Moisture Transport Mechanisms Research Articles

Experimental and numerical investigation of cast-in-situ concrete under external sulfate attack and drying-wetting cycles

In this paper, a numerical model was proposed to simulate the diffusion of sulfate and the strength degradation of cast-in-situ concrete under sulfate attack and drying-wetting cycles. The hysteresis of moisture transport was used to illustrate the moisture transport mechanisms during the wetting and drying process. The ion transport model was proposed according to Fick’s second law, and moisture transport, chemical reactions, advection and damage were taken into account. The cast-in-situ concrete cylinder specimens exposed to sulfate and drying-wetting conditions were conducted by the indoor experiments. The concentration of the sodium sulfate solutions varied at 0%, 3%, 5% and 10%, showing that the ingress of sulfate and strength degradation were significantly accelerated by the increase in the sulfate concentration. The compressive strength of the specimens exposed to sulfate increased before 6 months and then decreased. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were performed to observe and identify the corrosion products of concrete exposed to sulfates. The effectiveness of the proposed method was analyzed through a comparison between the numerical and experimental results for the sulfate distribution and the compressive strength loss. Numerical studies were performed to present the response of concrete to sulfate diffusion under different drying-wetting cycles and boundary conditions. The results were approximately close to the experimental data and indicated that advection accelerated the diffusion process. The loss of compressive strength could be predicted at the period of damage initially induced by sulfate attack, and the comparatively longer drying period and lower environmental moisture level could promote the diffusion of sulfate. In general, the proposed diffusion model coupled with moisture transfer could be capable of simulating sulfate diffusion and strength degradation due to external sulfate attack and drying-wetting cycles.

Simultaneous test and visual identification of heat and moisture transport in several types of thermal insulation

The utilization of thermal insulation is one of the most effective methods to reduce the heating and cooling loss through building envelopes. However, moisture ingress driven by the pressure gradient across the insulation may lead to a variation of the effective thermal conductivity, which highly deteriorates the thermal performance of building envelopes. In order to investigate moisture transport mechanism and its impact on the heat transfer procedure, a novel simultaneous test method based on the modified guarded hot box is proposed to investigate the variations of the moisture content and the effective thermal conductivity of thermal insulation, besides, several possible visual identification methods are applied to analyze the moisture transport paths. Based on the proposed methods, both aerogel blanket and organic foam are investigated on the variations of hygrothermal properties. Results indicate that compared to the foam insulation, the moisture content has a greater impact on aerogel blankets with the thermal conductivity increase to 3 times of the initial values. Both scanning electron microscopy (SEM) combined with crystal tracer and magnetic resonance imaging (MRI) are utilized to identify the moisture transport paths of phenolic and polyisocyanurate foams. Pinholes and capillary channels are two main paths for liquid permeation into phenolic.

Global atmospheric moisture transport associated with precipitation extremes: Mechanisms and climate change impacts

AbstractThe atmospheric moisture transport processes are of great importance to the occurrence and intensity of precipitation extremes. In this paper, we review the linkage between processes, including the large‐scale atmospheric circulation, atmospheric moisture transport, and extreme precipitation events. We first summarize the thermodynamic and dynamic processes and moisture transport trackings for historical precipitation extremes. We then focus on the contribution of three major atmospheric moisture transport pathways, that is, atmospheric rivers, low‐level jets, and tropical cyclones, to the occurrence and intensity of regional precipitation extremes. Studies on large‐scale atmospheric circulation driving water vapor transport for precipitation extremes over East Asia and North America were specifically reviewed for the understanding of physical mechanisms and predictability of moisture transport and extreme precipitation events. We then pay more attention to the effects of global warming on atmospheric moisture transport, and thus regional precipitation extremes from the perspectives of thermodynamic and dynamic changes of the atmosphere. In the end, we summarize future research challenges on the physical mechanisms of atmospheric moisture transport that are associated with regional precipitation extremes, especially under a warming climate.This article is categorized under: Science of Water > Water Extremes Science of Water > Hydrological Processes

Observation of the atmospheric water vapor stable isotopes in three dimensions over the Tibetan Plateau

The Third Pole with an average of 4 km above sea level, centered on the Tibetan Plateau and covered the surrounding regions, has gained growing attention as the Arctic and Antarctic, due to the largest store of water in the low latitude regions. In the past half-century, the fast and gigantic glacier retreating has changed the water supply for billions of people and increased the hazards of flood. The change of water in the interior third pole also alters the regional water cycle and ecological system. Moisture transport is a crucial process within the water cycle over the Tibetan Plateau. Atmospheric water vapor stable isotopes is a state-of-the-art tool to understand the moisture transport processes and mechanisms. In this paper, we describe the techniques for measuring water vapor stable isotopes and review the past studies of in-situ vertical profile measurements. With the strong support of the Second Tibetan Plateau Scientific Expedition and Research (STEP) project, we combine water vapor stable isotopes observations from a network of ground stations across the Third Pole (the largest observation network of instruments including nine running sites) with cutting edge tethered balloon measurements. In order to fully resolve the moisture transport across the Tibetan Plateau, collaboration among multiple disciplines is strongly needed in this study, and the extended monitoring of atmospheric water vapor stable isotopes in three dimensions (horizontally and vertically) is necessary for better understanding the moisture transport to the Tibetan Plateau. A new Earth System model that is appropriate for the water cycle over the Tibetan Plateau must be developed, representing the atmosphere, cryosphere, hydrosphere and biosphere of this unique region, and their interactions, at very high resolution, including the representation of water stable isotopes, aerosols and biogeochemical cycles. Such model would be able to quantify changes in Tibetan water cycle and servicing the regional strategies for water security and management.

Method for avoiding cracks during drying of masonry units made of illite raw material

Drying is one of the most important steps in the production process of masonry units. In order to prevent the formation of cracks during drying information’s about the moisture migration rate variability as well as the material strength variability through drying are necessary. The main goal of this paper was to find a solution how to prevent the crack formation at the beginning (during the first hour) of the drying for the drying sensitive illite raw material. The first step was to record a series of isothermal Deff – MR curves at different drying air temperatures and constant drying air velocity and humidity. As it was already reported all moisture transport mechanisms during isothermal drying are visible on those curves. Characteristic spots registered on these curves were then transposed on the experimentally registered figure material strength us moisture content. It was found that the material strength for the cracked masonry units at the beginning of drying was around 0.09 MPa and that the time of cracking was near the characteristic spot B. Registered material strength and the crack time position (spot B) has additionally confirmed that the drying sensitivity of the raw material are obviously related with the present clay mineral constituents structure and the initial moisture content of the green heavy clay units. Using the Deff values registered for each experiment in the spot B we were able to calculate the maximal moisture transport rate and consequently the proper drying air parameters which are safe and which will not initialize the formation of the cracks at the beginning of the drying.

Studying the effect of desiccation cracking on the evaporation process of a Luvisol – From a small-scale experimental and numerical approach

Cracking due to desiccation of the soil surface is a common phenomenon related to the interaction between soil and the atmosphere. Indeed, during dry seasons, high evaporation of pore water near the soil surface leads to an increase in soil suction in this region. Consequently, the suction results in compressive effective stress on the soil structure and produces shrinkage, including cracking. As the crack network forms, the initial soil structure is strongly modified, which provides preferential flow pathways for solute-water and influences the soil hydraulic behaviour in general. The work aims to study the formation of cracks during evaporation process of a Cutanic Luvisol and evaluate how cracking affects the soil evaporation process. Laboratory experiments were performed on one undisturbed soil sample. To do that, a small-scale environmental chamber was designed and equipped with sensors for measuring the ambient temperature and relative humidity, and a digital camera for investigating the initiation and propagation of cracks on the soil surface. In addition to a HYPROP device (UMS GmbH, Munich, Germany), the hydraulic properties and the kinetics of evaporation of soil samples were also determined through the test. Finally, numerical simulations were carried out by using the finite element code LAGAMINE developed at the University of Liege to emphasize the effect of desiccation cracking on the soil hydraulic conductivity and the moisture transport mechanisms in the soil, as well as exchanges with ambient atmosphere.

Generic algorithm to assess moisture susceptibility of simplified wall assemblies

A simplified wall assembly consisting of four material elements is considered. Optional vapor retarder and thermal resistances are located between the elements. A simplified numerical model is presented to perform probability-based risk analysis using the Monte Carlo method. The risk assessment of the wall assembly considers the interior and exterior climatic loads. The moisture transport mechanism accounted for in the analysis is diffusion. A prescribed rain load is assumed to be transferred directly to a chosen outer susceptible material element in the construction. A case study with a wooden wall structure is analyzed and the probability distribution of the time of wetness is presented for a case using 30 different climatic years, randomly varying indoor moisture excess, indoor temperature, and rain leakages. The results show that, in the case investigated in this paper, the simulations converged after 600 years. The proposed simplified model, combined with the use of Monte Carlo simulations, offers better opportunities for a systematic assessment of risk compared with the currently used simulation programs using only a design year.

Las corrientes superficiales de chorro del Chocó y el Caribe durante los eventos de El Niño y El Niño Modoki

El objetivo de este artículo fue analizar la influencia de dos tipos de eventos de El Niño, conocidos como Canónico y Modoki, en las corrientes superficiales de chorro del Chocó y el Caribe, importantes mecanismos de transporte de humedad y generación de precipitación en Colombia. La influencia de estos tipos de eventos de El Niño en las corrientes superficiales de interés se examinó con base en las diferencias entre las características en la climatología de estos chorros durante dichos eventos y el análisis de correlaciones entre estos fenómenos. Los resultados obtenidos indicaron que en los dos primeros trimestres del año, la intensidad de estas corrientes superficialesse ve disminuida durante ambos tipos de eventos El Niño, en tanto que en los dos últimos trimestres del año se observan efectos contrarios: un fortalecimiento del chorro del Caribe y un debilitamiento del chorro del Chocó. Sin embargo, la magnitud del impacto durante los eventos El Niño Modoki es menor que durante los eventos El Niño Canónico, aunque se observa un mayor número de eventos El Niño Modoki durante el periodo de análisis. Lo anterior es fundamental para la elaboración de pronósticos de precipitación en Colombia. © 2018. Acad. Colomb. Cienc. Ex. Fis. Nat.

Pressure measurement as a tool to identify moisture transport mechanisms in convective drying of non-shrinking material

The drying process is one of the most important stages in the production of building materials. The choice of the drying method affects the chemical and physical properties of the final product. The aim of this research is to measure and analyze the dynamic changes of internal pressure in non-shrinking, porous material during convective drying. In this work the problem will be discussed with special attention to the behavior of rewetted plaster. A commercial gypsum of company PIOTROWICE II (Alpol brand), typically used in construction and decorative plastering was applied. Gypsum was mixed with water in recommended proportion of 0.6 water/gypsum and drying experiments were performed at 50°C. The changes in sample overall mass as well as pressure and material temperature on the midpoint of sample axis were monitored. On the basis of the obtained experimental data of axial pressure, it is possible to perform a more detailed analysis of mass and heat transfer mechanisms than based on the drying kinetics alone. The pressure trends in the sample allow one to determine the moment of transition from the first to the second drying period, without the need to determine the kinetics of drying. The element of novelty consists of using a direct internal pressure measurement to provide information on the variation of the actual drying rate and mass transfer mechanisms.

An inverse analysis approach for the identification of the hygro-thermo-chemical model parameters of concrete

Hygro-thermo-chemical models provide useful representations of the mechanisms of moisture transport and temperature variations that take place in concrete structures and that can influence their durability and service behaviour. Several material parameters need to be specified when performing a hygro-thermo-chemical simulation. While some of these parameters can be evaluated based on the concrete mix specifications or from data reported in the literature, some other parameters are not readily available from the literature, partly because of their large variability and partly because they do not possess a precise physical meaning. In this context, this paper presents a robust inverse analysis procedure for the identification of this latter set of material parameters. The inverse analysis problem is formulated by using temperature and relative humidity profiles taking place within a concrete component as input. The proposed approach is applied to evaluate the minimum number of temperature and relative humidity measurements that are necessary to be performed for a successful identification of the sought material parameters. Representative results of an extensive sensitivity analysis are presented to gain insight into the most effective locations within the concrete component for the measurements and instants in time when these measurements should be collected. The inverse analysis procedure is then presented and validated against a set of pseudo-experimental results affected by different levels of noise, highlighting the robustness of the proposed methodology when applied with the arrangements suggested in terms of discrete relative humidity and temperature measurements and monitoring periods.

The South Carolina Flood of October 2015: Moisture Transport Analysis and the Role of Hurricane Joaquin

Abstract Record-setting rainfall occurred over the state of South Carolina in early October 2015, with maximum accumulations exceeding 500 mm. During the heavy rainfall, Hurricane Joaquin was located offshore to the southeast of the flooding event. Prior research, storm summaries, satellite imagery, and media accounts suggest that Joaquin played a major role in the flooding, mostly through the provision of additional water vapor. Here, numerical simulations are utilized to elucidate Joaquin’s role in the flooding and to diagnose moisture transport mechanisms. The South Carolina precipitation event and the track of Hurricane Joaquin are reasonably represented by two control simulations, a 36-km simulation without nesting and another with 12- and 4-km nests added; the latter improves upon a negative intensity bias for Joaquin. A band of intense moisture transport into the flooding region is associated with a narrow, diabatically produced cyclonic lower-tropospheric potential vorticity (PV) maximum. Simulations in which Joaquin is removed exhibit a similar moisture transport mechanism and also produce a band of heavy precipitation, though the axis of heaviest precipitation shifts northward into North Carolina, and there is a modest reduction (~7%) in area-averaged rainfall. Removing Joaquin produces negligible changes in regional total water vapor content but diminished upper-tropospheric diabatic outflow. The diminished outflow allows greater eastward progression of an upper-level trough, consistent with the northward precipitation shift and with weaker forcing for ascent. Changes in the upper jet associated with Joaquin appear to exert a greater influence on the flooding event than Joaquin’s contribution to water vapor content.

Towards understanding rain infiltration in historic brickwork

Historic masonry walls are typically afflicted with uncertainties and defects which might induce significant rain water infiltrations. Insight in these moisture transport mechanisms in brickwork is important because the moisture content of brickwork is the predominant parameter for damage functions for frost, mold, and corrosion, and also affects heat flux. Given the increased focus on low-energy retrofits, interior insulation is becoming more popular. However, this typically decreases the drying potential of the wall and might consequently induce even more moisture-related pathologies. The insulation causes the exterior surface of the brickwork to be saturated more often, and this renders it possible for a runoff water film to develop during heavy rainfall. Although it is well known that this water film can penetrate through defects in the brickwork, the amount of water that penetrates remains a big question mark. Consequently, this important transport mechanism is typically omitted in hygrothermal simulations of (historical) brickwork. To obtain quantitative information, a series of laboratory tests is conducted. In a test setup a runoff film is applied in combination with a range of air pressure differences on a surface with different types of deficiencies representing imperfections in the facade. In this first phase, as an abstraction of the real saturated masonry construction a PMMA surface was used. The applied runoff film is imposed precisely and the water infiltration rate through the deficiencies is measured gravimetrically. Finally, the laboratory results are analyzed and preliminary linear relations between pressure and infiltration rate are found.

The neglected Indo‐Gangetic Plains low‐level jet and its importance for moisture transport and precipitation during the peak summer monsoon

AbstractAccurately simulating the Indo‐Asian monsoon (IAM) using atmospheric general circulation models (AGCMs) is challenging but crucial. This study uses reanalysis products European Centre of Medium‐Range Forecast Interim reanalysis, Japanese Reanalysis year 55, and High Asia Reanalysis to highlight an easterly, low‐level barrier jet along the Indo‐Gangetic Plain (referred from here as IG LLJ), which we identify as the primary moisture transport mechanism for the northeastern branch of the IAM. We show that the NCAR family of AGCMs (Community Atmospheric Model (CAM)) does not capture this circulation until 1/2° or greater spatial horizontal resolution is used. The IG LLJ develops due to a persistent low‐pressure system centered over the Ganges basin and is enhanced by the Himalayas. Using diabatic heating rates and the moist Froude number as diagnostics, we find that in CAM, this branch of the IAM displays two different dynamical regimes as a function of resolution. At low resolution, the atmosphere near the Himalayas is statically unstable, diabatic heating is strong, and the moisture flow is southwesterly from the Arabian Sea and moves over the terrain (unblocked). At high resolution, the moist static stability near the Himalayan Mountains is stable, diabatic heating is weak, and the flow primarily enters easterly from the Bay of Bengal and moves parallel to the terrain (blocked). During the summer season, the low‐resolution CAM is locked into the unblocked mode, which has serious implications for interpreting topography‐monsoon interactions. For a broader context, we demonstrate that more than half of the CMIP5 models do not capture the IG LLJ, which further highlights model‐data mismatch across the IAM region.

Moisture Transport Mechanism and Drying Kinetic of Fresh Harvested Red Onion Bulbs under Dehumidified Air

Abstract Drying plays an important role in post-harvest handling of onion for prolonging storage time. This research focuses on the drying studies of red onion bulbs using dehumidified air. The works were carried out by observing the microscopic structure of red onion bulbs layer and followed by developing mass transfer model for moisture diffusion during drying. Moisture diffusivity was estimated from drying experiments conducted at various temperatures (40, 50 and 60 °C) using the unsteady state solutions of Fick’s second law model. The diffusivity values were used to predict the drying kinetics as well as effective drying time for different air relative humidity. Results showed that combination of low relative humidity and low drying temperature significantly improved drying rate and reduced drying time.

Diurnal Cycle of a Heavy Rainfall Corridor over East Asia

AbstractMoist convection occurred repeatedly in the midnight-to-morning hours of 11–16 June 1998 and yielded excessive rainfall in a narrow latitudinal corridor over East Asia, causing severe flood. Numerical experiments and composite analyses of a 5-day period are performed to examine the mechanisms governing nocturnal convection. Both simulations and observations show that a train of MCSs concurrently developed along a quasi-stationary mei-yu front and coincided with the impact of a monsoon surge on a frontogenetic zone at night. This process was regulated primarily by a nocturnal low-level jet (NLLJ) in the southwesterly monsoon that formed over southern China and extended to central China. In particular, the NLLJ acted as a mechanism of moisture transport over the plains. At its northern terminus, the NLLJ led to a zonal band of elevated conditionally unstable air where strong low-level ascent overcame small convective inhibition, triggering new convection in three preferred plains. An analysis of convective instability shows that the low-tropospheric intrusion of moist monsoon air generated CAPE of ~1000 J kg−1 prior to convection initiation, whereas free-atmospheric forcing was much weaker. The NLLJ-related horizontal advection accounted for most of the instability precondition at 100–175 J kg−1 h−1. At the convective stage, instability generation by the upward transport of moisture increased to ~100 J kg−1 h−1, suggesting that ascending inflow caused feedback in convection growth. The convection dissipated in late morning with decaying NLLJ and moisture at elevated layers. It is concluded that the diurnally varying summer monsoon acted as an effective discharge of available moist energy from southern to central China, generating the morning-peak heavy rainfall corridor.

Moisture transport in paper passing through the fuser nip of a laser printer

Paper curls in printers should be prevented since they cause paper jams and degradation of printing quality. Moisture transport due to high temperature in the fuser nip of a printer is one of main reasons for paper curls. In order to predict and control the paper curls, it is therefore essential to understand the mechanisms of heat and moisture transport in paper. In this paper, we derived a coupled heat and moisture transport model and applied it to the situation where a paper sheet moves between two rollers having different temperatures in a fuser nip. A series of simulations were carried out with varying parameters related to the characteristics of printer paper, the environment, and paper/environment interfaces. The simulation results provided a deep understanding of the moisture transport mechanisms and identified several key variables affecting changes of moisture profiles as a function of time. The time-dependent moisture contents data will be fed into a mechanical model to obtain the quantitative amount of the nip curl.

Case Studies of Rising Damp Treatment in Historical Buildings

Salt damage can affect the service life of numerous building structures, both historical and contemporary, in a significant way. Therefore, various conservation methods have been developed for the consolidation and protection of porous building materials exposed to the salt attack. As any successful treatment of salt damage requires a multidisciplinary attitude, many different factors such as salt solution transport and crystallization, presence and origin of salts in masonry, and salt-induced deterioration are to be taken into account. The importance of pre-treatment investigations is discussed as well; in a combination with the knowledge of salt and moisture transport mechanisms they can give useful indications regarding treatment options.Another important cause of building pathologies in buildings is the rising damp and this phenomenon it is particularly more severe with the presence of salts in water. The treatment of rising damp in historic building walls is a very complex procedure. At Laboratory of Building Physics (LFC-FEUP) a wall base hygro-regulated ventilation system was developed. This system patented, HUMIVENT, has been submitted to laboratorial monitoring and to in situ validation and a numerical simplified model was developed to facilitate the practical application. Having in mind the practical application of scientific and technological knowledge from Building Physics to practice, this paper presents the design of the system (geometry, ventilation rate and hygrothermal device), the detailing and technical specification of its different components and information about the implementation in three types of buildings: a church, a museum and a residential building.

Moisture origin and transport processes in Colombia, northern South America

We assess the spatial structure of moisture flux divergence, regional moisture sources and transport processes over Colombia, in northern South America. Using three independent methods the dynamic recycling model (DRM), FLEXPART and the Quasi-isentropic back-trajectory (QIBT) models we quantify the moisture sources that contribute to precipitation over the region. We find that moisture from the Atlantic Ocean and terrestrial recycling are the most important sources of moisture for Colombia, highlighting the importance of the Orinoco and Amazon basins as regional providers of atmospheric moisture. The results show the influence of long-range cross-equatorial flow from the Atlantic Ocean into the target region and the role of the study area as a passage of moisture into South America. We also describe the seasonal moisture transport mechanisms of the well-known low-level westerly and Caribbean jets that originate in the Pacific Ocean and Caribbean Sea, respectively. We find that these dynamical systems play an important role in the convergence of moisture over western Colombia.

Tropical precipitation anomalies and d -excess evolution during El Niño 2014-16

The last 2014-16 El Nino event was among the three strongest episodes on record. El Nino considerably changes annual and seasonal precipitation across the tropics. Here, we present a unique stable isotope data set of daily precipitation collected in Costa Rica prior to, during, and after El Nino 2014-16, in combination with Lagrangian moisture source and precipitation anomaly diagnostics. δ2H composition ranged from -129.4 to +18.1 (‰) while δ18O ranged from -17.3 to +1.0 (‰). No significant difference was observed among δ18O (P = 0.0186) and δ2H (P = 0.664) mean annual compositions. However, mean annual d-excess showed a significant decreasing trend (from +13.3 to +8.7 ‰) (P < 0.001) with values ranging from +26.6 to -13.9 ‰ prior to and during the El Nino evolution. The latter decrease in d-excess can be partly explained by an enhanced moisture flux convergence across the southeastern Caribbean Sea coupled with moisture transport from northern South America by means of an increased Caribbean Low Level Jet regime. During 2014-15, precipitation deficit across the Pacific domain averaged 46% resulting in a very severe drought; while a 94% precipitation surplus was observed in the Caribbean domain. Understanding these regional moisture transport mechanisms during a strong El Nino event may contribute to a) better understanding of precipitation anomalies in the tropics and b) re-evaluate past stable isotope interpretations of ENSO events in paleoclimatic archives within the Central America region. This article is protected by copyright. All rights reserved.

Major Mechanisms of Atmospheric Moisture Transport and Their Role in Extreme Precipitation Events

We review the major conceptual models of atmospheric moisture transport, which describe the link between evaporation from the ocean and precipitation over the continents. We begin by summarizing some of the basic aspects of the structure and geographical distribution of the two major mechanisms of atmospheric moisture transport, namely low-level jets (LLJs) and atmospheric rivers (ARs). We then focus on a regional analysis of the role of these mechanisms in extreme precipitation events with particular attention to the intensification (or reduction) of moisture transport and the outcome, in terms of precipitation anomalies and subsequent flooding (drought), and consider changes in the position and occurrence of LLJs and ARs with respect to any associated flooding or drought. We then conclude with a graphical summary of the impacts of precipitation extremes, highlighting the usefulness of this information to hydrologists and policymakers, and describe some future research challenges including the effects of possible changes to ARs and LLJs within the context of future warmer climates.