Thermal deformation of loaded concrete at low temperatures, 2: Transverse deformation

Transverse thermal deformation of concrete cylinders has been measured during a thermal cycle, up to −90°C. Oven dry concrete shows an almost linear, reversible and isotropic behaviour. Load-free water-saturated concrete exhibits a complex strain behaviour, characterized by dilatation during cooling (from −10°C to −60°C) and an irreversible expansion after reheating. Longitudinal loading (15 MPa) enhances the aforementioned behaviour. A qualitative explanation of such effects is outlined.     

Thermal deformation of loaded concrete at low temperatures. 3: Lightweight concrete

Longitudinal and transverse thermal deformation has been measured on lightweight concrete cylinders during a thermal cycle, from room temperature up to −90°C. Oven-dry samples show an almost linear, isotropic, reversible and load independent dilatational behaviour. Water-saturated samples behave in a different way; when load-free, thermal deformation is nearly isotropic, although highly non linear (between −10°C and −60°C) and ending with an irreversible expansion. When axially loaded (10 MPa), lightweight concrete shows a strongly anisotropic behaviour. Volumetric thermal deformation, for a given moisture content, does not seems to depend on applied load.     

Thermal deformation of loaded concrete during thermal cycles from 20°C to −165°C

Thermal deformation of concrete samples has been measured. Unloaded water-saturated concrete exhibits a complex strain behaviour during thermal cycles, characterized by dilatation cooling and an irreversible expansion reheating. Loaded water-saturated concrete behaves in a different way. The transition region id reversed -with respect to unloaded samples- and almost dissapears, when compressive stress of 15 MPa is applied. Also an irreversible compressive deformation is bound after reheating. A qualitative explanation of such effects, assuming that microcracking is the dominant deformation mechanism in the transition range, is offered. Further research is needed along this direction.     

A study on moisture diffusion in drying and drying shrinkage of concrete

It is the purpose of this paper to clarify the behavior of moisture existing in concrete as drying occurs, and the relationship between moisture loss and drying shrinkage. Assuming that the behavior of moisture existing in concrete when drying was governed by nonlinear diffusion equation, the diffusion coefficient was expressed as a function of the moisture content by the experiment. The surface factor was also determined by the experiment. The relationship between diffusion coefficient or surface factor and the water-cement ratio of concrete were given. It was shown that the shrinkage strain was closely related to moisture loss.     

Induced anisotropic permeability due to drying of concrete

Structural strength, porous space, and permeability of concrete are strongly affected by mechanical, hydrous, and thermal loading. These various loadings may lead to drying shrinkage, one of the main characteristics of this type of material, which has to be involved in the behaviour modelling and experimental investigations being the subjects of this paper. Experimental devices and principal parameters studied are first presented. Drying shrinkage and loss of mass in time were measured on prismatic samples while uniaxial compression tests were performed on cylindrical samples. Gas permeability tests, carried out on a concrete cylinder 30 mm in diameter, form the second part of this study. The samples used for these measurements were cored from each prismatic sample at the end of 10 months or 2 years of drying, either from the transverse direction of sample (privileged direction of drying) or from the longitudinal direction. Gas permeability procedure, using micropulse test technique, is described as well as the experimental process. Experimental results are finally commented on and discussed with a view on induced anisotropy due to desiccation. Such an anisotropy is clearly observable in permeability, which is also increasing with drying time.     

泡沫干燥塔变形原因及预防措施

通过分析和计算，认为氯气处理PVC材质泡沫干燥塔被氯泵抽变形的原因是1^#、2^#泡沫塔内氯气温度超过80℃。提出的防范措施包括：确保氯气干燥前温度为12—15℃；若发生事故，宜降低电解电流，减少氯气量，减轻氯气处理负荷，并降低Ⅱ级钛冷冻水温度、提高流量。     

Prediction equation of drying shrinkage of concrete based on composite model

In this study, the prediction equation of drying shrinkage of concrete is obtained with two-phase composite model as aggregate and matrix. In order to obtain the input values for this prediction equation easily, the experimental formula of drying shrinkage and Young's modulus of cement paste are obtained, and the estimation method of Young's modulus of aggregate are proposed with easy test using cement paste, mortar and concrete. According to the experimental results, this equation can predict the drying shrinkage at any age in error by less than about 100 μm.     

Test and simulation on moisture flow in earlyage concrete under drying

In this paper, moisture flow in two concrete with and without internal curing using presoaked lightweight aggregate (PSLWA) under surface drying is experimentally investigated by measuring the weight of the concrete samples. Mathematical modeling on moisture flow, especially moisture transfer coefficient between concrete and surrounding air, normally called surface factor is performed. The results show that moisture flow through the drying surface of concrete can be characterized by a constant moisture loss stage (I) followed by a gradually reducing moisture loss stage (II). For the given environmental condition, the length of stage I increases with a decrease in water-to-cement ratio. Internal curing with PSLWA will prolong the length of Stage I. Surface factor is a function of location along air flow direction, air flow speed, and temperature of air and concrete surface, while it is independent of mix proportions of concrete. Higher air flow rate, higher temperature of air and/or concrete will result in a larger surface factor. Element size along air flow direction significantly influences the value of surface factor. Smaller element size along the air flow direction will result in a larger surface factor. The comparison on surface factor between experimental determination and theoretical calculation is performed and a good agreement between them is obtained.     

Properties of concrete with and without calcium chloride during drying and wetting

The effect of calcium chloride on the direct tensile strength of hardened cement paste and concrete cylindrical and prismatic specimens are investigated. Two different techniques for the determination of the direct tensile strength i.e. cylinders with embedded bars and prisms with glued end blocks are used. The results indicate that the addition of 2% calcium chloride significantly increases the direct tensile strength and the dynamic modulus of hardened cement paste. It is concluded that the addition of calcium chloride can partially inhibit the cracking caused by drying and sorption induced microcracking in the concrete system.     

Experiment on and simulation of moisture transfer and rolling deformation during leaf drying

The drying characteristic and rolling deformation of eaglewood leaves are investigated experimentally, and a model is built based on Fick’s law and stress–strain relations to illustrate the leaf rolling rule. The leaves dehydrate free water and hardly roll during the initial drying period. Rolling deformation is induced by the shrink difference along leaf thickness and occurs when the moisture content reaches a critical level. The rolling index of a leaf that is dried on one side is greater than that of a leaf that is dried on both sides. In addition, the rolling index is influenced by drying temperature and leaf thickness. When a leaf is thick or when the drying temperature is high, the leaf rolls considerably.     

Production of silica aerogel microparticles loaded with ammonia borane by batch and semicontinuous supercritical drying techniques

Silica aerogel microparticles were prepared by supercritical drying and used as support for hydrogen-storing ammonia borane (AB). The formation of aerogel microparticles was done using two different processes: batch supercritical fluid extraction and a semicontinuous drying process. Silica aerogel microparticles with a surface area ranging from 400 to 800 m²/g, a volume of pores of 1 cm³/g, and a mean particle diameter ranging from 12 to 27 μm were produced using the two drying techniques. The particle size distribution (PSD) of the microparticles was influenced by shear rate, amount of catalyst, hydrophilic–hydrophobic solvent ratio and hydrophobic surface modification. In particular, irregular aerogel particles were obtained from hydrophilic gels, while regular, spherical particles with smooth surfaces were obtained from hydrophobic gels. AB was loaded into silica aerogel microparticles in concentrations ranging from 1% till 5% wt. Hydrogen release kinetics from the hydride-loaded aerogel was analyzed with a volumetric cell at 80 °C. By stabilization of AB into the silica aerogel microparticles, an improvement of the release rate of hydrogen from AB was observed.     

Comparison of concrete creep in tension and in compression: Influence of concrete age at loading and drying conditions

The relation between basic and drying creep in tension compared to basic and drying creep in compression was investigated. The results obtained can be summarized as follows. Basic creep in compression is significantly more important than basic creep in tension. This difference increases with decreasing concrete age at loading. Compression creep and tension creep are similar under drying conditions. Analysis of these results provides insight into the physical mechanism underlying basic creep in concrete: microcrack initiation generates additional strains related to the development of additional self-drying shrinkage. We thus propose that basic creep in concrete is mainly caused by additional self-drying shrinkage under stress.     

Investigation of surface deformation during drying of thin polymer films due to Marangoni convection

The investigation of surface deformation of thin polymer films (methanol–poly(vinyl acetate) solution with 67 wt.% methanol) due to inhomogeneous drying is presented. Of interest are the onset of fluid instability and the influence of drying kinetics in a system where surface tension depends on temperature and composition. An experimental method is used that correlates a virtual shift of a random dot pattern, located on the downside of a glass substrate, to a deformation of the free surface of a transparent film. Instability in the fluid is forced over a structured composite substrate with inhomogeneous thermal conductivity. We show that due to the inhomogeneous heat transfer a long-wave instability mode is triggered resulting in directed mass transport. To evaluate the driving forces, 1D simulations of the non-isothermal drying process have been performed. Furthermore the influence of drying kinetics on the developing surface topology is discussed.     

Autogenous shrinkage of high-strength concrete containing silica fume under drying at early ages

The present study investigated experimentally autogenous shrinkage of high-strength concrete containing silica fume under drying at early ages. The influence of drying on hydration of cementitious materials in the high-strength concrete with water-binder ratios of 0.25, 0.35 and 0.45 was evaluated based on bound water content (BWC), which was exposed to drying at the ages of 0.5, 1.0 and 3.0 days, respectively. By establishing the relationship between the BWC and autogenous shrinkage strain under sealed conditions, autogenous shrinkage strain under drying conditions and drying shrinkage strain were separated from total shrinkage strain, and, then, the contribution of autogenous shrinkage in total shrinkage was discussed. The results showed that the percentage of autogenous shrinkage was macroscopically 50-20% based on the present method, while that was 70-30% based on the conventional superposition principle (SP). The latter resulted in overestimating autogenous shrinkage strain under drying conditions.     

Effect of different drying methods on zein-based microcapsules loaded with Artemisia argyis essence obtained by anti-solvent precipitation

Microencapsulation is considered an efficient technique to protect functional materials from oxidization while enabling controlled release. In this study, anti-solvent precipitation was used to prepare zein-based microcapsules loaded with Artemisia argyis essence (AAE@ZMs). The impact of different drying methods, namely vacuum drying, freeze drying, and spray drying on AAE@ZMs was evaluated. Quality of AAE@ZMs was evaluated by the determination of color, moisture content, bulk density, chemical structure, and morphology evaluation. Freeze dried AAE@ZMs (F-AAE@ZMs) and vacuum dried microcapsules (V-AAE@ZMs) respectively showed stronger water and oil absorption capacities. The residual content of Artemisia argyis essence (AAE) in V-AAE@ZMs was higher than those in F-AAE@ZMs and spray dried microcapsules (S-AAE@ZMs) after continuously releasing for 120 h. Meanwhile, heated from 30 to 600°C, the residues of V-AAE@ZMs and F-AAE@ZMs were lower than those of S-AAE@ZMs. Therefore, drying methods greatly affected key quality parameters of AAE@ZMs. This study provides guidance on the use of drying methods in microcapsules delivery systems with zein or other materials.     

Creep strain versus residual strain of a concrete loaded under various levels of compressive stress

An experimental study of the impact of microcracking of creep behaviour of concrete is presented in this article. Creep and residual strain results (after unloading) observed on specimens loaded in compression at various stress levels (30, 50, and 70% of the compression strength of the studied concrete) are presented for both sealed and unsealed (drying) specimens.The principal conclusion of this experimental study is the following: the residual strain is a linear function of the creep strain. This function is independent of hygral exchange conditions, sealed or unsealed specimens.This conclusion is explained by the assumption of a physical mechanism related to the presence of microcracking during creep.     

The influence of premix polymer additives on the deformation behaviour of concrete

The influence of two premix polymer additives, Sarabond and methyl methacrylate, on the aggregate-mortar interface characteristics were studied. The effect that changes in adhesion and friction angle, the interface properties, have on the strength and deformation behaviour of concrete was also investigated. It was found that while adhesion was dependent on the type of premix polymer additive, friction angle was not. Further, multiple regression equations were established for the strength and elastic modulus of concrete in terms of the mortar properties, adhesion and the friction angle. The results indicated that while the compressive and splitting tension strengths of concrete were significantly influenced by the interface characteristics, the property of the mortar was the most dominant factor. This was particularly the case for elastic modulus where the contribution of the aggregate-mortar properties was very small.     

Prediction of cracking within early-age concrete due to thermal, drying and creep behavior

To predict potential early-age cracking after concrete placing, a numerical simulation procedure has been completed based on a micromechanical model and empirical formulas on the property development of young concrete. The numerical model could account for the effects of hydration, moisture transport and creep. Environmental influences, such as removal of formworks, curing conditions and variations of surrounding temperature and relative humidity, have been investigated. In calculating stress field with age caused by these synthetic physical-mechanical processes, three-dimensional finite element and finite difference (3D-FE-FD) methods are combined together.     

Evaluation of the effects of drying shrinkage on the behavior of concrete structures strengthened by overlays

The present contribution focuses on the experimental evaluation of the effects of drying shrinkage on the behavior of concrete structures strengthened by overlays. To this end a comprehensive laboratory test program is presented. Tests on thin concrete slices served for determining the water desorption isotherm and the ultimate drying shrinkage strains. The time and depth dependent mass water content distributions and the evolution of the drying shrinkage strains were measured on concrete prisms and on larger brick-shaped concrete specimens during two years of drying. After two years of drying the brick-shaped specimens were supplemented by a concrete overlay. Measurements of the mass water content distribution were continued during water jetting and subsequent wetting of the top surface. In addition, the shrinkage strains were recorded in the composite specimens during subsequent drying.