Hygrothermal Coupling Research Articles

Multi-scale study on the mechanical properties of concrete based on crack propagation under hygrothermal coupling

During the long-term service of structures, the environmental temperature and humidity fields can lead to the evolution of the pore structure within concrete materials, thereby affecting changes in mechanical properties. However, the mechanisms of their influence still need further clarification and are challenging to correlate for prediction. Addressing this specific issue, the current study focuses on evaluating 78 cubic concrete specimens with C30 and C40 strength grades. These specimens were subjected to a 95 % humidity environment at temperatures of 40 °C, 60 °C, and 80 °C for durations of 30, 60, and 90 days. The research involved conducting both microstructural and macroscopic mechanical tests to quantify the evolution of internal hydrates, pore structures, and pore distribution, as well as to assess the changes in the macroscopic mechanical properties of the specimens. To accurately predict the process, a multiscale finite element model (FEM) based on the bilinear continuum cohesive damage model was established. This model quantifies the changes in mechanical properties of mesoscopic three-phase concrete due to microcrack evolution under hygrothermal coupling. To facilitate multiscale simulations between the Representative Volume Element (RVE) and macroscopic scale models, relevant subroutines were written using Umath, and parallel computing was employed to reduce the computational burden. For the macroscopic model in this study, a homogenized concrete cube model with dimensions identical to the experimental specimens was selected. The effectiveness of the parallel computational model was validated by comparing and analyzing the simulation results under uniaxial compression with the experimental data. The findings indicate that hygrothermal action leads to the initiation and expansion of microcracks within the concrete, thereby impacting the alteration of its mechanical properties. In addition, the parallel computing-based multiscale FEM can more accurately quantify the typical damage process of crack propagation while reducing the computational load and may provide relatively accurate predictions of the mechanical characteristics of concrete under the influence of hygrothermal coupling, ensuring effective modeling of macroscopic structural specimens in various temperature and humidity environments.

Hygrothermal coupled modeling and behavior analysis of natural fiber-reinforced tubular composites

The mechanical properties of natural fiber-reinforced composites (NFRCs) are susceptible to the coupled effect between environmental temperature and humidity. However, the coupled system for the heat and moisture field in NFRCs remains unclear. In regard to this, this paper presents hygrothermal coupled modeling of natural fiber-reinforced tubular composites (NFRTCs), considering the correlation between natural fiber aggregation, matrix contents, and porosity. The transient hygrothermal field and distribution in NFRTCs are predicted using the differential quadrature method. The predicted results are verified by the comparison with the experimental data in the literature. Through the proposed model, the effect of hygrothermal coupling, boundary conditions, fiber aggregation, porosity, and structural dimensions on the hygrothermal responses and distributions are analyzed in detail. The results show that temperature and moisture fields decrease significantly when the hygrothermal coupling or the hygrothermal convection boundary condition is used. In addition, hygrothermal field distributions gradually decline with the increasing fiber weight fraction, outer-to-inner radius ratio, and porosity coefficient, as well as the smaller fiber aggregation parameter. These results can serve as a guideline for the design and manufacturing of reliable NFRTCs subjected to hygrothermal environments.

Thermo‐Mechanical Analysis of Epoxy Resins Under the Influence of Moisture‐Heat Coupling by Molecular Dynamics Simulation

AbstractEpoxy resins are used in electronic packaging. However, its low thermal conductivity is a disadvantage and the high moisture absorption leads to defects such as cracking and delamination. In this work, the effect of thermo‐mechanical properties of epoxy resins under moisture‐heat coupling is investigated by molecular dynamics (MD) simulations. The diglycidyl ether of bisphenol A (DGEBA) and polyetheramine D230 are used as epoxy resin matrix and curing agents, respectively. The degree of cross–linking is set from 0% to 98% and the moisture mass fraction is set from 0 wt.% to 10 wt.%. The radius distribution function(RDF) shows that water molecules tend to appear ≈2.75 Å from the epoxy resin. The thermal conductivity of the epoxy resin increases by 54.55% for the maximum cross–linking degrees studied. And it increases by 19.23% when the moisture mass fraction is increased from 0 wt.% to 10 wt.%. The deformation of the epoxy resin is calculated which includes the coefficient of thermal expansion(CTE), coefficient of moisture expansion(CME), and coefficient of linear expansion. It is found that moisture expansion predominated at low temperatures and thermal expansion dominates at high temperatures. The analysis provides a theoretical basis for the study of hygrothermal coupling effects.

Failure Study of BFRP Joints with Two Epoxy Adhesives under Hygrothermal Coupling.

Basalt Fibre Reinforced Polymer (BFRP)-bonded structures are lightweight, high strength, economical, and environmentally friendly, which is very advantageous in the civil sector. The aim of this paper is to provide a comprehensive account of the hygrothermal degradation and failure mechanisms of BFRP-bonded structures by comparing the residual properties of two epoxy adhesive BFRP single-lap joints after ageing for 240 h, 480 h, and 720 h in an extreme hygrothermal environment with pure water at 80 °C. The hydrophilicity and thermal stability of the two adhesives were firstly compared by water absorption and Thermogravimetric Analysis (TGA) tests, and the hygrothermal degradation of the molecular chains and the reduction in Tg were characterised by Fourier Transform Infra-Red (FTIR) spectroscopy and Differential Scanning Calorimetry (DSC) curves. The failure strength and load-displacement curves of the two joints were then compared, and it was found that the strength and stiffness had different trends, while the paired t-test was used to demonstrate the correlation between the failure strength and the adhesive Tg, as well as the difference in the failure mechanisms of the two joints caused by the water absorption rate. The analysis of macrosections and Scanning Electron Microscope (SEM) images summarised the process and reasons for the transition of the failure mode from fibre tearing to hybrid failure, and finally, the changes in elemental concentration and O/C values were analysed by Energy Dispersive X-ray Analysis (EDX), which proved that the degree of hydrolysis could not be used as a judgement of the degradation degree of the joint alone, and provided data support for the application of the BFRP-bonded structure in the humid and hot environment.

Thermal and Humidity Performance Test of Rammed-Earth Dwellings in Northwest Sichuan during Summer and Winter.

Rammed-earth dwellings have a long history in the construction field. It is a natural material that is both green and environmentally friendly. In recent years, the advantages of rammed earth, such as environmental protection, low cost, and recyclability, have attracted considerable attention. In this study, the thermal and humidity physical properties of rammed-earth materials in the northwest Sichuan region, the variation laws of thermal physical parameters, such as the thermal conductivity of rammed-earth under different moisture content conditions, and isothermal moisture absorption and desorption curves were investigated. The results indicated that the thermal physical parameters of the rammed earth measured in the experiment increased with an increase in moisture content, and its moisture absorption performance was better than the moisture release performance in the range of 11.31-97.3% relative humidity. The experimental site, Mianyang City, Sichuan Province, is a subtropical monsoon humid climate zone characterized by warm winters and hot summers with four distinct seasons. In this study, we investigated the hygrothermal coupling transfer of walls, as well as the indoor temperature and humidity changes in new rammed-earth buildings during summer and winter climates. During the test period, the maximum indoor temperature in summer was 35.08 °C, the minimum temperature was 33.76 °C, and the average daily temperature fluctuation was 3.62 °C. In winter, the maximum indoor temperature was 8.59 °C, the minimum temperature was 6.18 °C, and the average daily temperature fluctuation was 1.21 °C. An analysis was performed on the thermal insulation performance of rammed-earth buildings in an extremely high-temperature climate during summer, thermal insulation performance, the thermal-buffering capacity of walls in a low-temperature and high-humidity climate during winter, and thermal and humidity regulation of indoor environments provided by walls during summer and winter. The results showed that the rammed-earth buildings exhibited warmth in winter, coolness in summer, and a more stable and comfortable indoor environment.

Model Test Study on Natural Thawing Temperature Field of Artificial Ground Frozen Wall

In order to visualize the evolution and distribution law of the ground temperature field during artificial freezing construction, an indoor model test study was carried out based on the independently constructed hygrothermal coupling artificial ground freezing test platform. The test results show that the soil temperature in the freezing process went through the three stages of a steep drop, a slow drop, and stabilization, the earliest closure position of the frozen wall was the intermediate point between two freezing pipes, and the thickness of the frozen wall on different sections showed Section 1 > Section 2 > Section 3 after 61 min of positive freezing. The soil temperature in the natural thawing process went through the four stages of a rapid rise, short hysteresis, a second rapid rise, and a linear slow rise. By fitting the test data, the distribution function of the pipe wall temperature along the pipe length under natural thawing conditions was obtained. The research results can provide a valid basis for the numerical calculation model of a three-dimensional non-uniform natural thawing temperature field and can also provide a reference for the design of settlement grouting under natural thawing conditions.

Hygro-thermal coupling in earth building materials

Raw earth is emerging as a viable building material with lower carbon emissions than conventional concrete and fired bricks. Raw earth is as an excellent passive hygro-thermal regulator, which improves occupants’ comfort while reducing the need for active heating/cooling installations. The coupled hygro-thermal response of earth materials is investigated by exploiting the principles of the thermodynamics of porous media and unsaturated soil mechanics. The degree of coupling between temperature and relative humidity (or water content) depends on the adopted simplifying assumptions. Some of these assumptions are valid for traditional building materials but may not be applicable to raw earth characterised by relatively high levels of liquid/gas permeability. The validity of current approaches is here assessed with reference to earth building via a simple one-dimensional transfer model, which simulates the behaviour of an unbounded earth wall subjected to time-dependent boundary conditions on the two faces. For typical values of water and vapour permeability, the complexity of the governing equations can be greatly reduced by neglecting variations of vapour mass and the dependency of suction on temperature without significantly reducing accuracy. Results are also strongly influenced by both initial state and water retention properties of the earth material.

Dynamic response of a hollow cylinder under memory-dependent differential hygrothermal coupling

The dynamic response of a hollow cylinder subjected to a hygrothermal coupled shock is studied. The memory-dependent differential is introduced into the heat transfer equation and the diffusion equation, and a memory-dependent differential generalized hygrothermal coupling model is developed. In this article, Laplace transform and inverse transform are used to find the closed-form solutions of various physical quantities. The influence of the hysteresis factor and the kernel function on the distribution of each physical quantity is analyzed and the time history of the hygrothermal coupling is studied. The numerical results show that the larger the ratio of temperature gradient hysteresis factor to heat flux hysteresis factor, the faster the wave speed. Also, the hysteresis factor affects the wave crest. For the time course of hygrothermal, it is once again verified that the hygrothermal wave has a finite propagation speed. At the same time, we can get the conclusion that the wave is dominated by fluctuations in the early stage after the onset of hygrothermal shock. The above findings are of reference value for the structural design and optimization of tubes in a hygrothermal environment.

Effect of phase lags of moisture–heat flow on the hygrothermoelastic field of hollow cylinders with convective surfaces

In this article, by considering the Dufour effect and Soret effect, a hygrothermal coupling model based on phase delay of heat and moisture fluxes is established for convective surfaces. The convective surfaces are described by Robin boundary conditions. The Dirichlet boundary conditions (prescribed temperature and moisture on the surfaces) and Neumann boundary conditions (prescribed heat and moisture fluxes) are two extreme cases of the Robin conditions. Using Laplace transform, the effects of different relaxation times and moist-heat coupling on the temperature, moisture, and stress of the hollow cylinder are analyzed from two aspects of time and space. For convective boundary, the dependence of temperature, moisture, and stress on the phase lag of heat and moisture fluxes is examined. The results show that the temperature and moisture distributions along with the hygrothermal stress fields exhibit different behaviors, depending on the coefficient of Robin condition.

Effect of steam curing regimes on temperature and humidity gradient, permeability and microstructure of concrete

Steam curing has an adverse effect on the long-term performance of concrete, which is inseparable from the early hygro-thermal coupling action. The temperature-humidity field and performance of steam-cured concrete with different surface covering methods were characterized under the treatment temperature ranging from 45 to 75 ℃ by water permeability along with hydration characteristics, porosity, microhardness, and microstructure. The results show that steam-curing regimes directly affect the long-term performance of steam-cured concrete by changing internal temperature-humidity field. During the steam curing process, a higher treatment temperature often corresponds to a more complex temperature-humidity gradient, so that the steam-cured concrete exhibits low compactness and high permeability. In addition, the surface covering methods can reduce the surface porosity and interface transition zone width of steam-cured concrete. Meanwhile, with the increase of treatment temperature, the improvement effect of the surface covering methods on the impermeability of steam-cured concrete is more obvious.

Transient response of the hygro-thermo-elastic field in hollow cylinders under heat-moisture shock via hyperbolic non-Fourier heat conduction and non-Fick diffusion

The classical Fourier heat conduction theory as well as the widely used Fick’s diffusion law predicts an infinite wave velocity. This is physically unrealistic. By generalizing the classical Fourier’s and Fick’s laws, this paper presents a hyperbolic diffusion law to apply heat and moisture coupling. The transient response of the hygro-thermo-elastic field in infinitely long hollow cylinders subjected to sudden heat and moisture shock on the inner and outer surfaces is studied. With the aid of the Laplace transform and the decoupling technique, the closed-form solutions of temperature, moisture, elastic displacement and stresses are determined respectively. The analytical results show that the thermal and moisture relaxation time effect between temperature and moisture is significant for composites. Compared with the classic counterpart, the finite hygrothermal wave speed of the pipe is achieved and decreases with the relaxation time rising. The temperature, moisture, elastic displacement and stresses are calculated. Numerical results are displayed graphically to show the influence of the phase lag of heat/moisture flux on the response of the hygro-thermo-elastic fields. Non-Fourier and non-Fick effects are remarkable between the classic model and hyperbolic hygrothermal coupling model. Some drawbacks induced by the classical Fourier’s and Fick’s laws are averted.

Properties evolution of high-early-strength cement paste and interfacial transition zone during steam curing process

At present, concrete prefabricated elements commonly used in railway engineering in China were produced by steam curing. Steam curing is a complex hygro-thermal coupling process that has a significant impact on the hydration process and microstructure formation of cement-based materials. This paper investigated the macroscopic mechanical properties, hydration process, pore structure, microstructure formation and micromechanical properties of cement-based materials during steam curing process. It is discovered that the evolution of cement-based materials under steam curing is a dynamic process that can be divided into three stages. The fastest evolution stage of the performance is the rising stage and the first 2 h of treatment stage. At this time, the microstructure of the pastes is rapidly formed, and the mechanical properties are significantly improved. Porosity and interfacial transition zone widths also rapidly decrease. The reaction activity of cementitious material is one of the critical factors affecting the properties evolution of steam-cured paste. In addition, an evolution model of the pore structure was established based on the change in the hydration degree during steam curing, which is proved to be well consistent with the measured results.

Non-classical hygrothermal fracture behavior of a hollow cylinder with a circumferential crack

By extending the classical Fourier heat conduction law and Fick diffusion law, a hyperbolic hygrothermal coupled model is newly proposed to investigate the fracture problem of a hollow cylinder containing a circumferential crack. The cylindrical system is subjected to a hygrothermal shock at the inner surface, and its outer surface is insulated and damp-proof. The integral transform technique and decoupling technique are used to solve the hygrothermal and mechanical boundary problem. Finally, the effects of hygrothermal coupling, thermal and moisture relaxation time on the transient temperature, moisture, hygrothermal stress and stress intensity factors are calculated. It is found that the hyperbolic model gives more conservative results than the parabolic model for the safety design of structures against fracture under hygrothermal loads.

Evolution of multi-scale pore structure of concrete during steam-curing process

Abstract Steam-curing at elevated temperature involves a complex hygrothermal coupling action, which greatly affects the hydration process, microstructure buildup and thus the mechanical and long-term properties of cement-based materials. This paper investigated the multi-scale pore structure characteristics of concrete, paste and paste-aggregate interfacial zone during steam-curing process by a combined method of microscopic image analysis, scanning electron microscopy and mercury intrusion porosimetry. The evolution model of pore structure of steam-cured concrete during steam-curing stage and corresponding mechanism are also discussed. The results show that the multi-scale pore structure evolution of concrete is remarkably influenced by the steam-curing process, which can be divided into three stages. The initial free water content is one of the critical factors affecting the pore structure evolution of steam-cured concrete, especially in the interfacial transition zone. The morphology of hydrated cement pastes in interfacial transition zone is looser than that of matrix. The expansion pressure of vapor in the heating period of steam-curing process when water interior expands more severely in a higher initial free water content hinders the precipitation and diffusion of hydrates, bringing a relative weaker interfacial transition zone for steam-cured concrete. A hydration degree-dependent and water to cement ratio-related theoretical model is established to characterize the pore structure evolution during steam-curing process, which is proved to be well consistent with the experimentally measured results.

The hygro-thermal coupling model of concrete with water absorption characteristics is considered

Basing on the theories of compound mixtures, considering the development of the microscopic pore structure of concrete at early age, and the dynamic balance of adsorbed water, free water, and water vapor within the pores, the thermo-hygro-mechanical coupling model was established with temperature, humidity, hydration degree and capillary pressure as main variables.

Transient hygrothermoelastic response in a cylinder considering non-Fourier hyperbolic heat-moisture coupling

The classical theory of heat conduction (Fourier theory) predicts an infinite speed for thermal disturbance propagation, which is physically unrealistic. By extending the classical Fourier heat conduction and Fick’s diffusion, this paper presents a hyperbolic diffusion law with different phase lags of thermal and moisture fluxes to simulate coupled heat-moisture diffusion-propagation behavior. Transient hygrothermal and elastic response of an infinitely long cylinder subjected to sudden hygrothermal loadings at the surface is studied. By using Laplace transform and decoupling technique, a closed form solution of temperature, moisture, displacements and stresses is determined. The analytical results show that the thermal and moisture relaxation times or phase lags of heat and moisture fluxes play a significant role in the early stage of transient response after heat/moisture shock. The classical results corresponding to vanishing phase lags can be recovered from the present ones. For non-vanishing phase lags, hygrothermal waves have finite propagation speeds. Numerical results are calculated and displayed graphically to show the influence of the phase lags of heat and moisture fluxes on transient hygrothermoelastic fields. A comparison between classic model and hyperbolic hygrothermal coupling model is given. Based on the non-Fourier heat conduction and non-Fick diffusion, some shortcomings induced by the classical Fourier’s and Fick’s laws can be effectively avoided.

Hygrothermal coupling analysis of magneto-electroelastic beams using finite element methods

ABSTRACTIn this article, the finite element (FE) method has been used to assess the coupled static behavior of hygro-thermo-magneto-electroelastic (HTMEE) beam. Influence of externally applied hygrothermal loads on the direct (displacements, electric and magnetic potentials) and derived quantities (stresses, electric displacement and magnetic flux densities) of HTMEE beam have been studied in detail. The principle of total potential energy and the coupled constitutive equations of HTMEE material are used for the FE formulation. A generalized condensation technique is adopted to solve the global FE equations of motion. Numerical examples are discussed to examine the effect of hygrothermal loads and distinct effect of moisture concentration on the behavior of the beam. Particular emphasis has been placed to analyze the influence of temperature and moisture dependent elastic stiffness coefficients associated with empirical constants. Considering the independent effect of temperature and moisture on the coupled static responses, the most significant combination of the empirical constants corresponding to temperature dependency and moisture dependency are explored. Extensive computational examples are considered to examine the significant effect of boundary conditions, temperature gradient, moisture concentration gradient and empirical constants on the static behavior of HTMEE beam. It is observed that the static behavior of HTMEE beam is significantly influenced by the hygrothermal loads and empirical constants. The results presented in this article would serve as a benchmark results in design and analysis of HTMEE structures for sensors and actuators applications.

Characterization of hygrothermal transfers in the unfired earth

In front of the building energy issues and environmental impact bound, it appears that the earth envelopes are a promising track in terms of improving thermal comfort, indoor air quality, energy consumption and indoor humidity regulation. For this purpose, a characterization of hygrothermal transfers in the unfired earth was been conducted with experimental and numerical approaches. Thus, the studies at the material and wall scales were necessary to check dissociate and coupled hygrothermal behaviour. The material scale has been used to characterize properties related to heat and mass transfers. The hygrothermal coupling has been the subject of a specific study via a dual-climate test chamber at an earth wall scale. Finite Differences (FD) and Finite Elements (FE) implementations have resulted in a detailed analysis of transfers across the tested earth wall with a reduction work of order required to limit the calculation time. Emphasis is placed on the effects of moisture in the hygrothermal transfer in order to validate a digital tool developed in this work.

A review on the buffering capacity of earth building materials

Unfired clay building materials are recognised for their ability to regulate indoor humidity levels through their moisture buffering capacity. Research is being conducted on the moisture buffering capacity of a variety of building materials with natural materials, such as clay, and organic materials, such as hemp or straw, presenting a greater potential to regulate indoor humidity than industrial building materials. Due to their high affinity to water, which is usually regarded as detrimental, clay materials present complex hygrothermal coupling phenomena, which are still under investigation. This paper summarises some recent investigations into the dynamic water adsorption process within clay materials in relation to their ability to regulate indoor air humidity levels. First, a review of the experimental methods to characterise this behaviour is provided. A review of experimentally measured results on the material scale using compressed earth block, rammed earth or plaster samples is then provided, followed by some larger and whole building measurements.

A coupled hygro-thermal model of early-age concrete based on micro-pore structure evolution

In order to analyze the hygro-thermal coupling effect in early-age concrete, this research establishes a coupling hygro-thermal model based on hybrid mixture theory (HMT) considering the characteristic of the microstructure evolution of early-age concrete. The adsorption curve of early-age concrete is acquired based on the microstructure evolution through establishing the relationship between saturation and several factors, such as hydration degree, temperature, capillary pressure and so on. An innovative coefficient for counting the influence of humidity on hydration rate is proposed, which takes the influence of humidity on hydration rate under self-desiccation as a reference based on the existing test. The research results indicate that the coupled hygro-thermal model constructed based on the micro-pore structure evolution can accurately predict the time-dependent development of temperature and humidity field inside the early-age concrete. The research can theoretically support the investigation on the deformation and cracking of the early-age concrete.