Identification of viscoelastic C-S-H behavior in mature cement paste by FFT-based homogenization method

A powerful and robust numerical homogenization method based on fast Fourier transform (FFT) is formulated to identify the viscoelastic behavior of calcium silicate hydrates (C-S-H) in hardened cement paste from its heterogeneous composition. The identification is contingent upon the linearity of the creep law. To characterize cement paste microstructure, the model developed by Bentz at the National Institute of Standards and Technology, which has the resolution of 1 μm, is adopted. Model B3 for concrete creep is adapted to characterize the creep of C-S-H in cement paste. It is found that the adaptation requires increasing the exponent of power law asymptote of creep compliance. This modification means that the rate of attenuation of creep with time is lower in C-S-H than in cement paste, and is explained by differences in stress redistribution. In cement paste, the stress is gradually transferred from the creeping C-S-H to the non-creeping components. The viscoelastic properties of C-S-H at the resolution of 1 μm were identified from creep experiments on cement pastes 2 and 30 years old, having the water-cement ratio of 0.5. The irreversible part of C-S-H creep, obtained from these old specimens at almost saturated state, is found to be negligible unless the specimens undergo drying and resaturation prior to the creep test.     

Nanoindentation investigation of creep properties of calcium silicate hydrates

The creep properties of calcium silicate hydrates (C-S-H) are assessed by means of nanoindentation creep experiments on a wide range of substoichiometric cement pastes. We observe that, after a few seconds, the measured creep compliance of C-S-H is very well captured by a logarithmic time function. The rate of the logarithmic creep is found to scale in a unique manner with indentation modulus, indentation hardness, and packing density, independent of processing, mix proportions, indenter geometry and load history. The comparison with macroscopic creep experiments on concrete shows that minutes-long nanoindentations enable a quantitative assessment of the long-term creep properties of cementitious materials, orders of magnitude faster than macroscopic testing. Finally, we show that a strong analogy exists between this logarithmic creep behavior of C-S-H and that of soils, which suggests a granular origin of creep of geomaterials.     

Time,temperature, and strain effects on viscoelastic Poisson's ratio of epoxy resins

Poisson's ratio of polymeric materials, although generally assumed as a constant, is known to display a viscoelastic dependence on time, temperature, and strain. This article investigates the phenomenology of this dependence on two crosslinked epoxy systems with different glass transition temperatures. Poisson's ratio measurements are performed by contact extensometers simultaneously measuring the axial and transverse deformations under two different tensile testing conditions: (i) constant deformation rate, in which the effects of strain, strain rate, and temperature are highlighted; (ii) stress relaxation (or constant deformation), where the dependence of Poisson's ratio on time is studied at various strain levels. The viscoelastic Poisson's ratio increases as strain, temperature, and time increases, with trends markedly depending on the materials glass transition. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Study on the Viscoelastic Poisson‘s Ratio of Solid Propellants Using Digital Image Correlation Method

Digital image correlation methods were used for further studies of the viscoelastic Poisson's ratio of solid propellants. The Poisson's ratio and the Young's relaxation modulus of solid propellants were separately determined in a single stress relaxation test. In addition, the effects of temperature, longitudinal strain, preload and storage time on the Poisson's ratio of solid propellants were discussed. The Poisson's ratio master curve and the Young's relaxation modulus master curve were constructed based on the time‐temperature equivalence principle. The obtained results showed that the Poisson's ratio of solid propellants is a monotone non‐decreasing function of time, the instantaneous Poisson's ratio increased from 0.3899 to 0.4858 and the time of the equilibrium Poisson's ratio occurred late when the temperature was varied from −30 °C to 70 °C. The Poisson's ratio increased with temperature and longitudinal strain, decreased with preload and storage time, while the amplitude Poisson's ratio increased with preload, decreases with longitudinal strain and storage time. The time of the equilibrium Poisson's ratio occurred in advance with the increase of longitudinal strain, preload and storage time.     

On the complex poisson's ratio of a urethane rubber compound

The complex Poisson's ratio of a urethane rubber compound was determined for frequencies up to 700 cps. It is shown that the assumption made by earlier workers using this material, that Poisson's ratio is a numerical constant slightly less than 1/2, while approximately correct at low (creep) frequencies is definitely invalid in certain more elevated frequency bands.     

氧化石墨烯复合水泥基材料的分子动力学模拟

氧化石墨烯（GO）边缘的大量含氧官能团具有更好的反应活性，能够有效的增强水泥基材料的性能。本文为探究氧化石墨烯中各含氧官能团对水泥基材料性能的影响，建立了GO/C-S-H、GO羧基/C-S-H、GO羟基/C-S-H和GO环氧基/C-S-H模型。采用分子动力学的方法对复合材料的力学参数、界面吸附能、动力学特性进行研究。结果表明：GO的加入使复合材料的杨氏模量提高了10.7%，泊松比提高了12%。GO中的环氧基有助于复合材料界面的稳定性，羟基有助于复合材料内部原子间作用更紧密。含氧官能团与钙原子间的钙氧键是粘聚力的来源之一，钙氧键键长的顺序为Ca-OH相似文献   

Investigation of the effect of aggregates' morphology on concrete creep properties by numerical simulations

Prestress losses due to creep of concrete is a matter of interest for long-term operations of nuclear power plants containment buildings. Experimental studies by Granger (1995) have shown that concretes with similar formulations have different creep behaviors. The aim of this paper is to numerically investigate the effect of size distribution and shape of elastic inclusions on the long-term creep of concrete. Several microstructures with prescribed size distribution and spherical or polyhedral shape of inclusions are generated. By using the 3D numerical homogenization procedure for viscoelastic microstructures proposed by Šmilauer and Bažant (2010), it is shown that the size distribution and shape of inclusions have no measurable influence on the overall creep behavior. Moreover, a mean-field estimate provides close predictions. An Interfacial Transition Zone was introduced according to the model of Nadeau (2003). It is shown that this feature of concrete's microstructure can explain differences between creep behaviors.     

Properties of Adhesives and their Applications

The stress analysis of an adhesively bonded lap joint requires more information on the mechanical properties of adhesives than it is normally furnished by the manufacturers. For this reason the tests were performed on the three types of adhesives covering a large range of properties. In order to get the true stress-strain curves in tension and compression the change in the Poisson's Ratio with strain was investigated. It was found that the Poisson's Ratio increases almost to the constant volume deformation value until the nonrecoverable deformation sets in. From that point the Poisson's Ratio begins to decrease. Considering only the range of the recoverable deformation, the computer programs developed for the stress analysis of metallic materials can be used for an adhesively bonded lap joint. The recoverable viscoelastic deformation was considered non linear elastic and by applying an effective stress-effective strain relationship the analysis was performed.     

Mechanical Response of Thermoset Polymers under High Compressive Loads, 2

Summary: A nonlinear viscoelastic material model was used to describe the experimental behaviour of thin vinyl ester specimens subjected to compression in thickness direction. The stress‐dependent material functions in the model were found in creep and strain recovery tests on thick cylindrical specimens. The elastic and creep response of thin thermoset polymer specimens subjected to compressive loads was simulated while varying the geometry of the test set samples. The calculated increase in the apparent elastic modulus and decrease of the creep‐strain rate due to reduced thickness‐to‐width ratio is in a good qualitative correlation with experimental results for corresponding geometries. The constraint due to friction and interaction with the material outside the loaded surface area were identified as the cause for high apparent stiffness, which converges with decreasing thickness to an asymptotic value dependent on the modulus and Poisson's ratio of the material.

The shape of a 2 mm‐thick specimen under compression.     

Introductory behavior of rubber concrete

This article introduces a new type of concrete, the so‐called rubber concrete, and thereupon presents a way of modification of waste rubber to construction articles. The conventional cement concrete is made by mixing cement with sand and pebbles, but the rubber concrete proposed here virtually excludes cement completely. The manufacturing process of rubber concrete can be divided into two methods, which are designated for dry and wet processes, but this article focused just on the dry process. The physical properties of rubber composite increased with the silane treatment of added aggregates, but the volume of the aggregate might not be a critical factor affecting the compressive strength in the range of the aggregate contents used in this study, that is, the interfacial adhesion between the matrix rubber and the aggregates was a key factor to improve the mechanical properties of rubber concrete. The compressive strength of rubber concrete was about 89 MPa and the Poisson's ratio, which is the ratio of compressive‐to‐tensile strength, was 5.5%. From the viewpoint of the compressive strength and the Poisson's ratio, rubber concrete had better properties than those of conventional cement concrete. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 35–40, 1999     

Mechanical,hygroscopic, and thermal properties of metal particle and metal evaporated tapes and their individual layers

Mechanical and thermal properties of magnetic tapes and their individual layers strongly affect the tribology of magnetic head–tape interface and reliability of tape drives. Dynamic mechanical analysis, longitudinal creep, lateral creep, Poisson's ratio, the coefficient of hygroscopic expansion (CHE), and the coefficient of thermal expansion (CTE) tests were performed on magnetic tapes, tapes with front coat or back coat removed, substrates (with front and back coats removed), and never‐coated virgin films of the substrates. Storage modulus and loss tangent values were obtained at a frequency range from 0.016 to 28 Hz, and at a temperature range from ?50 to 150 or 210°C. Longitudinal creep tests were performed at 25°C/50% RH, 40°C/25% RH, and 55°C/10% RH for 50 h. The Poisson's ratio and lateral creep were measured at 25°C/50% RH. CHE was measured at 25°C/15–80% RH. CTE values of various samples were measured at a temperature range from 30 to 70°C. The tapes used in this research included two magnetic particle (MP) tapes and two metal evaporated (ME) tapes that were based on poly(ethylene terephthalate) and poly(ethylene naphthalate) substrates. The master curves of storage modulus and creep compliance for these samples were generated for a frequency range from 10^?20 to 10¹⁵ Hz. The effect of tape manufacturing process on the various mechanical properties of substrates was analyzed by comparing the data for the substrates (with front and back coats removed) and the never‐coated virgin films. A model based on the rule of mixtures was developed to determine the storage modulus, complex modulus, creep compliance, and CTE for the front coat and back coat of MP and ME tapes. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1319–1345, 2004     

3D Nanotomography of calcium silicate hydrates by transmission electron microscopy

Calcium silicate hydrate (C-S-H), is the principal hydration product of Portland cement that mainly contributes to the physical and mechanical properties of concrete. This paper aims to investigate the three-dimensional structure of C-S-H with Ca/Si ratios of 1.0 and 1.6 at the nanoscale using electron tomography. The 3D reconstructions and selected region of interest analysis confirm that the morphology of both C-S-H materials are foil-like structures. The difference between the two materials is the density of elongated structures. C-S-H with Ca/Si ratio 1.6 is clearly composed of denser particles compared to the other C-S-H material due to overlapping of the foil-like structure. Pore analysis shows that C-S-H 1.0 and C-S-H 1.6 have porosities 69.2% and 49.8% respectively. Pore size distribution also reveals that C-S-H 1.0 has pore size range between 0-250 nm and C-S-H 1.6 between 0-100 nm. The pore network's size of C-S-H 1.0 is significantly larger than 1.6. This study illustrates the capability of using electron tomography to determine the 3D nanoscale structure of cementitious products and to distinguish between C-S-H 1.0 and 1.6.     

The dependence of the elastic properties of silica/alumina materials on the conditions used for firing

The dependence of the elastic properties of a range of powder compact samples has been measured as a function of firing variables. It was found that both Young's modulus and Poisson's ratio are particularly sensitive to the peak temperature and the time for which the peak temperature is maintained, over a range of these variables for which density is not significantly affected. The material investigated is used industrially for the manufacture of wall tiles. Firing trials conducted in an industrially operated tunnel kiln have indicated that sufficient variation in firing conditions exists, in the cross-section of the tunnel kiln, to cause significant variation in the values of Young's modulus and Poisson's ratio of bodies fired in different positions in the kiln. Microstructural examination of bodies produced to have very similar densities but vastly different values of Young's modulus and Poisson's ratio has indicated that the dependence of Young's modulus and Poisson's ratio on firing conditions can be explained by the extent of sintering within the ceramic matrix.     

An approximate method for the determination of poisson's complex ratio in harmonic viscoelastic behavior

To improve the simulation of viscoelastic behavior of composites, an approximate and incremental method for the determination of Poisson's complex ratio of the polymer matrix is proposed. This method is based on the fact that many polymers exhibit a slight variation of their bulk modulus throughout, at their main relaxation temperature. We examine different parameters that affect the proposed method. It is shown that (i) the Poisson's complex ratio of the matrix and the complex modulus of the composite depend significantly on the initial values of the incremental method and (ii) the Poisson's complex ratio of the matrix allows for the effects of filler content on the magnitude of the mechanical relaxation.     

Poisson's ratio and volume change accompanying deformation of randomly oriented electrospun nanofibrous membranes

ABSTRACT

The present work focuses on the determination of volume change accompanying deformation and Poisson's ratio for electrospun nanofibrous membranes. For this purpose, polyurethane (PU) is considered for the fabrication of electrospun nanofibrous membranes. Three different sample thicknesses are fabricated. Following this, surface morphology analysis and fibre orientation analysis are conducted to investigate the variation of properties between electrospun PU membranes of different thicknesses. Subsequently, PU specimens are subjected to uniaxial extension test where the changes in sample width and thickness are recorded as a function of applied strain. Volume changes are computed while further analysis on the relationship between transverse strains and axial strain provided the values of Poisson's ratio. For all three electrospun PU samples investigated, significant volume changes are observed while the in-plane Poisson's ratio is found to be around 0.55. However, the out-of-plane Poisson's ratio of electrospun PU membranes are not classical and remains undetermined.     

Effective volume changes during fatigue and fracture of polyacetal

Conventional tensile dilatometry techniques are extended to cyclic fatigue applications to study volume changes that occur during controlled-load cyclic fatigue of polyacetal. During fatigue, in-situ measures of the irreversible and elastic volume change are monitored together with dynamic viscoelastic parameters (E′, E″, and Tan δ), and changes in the energy densities (strain energy, potential energy, and irreversible work). The results show that the effective irreversible volume of the polyacetal gradually increases over a wide range of applied cyclic stress. However, at high stress levels and/or frequencies (i.e., low-cycle, thermally dominated regime), the effective Poisson's ratio of the polyacetal increases as it softens (evidenced by the dynamic viscoelastic data). Conversely, at lower stress levels, the Poisson's ratio continually decreases coincident with decreases in the loss modulus (E″) and the irreversible work density. These results are indicative of entirely different mechanisms governing the low-cycle (high stress level) and high-cycle (low stress) regimes. Also, comparisons between tensile and fatigue dilatometry studies show that the dilational-strain response of samples fatigued at high stress levels are similar to data obtained from monotonic tensile dilatometry. However, the dilationstrain response of samples fatigued at lower stress levels are distinctly different from low-cycle fatigue and tensile dilatometry.     

Characterization of creep and crack growth interactions in the fracture behavior of concrete

Creep and crack growth govern the long-term deformability of concrete and thus its service behavior and its durability. The mechanisms involved in the couplings between these effects are not yet clearly identified. The aim of this research was to investigate experimentally and numerically at both the macro and the meso-level (heterogeneous material) the interactions between creep and crack growth in concrete and identify the main phenomena acting on the overall viscoelastic response of concrete, at various stages of fracture, during relaxations.     

锂渣对混凝土徐变的影响

通过测试不同龄期的锂渣混凝土徐变,研究了锂渣掺量对混凝土徐变的影响,结合其孔结构、微观形貌和纳观结构,基于微纳观尺度揭示了其影响机理.结果表明:锂渣显著降低了混凝土徐变,且随着其掺量(≤20%)地增加,降低效果越来越好;锂渣可以细化混凝土孔隙,改善其内部结构;锂渣降低了LD C-S-H含量,增加了HD C-S-H含量;混凝土徐变度与体积孔隙率有着较好的线性关系.     

Macroscopic effective mechanical properties of porous dry concrete

The pores and voids within concrete have a great influence on the macroscopic elastic modulus, strength and other mechanical properties of concrete. The present study determines the macroscopic mechanical properties of porous dry concrete composed of hollow void inclusions embedded in a concrete matrix. Based on a three-phase sphere model, the effective bulk modulus of a concrete composite with a hollow sphere in concrete matrix is obtained. The investigation on the influence of porosity on the effective shear modulus of porous dry concrete is carried out by using a hollow cylindrical tube model. Based on the assumption that the material is isotropic, homogeneous and elastic, the effective Young's modulus and Poisson's ratio of porous concrete composites are derived. A comparison between the elastic modulus and Poisson's ratio derived from the present model and those from open literature indicates a good agreement. Furthermore, on the basis of the developed simplified analytical model, the quantitative effect of porosity on the macroscopic effective tensile and shear strengths of porous concrete in dry state are studied. Besides, their corresponding effective peak strains when porous dry concrete reaches its macroscopic effective tensile/shear strengths are also investigated and discussed. Also a comparison between the available tensile strength and two classical solutions is made to verify the rationality and accuracy of the present approach. The consistent results indicate that the proposed approach can predict the effective mechanical properties of porous dry concrete well, and the formulas are simple and convenient to use.