Hygrothermal Conditions Research Articles

Impact on a CLT structure concerning moisture and mould growth using weather protection

Timber buildings, including cross laminated timber (CLT), are gaining market shares globally, mainly due to anticipated environmental benefits, but a new technical solution also raises new questions. Durability is critical to obtain real sustainable constructions built for the future. There are field studies concerning hygrothermal conditions of timber structures, however, there is a lack of documented experiences combining hygrothermal conditions, mould growth potential and weather protection during construction using CLT. The use of full weather protection is being debated in the building industry as well as in the research community, due to lack of knowledge of the combined effects. How does weather protection during the construction affect hygrothermal conditions and risk of mould growth in a CLT structure? A case study using a weather protected six-storey CLT building was performed. The hygrothermal conditions – indoors and outdoors – were monitored during construction and samples from CLT were analysed with respect to mould. The results were analysed together with simulations of mould growth using actual hygrothermal conditions. Theoretical conclusions show the weather protection gives significantly improved conditions resulting in lower potential of mould growth compared to outdoor conditions. The results also show lessons to be learned concerning planning of the construction site.

Viscoelastic and mechanical properties of CNT-reinforced polymer-based hybrid composite materials using hygrothermal creep

In the present work, a combination of experimental and numerical procedure is proposed to study the effects of different hygrothermal conditions on the creep strain, viscoelastic properties of nanocomposites, and mechanical properties of such nanocomposite-based carbon fiber–reinforced polymer (CFRP) hybrid composite materials. Ultrasonic probe sonicator is used to randomly disperse the multiwalled carbon nanotubes into an epoxy to minimize agglomerations. Dynamic mechanical analysis is employed to conduct the creep tests under different hygrothermal conditions of such nanocomposite samples. The Findley power law is used to obtain the long-term creep behavior of nanocomposite materials. Prony series is used to determine the viscoelastic properties of nanocomposite material in the frequency domain. Coefficient of moisture expansion (CME) is independent of moisture concentration; thus, CME of the nanocomposite is also determined. Strength of materials and Saravanos–Chamis micromechanics (SCM) have also been utilized to obtain the mechanical properties of such hybrid composite materials under different hygrothermal conditions. It has been found that the inclusion of multiwalled carbon nanotubes in the nanocomposite and hybrid composites improves storage modulus and loss factor (i.e., tan δ) compared to the conventional CFRP-based composite materials under hygrothermal conditions.

Buckling and Free Vibration Analysis of Laminated Sandwich Beams under Hygrothermal Conditions

In present work, an attempt has been made for carrying out free vibration and buckling analysis of laminated sandwich beams under hygrothermal conditions. The analysis is carried out using fourth order zigzag theory based on finite element formulation. The efficiency of proposed model is validated by comparing the present results with those available in literature. Geometric properties and loading condition widely affect the behavior of the laminated sandwich beams.

Hygrothermal assessment of internally insulated historic solid masonry walls with focus on the thermal bridge due to internal partition walls

This study investigated the hygrothermal performance of five insulation systems for internal retrofitting purposes. Focus was on the hygrothermal performance near partition brick walls compared to the middle of the wall. The setup comprised two insulated reefer containers with controlled indoor climate, reconfigured with several holes containing solid masonry walls with interior embedded wooden elements, an internal brick partition wall and different internal insulation systems, with and without exterior hydrophobisation. Relative humidity and temperature were measured over five years in the masonry/insulation interface and near the interior surface, in the centre of the test field and near the partition wall. In addition, calibrated numerical simulations were performed for further investigation of the thermal bridge effect. Findings for the masonry/insulation interface showed higher temperatures and lower relative humidity near the partition wall in comparison with the central part of the wall. Near the interior surface, measurements showed only minor differences between the two locations. The relative effect of the thermal bridge was smaller in the case of a high driving rain load on the exterior surfaces. The numerical simulations showed that the hygrothermal conditions were affected further away from the partition wall than what could be measured in the experimental setup.

Experimental study of hygrothermal conditions in a wooden room for numerical comparisons

The use of hygroscopic materials indoors has a significant impact on the hygrothermal balance of a room air. It affects both the temperature and the relative humidity. Numerical tools still lack of accuracy in predicting these parameters and some discrepancies are observed between their predictions and experimental measurements. It may be caused by the model itself or by incorrect inputs data (materials properties, occupancy schedule, ventilation rate, etc…) Therefore, an experimental study has been carried out at the room scale under real climate to obtain an experimental dataset as a basis for numerical comparisons. The hygrothermal parameters of the room air have been measured for different loads while all the inputs (heat and moisture generation, air exchange and materials properties) have been properly quantified. This article presents the experimental setup and some of the experimental data obtained.

Influence of the visco-Pasternak foundation parameters on the buckling behavior of a sandwich functional graded ceramic–metal plate in a hygrothermal environment

This study presents the buckling response of functionally graded (FG) “sandwich plate” on a viscoelastic foundation and exposed to hygrothermal conditions. An accurate solution is developed using higher-order shear deformation theory (HSDT), with only four unknowns being placed to reach the solution. The displacement fields first utilize an indeterminate integral accompanied by a sinusoidal shape function to simulate the transverse shear deformation theory. The foundation’s mathematical model followed the two-Pasternak coefficient model, with one more term being added to represent the damping effect. The sandwich plate is essentially composed of three layers. This study presented three different FG sandwich plate geometric analytical solutions regarding layer orders and composition. The equations of motion were generated according to Hamilton’s principle. Thereafter, the analytical solution was based on Navier’s principle to solve the buckling temperature of a simply supported FG sandwich plate seated on a viscoelastic foundation. This paper shows a parametric study of the effect of the damping coefficient along with the aspect ratio, moisture condition, power-law index, and temperature variation over the buckling temperature of the FG “sandwich plate” on the viscoelastic foundation.

Evaluation of Multiyear Weather Data Effects on Hygrothermal Building Energy Simulations Using WUFI Plus

Transient building energy simulations are powerful design tools that are used for the estimation of HVAC demands and internal hygrothermal conditions of buildings. These calculations are commonly performed using a (often dated) typical meteorological year, generated from past weather measurements excluding extreme weather conditions. In this paper the results of multiyear building simulations performed considering coupled Heat and Moisture Transfer (HMT) in building materials are presented. A simple building is simulated in the city of Udine (Italy) using a weather record of 25 years. Performing a multiyear simulation allows to obtain a distribution of results instead of a single number for each variable. The small therm climate change is shown to influence thermal demands and internal conditions with multiyear effects. From this results it is possible to conclude that weather records used as weather files have to be periodically updated and that moisture transfer is relevant in energy and comfort calculations. Moreover, the simulations are performed using the software WUFI Plus and it is shown that using a thermal model for the building envelope could be a non negligible simplification for the comfort related calculations.

Acid Aging of CFRP Composite Materials for Solar UAV Structure

The purpose of this investigation is to study the performance degradation mechanism of CRRP composite materials used in the structure of a solar UAV under acid rain environment and to provide references for the structural design of the solar UAV. An aging test was designed according to the actual working conditions of the solar unmanned aerial vehicle and was carried out by continuously immersing the composite material in both deionized water and an acidic solution with a pH of 2.0 at 60°C. The mechanical behavior of the composites was studied through a three-point bending test. The results showed that after 120 days of aging test, the composite material exhibits different characteristics in the two conditions. The composite material under the hygrothermal conditions finally reached the equilibrium moisture absorption content of 1.07%, and the flexural strength decreased by 8.77%. The composite material under acidic conditions deviated from the Fick behavior in the final stage of the test, and the final moisture content was 2.88%, and the bending strength decreased by 26.43%. Several typical empirical models were analyzed, and a hyperbolic tangent function empirical model with moisture content as the main variable was used to predict the CFRP flexural strength degradation process, and good results were obtained. The effect of aging conditions on the microstructure of composite materials was observed by SEM, and the aging mechanism of composite materials was analyzed. The corrosion of the polymer matrix and the debonding of the fiber/matrix interface are the root causes of the performance degradation of composite materials.

Effects of the preparation, curing and hygrothermal conditions on the viscoelastic response of a structural epoxy adhesive

This paper addresses the viscoelastic behaviour of a commercially available cold-curing structural epoxy adhesive, under different preparation, curing and hygrothermal conditions. The main parameters studied were the preparation method (influence of the degassing and the temperature of the initial curing), the creep stress level, and the hygrothermal conditions. Tensile creep tests last up to 2400 h. Test results revealed that the preparation method has great influence on the tensile properties of the adhesive, particularly on the viscoelastic response where degassing and curing at 20 °C showed lower creep deformations. Specimens under 98% of relative humidity faced tertiary creep and then rupture. For the adopted levels of creep stress, the adhesive shows a linear creep behaviour, being parameterized using the Burgers and the modified Burgers equations.

Durability Prediction of FRP composite materials under Hygrothermal Environment

This paper proposes a novel degradation model of mechanical properties of fiber-reinforced-polymer(FRP) composites under hygrothermal environment. The influence of moisture concentration, temperature and the long-term effects of hydrolysis are considered in the degradation model. The experimental data of E-glass/Vinylester under five hygrothermal environment conditions is used for the model validation and comparative study with traditional models. The results show that the prediction results correspond well with the experimental data. Comparative study shows that the proposed model has better accuracy in predicting the FRP durability for close-to-room temperature conditions, and the required number of fitting parameters is less.

Deep renovation of an old single-family house including application of an water repellent agent – a case story

The 145 year old rural case building presented in this paper has undergone a deep renovation including internal insulation of the external walls to reduce the heat loss and improve the indoor thermal comfort. The internal insulation was a PUR-based insulation with channels of calcium silicate, experiencing to some extent capillary active behaviour. Sensors were installed between the existing wall and the internal insulation to monitor the development of hygrothermal conditions. The external façade was later hydrophobized with a water repellent agent to minimize the wind driven rain load. Measurements show that it takes time to get rid of the built-in moisture due to the application of internal insulation, however the moisture content expressed in relative humidity is slowly decreasing, although still high about two years after hydrophobizing the wall. Simulations show that the order of hydrophobizing the wall and applying internal insulation is important to promote drying of the wall.

Mapping the postharvest life of imported fruits from packhouse to retail stores using physics-based digital twins

Controlling the hygrothermal conditions around fresh fruit and vegetables is vital for their preservation. Therefore, cold chain stakeholders often measure temperature along the supply chain of fresh produce. However, the temperature is typically monitored only in one segment of the entire cold chain, namely from the supplier until the distribution center. Besides, such measured data are rarely used for decision-making because they are not translated into the impact on the quality of the products. We provide a solution by extending the monitoring until the retail stores and upcycling these thermal data into actionable metrics. To do so, we use physics-based digital twins, namely virtual representations of the food products. This study focuses on 331 cold chain shipments of cucumber, eggplant, strawberry, and raspberry imported from Spain to Switzerland. We followed these fruits through pre-cooling, thermally stable conditions at the distribution center, and the temperature ramp-up phase before arriving at the retail store. The temperature performance of each carrier and flow analysis of the shipment enabled us to map the complex logistic system better. The digital twins detected that the fruits lost 43 - 85% of their quality before being displayed at the retail store. This quality loss remains invisible to the retailer. Additionally, we found a strong correlation between fruit quality and shipment duration (i.e., for cucumber r = -0.95 (P < 0.001)), which emphasizes the importance of shortening the shipment to prolong the freshness of the fruit. The digital twins have shown a large potential to help further maximize shelf life and uniform product quality.

Free vibration and modal stress analysis of FG-CNTRC beams under hygrothermal conditions using zigzag theory

The present study aims to carry out free vibration analysis of functionally graded single-walled carbon nanotube-reinforced composite (FG-CNTRC) beams under hygro-thermal conditions. The study is carried out using the recently proposed C-0 finite element-based higher-order zigzag theory. The study is carried out on five different CNTRC graded beams. Temperature and moisture-dependent material properties are used. Constant temperature or moisture distribution across the thickness of the beam is taken. Hamilton's principle has been used for defining the governing differential equations. Modal stresses are also studied for the first six mode shapes. Stresses at the higher mode of vibration are found to be more affected by temperature or moisture concentration as compared to the stresses observed for the fundamental mode of vibration. It is observed that the nature of stress distribution across the beam is widely affected by the gradation law along with moisture or temperature values to which the beam is subjected. FG-O beam is found to be least sensitive under thermal conditions whereas FG-X beam is found to be the most.

Performance improvements of a short glass fiber-reinforced PA66 composite

Abstract A plate used in a medium voltage switch, made by a 25 wt% glass fiber-reinforced polyamide 66 composite filled with flame-retardant red phosphorus (RP) (PA66-GF25 FR (RP)), was injection molded. To satisfy the relatively high dimensional accuracy requirement, a “supporting mold” was used to compensate for the difference between the transverse and longitudinal linear shrinkage of the PA66-GF25 FR (RP). To reduce the internal residual stress caused by the “supporting mold,” hygrothermal conditioning treatments, including boiling water bath at 90–100°C and re-drying at 110°C, were used. To determine the effects of boiling water bath and re-drying on the dimensional accuracy and mechanical properties of the product, three treatment routes were applied. It was found that the route in which the boiling water bath is applied after the “supporting mold” is preferred to ensure the dimensional accuracies and the mechanical properties as a whole. Using a boiling water bath as a hygrothermal conditioning treatment can improve the mechanical properties and increase the dimensional accuracy of the product. In addition, by using the preferred route, re-drying can further improve the tensile, bending, and even impact strength.

An “inner soft external hard”, scratch-resistant, self-healing waterborne poly(urethane-urea) coating based on gradient metal coordination structure

To reduce pollution and maintain sustainable development, environmental friendly waterborne coatings as a replacement for conventional oil-based coatings have drawn lots of attention and been widely studied recently. It is a challenge to overcome the inherited weakness of waterborne coatings including poor scratch-resistance, low hardness and short lifespan. Herein, we have developed a scratch-resistant and self-healing waterborne coating of poly(urethane-urea), which contains a dual metal coordination structure of Zn(II)-carboxyl and Zn(II)-diamidepyridine by using the method of ions penetration. The addition of zinc ions has given dense metal coordination bonds on the coating surface, which has greatly increased the surface hardness (from B to 4H) and scratch-resistance, while the inner layer keeps excellent adhesion (>1.5 MPa) to substrates because of less metal ion penetration to the inner. What's more, the coating exhibits outstanding self-repairing performance, and the repair ability is better under hygrothermal condition. Atomic force microscope (AFM), dynamic mechanical analysis (DMA) and thermal analysis were used to characterize the self-healing and hardening mechanisms of the coatings. These results are expected to provide a useful method to improve the overall strength of waterborne coating.

Adjustable positive and negative hygrothermal expansion metamaterial inspired by the Maltese cross.

A metamaterial that can manifest both positive and negative coefficients of moisture and thermal expansion is presented herein, based on inspiration from the Maltese cross. Each unit of the metamaterial consists of a pair of equal-armed crosses pin-joined at their junctions to permit rotation, but elastically restrained by a bimaterial spiral spring, and four pairs of hinge rods to translate the relative rotational motion of the pair of equal-armed crosses into translational motion of the connecting rods. The effective coefficients of moisture and thermal expansion models were developed for small and large changes in the hygrothermal conditions using infinitesimal (approximate) and finite (exact) motion analyses, respectively, with the former giving constant effective coefficients with respect to environmental changes. Results indicate that the approximate method underestimates the magnitude of both the effective expansion coefficients under cooling and drying but overestimates magnitudes of both coefficients during heating and moistening, and that the change in both expansion coefficients is more drastic during cooling and drying than during heating and moistening. In addition to providing another micro-lattice geometry for effecting expansion coefficients of either signs, this metamaterial exhibits auxetic property.

Modelling hygrothermal conditions of unsaturated substrate and drainage layers for the thermal resistance assessment of green roof: Effect of coarse recycled materials

Alternative materials exist for green roof layers: secondary resources like coarse or fine recycled aggregates may be used as a substitute to natural materials. For using these new types of materials, it is needed to assess their heat resistance which is performed according to ISO 9869-1 standard. Moreover, the initial hygrothermal conditions of unsaturated substrate and drainage layers have also to be modelled and assessed for optimizing the layers’ thickness. In this study, the green roofs with unsaturated substrate and drainage layers incorporating coarse recycled materials were tested and assessed. The hygrothermal conditions of unsaturated substrate and drainage layers were simulated using WUFI software. A small difference (4.2%) was observed between the Rc-value of the green roofs with and without coarse recycled materials, confirming that these materials provided a sufficient thermal resistance, similar to soil particles for the substrate layer. Considering a constant thickness for the substrate layer (15 cm), a 6-cm drainage layer with coarse aggregates was considered as the optimum design for green roof systems. Besides, 18-cm unsaturated substrate layer was the optimum design when the drainage layer’ thickness was considered constant (5 cm). The 6-cm drainage layer and 18-cm unsaturated substrate layer were definitely the best design for the roofing systems with the simultaneous change in the substrate and drainage layers’ thickness.

Indoor Air Quality in Naturally Ventilated Classrooms. Lessons Learned from a Case Study in a COVID-19 Scenario

This paper describes the implementation of a series of ventilation strategies in a nursery and primary school from September 2020, when the government decided to resume the students’ face-to-face activity in the middle of a COVID scenario. Air quality and hygrothermal comfort conditions were analysed before the pandemic and compared for different ventilation configurations in a post-COVID scenario. Ventilation strategies included the protocols issued by the Public Administration, while others were developed based on the typological configuration and use of the school. Results revealed that it is advisable to implement certain strategies that reduce the risk of infection among the occupants of the spaces, without a significant decrease in hygrothermal comfort. Given the importance of maintaining better IAQ in the future within classrooms, and regarding the pre-COVID situation, these strategies may be extended beyond this pandemic period, through a simple protocol and necessary didactic package to be assumed by both teachers and students of the centre.

Assessing the Effects of Vineyard Soil Management on Downy and Powdery Mildew Development

Advantages of vineyard cover crops include soil conservation, nutrient and water cycling, and a better growth–yield balance. They naturally promote agro-system biological multifunctionality, soil biocenosis, and biological pest control. However, for the role of cover crops on disease control, there is still a lack of information. We performed a systematic evaluation of the soil management effects on the development of two main pathogens: Plasmopara viticola and Erysiphe necator. Conventional soil tillage was compared to grass and legume cover crops during a three-season trial in the Nurra wine region (Sardinia, Italy). Disease and grapevine development were assessed in relation to the weather, leaf area growth, leaf nitrogen, canopy density, and favorable microclimatic conditions for each disease, to weight their importance on disease susceptibility in each treatment. Higher infection percentages were observed in plots subjected to soil tillage. Disease development was better understood in relation to leaf area, leaf nitrogen, and canopy density. The main role of weather conditions on downy mildew infections was ascertained, yet high canopy sunlight levels reduced disease spread under grass cover. For powdery mildew, leaf nitrogen had a crucial role in disease development under soil tillage, and canopy light and hygrothermal conditions had the most relevant function on disease development in cover-cropped vineyards.

Neural Network Optimization Algorithms to Predict Wind Turbine Blade Fatigue Life under Variable Hygrothermal Conditions

Moisture and temperature are the most important environmental factors that affect the degradation of wind turbine blades, and their influence must be considered in the design process. They will first affect the resin matrix and then, possibly, the interface with the fibers. This work is the first to use a series of metaheuristic approaches to analyze the most recent experimental results database and to identify which resins are the most robust to moisture/temperature in terms of fatigue life. Four types of resin are compared, representing the most common types used for wind turbine blades manufacturing. Thermoset polymer resins, including polyesters and vinyl esters, were machined as coupons and tested for the fatigue in air temperatures of 20 °C and 50 °C under “dry” and “wet” conditions. The experimental fatigue data available from Sandia National Laboratories (SNL) for wind turbine-related materials have been used to build, train, and validate an artificial neural network (ANN) to predict fatigue life under different environmental conditions. The performances of three algorithms (Backpropagation BP, Particle Swarm Optimization PSO, and Cuckoo Search CS) are compared for adjusting the synaptic weights of the ANN and evaluating the efficiency in predicting the fatigue life of the materials studied, under the conditions mentioned above. For accuracy evaluation, the mean square error (MSE) is used as an objective function to be optimized by the three algorithms.