Impact of wetting–drying cycles on hydro-mechanical behavior of an unsaturated compacted clay

To further our understanding on the impact of wetting–drying cycles on the hydro-mechanical behavior of unsaturated soils, this paper presents experimental results from suction-controlled isotropic compression tests on an unsaturated compacted clay subjected to different wetting–drying histories. This clay exhibited complicated volumetric response to wetting–drying cycles such as irreversible swelling upon wetting, irreversible shrinkage upon subsequent drying and accumulated swelling after a wetting–drying–wetting cycle. The wetting-induced irreversible swelling contributes to a significant reduction in pre-consolidation stress. It was observed that a wetting–drying cycle leads to a smaller pre-consolidation stress and downward shifting of the post-yield compression curve at a given suction, whereas a wetting–drying–wetting cycle shows an opposite effect. These observations are attributed to both irreversible swelling and irreversible change in the degree of saturation resulting from wetting–drying cycles. It was found that irreversible swelling or an irreversible increase in degree of saturation makes the soil more susceptible to yield, exhibiting a softening effect. Regarding water phase, its response to isotropic compression is mostly related to the recent wetting–drying history rather than the overall wetting–drying history.     

The effect of rate of drying on the drying/wetting behaviour of hardened cement paste

Miniature hcp specimens were dried at various rates from saturation to equilibrium at 53% rh, and were then rewet. Strain and weight changes were measured. The results show: a) recoverable and irrecoverable shrinkage was not affected appreciably by rate of drying; b) after the first 15% of the pores had emptied, rate of drying did not influence shrinkage vs. moisture loss; and c) the rate at which specimens were dried had no effect on the subsequent swelling vs. time or swelling vs. moisture gain relationships. It is concluded that spurious effects caused by internal moisture gradients were not significant; for a wide range of drying rates, miniature specimens can be used to measure the unrestrained response of the material.     

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.     

Total deformation of loaded drying concrete

Usually the total time-dependent deformation of a loaded drying concrete specimen is subdivided into two components, these two being creep and shrinkage. But it turns out that the sum of both, pure creep and pure shrinkage, is always less than the deformation under load and simultaneous drying. Until now the question remains whether there does exist a special mechanism of drying creep or load induced shrinkage. Creep under sealed conditions can be analytically expressed by means of rate theory. It is shown here that tensile stresses in the drying outer shell usually overcome tensile strength of concrete. Thus crack formation takes place and the internal stress is redistributed. Theoretical predictions are compared with experimental results. It may be concluded that creep and shrinkage of a loaded drying specimen cannot be separated. The total deformation is a consequence of the superposition of internal and external state of stress.     

Separation of drying strains and the calculation of drying stresses considering the viscoelasticity of red pine wood during drying

The objective of this study was to separate drying strains into elastic, viscoelastic, and viscous strains using free shrinkage and recovery from the deformation of slices at stress relief. The apparent shrinkage deformation was obtained by measuring the change in width of drying specimens during drying. Using the slice method, elastic and viscoelastic deformation were defined as the instantaneous change in width of a slice right after cutting and the change in the width of a slice with constant moisture content during 48?h, respectively. Viscous deformation, permanent and nonrecoverable deformation of wood, was defined as the difference in deformation between free shrinkage and the sum of the apparent, elastic, and viscoelastic deformations. These elastic, viscoelastic, and viscous strains were applied to a viscoelastic model, and coefficients of viscoelasticity and viscosity were derived. The drying stress and deformation of red pine wood at specific times during the drying process can be predicted using each coefficients and modulus of elasticity obtained by experiment.     

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.     

The roles of interlayer spacing and water in the drying shrinkage of tobermorite based materials

The roles of the interlayer spacing and water in the drying shrinkage of a hydrothermal lime-silica product have been investigated. In this work, changes in the length and weight of the specimens and the interlayer spacing of tobermorite crystals in them, have been recorded at varying drying conditions. The 11.3Å line of the tobermorite remained unaltered even when heated at 130°C. The major part of the drying shrinkage occured between 43% RH and drying at 105°C, though accompanied by a minor part of the water loss. The results show that the measured drying shrinkage of the material is not determined by any change in the interlayer characteristics of tobermorite. The results do not support the drying shrinkage mechanism proposed by Feldman and Sereda.     

Drying shrinkage of portland cement pastes I. Microcracking during drying

The occurrence of microcracking of portland cement pastes during drying has been studied by comparing the effects of specimen thickness on shrinkage and cracking using light microscopy. Increases in specimen thickness tended to impede drying and wetting, but there were only slight changes (less than experimental errors) in total and reversible shrinkage once equilibrium was attained. Although microcracking occurred at the beginning of drying whenever the thin specimen (thickness <2mm) was suddenly exposed to low relative humidity (～50%), the cracks eventually closed up. It was concluded that no matter whether or not this microcracking happened, the shrinkage of the specimen after reaching equilibrium was unrestrained.     

Creep and drying shrinkage of calcium silicate pastes I. Specimen preparation and mechanical properties

In this first paper of a series dealing with the creep and shrinkage of calcium silicate pastes, the materials, specimen preparation methods, mechanical test procedures and results are discussed. Pastes were prepared with a w/s ratio of 0.4 using C₃S, β-C₂S or a C₃S/C₂S blend. Thin-wall, hollow-cylinder specimens were cast and subjected to various conditions of load and drying. The structural and chemical modifications resulting from these treatments will be covered in subsequence papers.     

Effect of cracking in drying and shrinkage specimens

The significance of cracking and microcracking caused by nonuniform drying shrinkage of test specimens is analyzed. To assure that no cracks are produced by drying in load-free specimens, one must lower the environmental humidity gradually and sufficiently slowly, and use very thin specimens (about 1 mm thick). Graphs for the maximum admissible rate of change of environmental humidity, calculated from both linear and nonlinear diffusion theories, are provided. The spacing and width of parallel cracks due to drying are estimated from fracture mechanics considerations. In normal size specimens the drying cracks are usually too narrow to be visible. Drying leads to discontinuous microcracking rather than continuous macrocracks and is represented better as strain softening than as an abrupt stress drop. Shrinkage cracking can increase drying diffusivity by several orders of magnitude.     

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.     

Effects of drying conditions, admixtures and specimen size on shrinkage strains

The paper presents the results of an experimental investigation on the effects of drying conditions, specimen size and presence of plasticizing admixture on the development of shrinkage strains. The measurements are taken in a harsh (50 °C and 5% R.H.) and a moderate environment (28 °C and 50% R.H.). The results include strain development at various levels of cross sections of concrete prisms. The drying conditions are found to be the dominant parameter affecting the shrinkage strain development particularly in specimens of smaller sizes. The effect of plasticizing admixture on shrinkage strains is negligible.     

Early-age deformation, drying shrinkage and thermal dilation in a new type of dental restorative material based on calcium aluminate cement

Dimensional changes during the first hour of hydration for small specimens of a dental material based on calcium aluminate cement (CAC) was examined. The study was conducted on specimens prepared in two different ways. First, intact tablets (three pieces per test) dipped in water were measured. Second, compacted specimens from four tablets were measured after 10 min of hydration. The dimensional changes were studied in both wet and dry conditions at 37 °C and in a dry condition at 25 °C. In the wet environment at 37 °C no dimensional change of the samples was observed. At normal room humidity (RH 55%) at both temperatures, shrinkage of 0.35-0.40% was observed. For comparison to the early-age drying shrinkage, a study of the drying shrinkage in mature material, hydrated for 50 and 100 days, respectively, was conducted. Furthermore the thermal expansion coefficient was determined and found to be close to that of tooth substance.     

Preliminary investigations of the dimensional stability of super-critically carbonated glass fibre reinforced cement

The dimensional changes occurring during the super-critical carbonation of glass fibre reinforced cement (GRC) and its subsequent environmental exposure have been investigated. Sheet samples, with embedded stainless steel pins 200 mm apart, were fabricated and then subjected to super-critical carbonation. Test coupons were subsequently exposed to a range of environments including continuous immersion in water, cyclic wetting/drying and outdoor exposure. The super-critical carbonation process resulted in a slight expansion of GRC. This is contrary to the behaviour observed during natural carbonation where irreversible shrinkage normally occurs. Exposure to the various environments mentioned above showed that super-critically carbonated samples had much greater resistance to swelling and shrinkage than uncarbonated specimens. These observations are particularly significant in relation to the practical application of GRC in environments of fluctuating moisture content.     

Basic creep,drying creep and shrinkage of a mature cement paste after a heat cycle

Cement paste that was heated in a saturated state at an age of 28 days exhibited reductions in basic creep and shrinkage and an increase in the extent of polymerisation of the hydrated silicates. This behaviour was similar to that of cement pastes heated at early ages. Drying creep was also reduced by the heat treatment, the reductions in drying creep being closely related to the reductions in shrinkage. Measured changes in properties that were related to the structure of the cement paste assisted interpretation of the present deformation and strength observations and of transitional thermal creep data reported in the literature.     

Concrete creep modelling for structural applications: non-linearity,multi-axiality,hydration, temperature and drying effects

Concrete creep models have to consider several important phenomena (non-linearity, multi-axiality, hydration, and thermal and drying effects) to be relevant in structural applications. A selection of experimental results of creep tests found in the scientific literature are used to highlight these phenomena. Firstly, regarding the creep rate in different directions of a specimen under various loads, it is shown that creep rate under moderate loading can derive from elastic strains. Secondly, the reason why a Drucker Prager criterion can be chosen to model non-linear creep is discussed. Thirdly the interest of resorting to a creep theory able to decouple ageing (or hydration) effects and consolidation effects is explained. Moreover, interest using a poro-mechanical formulation, in which Biot coefficient depends on stress state, to model drying creep and shrinkage is discussed in the light of short meso-scopic analysis. The effect of temperature on creep is also addressed. The numerical implementation of the proposed modelling is briefly exposed and the model responses are confronted with experimental results.     

The creep of hydrated cement paste

Length and weight changes were measured for six series of mortar specimens subjected to various compressive stress histories during desorption and sorption cycles.It was found that complete drying of specimens subjected to compressive stress induces a deformation in excess of that obtainable by shrinkage alone. Appreciable creep occurs with the macropore water frozen at ?18°C. For originally saturated specimens no significant difference in water content due to creep could be detected. Prior removal of the macropore water revealed that the rate of loss of water on drying is reduced by compressive stress due to closure of micropore spaces. For previously dried specimens expansion against load required 1.25 times the weight of water compared with unloaded specimens for the initial sorption region.     

A new approach to modelling of single droplet drying

A model was developed to predict the change in droplet mass and temperature when it is exposed to hot air, as in spray drying of droplets containing solids. The droplet was assumed first to undergo sensible rapid heating with no mass change. Then the droplet experiences some shrinkage, with no temperature change but rapid mass losses, followed by a period of crust formation with a significant change in droplet mass and temperature and finally a short period of sensible heating of the dried particle. The model, unlike previous models, accounts for shrinkage and for the temperature distribution in the droplet. It provided a good prediction for the change in droplet temperature and mass for some of the experimental measurements available in the literatures.     

Creep and drying shrinkage of calcium silicate pastes II. Induced microstructural and chemical changes

Structural changes occurring during creep and shrinkage of pure calcium silicate pastes were investigated. The tests included determination of pore structure and surface area by adsorption of H₂O and N₂. Helium inflow tests were also carried out. The degree of silicate polymerization was analyzed using the trimethylsilyl method. Penetration of He and H₂O into the pore system is unaffected by the creep and shrinkage of the pastes. At low degrees of hydration drying decreases the N₂ surface area S(N₂), and increases the amount of highly polymerized silicate, while loading does not affect S(N₂) but inhibits the additional polymerization that can occur on drying. At high degrees of hydration, drying causes a slight reduction in S(N₂) while loading increases it. Drying, and especially loading and drying, promote silicate polymerization while loading without any drying does not induce significant changes.