Stiff Aggregates Research Articles

Molecular Dynamics Simulations of Different Nanoparticles at Substrates.

We report the results of large-scale molecular dynamics simulations of adsorption nanoparticles on solid surfaces. The particles were modeled as stiff aggregates of spherical segments. Three types of particles were studied: rods, rectangles, and triangles built of the same number of segments. We show how the particle shape affects the adsorption, the structure of the surface layer, and the degree of the removal of particles from the solvent. The systems with different segment-segment and segment-surface interactions and different concentrations of particles were investigated. The ordered structures formed in adsorption monolayers were also analyzed. The results are consistent with experimental observations.

Colorless, transparent, and healable silicone elastomers by introducing Zn(II)–carboxylate interactions via aza-Michael reaction

It is a challenge to fabricate smart materials with excellent optical, mechanical, and healable properties simultaneously. Herein, colorless, transparent, and healable polydimethylsiloxane (PDMS) elastomers were successfully prepared by introducing Zn(II)–carboxylate interactions via aza-Michael reaction of amino polysiloxane and zinc acrylate. The mechanical properties of these polymers can be easily tuned in a wide range between stretchability and rigidity by tailoring the ion content simply. The sample with medium ion content has the best integrated mechanical properties (breaking strain: ~ 1000%; breaking stress: ~ 3 MPa), which can also be recovered with high efficiency (~ 90%) after healed or reprocessed at mild temperature (80–100 °C). Furthermore, based on the results of stress relaxation and scanning electron microscopy, the excellent mechanical properties are considered to be due to the gradient ionic aggregates, as the small and weak ionic aggregates can preferentially sacrifice to dissipate energy under external loading, while the large and stiff aggregates impart the elastomers with high strength and elasticity. Finally, due to the colorless appearance, good transparency, healing ability, and adhesive strength for glass substrates, these ionic PDMS elastomers can also be applied as reusable adhesives in optical devices.

N-Phase micromechanical framework for the conductivity and elastic modulus of particulate composites: Design to microencapsulated phase change materials (MPCMs)-cementitious composites

The smart design of microencapsulated phase change materials (MPCMs) in cementitious composites requires an explicit understanding of effects of soft microcapsule particles, stiff aggregates and their surrounding weak interfaces on the physico-mechanical properties of particulate composites. This paper devises a n-phase micromechanical framework to predict the effective thermal conductivity and elastic modulus of multicomponent particulate composites that consist in stiff and soft anisotropic-shaped inclusions, their surrounding weak interfaces and matrix. In this micromechanical model, the volume fraction of weak interfaces treated as the interphase model is quantified and incorporated into the n-phase differential effective medium model. It is found that the structural configuration of interfaces has a significant effect on the effective physico-mechanical properties of particulate composites. The micromechanical model leads to predictions of the effective conductivity and elastic modulus of multicomponent particulate composites to a good accuracy by comparing with available experimental data for regular concrete, quartz mortar and MPCMs-cementitious composites. By utilizing the micromechanical model, the authors further develop a theoretical design rule for the robust overall performance of MPCMs-cementitious composites with the better thermal resistance and elastic modulus. These results can also be used to design other multiphase particulate composites and porous media with the cherry-pit structure.

3‐D mesoscale simulation of crack‐permeability coupling in the Brazilian splitting test

SummaryIn this paper, mesoscale hydromechanical simulations are performed to study (1) fracture features and (2) crack‐gas permeability coupling evolution in the context of the tensile splitting test. The mesostructure is based on a 2‐phase 3‐D representation of heterogeneous materials, such as concrete, where stiff aggregates are embedded into a mortar matrix. To take into account these heterogeneities without any mesh adaptation, a weak discontinuity is introduced into the strain field. In addition, a strong discontinuity is also added to take into account microcracking. This mechanical model is cast into the framework of the enhanced finite element method. Concerning the coupling with gas permeability, a double‐porosity method is used to simulate the flow through the cracks and the porosity. The apparent gas permeability is afterwards evaluated by a homogenization method. On the basis of finite element simulations, influence of aggregate size on ultimate crack opening, macroscopic ultimate tensile stress, total dissipated energy, and gas permeability evolution is numerically investigated. Furthermore, gas permeability evolution is also compared with experimental results from the literature. In addition, in the spirit of a sequential multiscale approach, macroscale gas permeability equations are identified from the hydromechanical results coming from the mesoscale computations. These equations lead to a relation between macroscale gas permeability evolution and crack opening. Besides, we show how the aggregate size influences the percolation threshold and that after this threshold, a cubic relation between macroscale gas permeability and crack opening is obtained.

FE design for the numerical modelling of failure induced by differential straining in meso-scale concrete: Algorithmic implementation based on operator split method

The present paper focuses on the design of FE for the numerical modelling of failure induced by differential straining in concrete at the meso-scale. The differential straining is here considered as a deformation field added to the mechanical strain field and coming from the solution of a mass transport problem for instance. This differential straining can be induced by changes in degree of saturation, drying shrinkage or fluid overpressure for example. The meso-structure is based on a two-phase 3D representation of concrete, where stiff aggregates are embedded into a mortar matrix. In order to explicitly take into account these aggregates without any mesh adaptation, a weak discontinuity is introduced into the strain field. In addition, a strong discontinuity is also added to take into account cracking. The FE implementation is performed within the Enhanced Finite Element Method (E-FEM). An operator split method equipped with a return mapping algorithm accounting for differential straining is implemented for the computation of the discontinuities values, treated as local variables. In regards to the operator split method proposed in this paper, a broad range of problems can be solved such as basic mass transport problems or failure problem induced both by mechanical loading and differential straining. This feature is an advantage for the design of FE for durability issues. Finally 1D simulations are presented to assess the FE implementation and the performance of the numerical method. We also demonstrate the capability of the method for solving and analyzing 3D mesoscale problems.

Embedded finite element formulation for the modeling of chloride diffusion accounting for chloride binding in meso-scale concrete

This paper presents the FE formulation for the numerical modelling of chloride ions diffusion accounting for the chloride binding capacity in meso-scale concrete. The finite element formulation is built through a discrete (lattice) finite element model and the meso-structure is based on a two-phase 3D representation of heterogeneous materials, such as concrete, where stiff aggregates are embedded into a mortar matrix. In order to take into account these heterogeneities in the mesh, we turn to the Embedded Finite Element Method (E-FEM). The E-FEM allows to use meshes not necessarily matching the physical interface (aggregates/mortar matrix) while retaining the accuracy of the classical finite element approach. This is achieved by introducing a weak discontinuity in the chloride concentration field for finite elements where the physical interface is present. A numerical solving strategy is presented to efficiently resolve the FE problem both in terms of chloride concentration field variables and weak discontinuity parameters. We investigate through 1D simulations the influence of the microstructure properties and the chloride binding capacity on the chloride concentration profiles. We show that taking explicitly into account the diffusion coefficients of the microstructure and the chloride binding capacity are important to consider for a better prediction of the time span until corrosion initiation. This is important for accurately estimating the service life of a structure. Numerical homogenization experiments are also performed on 3D meso-structures to compute macroscopic diffusivity tensors accounting for two-phase material. Comparison with Maxwell's equation are performed to show the accuracy of the proposed numerical approach.

A meso-macro numerical approach for crack-induced diffusivity evolution in concrete

In this paper, the authors present a meso-macro numerical approach in order to determine macroscopic diffusivity tensors in heterogeneous quasi-brittle materials such as concrete. The meso-structure is based on a two-phase 3D representation of heterogeneous materials with stiff aggregates embedded in a mortar matrix. In order to take into account these heterogeneities without any mesh adaptation, a weak discontinuity is introduced into the strain field of those finite elements containing more than a single material. In addition, a strong discontinuity is also added to take into account the crack formation and evolution. The mesoscale coupling with the mass transport part is based on Fick’s Law with a modified diffusion coefficient accounting for crack opening and aggregates. Then, by means of an homogenization procedure some macro values for the macroscopic diffusivity tensor are computed. The main original contribution of this paper is that the macroscopic diffusivity tensor integrates more complex features. Such features coming as a by-product results of the meso-macro analysis such as the cracking process – including evolution from diffuse cracks in the bulk to localized macro-crack(s) –, tortuosity of the crack’s path, induced-anisotropy and presence of aggregates. The defined tensor is used afterwards in order to estimate the service-life of concrete structures, including the effect of the cracking and the internal meso structure.

Influence of the elastic mismatch on crack propagation in a silicate-based composite

Fracture behaviour (crack deflection) of a crack in a silicate-based composite is investigated. The three-point bending test is simulated numerically by means of the finite element method and the influence of existence of a stiff aggregate ahead of the crack tip is studied and discussed. The maximum tangential stress criterion is applied in order to calculate the initial crack propagation angle. Various geometrical configurations are investigated (the aggregate eccentricity and depth are varied) and the generalized MTS criterion based on the Williams expansion is tested considering various numbers of initial terms of the series. Unfortunately, the results show that using the generalized MTS criterion is strongly limited by the elastic mismatch caused by the presence of the stiff aggregate near the crack tip.

Meso-scale investigation of failure in the tensile splitting test: Size effect and fracture energy analysis

In this paper, a meso-scale analysis is performed (1) to study the size effect on the nominal stress at failure and, (2) to quantify the evolution of the fracture process zone (FPZ) in the context of the tensile splitting test. The meso-structure is based on a two-phase 3D representation of heterogeneous materials, such as concrete, where stiff aggregates are embedded into a mortar matrix. In order to take into account these heterogeneities without any mesh adaptation, a weak discontinuity is introduced into the strain field. In addition, a strong discontinuity is also added to take into account micro-cracking. This model is cast into the framework of the Enhanced Finite Element Method (E-FEM). Based on the Finite Element simulations, size effect on the nominal stress at failure is numerically investigated and then compared to the so-called Bažant size effect law. In addition, an analysis based on the spatial distribution of the fracture energy is also regarded, leading to the 3D representation of the FPZ and to its volume value estimation.

Influence of Aggregate Stiffness on Fracture-Mechanical Properties of Lime-Based Mortars

Addition of relatively stiff aggregates into lime-based mortars is responsible for an increase of the effective mortar stiffness and stress concentrations around aggregates during mechanical loading. To observe the damage development during the three-point bending and splitting tests a 2D plane-stress nonlinear finite element analysis utilizing isotropic damage model was carried out and the results were validated against experimentally obtained data. The study revealed that the finite element model is able to capture the trends observed during the experimental investigation. The results of the numerical modeling and experimental investigation show the advantages of the use of relatively compliant crushed brick aggregates in ancient structures.

Fracture-Micromechanics Based Model of Mortars Susceptible to Shrinkage

The addition of stiff aggregates into a relatively compliant matrix results, up to a certain limit, in an increase of the effective mortar stiffness. Conventional micromechanical models are not able to capture this limit and must be coupled with a fracture mechanics model predicting the formation of microcracks between aggregates due to shrinkage of matrix. The proposed model utilizes the Mori-Tanaka (MT) scheme together with an estimation of a crack density, based on requirements for the crack formation determined by a numerical analysis. As a result, the model involves a single phenomenological constant, and provides significant improvement over the basic MT scheme.

Influence of cold weather during casting and curing on the stiffness and strength of concrete

The supposition that casting and curing at near freezing temperatures leads to the loss of stiffness of the hardened concrete and causes microstructure disruption and abnormalities was investigated. The research programme reported here involved 25 concrete slabs, 750×750×300mm, which were subjected to various curing regimes in the range of 20°C to −5°C. The extent of loss of stiffness and the degree of microcracking were quantified using the Stiffness Damage Test (SDT). This test method has previously been shown by one of the authors to be very effective in quantifying the extent of disruption to the microstructure of concrete caused by various damage mechanisms. In addition to the SDT, the cold-cured concrete was evaluated by the full stress–strain test as well as ultrasonic, dynamic, and petrographic methods. Concreting at temperatures near freezing resulted in approximately 20% reduction of the 28day stiffness and a Damage Index of 5 as determined by the SDT. Cold weather curing resulted in the development of microcracks in the paste and adversely affected the interfacial zone. Storage of the constituents of concrete at near freezing conditions prior to mixing adversely affects the long term stiffness and strength of concrete. The water absorption of hardened concrete increased as a result of cold-curing. The loss of stiffness because of low temperature curing is not remedied by using stiff aggregate such as granite nor by PFA cement replacement.

Multi-scale modeling of fracture in concrete composites

We study non-linear fracture behavior of concrete using a concurrent multi-scale method. Therefore, a mesoscopic description is exploited for domains around the fracture process zone while a macroscopic description is used far away from the fracture process zone. The composite concrete on the mesoscopic scale consists of stiff aggregates and cement matrix. Linear elastic material behavior is considered for the aggregates while a cohesive crack approach is used for the cement matrix. The extended element-free Galerkin method is used to model fracture of the concrete composite. We consider both, interface models and rigid bond between the aggregate and the matrix material and show that the interface model has a significant influence on the results. The method is validated by comparison to experimental data for different failure modes.

Meso-scale modelling of the size effect on the fracture process zone of concrete

The size effect on the fracture process zone in notched and unnotched three point bending tests of concrete beams is analysed by a meso-scale approach. Concrete is modelled at the meso-scale as stiff aggregates embedded in a soft matrix separated by weak interfaces. The mechanical response of the three phases is modelled by a discrete lattice approach. The model parameters were chosen so that the global model response in the form of load-crack mouth opening displacement curves were in agreement with experimental results reported in the literature. The fracture process zone of concrete is determined numerically by evaluating the average of spatial distribution of dissipated energy densities of random meso-scale analyses. The influence of size and boundary conditions on the fracture process zone in concrete is investigated by comparing the results for beams of different sizes and boundary conditions.

Linear aggregation and liquid-crystalline order: comparison of Monte Carlo simulation and analytic theory

Many soft-matter and biophysical systems are composed of monomers that reversibly assemble into rod-like aggregates. The aggregates can then order into liquid-crystal phases if the density is high enough, and liquid-crystal ordering promotes increased growth of aggregates. Systems that display coupled aggregation and liquid-crystal ordering include wormlike micelles, chromonic liquid crystals, DNA and RNA, and protein polymers and fibrils. Coarse-grained molecular models that capture key features of coupled aggregation and liquid-crystal ordering common to many different systems are lacking; in particular, the roles of monomer aspect ratio and aggregate flexibility in controlling the phase behavior are not well understood. Here we study a minimal system of sticky cylinders using Monte Carlo simulations and analytic theory. Cylindrical monomers interact primarily by hard-core interactions but can stack and bind end to end. We present results for several different cylinder aspect ratios and a range of end-to-end binding energies. The phase diagrams are qualitatively similar to those of chromonic liquid crystals, with an isotropic-nematic-columnar triple point. The location of the triple point is sensitive to the monomer aspect ratio. We find that the aggregate persistence length varies with temperature in a way that is controlled by the interaction potential; this suggests that the form of the interaction potential affects the phase behavior of the system. Our analytic theory shows improvement compared to previous theory in quantitatively predicting the I–N transition for relatively stiff aggregates, but requires a better treatment of aggregate flexibility.

Meso-scale approach to modelling the fracture process zone of concrete subjected to uniaxial tension

A meso-scale analysis is performed to determine the fracture process zone of concrete subjected to uniaxial tension. The meso-structure of concrete is idealised as stiff aggregates embedded in a soft matrix and separated by weak interfaces. The mechanical response of the matrix, the inclusions and the interface between the matrix and the inclusions is modelled by a discrete lattice approach. The inelastic response of the lattice elements is described by a damage approach, which corresponds to a continuous reduction of the stiffness of the springs. The fracture process in uniaxial tension is approximated by an analysis of a two-dimensional cell with periodic boundary conditions. The spatial distribution of dissipated energy density at the meso-scale of concrete is determined. The size and shape of the deterministic FPZ is obtained as the average of random meso-scale analyses. Additionally, periodicity of the discretisation is prescribed to avoid influences of the boundaries of the periodic cell on fracture patterns. The results of these analyses are then used to calibrate an integral-type nonlocal model.

Phase behavior of a reversibly assembling system in two dimensions: A Monte Carlo simulation study

Using Monte Carlo simulation, we investigate the structural phase behavior of a continuum molecular model for self-assembling semiflexible equilibrium polymers in two dimensions. Particle-particle interaction is modeled via a Lennard-Jones potential with tunable anisotropic attraction. Depending on the strength of the anisotropy, we find the formation of reversible networks as well as stiff rodlike aggregates. The phase transition observed in the presence of the network structures is compared to predictions of the Tlusty-Safran defect model.

New Class of Aggregates in Aqueous Solution: An NMR, Thermodynamic, and Dynamic Light Scattering Study

We investigated the aggregation properties of two classes of aromatic and hydrophobic compounds, namely chloroacetamides and ethyl 3-phenyl-2-nitropropionates, in moderately concentrated aqueous solution (millimolar range). The identification of all species present in solution under specific experimental conditions was performed by 1D and 2D NMR, pulsed gradient spin-echo NMR, and dynamic light scattering techniques. Some physical-chemical properties (viscosity, surface tension, and colligative properties) of the aqueous solutions were also determined. Both classes of compounds behave quite similarly: in solution, three distinct species, namely a monomeric species, small and mobile aggregates, and large and stiff aggregates, are observed. The results give insight into a new class of aggregates, held together by pi-pi interactions, which show an unusual associative behavior in water.

Soil structure of a highly weathered old alluvium

The old alluvium in San Juan, Puerto Rico, is an early Pleistocene deposit that has undergone substantial post-depositional weathering in a tropical climate. Preliminary geotechnical laboratory testing found that the soil exhibits some unusual index and consolidation properties compared with stereotypical sedimentary soils. For instance, both the swelling index and the coefficient of consolidation are greatly affected by the maximum past stress applied to the soil. In order to understand its unusual behaviour, a variety of experiments were performed to characterise its composition and microstructure. Direct observations of the microstructure used environmental scanning electron microscopy, and indirect examinations included slaking, cation exchange capacity, selective chemical dissolution, particle size analyses and Atterberg limits. The results clearly indicate that the weathered deposit possesses a complex microstructure. The intact material consists of a stiff network of aggregates interconnected by Fe oxides. These relatively large and stiff aggregates actually comprise groups of clay platelets, which in turn contain kaolinite and smectite particles, arranged in a parallel configuration, that are coated and connected by Fe oxides. Based on the results, a conceptual microstructure model was developed that links the unusual macroscopic engineering properties to the particle-level phenomena.

Soil structure of a highly weathered old alluvium

The old alluvium in San Juan, Puerto Rico, is an early Pleistocene deposit that has undergone substantial post-depositional weathering in a tropical climate. Preliminary geotechnical laboratory testing found that the soil exhibits some unusual index and consolidation properties compared with stereotypical sedimentary soils. For instance, both the swelling index and the coefficient of consolidation are greatly affected by the maximum past stress applied to the soil. In order to understand its unusual behaviour, a variety of experiments were performed to characterise its composition and microstructure. Direct observations of the microstructure used environmental scanning electron microscopy, and indirect examinations included slaking, cation exchange capacity, selective chemical dissolution, particle size analyses and Atterberg limits. The results clearly indicate that the weathered deposit possesses a complex microstructure. The intact material consists of a stiff network of aggregates interconnected by Fe oxides. These relatively large and stiff aggregates actually comprise groups of clay platelets, which in turn contain kaolinite and smectite particles, arranged in a parallel configuration, that are coated and connected by Fe oxides. Based on the results, a conceptual microstructure model was developed that links the unusual macroscopic engineering properties to the particle-level phenomena.