Uniform ultra-short (< 100 nm) single-walled carbon nanotubes (SWCNTs) with properties of rich chemistry, high dispersity and easy manipulation, are of great importance for constructing novel nanostructures and highly integrated miniature devices. Herein, we report a recent breakthrough in cutting long SWCNTs into uniform ultra-short segments in gas-phase using CO2 as a 'blade.' SWCNTs synthesized by an aerosol reactor are directly introduced into a second reactor for gaseous cutting. The quality and the number concentration of shortened SWCNTs are enhanced after the gas-phase cutting process. Moreover, the growth and cutting of SWCNTs are accomplished in a continuous gas-phase process, thus allows direct dry deposition of ultra-short SWCNTs as individual macromolecules or thin films onto various substrates for multiple applications. (C) 2018 Elsevier Ltd. All rights reserved.

VL - 143 UR - https://linkinghub.elsevier.com/retrieve/pii/S0008622318310558 JO - Carbon ER - TY - JOUR T1 - Computing the linear viscoelastic properties of soft gels using an optimally windowed chirp protocol JF - Journal of Rheology Y1 - 2018 A1 - Bouzid, Mehdi A1 - Keshavarz, Bavand A1 - Geri, Michela A1 - Divoux, Thibaut A1 - Emanuela Del Gado A1 - McKinley, Gareth H. AB -We use molecular dynamics simulations to investigate the linear viscoelastic response of a model three-dimensional particulate gel. The numerical simulations are combined with a novel test protocol (the optimally windowed chirp or OWCh), in which a continuous exponentially varying frequency sweep windowed by a tapered cosine function is applied. The mechanical response of the gel is then analyzed in the Fourier domain. We show that (i) OWCh leads to an accurate computation of the full frequency spectrum at a rate significantly faster than with the traditional discrete frequency sweeps, and with a reasonably high signal-to-noise ratio, and (ii) the bulk viscoelastic response of the microscopic model can be described in terms of a simple mesoscopic constitutive model. The simulated gel response is in fact well described by a mechanical model corresponding to a fractional Kelvin-Voigt model with a single Scott-Blair (or springpot) element and a spring in parallel. By varying the viscous damping and the particle mass used in the microscopic simulations over a wide range of values, we demonstrate the existence of a single master curve for the frequency dependence of the viscoelastic response of the gel that is fully predicted by the constitutive model. By developing a fast and robust protocol for evaluating the linear viscoelastic spectrum of these soft solids, we open the path toward novel multiscale insight into the rheological response for such complex materials. (C) 2018 The Society of Rheology.

VL - 62 UR - http://sor.scitation.org/doi/10.1122/1.5018715 IS - 4 JO - Journal of Rheology ER - TY - JOUR T1 - Concrete material science: Past, present, and future innovations JF - Cement and Concrete Research Y1 - 2018 A1 - Henri Van Damme AB -Concrete is flying off, but it is simultaneously facing tremendous challenges in terms of environmental impact, financial needs, societal acceptance and image. Based on an historical approach of the science of concrete and reinforced concrete in particular, this paper calls for the exploration of radical changes in three key aspects of concrete use: reinforcement, binder content, and implementation methods. More precisely, it is suggested that, in parallel to the introduction of robotic fabrication methods, digital technologies may be key for the introduction several innovations like (i) rebar-free reinforcement using non-convex granular media; (ii) compression-optimized concrete structures, using topology optimization, architectural geometry, and 3D-printing or origami-patterned formworks; (iii) truly digital concrete through the coupling of massive data collection and deep learning.

As a crucial part of the transportation system, roadway network provides mobility to the society and is vital for the economy. At the same time it contributes significantly to the environmental footprint during its construction, operation and maintenance. Hence, the sustainable development of our Nation's roadway system requires quantitative means to link infrastructure performance to lifecycle energy use and greenhouse gas emissions. Recent developments in mechanistic models of roughness- and deflection-induced pavement-vehicle interaction aim at providing such engineering estimates. Herein, it is demonstrated that these models when implemented at a network scale are a powerful basis for big data analytics of excess-energy consumption and carbon dioxide emissions by integrating spatially and temporally varying road conditions, pavement properties, traffic loads and climatic conditions. A novel ranking algorithm is proposed, that allows upscaling of the local carbon dioxide emissions due to pavement vehicle interaction to the size of state-wide or national sustainability goals. Implemented for 5157 lane-miles of the interstate highway system in the State of Virginia, sections contributing significantly to carbon dioxide emissions are identified. It is shown that the proposed ranking algorithm based on the inferred emission that exhibits a power-law distribution, provides the shortest path for greenhouse gas emissions savings per maintenance at network scale. That is, maintaining a few lane miles allows for a significant synergetic improvement of both infrastructure performance and environmental impact of the interstate network and helps transportation agencies in making economic and environmentally sustainable decisions.

VL - 142 IS - Special Issue: SI JO - Journal of Cleaner Production ER - TY - JOUR T1 - Cohesive strength of iron ore granules JF - EPJ Web of Conferences Y1 - 2017 A1 - Contreras, Rafael Jaimes A1 - Berger, Nicolas A1 - Izard, Edouard A1 - Douce, Jean-François A1 - Koltsov, Alexey A1 - Jean-Yves Delenne A1 - Emilien Azéma A1 - Saeid Nezamabadi A1 - van Loo, Frédéric A1 - Roland Jean-Marc Pellenq A1 - Farhang Radjaï ED - Luding, S. AB -We present an experimental and numerical investigation of the mechanical strength of crude iron ore (Hematite) granules in which capillary bonds between primary particles are the source of internal cohesion. The strength is measured by subjecting the granules to vertical compression between two plates. We show that the behavior of the granules is ductile with a well-defined plastic threshold which increases with the amount of water. It is found that the compressive strength scales with capillary cohesion with a pre-factor that is nearly independent of size polydispersity for the investigated range of parameters but increases with friction coefficient between primary particles. This weak dependence may be attributed to the class of fine particles which, due to their large number, behaves as a cohesive matrix that controls the strength of the granule.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714008020 IS - 10141146 JO - EPJ Web Conf. ER - TY - JOUR T1 - Compaction of granular materials composed of deformable particles JF - EPJ Web of Conferences Y1 - 2017 A1 - Thanh Hai Nguyen A1 - Saeid Nezamabadi A1 - Jean-Yves Delenne A1 - Farhang Radjaï ED - Luding, S. AB -In soft particle materials such as metallic powders the particles can undergo large deformations without rupture. The large elastic or plastic deformations of the particles are expected to strongly affect the mechanical properties of these materials compared to hard particle materials more often considered in research on granular materials. Herein, two numerical approaches are proposed for the simulation of soft granular systems: (i) an implicit formulation of the Material Point Method (MPM) combined with the Contact Dynamics (CD) method to deal with contact interactions, and (i) Bonded Particle Model (BPM), in which each deformable particle is modeled as an aggregate of rigid primary particles using the CD method. These two approaches allow us to simulate the compaction of an assembly of elastic or plastic particles. By analyzing the uniaxial compaction of 2D soft particle packings, we investigate the effects of particle shape change on the stress-strain relationship and volume change behavior as well as the evolution of the microstructure.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714005013 JO - EPJ Web Conf. ER - TY - JOUR T1 - Conformal field theory of critical Casimir forces between surfaces with alternating boundary conditions in two dimensions JF - Journal of Statistical Mechanics: Theory and Experiment Y1 - 2017 A1 - Dubail, Jerome A1 - Santachiara, Raoul A1 - Emig, Thorsten KW - Casimir effect; conformal field theory; critical exponents and amplitudes AB -Systems as diverse as binary mixtures and inclusions in biological membranes, and many more, can be described effectively by interacting spins. When the critical fluctuations in these systems are constrained by boundary conditions, critical Casimir forces (CCF) emerge. Here we analyze CCF between boundaries with alternating boundary conditions in two dimensions, employing conformal field theory (CFT). After presenting the concept of boundary changing operators, we specifically consider two different boundary configurations for a strip of critical Ising spins: (I) alternating equi-sized domains of up and down spins on both sides of the strip, with a possible lateral shift, and (II) alternating domains of up and down spins of different size on one side and homogeneously fixed spins on the other side of the strip. Asymptotic results for the CCF at small and large distances are derived. We introduce a novel modified Szego formula for determinants of real antisymmetric block Toeplitz matrices to obtain the exact CCF and the corresponding scaling functions at all distances. We demonstrate the existence of a surface renormalization group flow between universal force amplitudes of different magnitude and sign. The Casimir force can vanish at a stable equilibrium position that can be controlled by parameters of the boundary conditions. Lateral Casimir forces assume a universal simple cosine form at large separations.

UR - http://stacks.iop.org/1742-5468/2017/i=3/a=033201?key=crossref.e58e04fb8593248b573c53589bed0f1d JO - J. Stat. Mech. ER - TY - JOUR T1 - Crystal-chemistry control of the mechanical properties of 2:1 clay minerals JF - Applied Clay Science Y1 - 2017 A1 - Berthonneau, Jeremie A1 - Christian G. Hoover A1 - Grauby, Olivier A1 - Alain Baronnet A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm AB -Clay minerals are the main constituents of the clay matrix of a wide variety of sedimentary deposits. When subjected to burial, some of these minerals undergo phase transitions accompanied by atomic substitutions which ultimately impact the cohesive interactions between their constitutive layers. The most common among such transitions is smectite illitization, which is also highly relevant for oil and gas exploration and production from source rocks. The impact of this transition on the mechanical properties of clay minerals as well as clay bearing rocks remains, however, to be properly addressed. To this end, a set of macroscopic single 2:1 clay minerals (pyrophyllite, talc, vermiculite, phlogopite, muscovite, and clintonite) representative of the two octahedral fillings and the variation in surface charge densities was investigated. A hybrid experimental-modeling approach is proposed, which combines nanoindentation in orthogonal directions (*x*1 and *x*3-directions) with analytical derivations of the cohesive energy (*U*_{i}) using XRD and TEM-EDS measurements. The results highlight that the interlayer energy defines the elasticity (modulus *M*), strength (hardness *H*) and ductility behavior (*M*/*H*) of these materials; and not the bond energy stored into the constituent layers. This finding permits the derivation of predictive stiffness and strength functional relations as derivatives of the interlayer energy that account for the arrangement of nanoscale layers and the interlayer composition. These relations suggest that as the cohesion increases with the coulombic interactions through progressive smectite illitization, the system loses its capacity to dissipate applied energy by dislocation mechanisms in between the layers, entailing an increase in brittleness of the clay particles with burial. By way of conclusion, the geophysics and geochemistry implications of these results for predicting the macroscopic mechanical performance of clay bearing geomaterials, such as economically valuable source rocks, are discussed.

One of the main challenges faced by the nuclear industry is the long-term confinement of nuclear waste. Because it is inexpensive and easy to manufacture, cement is the material of choice to store large volumes of radioactive materials, in particular the low-level medium-lived fission products. It is therefore of utmost importance to assess the chemical and structural stability of cement containing radioactive species. Here, we use ab initio calculations based on density functional theory (DFT) to study the effects of Sr-90 insertion and decay in C-S-H (calcium-silicate-hydrate) in order to test the ability of cement to trap and hold this radioactive fission product and to investigate the consequences of its beta-decay on the cement paste structure. We show that Sr-90 is stable when it substitutes the Ca2+ ions in C-S-H, and so is its daughter nucleus Y-90 after beta-decay. Interestingly, Zr-90, daughter of Y-90 and final product in the decay sequence, is found to be unstable compared to the bulk phase of the element at zero K but stable when compared to the solvated ion in water. Therefore, cement appears as a suitable waste form for Sr-90 storage.

VL - 49 UR - http://pubs.acs.org/doi/10.1021/acs.est.5b02609 IS - 22 JO - Environ. Sci. Technol. ER - TY - Generic T1 - Contribution of mechanical factors to the variability of root architecture: Quantifying the past history of interaction forces between growing roots and soil grains T2 - 2016 IEEE INTERNATIONAL CONFERENCE ON FUNCTIONAL-STRUCTURAL PLANT GROWTH MODELING, SIMULATION, VISUALIZATION AND APPLICATIONS (FSPMA) Y1 - 2016 A1 - Fakih, Mahmoud A1 - Jean-Yves Delenne A1 - Farhang Radjaï A1 - Fourcaud, Thierry AB -The relation between a growing root and the soil movement has often been under-estimated. The present work aims to determine how grains in granular soils are reorganized by the action of growing roots, and in turn how the resulting forces acting on root tips modify their development. For this purpose, we have developed a 2D Discrete Element Model (DEM) able to compute a numerical growth of a single root inside a granular medium, taking into account the grain-grain and the root-grain contact forces during the growth. First in silico simulations were carried out in order to : 1-quantify the influence of the granular structure (grain diameter distribution and gaps) and root mechanical properties (root bending stiffness) on the evolution of reaction forces applied to a single root during its growth; 2-highlight “group effects”, e.g. how the reorganization of grains and their interaction forces due to a given growing root can affect the mechanical signal perceived by its near neighbours; 3-investigate how the presence of initial channels within the granular medium can effect the growth trajectory and minimize the resistance to penetration. All simulations were carried out assuming that root growth direction was only driven by external forces. Simlation results allowed the extraction of general physical laws that will be used further to provide mechanoperceptive indicators and analyze experimental data provided by phenotyping platforms. The final objective will be to quantify the response of plants to mechanical stresses in terms of root elongation rate, root straightness and ramification.

JF - 2016 IEEE INTERNATIONAL CONFERENCE ON FUNCTIONAL-STRUCTURAL PLANT GROWTH MODELING, SIMULATION, VISUALIZATION AND APPLICATIONS (FSPMA) PB - I E E E CY - NOV 07-11, 2016, Qingdao, PEOPLES R CHINA ER - TY - JOUR T1 - The crucial effect of early-stage gelation on the mechanical properties of cement hydrates JF - Nature Communications Y1 - 2016 A1 - Katerina Ioannidou A1 - Matej, Kanduč A1 - Li, Lunna A1 - Frenkel, Daan A1 - Dobnikar, Jure A1 - Emanuela Del Gado AB -Gelation and densification of calcium–silicate–hydrate take place during cement hydration. Both processes are crucial for the development of cement strength, and for the long-term evolution of concrete structures. However, the physicochemical environment evolves during cement formation, making it difficult to disentangle what factors are crucial for the mechanical properties. Here we use Monte Carlo and Molecular Dynamics simulations to study a coarse-grained model of cement formation, and investigate the equilibrium and arrested states. We can correlate the various structures with the time evolution of the interactions between the nano-hydrates during the preparation of cement. The novel emerging picture is that the changes of the physicochemical environment, which dictate the evolution of the effective interactions, specifically favour the early gel formation and its continuous densification. Our observations help us understand how cement attains its unique strength and may help in the rational design of the properties of cement and related materials.

VL - 7 UR - http://www.nature.com/articles/ncomms12106 JO - Nat Commun ER - TY - JOUR T1 - Capturing material toughness by molecular simulation: accounting for large yielding effects and limits JF - International Journal of Fracture Y1 - 2015 A1 - Brochard, Laurent A1 - György Hantal A1 - Hadrien Laubie A1 - Franz-Josef Ulm A1 - Roland Jean-Marc Pellenq A1 - Benoit A. Coasne AB -The inherent computational cost of molecular simulations limits their use to the study of nanometric systems with potentially strong size effects. In the case of fracture mechanics, size effects due to yielding at the crack tip can affect strongly the mechanical response of small systems. In this paper we consider two examples: a silica crystal for which yielding is limited to a few atoms at the crack tip, and a nanoporous polymer for which the process zone is about one order of magnitude larger. We perform molecular simulations of fracture of those materials and investigate in particular the system and crack size effects. The simulated systems are periodic with an initial crack. Quasi-static loading is achieved by increasing the system size in the direction orthogonal to the crack while maintaining a constant temperature. As expected, the behaviors of the two materials are significantly different. We show that the behavior of the silica crystal is reasonably well described by the classical framework of linear elastic fracture mechanics (LEFM). Therefore, one can easily upscale engineering fracture properties from molecular simulation results. In contrast, LEFM fails capturing the behavior of the polymer and we propose an alternative analysis based on cohesive crack zone models. We show that with a linear decreasing cohesive law, this alternative approach captures well the behavior of the polymer. Using this cohesive law, one can anticipate the mechanical behavior at larger scale and assess engineering fracture properties. Thus, despite the large yielding of the polymer at the scale of the molecular simulation, the cohesive zone analysis offers a proper upscaling methodology.

VL - 194 IS - 2 JO - Int J Fract ER - TY - JOUR T1 - Casimir-Polder force between anisotropic nanoparticles and gently curved surfaces JF - Physical Review D Y1 - 2015 A1 - Bimonte, Giuseppe A1 - Emig, Thorsten A1 - Kardar, Mehran AB -The Casimir--Polder interaction between an anisotropic particle and a surface is orientation dependent. We study novel orientational effects that arise due to curvature of the surface for distances much smaller than the radii of curvature by employing a derivative expansion. For nanoparticles we derive a general short distance expansion of the interaction potential in terms of their dipolar polarizabilities. Explicit results are presented for nano-spheroids made of SiO2 and gold, both at zero and at finite temperatures. The preferred orientation of the particle is strongly dependent on curvature, temperature, as well as material properties.

VL - 92 IS - 2 JO - Phys. Rev. D ER - TY - Generic T1 - Is cement a glassy material? T2 - Euro-C Conference Y1 - 2015 A1 - Mathieu Bauchy A1 - Mohammad Javad Abdolhosseini Qomi A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm ED - Bicanic, N ED - Mang, H ED - Meschke, G ED - DeBorst, R AB -The nature of Calcium-Silicate-Hydrate (C-S-H), the binding phase of cement, remains a controversial question. In particular, contrary to the former crystalline model, it was recently proposed that its nanoscale structure was actually amorphous. To elucidate this issue, we analyzed the structure of a realistic simulation of C-S-H, and compared the latter to crystalline tobermorite, a natural analogue to cement, and to an artificial ideal glass. Results clearly support that C-S-H is amorphous. However, its structure shows an intermediate degree of order, retaining some characteristics of the crystal while acquiring an overall glass-like disorder. Thanks to a detailed quantification of order and disorder, we show that its amorphous state mainly arises from its hydration.

JF - Euro-C Conference PB - CRC PRESS-TAYLOR & FRANCIS GROUP CY - MAR 24-27, 2014, St Anton am Alberg, AUSTRIA VL - COMPUTATIONAL MODELLING OF CONCRETE STRUCTURES, VOL 1 SN - 978-1-138-02641-4; 978-1-315-76203-6 ER - TY - JOUR T1 - Cement As a Waste Form for Nuclear Fission Products: The Case of 90 Sr and Its Daughters JF - Environ Sci Technol Y1 - 2015 A1 - Dezerald, Lucile A1 - Kohanoff, Jorge J A1 - Alfredo A. Correa A1 - Caro, Alfredo A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm A1 - Andres Saùl AB -One of the main challenges faced by the nuclear industry is the long-term confinement of nuclear waste. Because it is inexpensive and easy to manufacture, cement is the material of choice to store large volumes of radioactive materials, in particular the low-level medium-lived fission products. It is therefore of utmost importance to assess the chemical and structural stability of cement containing radioactive species. Here, we use ab initio calculations based on density functional theory (DFT) to study the effects of (90)Sr insertion and decay in C-S-H (calcium-silicate-hydrate) in order to test the ability of cement to trap and hold this radioactive fission product and to investigate the consequences of its β-decay on the cement paste structure. We show that (90)Sr is stable when it substitutes the Ca(2+) ions in C-S-H, and so is its daughter nucleus (90)Y after β-decay. Interestingly, (90)Zr, daughter of (90)Y and final product in the decay sequence, is found to be unstable compared to the bulk phase of the element at zero K but stable when compared to the solvated ion in water. Therefore, cement appears as a suitable waste form for (90)Sr storage.

VL - 49 IS - 22 ER - TY - JOUR T1 - Control of the Pore Texture in Nanoporous Silicon via Chemical Dissolution JF - Langmuir Y1 - 2015 A1 - Secret, Emilie A1 - Wu, Chia-Chen A1 - Chaix, Arnaud A1 - Anne Galarneau A1 - Gonzalez, Philippe A1 - Cot, Didier A1 - Sailor, Michael J. A1 - Jestin, Jacques A1 - Zanotti, Jean-Marc A1 - Cunin, Frédérique A1 - Benoit A. Coasne AB -

The surface and textural properties of porous silicon (pSi) control many of its physical properties essential to its performance in key applications such as optoelectronics, energy storage, luminescence, sensing, and drug delivery. Here, we combine experimental and theoretical tools to demonstrate that the surface roughness at the nanometer scale of pSi can be tuned in a controlled fashion using partial thermal oxidation followed by removal of the resulting silicon oxide layer with hydrofluoric acid (HF) solution. Such a process is shown to smooth the pSi surface by means of nitrogen adsorption, electron microscopy, and small-angle X-ray and neutron scattering. Statistical mechanics Monte Carlo simulations, which are consistent with the experimental data, support the interpretation that the pore surface is initially rough and that the oxidation/oxide removal procedure diminishes the surface roughness while increasing the pore diameter. As a specific example considered in this work, the initial roughness ξ ∼ 3.2 nm of pSi pores having a diameter of 7.6 nm can be decreased to 1.0 nm following the simple procedure above. This study allows envisioning the design of pSi samples with optimal surface properties toward a specific process.

Understanding the physical origin of creep in calcium–silicate–hydrate (C–S–H) is of primary importance, both for fundamental and practical interest. Here, we present a new method, based on molecular dynamics simulation, allowing us to simulate the long-term visco-elastic deformations of C–S–H. Under a given shear stress, C–S–H features a gradually increasing shear strain, which follows a logarithmic law. The computed creep modulus is found to be independent of the shear stress applied and is in excellent agreement with nanoindentation measurements, as extrapolated to zero porosity.

JF - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 PB - American Society of Civil Engineers CY - September 21–23, 2015, Vienna, AustriaReston, VA VL - CONCREEP 10: MECHANICS AND PHYSICS OF CREEP, SHRINKAGE, AND DURABILITY OF CONCRETE AND CONCRETE STRUCTURES ER - TY - Generic T1 - Creep of Clay: Numerical Results at the Scale of a Layer and Experimental Results at the Scale of Thin Self-Standing Films T2 - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 Y1 - 2015 A1 - Benoit Carrier A1 - Matthieu Vandamme A1 - Roland Jean-Marc Pellenq A1 - Henri Van Damme ED - Hellmich, Christian ED - Pichler, Bernhard ED - Kollegger, Johann AB -This work focuses on the creep of clay-based materials, which exhibit significant analogies with cement-based materials. Here, we studied the creep of clay at two scales and with two techniques: numerically (with molecular simulations) at the scale of a clay layer (nm), and experimentally at the scale of thin self-standing clay films (few dozen *μ*m). At the scale of the clay layer, numerical simulations showed that the shear rate was constant over time and an affine function of the shear stress. Creep experiments showed that, after a transient period, the creep function of our thin self-standing clay films was a logarithmic function of time. A comparison of the results obtained at the two scales shows that the origin of the logarithmic feature of clay creep must at least partly originate from a scale greater than that of an individual clay layer. By analogy, such result is likely to hold for cementitious materials, which are also known to creep logarithmically with respect to time in the long term: the origin of this logarithmic feature is likely to stem at least partly from a scale greater than the scale of an individual C-S-H layer.

To study the critical Casimir force between chemically structured boundaries immersed in a binary mixture at its demixing transition, we consider a strip of Ising spins subject to alternating fixed spin boundary conditions. The system exhibits a boundary induced phase transition as function of the relative amount of up and down boundary spins. This transition is associated with a sign change of the asymptotic force and a diverging correlation length that sets the scale for the crossover between different universal force amplitudes. Using conformal field theory and a mapping to Majorana fermions, we obtain the universal scaling function of this crossover, and the force at short distances.

VL - 112 IS - 6 JO - EPL ER - TY - Generic T1 - C-S-H across Length Scales: From Nano to Micron T2 - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 Y1 - 2015 A1 - Mohammad Javad Abdolhosseini Qomi A1 - Enrico Masoero A1 - Mathieu Bauchy A1 - Franz-Josef Ulm A1 - Emanuela Del Gado A1 - Roland Jean-Marc Pellenq ED - Hellmich, Christian ED - Pichler, Bernhard ED - Kollegger, Johann AB -Despite their impact on longevity, serviceability, and environmental footprint of our built infrastructure, the chemo-physical origins of nanoscale properties of cementitious materials, and their link to macroscale properties still remain rather obscure. Here, we discuss a multi-scale approach that describes different aspects of physical properties of C-S-H at the nano- and meso-scales. These include dynamics of water, thermal properties and mechanical behavior of C-S-H and its effect on properties of cement paste at different scales.

JF - 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete StructuresCONCREEP 10 PB - American Society of Civil Engineers CY - September 21–23, 2015, Vienna, AustriaReston, VA VL - CONCREEP 10: MECHANICS AND PHYSICS OF CREEP, SHRINKAGE, AND DURABILITY OF CONCRETE AND CONCRETE STRUCTURES UR - http://ascelibrary.org/doi/book/10.1061/9780784479346http://ascelibrary.org/doi/pdf/10.1061/9780784479346http://ascelibrary.org/doi/10.1061/9780784479346.006http://ascelibrary.org/doi/pdf/10.1061/9780784479346.006 ER - TY - JOUR T1 - Casimir-Polder interaction for gently curved surfaces JF - Physical Review D Y1 - 2014 A1 - Bimonte, Giuseppe A1 - Emig, Thorsten A1 - Kardar, Mehran AB -We use a derivative expansion for gently curved surfaces to compute the leading and the next-to-leading curvature corrections to the Casimir-Polder interaction between a polarizable small particle and a nonplanar surface. While our methods apply to any homogeneous and isotropic surface, explicit results are presented here for perfect conductors. We show that the derivative expansion of the Casimir-Polder potential follows from a resummation of its perturbative series, for small in-plane momenta. We consider the retarded, nonretarded and classical high-temperature limits.

VL - 90 ER - TY - JOUR T1 - Chemoelastic Fracture Mechanics Model for Cement Sheath Integrity JF - Journal of Engineering Mechanics Y1 - 2014 A1 - Ardakani, Sina Moeini A1 - Franz-Josef Ulm AB -A linear elastic fracture mechanics (LEFM) model for the engineering fracture design of cement sheath integrity at early ages in oil and gas well applications is proposed. The model considers the specific worst-case scenario of a single radial crack and estimates the energy release rate (stress intensity factor) that potentially develops in a cement sheath as a result of the buildup of eigenstresses at early ages, considering the loss of axisymmetry. Specifically, the model involves a two-step solution procedure: a chemoelastic stress solver and a LEFM solver using an Airy-stress function approach together with the method of complex variables. For the first, an appropriate constitutive model for cement slurries at early ages is required as a backbone for chemoelastic stress development. Second, the LEFM solver uses the stresses to estimate the energy release rate and the stress intensity, and thus provide a means to evaluate the driving force of fracture propagation and the fracture risk as a function of the degree of hydration. The functional relationships thus established between the LEFM quantities and the degree of the chemical reaction are expected to become an indispensable tool in safely designing well cements for operation under extreme bore hole conditions.

VL - 140 IS - 4 JO - J. Eng. Mech. ER - TY - JOUR T1 - Combinatorial molecular optimization of cement hydrates JF - Nat Commun Y1 - 2014 A1 - Mohammad Javad Abdolhosseini Qomi A1 - Konrad J. Krakowiak A1 - Mathieu Bauchy A1 - Stewart, K.L. A1 - Rouzbeh Shahsavari A1 - Jagannathan, D. A1 - Brommer, D.B. A1 - Alain Baronnet A1 - Markus J Buehler A1 - Sidney Yip A1 - Franz-Josef Ulm A1 - Krystyn J. Van Vliet A1 - Roland Jean-Marc Pellenq AB -Despite its ubiquitous presence in the built environment, concreteâ€$(1s (Bmolecular-level properties are only recently being explored using experimental and simulation studies. Increasing societal concerns about concreteâ€$(1s (Benvironmental footprint have provided strong motivation to develop new concrete with greater specific stiffness or strength (for structures with less material). Herein, a combinatorial approach is described to optimize properties of cement hydrates. The method entails screening a computationally generated database of atomic structures of calcium-silicate-hydrate, the binding phase of concrete, against a set of three defect attributes: calcium-to-silicon ratio as compositional index and two correlation distances describing medium-range silicon-oxygen and calcium-oxygen environments. Although structural and mechanical properties correlate well with calcium-to-silicon ratio, the cross-correlation between all three defect attributes reveals an indentation modulus-to-hardness ratio extremum, analogous to identifying optimum network connectivity in glass rheology. We also comment on implications of the present findings for a novel route to optimize the nanoscale mechanical properties of cement hydrate.

VL - 5 ER - TY - JOUR T1 - Controlling local packing and growth in calcium-silicate-hydrate gels JF - Soft Matter Y1 - 2014 A1 - Katerina Ioannidou A1 - Roland Jean-Marc Pellenq A1 - Emanuela Del Gado AB -We investigate the development of gels under out-of-equilibrium conditions, such as calcium–silicate–hydrate (C–S–H) gels that form during cement hydration and are the major factor responsible for cement mechanical strength. We propose a new model and numerical approach to follow the gel formation upon precipitation and aggregation of nano-scale colloidal hydrates, whose effective interactions are consistent with forces measured in experiments at fixed lime concentrations. We use Grand Canonical Monte Carlo to mimic precipitation events during Molecular Dynamics simulations, with their rate corresponding to the hydrate production rate set by the chemical environment. Our results display hydrate precipitation curves that indeed reproduce the acceleration and deceleration regime typically observed in experiments and we are able to correctly capture the effect of lime concentration on the hydration kinetics and the gel morphology. Our analysis of the evolution of the gel morphology indicates that the acceleration is related to the formation of an optimal local crystalline packing that allows for large, elongated aggregates to grow and that is controlled by the underlying thermodynamics. The defects produced during precipitation favor branching and gelation that end up controlling the deceleration. The effects on the mechanical properties of C–S–H gels are also discussed.

VL - 10 IS - 8 ER - TY - Generic T1 - Capillary states of granular materials in the funicular state T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Jean-Yves Delenne A1 - Vincent Richefeu A1 - Farhang Radjaï ED - Yu, A ED - Dong, K ED - Yang, R AB -

Using a multi-phase lattice Boltzmann model, we investigate the capillary states of a 2D granular packing gradually saturated by condensation from a homogeneously injected vapor phase. The internal stresses induced by surface tension and Laplace pressure are directly calculated from the forces acting on the grains with increasing amount of liquid. The evolution of cohesive strength with the amount of liquid reveals four different states reflecting the connectivity of the liquid phase and local grain environments. It increases in the pendular state, characterized by binary liquid bridges holding the grains together, and within the funicular state with an increasing number of liquid clusters connected to several grains. Beyond 40% of saturation, the cohesive strength falls off due to a decreasing Laplace pressure of liquid clusters.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - JOUR T1 - Cohesive granular materials composed of nonconvex particles JF - Physical Review E Y1 - 2013 A1 - Baptiste Saint-Cyr A1 - Farhang Radjaï A1 - Jean-Yves Delenne A1 - Philippe Sornay AB -The macroscopic cohesion of granular materials made up of sticky particles depends on the particle shapes. We address this issue by performing contact dynamics simulations of 2D packings of nonconvex aggregates. We find that the macroscopic cohesion is strongly dependent on the strain and stress inhomogeneities developing inside the material. The largest cohesion is obtained for nearly homogeneous deformation at the beginning of unconfined axial compression and it evolves linearly with nonconvexity. Interestingly, the aggregates in a sheared packing tend to form more contacts with fewer neighboring aggregates as the degree of nonconvexity increases. We also find that shearing leads either to an isotropic distribution of tensile contacts or to the same privileged direction as that of compressive contacts.

VL - 87 IS - 5 JO - Phys. Rev. E ER - TY - Generic T1 - Comparison of the effects of rolling resistance and angularity in sheared granular media T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Estrada, Nicolas A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Taboada, Alfredo ED - Yu, A ED - Dong, K ED - Yang, R KW - angularity KW - anisotropy KW - force distribution KW - Granular material KW - rolling resistance KW - Shear strength KW - solid fraction AB -In this paper, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. For this purpose, we use contact dynamics simulations. By means of a simple shear numerical device, we investigate the mechanical behavior of these materials in the steady state in terms of shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. We find that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 UR - https://hal.archives-ouvertes.fr/hal-00842799 ER - TY - JOUR T1 - Collapse Dynamics and Runout of Dense Granular Materials in a Fluid JF - Physical Review Letters Y1 - 2012 A1 - Vincent Topin A1 - Yann Monerie A1 - Perales, F. A1 - Farhang Radjaï VL - 109 IS - 18 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Comparison of computational water models for simulation of calcium-silicate-hydrate JF - Computational Materials Science Y1 - 2012 A1 - Qing Ji A1 - Roland Jean-Marc Pellenq A1 - Krystyn J. Van Vliet KW - C-S-H KW - Cement KW - Simulation KW - Water model AB -Calcium silicate hydrate, or C-S-H, is the chief hydration product of Portland cement. The structure of the C-S-H phase within cement has been proposed and developed via molecular simulations. In such simulations, empirical interatomic potentials for water molecules within C-S-H are adopted to govern the position and relative motion of this key constituent. Initial simulations and force fields of C-S-H have assumed the simplest molecular model of H2O termed ``single point charge{''} or SPC, but this choice has not been validated by comparison with other computational models of water that confer additional bond flexibility or charge distribution. To enable efficiently computational modeling of C-S-H and to explore the role that H2O plays in maintaining C-S-H structure and properties, the choice of an efficient and accurate water model is critical. Here, we consider five distinct, classical atomistic water models (SPC, TIP3P, TIP4P, TIP4P05, and TIP5P) to determine the effects of these computational simplifications on C-S-H properties. Quantitative comparison of all five water models shows that the appropriate water model depends on the C-S-H characteristics of interest. Among these models, both SPC and TIP5P models successfully predict key properties of the structure and elastic constants of C-S-H, as well as the dynamics of water molecules within C-S-H. (C) 2011 Elsevier B. V. All rights reserved.

VL - 53 IS - 1 ER - TY - JOUR T1 - Concrete Innovation Potential: From Atoms to Green Infrastructure JF - Beton- UND Stahlbetonbau Y1 - 2012 A1 - Franz-Josef Ulm AB -Concrete Innovation Potential: From Atoms to Green Infrastructure The sustainable development of concrete opens new possibilities for concrete engineers. This contribution aims at identifying the innovation potential of concrete for a sustainable development. It is shown that breakthroughs in Materials Science of concrete release new degrees of freedom for design and construction of concrete infrastructure that can contribute to reducing the material's ecological footprint. Integrated in a Life-Cycle Analysis of concrete structures, which considers both the embodied energy and the use phase, acomprehensive picture is obtained how molecular and nanometric modifications can contribute to a sustainable development of concrete. Given the more than 20 million tons of concrete produced annually, it is readily understood that new intellectual challenges are opening for concrete engineers to design buildings not only for function and safety but as well for ecological soundness.

VL - 107 ER - TY - JOUR T1 - Confined water dissociation in microporous defective silicates: Mechanism, dipole distribution, and impact on substrate properties JF - Journal of the American Chemical Society Y1 - 2012 A1 - Hegoi Manzano A1 - Moeini, Sina A1 - Marinelli, Francis A1 - Adri CT Van Duin A1 - Franz-Josef Ulm A1 - Roland Jean-Marc Pellenq AB -Interest in microporous materials has risen in recent years, as they offer a confined environment that is optimal to enhance chemical reactions. Calcium silicate hydrate (C-S-H) gel, the main component of cement, presents a layered structure with sub-nanometer-size disordered pores filled with water and cations. The size of the pores and the hydrophilicity of the environment make C-S-H gel an excellent system to study the possibility of confined water reactions. To investigate it, we have performed molecular dynamics simulations using the ReaxFF force field. The results show that water does dissociate to form hydroxyl groups. We have analyzed the water dissociation mechanism, as well as the changes in the structure and water affinity of the C-S-H matrix and water polarization, comparing the results with the behavior of water in a defective zeolite. Finally, we establish a relationship between water dissociation in C-S-H gel and the increase of hardness due to a transformation from a two- to a three-dimensional structure.

VL - 134 IS - 4 ER - TY - Generic T1 - Compressive strength of an unsaturated granular material during cementation T2 - Symposium on Science and Technology of Powders and Sintered Materials (STPMF 2009) Y1 - 2011 A1 - Jean-Yves Delenne A1 - Soulié, Fabien A1 - Moulay Saïd El Youssoufi A1 - Farhang Radjaï AB -The cohesive behaviour of unsaturated granular materials is due to the presence of cohesive bonds between grains. These bonds can have various physico-chemical characteristics and may evolve with environmental conditions. We study the case of a granular material partially saturated by an aqueous solution. The bonds are thus initially of capillary type and the mechanical strength is weak. At low relative humidity, the phase change of water involves crystallization of the solute at the contact points between grains, generating thus solid bonds. The mechanical strength of the material is then enhanced. An experimental study of the evolution of the mechanical strength during crystallization of the solute shows clearly the transition from capillary regime to cemented regime. This transition is not correlated with the mass of the crystallized solute, but rather with the residual degree of saturation. This behavior is analyzed here in the light of discrete element simulations. We introduce a local cohesion law that accounts for transition from capillary to cemented bonding. This law is formulated in terms of the degree of crystallization as a result of the evaporation of water at the boundary of the sample. The cohesion of the packing is initially of capillary type. A crystallization front then spreads from the sample boundaries to the center of the sample, and the strength increases as a result. Uniaxial compression allows us to determine the strength at different times. The numerical strength agrees well with the experimental data, and reveals strength enhancement as the solute crystallizes, as well as the transition from capillary to cementation regime.

JF - Symposium on Science and Technology of Powders and Sintered Materials (STPMF 2009) CY - MAY 25-27 2009 Montpellier, FRANCE VL - POWDER TECHNOLOGY JO - Powder Technology ER - TY - Generic T1 - Creep behaviour of confined layers of polyhedral grains T2 - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) Y1 - 2011 A1 - Quezada, Juan Carlos A1 - Farhang Radjaï A1 - Breul, Pierre A1 - Saussine, Gilles ED - Onate, E ED - Owen, DRJ AB -By means of contact dynamics simulations, we investigate the creep deformation of a thin granular layer composed of irregular polyhedral particles under the action of a constant vertical overload applied on a horizontal wall on top of the layer. We show that the total deformation induced by the overload increases with the ratio between the vertical and confining horizontal stresses and the aspect ratio of the sample. The effect of the aspect ratio is a consequence of the mobilized wall-grain friction forces at the top and bottom boundaries, that lead to enhanced strength by stabilizing strong force chains at the center of the sample. We also evidence the influence of loading history due to strain-induced fabric change or inertial effects resulting from the instant application of the overload. The topology of the contact network evolves in close correlation with creep. In particular, the face/face contacts between polyhedral particles concentrate largest force chains and their number is an increasing function of creep. A crucial feature of a confined granular system is the statistical variability of the mechanical response that we analyzed for creep deformations by performing a large number of simulations for independent initial configurations. Our data indicate that the distribution of fluctuations with respect to the mean creep falls off exponentially.

JF - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) CY - OCT 26-28 2011 Barcelona, SPAIN VL - PARTICLE-BASED METHODS II: FUNDAMENTALS AND APPLICATIONS UR - https://hal.archives-ouvertes.fr/hal-00761094 ER -