is the most common cause of degradation of reinforced concrete. The repair work is always associated with high costs which is mainly due to the lack of fundamental understanding of the processes of corrosion and cracking. In the literature, most degradation models are empirical and based on the assumptions, such as homogeneous concrete structure and uniform corrosion products growth from rebar. According to the steps of the corrosion-related degradation, this paper proposes a framework for modeling corrosion-related degradation in reinforced concrete. Models that can be related to each step in this framework are reviewed. The proposed framework can enhance the understanding of corrosion-related degradation for researchers and engineers.

In this research, a model of the couplings between hydration and moisture transfer within cementi... more In this research, a model of the couplings between hydration and moisture transfer within cementitious materials is proposed. Based on taking into account the microstructure properties and the moisture transfer properties due to hydration, the model describes the couplings between the moisture transfer at early-age owing to external drying and the internal water depletion by hydration. Furthermore, the slowing down of the hydration kinetics caused by the decrease of relative humidity prevailing in the pores is also investigated. Experimental data, such as drying kinetics, as well as porosity and degree of hydration profiles, are used to verify the proposed modelling. The materials used here are three OPC cement pastes with different water-to-cement ratios. The agreement between observed and modelled results is considered satisfactory and confirms the possibilities of using the coupling model to simulate moisture transfer for cementitious materials at early-age.

Water vapour sorption isotherms (WVSIs) are one of main material properties that are important fo... more Water vapour sorption isotherms (WVSIs) are one of main material properties that are important for modelling of moisture transport in porous medium, such as cementitious materials. However, measurements of WVSIs are time-consuming. In this paper, we have proposed a method that uses the fewest measured data to predict both desorption and adsorption isotherms. Experimental data have been collected from the literature to verify the proposed simplified method. We concluded that for OPC materials three points on the desorption isotherm and two points on the adsorption isotherm are enough to calculate both curves. For high performance materials, one or more points are needed to predict the desorption curve. One statistical criterion developed based on and MME were used to quantify the proposed method. Results show that this new method has the same prediction accuracy was using the original experimental data. The proposed method not only can save experimental time but also can reduce the m...

Cement and Concrete Research, 2020

The microstructure of the steel-concrete interface (SCI) in reinforced concrete is closely relate... more The microstructure of the steel-concrete interface (SCI) in reinforced concrete is closely related to corrosion of reinforcing steel bars. Accordingly, characterization of the SCI is receiving increasing research attention. For microscopical observations of the SCI, a cutting process is needed to create a flat cross-section. Cutting carries the risk of damaging the SCI because of the considerable difference of hardness between concrete and steel. However, studies on characterizing the microstructure of the SCI rarely consider the damage induced by the potentially inappropriate cutting process. This study investigated the damage created by three cutting methods, namely, mechanical sawing, laser cutting, and combined laser-water cutting by the Laser MicroJet technology (LMJ). The SCI of the cut sections was imaged by scanning electron microscopy equipped with a backscattered electron detector. Additionally, the specimens were non-invasively studied by X-ray microtomography before and after cutting, to compare the impact of various cutting techniques on inducing damage to the SCI beneath the cutting surface. The results showed that if a bleed water zone (BWZ) is present, the cutting technique and protocol can significantly influence the morphology of this zone and adjacent regions. This study recommends an optimized mechanical sawing protocol with low feed speed as this led to considerably less SCI damage than laser and LMJ cutting. Moreover, it was found that adjusting the cutting direction can further significantly reduce the damage created during cutting. The least damage was found when the saw blade cut through the steel before cutting the BWZ. The main problem with laser cutting was heat generated even for a relatively low laser power; therefore, a heat-affected zone was created which significantly altered the microstructural features of the SCI not only on the cutting surface but also a certain depth below the surface. In LMJ cutting, this thermal effect was significantly reduced, however, the high-pressure water eroded the porous SCI and caused cracks. These effects can penetrate along the BWZ into the interior material. To complete this study, two applications demonstrate that the optimized mechanical sawing protocol is applicable to concrete specimens with rebars of actual size and corroded rebars.

Transport in Porous Media, 2020

Anomalous moisture transport in cement-based materials is often reported in the literature, but t... more 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 ...

International Journal of Molecular Sciences, 2020

The durability of reinforced concrete structures is closely related to moisture state in cement-b... more 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 we...

Universite de Paris-Est, 2014

The durability of reinforced concrete structures and their service life are closely related to th... more The durability of reinforced concrete structures and their service life are closely related to the simultaneous occurrence of many physical and chemical phenomena. These phenomena are diverse in nature, but in common they are dependent on the moisture properties of the material. Therefore, the prediction of the potential degradation of cementitious materials requires the study of the movement of liquid-water and gas-phase transport in the material which is considered as a porous medium. In a natural environment, structures are always affected by periodic variations of external relative humidity (RH). However, most moisture transport models in the literature only focus on the drying process. There are few studies considering both drying and wetting, although these conditions represent natural RH variations. Much fewer studies take into account hysteresis in moisture transport. Therefore, this work is devoted to a better understanding of how the moisture behaviour within cementitious materials responds to the ambient RH changes through both experimental investigations and numerical modelling. In particular, the consideration of hysteresis will be included in numerical modelling. In the research, a multiphase continuum model was recalled firstly. According to theoretical analysis and experimental verifications, a simplified model was obtained for the case that the intrinsic permeability to liquid-water is much smaller than the intrinsic permeability to gas-phase. The review of commonly-used hysteresis models enabled to conclude a set of best models for the prediction of water vapour sorption isotherms and their hysteresis. The simplified model was coupled with the selected hysteresis models to simulate moisture transport under drying and wetting cycles. Compared with experimental data, numerical simulations revealed that modelling with hysteretic effects provided much better results than non-hysteresis modelling. Among different hysteresis models, the use of the conceptual hysteresis model, which presents a closed form of scanning loops, showed more accurate predictions. Further simulations for different scenarios were also performed. All comparisons and investigations enhanced the necessity of considering hysteresis to model moisture transport for varying RH at the boundary. The investigation of moisture penetration depth could provide a better understanding of how deep moisture, as well as ions, can move into the material. Furthermore, this research investigated different methods to determine the liquid-water intrinsic permeability, including the inverse analysis with different boundary conditions and fitting measured apparent diffusivity values for different equations to calculate relative permeabilities.