Matthieu Vandamme

Nanogranular origin of concrete creep

Concrete, the solid that forms at room temperature from mixing Portland cement with water, sand, and aggregates, suffers from time-dependent deformation under load. This creep occurs at a rate that deteriorates the durability and truncates the lifespan of concrete structures. However, despite decades of research, the origin of concrete creep remains unknown. Here, we measure the in situ creep behavior of calcium–silicate–hydrates (C–S–H), the nano-meter sized particles that form the fundamental building block of Portland cement concrete. We show that C–S–H exhibits a logarithmic creep that depends only on the packing of 3 structurally distinct but compositionally similar C–S–H forms: low density, high density, ultra-high density. We demonstrate that the creep rate (≈1/t) is likely due to the rearrangement of nanoscale particles around limit packing densities following the free-volume dynamics theory of granular physics. These findings could lead to a new basis for nanoengineering concrete materials and structures with minimal creep rates monitored by packing density distributions of nanoscale particles, and predicted by nanoscale creep measurements in some minute time, which are as exact as macroscopic creep tests carried out over years.

Adsorption-Induced Deformation of Microporous Materials: Coal Swelling Induced by CO2–CH4 Competitive Adsorption

Carbon dioxide injection in coal seams is known to improve the methane production of the coal seam, while ensuring a safe and long-term carbon sequestration. This improvement is due to the preferential adsorption of CO(2) in coal with respect to CH(4): an injection of CO(2) thus results in a desorption of CH(4). However, this preferential adsorption is also known to cause a differential swelling of coal, which results in a significant decrease in the reservoir permeability during the injection process. Recent studies have shown that adsorption in coal micropores (few angströms in size) is the main cause of the swelling. In this work, we focus on the competitive adsorption behavior of CO(2) and CH(4) in micropores. We perform molecular simulations of adsorption with a realistic atomistic model for coal. The competitive adsorption is studied at various temperatures and pressures representative of those in geological reservoirs. With the help of a poromechanical model, we then quantify the subsequent differential swelling induced by the computed adsorption behaviors. The differential swelling is almost insensitive to the geological temperatures and pressures considered here and is proportional to the CO(2) mole fraction in the coal.

ESEM study of the humidity-induced swelling of clay film.

We measured the humidity-induced swelling of thin self-standing films of montmorillonite clay by a combination of environmental scanning electron microscopy (ESEM) and digital image correlation (DIC). The films were about 40 μm thick. They were prepared by depositing and evaporating a suspension of clay and peeling off the highly oriented deposits. The rationale for creating such original samples was to obtain mesoscopic samples that could be used to bridge experimentally the gap between the scale of the clay layer and the engineering scale of a macroscopic clay sample. Several montmorillonite samples were used: the reference clay Swy-2, the same clay homoionized with sodium or calcium ions, and a sodium-exchanged Cloisite. The edges of the clay films were observed by ESEM at various relative humidity values between 14% and 95%. The ESEM images were then analyzed by DIC to measure the swelling or the shrinkage of the films. We also measured the adsorption/desorption isotherms by weighing the film samples in a humidity-controlled environment. In order to analyze our results, we compared our swelling/shrinkage and adsorption/desorption data with previously published data on the interlayer spacing obtained by X-ray diffraction and with numerical estimates of the interlayer water obtained by molecular dynamics simulation. The swelling and the hysteresis of this swelling were found to be comparable for the overall macroscopic films and for the interlayer space. The same correspondence between film and interlayer space was observed for the amount of adsorbed water. This suggests that, in the range of relative humidities values explored, the films behave like freely swelling oriented stacks of clay layers, without any significant contribution from the mesoporosity. The relevance of this result for the behavior of clayey sedimentary rocks and the differences with the behavior of nonoriented samples (powders or compacted powders) are briefly discussed.

Effect of Water on Elastic and Creep Properties of Self-Standing Clay Films

We characterized experimentally the elastic and creep properties of thin self-standing clay films, and how their mechanical properties evolved with relative humidity and water content. The films were made of clay montmorillonite SWy-2, obtained by evaporation of a clay suspension. Three types of films were manufactured, which differed by their interlayer cation: sodium, calcium, or a mixture of sodium with calcium. The orientational order of the films was characterized by X-ray diffractometry. The films were mechanically solicited in tension, the resulting strains being measured by digital image correlation. We measured the Youngs modulus and the creep over a variety of relative humidities, on a full cycle of adsorption-desorption for what concerns the Youngs modulus. Increasing relative humidity made the films less stiff and made them creep more. Both the elastic and creep properties depended significantly on the interlayer cation. For the Youngs modulus, this dependence must originate from a scale greater than the scale of the clay layer. Also, hysteresis disappeared when plotting the Youngs modulus versus water content instead of relative humidity. Independent of interlayer cation and of relative humidity greater than 60%, after a transient period, the creep of the films was always a logarithmic function of time. The experimental data gathered on these mesoscale systems can be of value for modelers who aim at predicting the mechanical behavior of clay-based materials (e.g., shales) at the engineering macroscopic scale from the one at the atomistic scale, for them to validate the first steps of their upscaling scheme. They provide also valuable reference data for bioinspired clay-based hybrid materials.

Probing Nano-structure of C-S-H by Micro-mechanics Based Indentation Techniques

This paper summarizes recent developments in the field of nanoindentation analysis of highly heterogeneous composites. The fundamental idea of the proposed approach is that it is possible to assess nanostructure from the implementation of micromechanics-based scaling relations for a large array of nanoindentation tests on heterogeneous materials. We illustrate this approach through the application to Calcium-Silicate-Hydrate (C-S-H), the binding phase of all cementbased materials. For this important class of materials we show that C-S-H exists in at least three structurally distinct but compositionally similar forms: Low Density (LD), High Density (HD) and Ultra-High-Density (UHD). These three forms differ merely in the packing density of five nano-meter sized particles. The proposed approach also gives access to the solid particle properties of C-S-H, which can now be compared with results from atomistic simulations. By way of conclusion, we show how this approach provides a new way of analyzing complex hydrated nanocomposites, in addition to classical microscopy techniques and chemical analysis.

Hydration phase diagram of clay particles from molecular simulations

Adsorption plays a fundamental role in the behavior of clays. Because of the confinement between solid clay layers on the nanoscale, adsorbed water is structured in layers, which can occupy a specific volume. The transition between these states is intimately related to key features of clay thermo-hydro-mechanical behavior. In this article, we consider the hydration states of clays as phases and the transition between these states as phase changes. The thermodynamic formulation supporting this idea is presented. Then, the results from grand canonical Monte Carlo simulations of sodium montmorillonite are used to derive hydration phase diagrams. The stability analysis presented here explains the coexistence of different hydration states at clay particle scale and improves our understanding of the irreversibilities of clay thermo-hydro-mechanical behavior. Our results provide insights into the mechanics of the elementary constituents of clays, which is crucial for a better understanding of the macroscopic behavior of clay-rich rocks and soils.

Transient effects of drying creep in nanoporous solids: understanding the effects of nanoscale energy barriers.

The Pickett effect is the phenomenon of creep enhancement during transient drying. It has been observed for many nanoporous solids, including concrete, wood and Kevlar. While the existing micromechanical models can partially explain this effect, they have yet to consider nanoscale dynamic effects of water in nanopores, which are believed to be of paramount importance. Here, we examine how creep deformations in a slit pore are accelerated by the motion of water due to drying forces using coarse-grained molecular dynamics simulations. We find that the drying that drives water flow in the nanopores lowers both the activation energy of pore walls sliding past one another and the apparent viscosity of confined water molecules. This lowering can be captured with an analytical Arrhenius relationship accounting for the role of water flow in overcoming the energy barriers. Notably, we use this model and simulation results to demonstrate that the drying creep strain is not linearly dependent on the applied creep stress at the nanopore level. Our findings establish the scaling relationships that explain how the creep driving force, drying force and fluid properties are related. Thus, we establish the nanoscale origins of the Pickett effect and provide strategies for minimizing the additional displacements arising from this effect.

On the Shrinkage and Stiffening of a Cellulose Sponge Upon Drying

Justine Rey Undergraduate Student e-mail: justinerey.1@gmail.com Matthieu Vandamme 1 Assistant Professor e-mail: matthieu.vandamme@enpc.fr Laboratoire Navier (E´cole des Ponts ParisTech/ IFSTTAR/CNRS), E´cole des Ponts ParisTech, Universite´ Paris-Est, 77420 Champs-sur-Marne, France On the Shrinkage and Stiffening of a Cellulose Sponge Upon Drying Everyone can observe the peculiar effect of water on a sponge: upon drying, a sponge shrinks and stiffens; it swells and softens upon wetting. In this work, we aim to explain and model this behavior by using the Biot–Coussy poromechanical framework. We measure the volume and the bulk modulus of sponges at different water contents. Upon drying, the volume of the sponge decreases by more than half and its bulk modulus increases by almost two orders of magnitude. We develop a partially saturated micropor- omechanical model of the sponge undergoing finite transformations. The model compares well with the experimental data. We show that about half of the stiffening of the sponge upon drying is due to geometrical nonlinearities induced by a closing of the pores under the action of capillary pressure. The other half of the stiffening can be explained by the nonlinear elastic properties of the cellulose material itself. [DOI: 10.1115/1.4007906] Introduction A kitchen sponge is a common object of daily use. Neverthe- less, its behavior can be surprising: a wet sponge left overnight beside the sink will be shrunken and hard in the morning. When immersed in water, the same sponge will swell back and soften tremendously. Such a peculiar response to an intake of water is also observed for other natural or man-made materials: soil, con- crete, bread, etc. Most kitchen sponges are made of cellulose. Cellulose is the most abundant organic compound on Earth [1]. It is a naturally occurring polymer, which is the main constituent of plants. In industry, cellulose is mostly used to produce paper or paperboard. Cellulose is a polysaccharide composed of b-D-glucopyranose units linked by glycosidic bonds [2]. The degree of polymeriza- tion of cellulose found in nature can be greater than ten thousand [3]. Despite decades of intensive research, the exact structure of cellulose still remains unresolved. Cellulose can exist in more or less crystalline forms and at least six different polymorphs of its crystalline structure have already been identified [1]. Due to the presence of hydroxyl groups and oxygen atoms, there exists a significant amount of hydrogen bonds within a cellulose chain, and between neighboring chains: those hydrogen bonds play a significant role in the structure and on the mechanical properties of cellulosic materials [4–7]. When immersed in water, cellulose swells [8]. This swelling depends on the chemistry of the surrounding fluid [9–11]. The sur- rounding liquid also has an effect on the mechanical properties of the cellulose [12]. In the presence of moisture, the intake of water depends on the structure of the hydrogen bonds [13]. Moisture has an effect on the mechanical properties of cellulose [14–16]. At the structural level, sponge is a porous material. In a manner similar to any porous material, upon drying, water will leave the pores and the material will become partially saturated. For surface energy reasons (for an in-depth explanation, see, for instance, Ref. [17]), at equilibrium at a given relative humidity, the larger pores will be empty (full of air), while the smaller pores will still be saturated with liquid water. The lower the relative humidity, the smaller the critical radius below which pores remain saturated. Liquid water can be in thermodynamic equilibrium with air with a Corresponding author. Manuscript received December 24, 2010; final manuscript received April 11, 2012; accepted manuscript posted October 25, 2012; published online February 6, 2013. Assoc. Editor: Younane Abousleiman. Journal of Applied Mechanics relative humidity below 100% only if the pressure in the liquid water drops below that of the surrounding air. The drier the air, the greater the drop in the pressure of the liquid water. This depressed water will pull on the saturated pores and, thus, lead to a global shrinkage of the material. The same mechanism contrib- utes, for instance, to the drying shrinkage of concrete [18]. In this work, we aim to determine whether the same mechanism can explain why a sponge shrinks and stiffens upon drying. In other words, can the mechanical behavior of a cellulose sponge upon drying be explained and modeled by considering it as a regular partially saturated porous solid? Our study begins with an experimental campaign of measure- ments of the shrinkage and stiffening of a cellulose sponge at several stages of the drying process. We then develop a partially saturated poromechanical model of a sponge in finite transforma- tions. In a later section, the developed model is compared with the experimental data and the results are discussed. 2 Experimental Study of the Effect of Water on the Volume and on the Elastic Properties of a Sponge In this section, we present the experimental study we performed in order to characterize the effect of drying on the volume and the elastic properties of a cellulose sponge. 2.1 Materials and Methods. The experimental study was performed on four parallelepipedic Nicols heavy-duty sponges. Their dry mass, measured after oven drying, was 23:7g 6 5:6%. Their volume upon full wetting was 744cm 3 6 2:3%. The small coefficients of variation suggest that the four sponges were similar. We performed measurements of the volume and elastic proper- ties at various drying stages. The sponges were first completely wet. After each measurement, we dried them with a hairdryer. We then hermetically wrapped them with silver foil and waited half a day so that the remaining water diffused and was uniformly dis- tributed in the sponge. At each stage of the drying process, the mass of the sponges was measured with a balance with an accuracy of 0.01 g. Upon drying, the sponges remained reasonably parallelepipedic: their dimensions, and thus their volume, were measured with a caliper. Then the bulk modulus of the sponges was measured. In order to do so, we performed compression tests in each of the three principal directions of the sponge with an MTS tensile/compres- sion machine. A ramp displacement was applied at a velocity of C 2013 by ASME Copyright V MARCH 2013, Vol. 80 / 020908-1 Downloaded From: http://energyresources.asmedigitalcollection.asme.org/ on 07/16/2014 Terms of Use: http://asme.org/terms

Stability of Hydrated Clay Layers from Molecular Simulations

Clays adsorb water following discrete steps in which water is roughly structured in layers. The resulting discrete hydration states can be viewed as phases separated by unstable domains and the change of hydration state can be viewed as a phase transformation. Here, we identify stable basal spacings and the domains of (meta-) stability through the analysis of confining pressure isotherms obtained from molecular simulations performed in grand canonical ensemble. This ensemble mimics drained conditions. Through thermodynamic analysis, we can explain the coexistence of different hydration states at clay particle scale and provide kinetics arguments to the hysteresis observed with respect to traction and compression. The gained insights into the mechanical behavior of the elementary constituents of clays may lead to a better understanding of the behavior of clay-rich rocks and soils.

Creep Properties of Cementitious Materials from Indentation Testing: Significance, Influence of Relative Humidity, and Analogy Between C-S-H and Soils

Concrete creeps, and this creep must be well characterized and modeled to properly design civil engineering infrastructures. Here, we present some results on the creep properties of cementitious materials obtained by using the indentation technique. Firstly, we show that minutes-long microindentations on cement paste yield a quantitative measurement of their long-term logarithmic creep kinetics, which can be used to predict the rate of long-term creep of concrete. Then, by performing microindentations, we study the effect of relative humidity on the long-term creep properties of hydrated C 3 S samples, compacted samples made of pure C-S-H, and compacted samples made of pure CH. The last part is dedicated to a study by nanoindentation of the creep properties of C-S-H phases directly within a hydrated cement paste. We identify scaling relations independent of mix proportion and heat treatment, which we can explain by drawing a thorough analogy between the mechanical behavior of C-S-H and that of clays.