210 research outputs found
Nano-Engineering of Concrete
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 cement-based 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, high density and ultra-high density. These three forms differ merely in the packing density of 5-nm 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. This approach will turn out invaluable in our quest of adding the necessary âgreenâ value to a commodity, concrete, by nano-engineering higher strength and toughness from first principles.Lafarge Corporatio
Creep of Bulk C--S--H: Insights from Molecular Dynamics Simulations
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
Early-Age Stress and Pressure Developments in a Wellbore Cement Liner: Application to Eccentric Geometries
his paper introduces a predictive model for the stress and pressure evolutions in a wellbore cement liner at early ages. A pressure state equation is derived that observes the coupling of the elastic changes of the solid matrix, the eigenstress developments in the solid and porespaces, and the mass consumption of water in course of the reaction. Here, the transient constitution of the solid volume necessitates advancing the mechanical state of the poroelastic cement skeleton incrementally and at constant hydration degree. Next, analytic function theory is employed to assess the localization of stresses along the steelâcement (SC) and rockâcement (RC) interfaces by placing the casing eccentrically with respect to the wellbore hole. Though the energy release rate due to complete debonding of either interface is only marginally influenced by the eccentricity, the risk of evolving a microcrack along the thick portion of the sheath is substantially increased. Additionally, it is observed that the risk of microannulus formation is principally affected by the pressure rebound, which is engendered by the slowing reaction rate and amplified for rock boundaries with low permeability.National Science Foundation (U.S.). Graduate Research Fellowship ProgramSchlumberger Limite
Rate-dependent toughness in soft materials via microscopic scratch testing
Although fracture processes are rate-sensitive in a wide variety of biological and engineering systems, such as protein materials and bulk metallic glasses, the exact contribution of the prescribed mechanical loading rate to the measured fracture resistance is not fully understood. In this study, we formulate a novel energy-based framework for crack propagation in nonlinear viscous solids, using microscopic scratch tests. The scratch test consists in pushing a tool across the surface of a weaker material at a given penetration depth, and is relevant to several fields of science and engineering ranging from quality control of thin films and coatings to fracture characterization of cementitious materials. A hybrid experimental and theoretical study on amorphous and semicrystalline polymers shows that the apparent fracture toughness increases with the prescribed scratching speed up to an asymptotic value that is independent on the prescribed loading rate. Nonlinear viscoelastic fracture mechanics reveals that, because of the bulk viscous dissipation, the crack propagation processes can inhibited or delayed, resulting in a coupling between the intrinsic fracture energy and the material viscoelastic properties. Moreover, by combining indentation and scratch tests to decouple creep and fracture, it becomes possible to represent with a single master curve the evolution of the apparent fracture toughness for three loading rates, 0, 45, and 90 N/min, and for scratching speeds ranging from 0 to 20 mm/min. Overall, by considering a dual dissipation mode, viscous and fracture dissipation, we can capture the scaling of the scratch forces over a wide range of loading rates and scratching speeds in order to assess the intrinsic rate-independent and geometry independent fracture toughness of the tested material. Given the scalability of scratch tests, this new development open new venues for the characterization of the fracture toughness of soft materials at the microscopic scale
FlĂŒggeâs Conjecture: Dissipation- versus Deflection-Induced PavementâVehicle Interactions
The dissipation occurring below a moving tire in steady-state conditions in contact with a viscoelastic pavement is expressed using two different reference frames: a fixed observer attached to the pavement and a moving observer attached to the pavementâtire contact surface. The first approach is commonly referred to as dissipation-induced pavementâvehicle interaction (PVI), the second as deflection-induced PVI. Based on the principle of frame independence, it is shown that both approaches are strictly equal, from a thermodynamic point of view, and thus predict the same amount of dissipated energy. This equivalence is illustrated through application to two pavement systems: a viscoelastic beam and a viscoelastic plate both resting on an elastic foundation. The amount of dissipated energy in the pavement structure needs to be supplied by the vehicle to maintain constant speed, thus contributing to the rolling resistance, associated excess fuel consumption, and greenhouse gas emissions. The model here proposed can be used to quantify the dissipated energy and contribute to the development of engineering methods for the sustainable design of pavements.Portland Cement AssociationReady Mixed Concrete Research & Education Foundatio
Roughness-Induced Vehicle Energy Dissipation: Statistical Analysis and Scaling
The energy dissipated in a vehicle suspension system due to road roughness affects rolling resistance and the resulting fuel consumption and greenhouse gas emission. The key parameters driving this dissipation mechanism are identified via dimensional analysis. A mechanistic model is proposed that relates vehicle dynamic properties and road roughness statistics to vehicle dissipated energy and thus fuel consumption. A scaling relationship between the dissipated energy and the most commonly used road roughness index, the International Roughness Index (IRI), is also established. It is shown that the dissipated energy scales with IRI squared and scaling of dissipation with vehicle speed V depends on road waviness number w in the form of V[superscript wâ2]. The effect of marginal probability distribution of the road roughness profile on dissipated energy is examined. It is shown that although the marginal distribution of the road profile does not affect the identified scaling relationships, the multiplicative factor in these relationships does change from one distribution to another. As an example of practical application, the model is calibrated with the empirical HDM-4 model for different vehicle classes
Multiscale Poromechanics of Wet Cement Paste
Capillary effects such as imbibition-drying cycles impact the mechanics of
granular systems over time. A multiscale poromechanics framework was applied to
cement paste, that is the most common building material, experiencing broad
humidity variations over the lifetime of infrastructure. First, the liquid
density distribution at intermediate to high relative humidities is obtained
using a lattice gas density functional method together with a realistic
nano-granular model of cement hydrates. The calculated adsorption/desorption
isotherms and pore size distributions are discussed and compare well to
nitrogen and water experiments. The standard method for pore size distribution
determination from desorption data is evaluated. Then, the integration of the
Korteweg liquid stress field around each cement hydrate particle provided the
capillary forces at the nanoscale. The cement mesoscale structure was relaxed
under the action of the capillary forces. Local irreversible deformations of
the cement nano-grains assembly were identified due to liquid-solid
interactions. The spatial correlations of the nonaffine displacements extend to
a few tens of nm. Finally, the Love-Weber method provided the homogenized
liquid stress at the micronscale. The homogenization length coincided with the
spatial correlation length nonaffine displacements. Our results on the solid
response to capillary stress field suggest that the micronscale texture is not
affected by mild drying, while local irreversible deformations still occur.
These results pave the way towards understanding capillary phenomena induced
stresses in heterogeneous porous media ranging from construction materials,
hydrogels to living systems.Comment: 6 figures in main text, 4 figures in the SI appendi
Unravelling CSH atomic structure via computational and experimental physical chemistry
Calcium Silicate Hydrate (CSH) is the main binding phase for the cement paste, which is responsible for its strength and creep behavior. This is a nonstoichiometric hydration phase with calcium to silicon ratio (C/S) ranging from 1 to 2.2. At low C/S ratios, the molecular structure of CSH resembles to that of Tobermorite minerals, whereas in high C/S ratios it mostly looks like disordered glasses. By taking advantage of tools of statistical physics, it is shown that CSH at a given C/S can be associated with degenerate molecular structures called CSH polymorphs. Polymorphs are energetically competitive, i.e., they have the same free energy content, which means they can coexist under equilibrium conditions. To start, SiO2 groups are randomly removed from the layered atomic structure Tobermorite 11A. One hundred and fifty structures are created. Grand Canonical Monte Carlo simulation of water adsorption is performed to adsorb water in the interlayer spacing and nanoscale porosities in defected CSH structures. The amount of adsorbed water scales linearly with the number of defects in the calciumâsilicate layer. Samples are relaxed using a reactive potential in canonical and isothermalâisobaric ensembles. We observe that the confined water reacts with the free interlayer calcium atoms and nonbridging oxygen to form hydroxyl groups. The number of hydroxyl groups scales linearly with the amount of defects. The amount of water in CSH and CaâOH content match well with drying and Neutron Scattering experiment. Although the reactive modeling of CSH impacts the water molecules in CSHâs nanoconfinement environment, it does not significantly affect the silica chain length. This means that the reactive atomistic modeling does not affect the calico-silicate backbone of CSH structures. The silica mean chain length from atomistic simulation aligns perfectly with experimental NMR data. The elastic properties and hardness of all CSH polymorphs are measured at a given C/S and are directly compared with nano-chemo-mechanical testing via coupled nanoindentation and X-ray WDS. Atomistic simulation matches with the experimental data in both elastic and plastic regimes. The correlation of mechanical properties to structural observables of the molecular structures such as dimer content, mean silicate chain length, density, basal distance, water content, number of hydroxyl groups, and topological constraints parameter are calculated. No direct correlations were found at short ranges. The search was extended to the medium range order analysis and it is found that the polymorphism is closely related to the medium range order of SiâO bonds
Use of UHPC in Bridge Structures: Material Modeling and Design
Ultra-high-performance concrete (UHPC) is a promising new class of concrete material that is likely to make a significant contribution to addressing the challenges associated with the load capacity, durability, sustainability, economy, and environmental impact of concrete bridge infrastructures. This paper focuses on the material modeling of UHPC and design of bridge girders made of UHPC. A two-phase model used for modeling the behavior of UHPC was briefly discussed, and the model was implemented in a preliminary design case study. Based on the implemented design and the reported use of UHPC in bridge applications, the advantages, limitations, and future prospects of UHPC bridges were discussed, highlighting the need for innovative research and design to make optimum use of the favorable properties of the material in bridge structures
Physical Origins of Thermal Properties of Cement Paste
Despite the ever-increasing interest in multiscale porous materials, the chemophysical origin of their thermal properties at the nanoscale and its connection to the macroscale properties still remain rather obscure. In this paper, we link the atomic- and macroscopic-level thermal properties by combining tools of statistical physics and mean-field homogenization theory. We begin with analyzing the vibrational density of states of several calcium-silicate materials in the cement paste. Unlike crystalline phases, we indicate that calcium silicate hydrates (CSH) exhibit extra vibrational states at low frequencies (<2ââTHz) compared to the vibrational states predicted by the Debye model. This anomaly is commonly referred to as the boson peak in glass physics. In addition, the specific-heat capacity of CSH in both dry and saturated states scales linearly with the calcium-to-silicon ratio. We show that the nanoscale-confining environment of CSH decreases the apparent heat capacity of water by a factor of 4. Furthermore, full thermal conductivity tensors for all phases are calculated via the Green-Kubo formalism. We estimate the mean free path of phonons in calcium silicates to be on the order of interatomic bonds. This satisfies the scale separability condition and justifies the use of mean-field homogenization theories for upscaling purposes. Upscaling schemes yield a good estimate of the macroscopic specific-heat capacity and thermal conductivity of cement paste during the hydration process, independent of fitting parameters.Portland Cement AssociationNational Ready Mixed Concrete Association (Research and Education Foundation
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