Yield stress and viscosity equations for mortars and self-consolidating concrete

The rheological behavior of flowable concrete, such as self consolidating concrete is closely influenced by concreting temperature and the elapsed time. The variation of the plastic viscosity and the yield stress with the elapsed time and temperature must be accurately quantified in order to forecast the variation of workability of cement-based materials. A convenient method to study the variation of these rheological parameters is proposed, using the mortar of the concrete. This latter is designed from the concrete mixture, taking in account the liquid and solid phases with a maximum granulometry of 315 μm. Different SCC and mortars proportioned with two types of high range water reducing admixtures (HRWRA) were prepared at temperatures ranging from 10 to 33 °C. Test results indicates that the yield stress and the plastic viscosity of the mortar mixtures vary in a linear way with the elapsed time while an exponential variation of these rheological parameter is seen on SCC. In order to enhance robotization of concrete, general equations to predict the variations of the yield stress and plastic viscosity with time are proposed, using the corresponding mortar initial yield stress and plastic viscosity. Such equations, derived from existing models, can easily be employed to develop concrete design software. Experimental constants which are related to the paste fluidity or the aggregates proportioning can be extracted from a database created with either mortar or aggregates test results.     

基于流变特性的粗骨料级配优化设计

采用ICAR流变仪测试了三级配粗骨料不同比例组成时新拌混凝土的性能,在宾汉姆模型的基础上,得到了流变参数.探究了粗骨料级配对扩展度、屈服应力和塑性粘度的影响,进而综合评述屈服应力和塑性粘度的最优范围,对混凝土三级配粗骨料组成进行了优化.结果表明,改变粗骨料的级配组成能够明显影响混凝土的流动性和流变参数,另一方面,通过控制新拌混凝土流变参数,可以确定不同级配粗骨料的最优组配.因此,通过限定适宜的流变参数范围,可以指导混凝土的组成设计.     

Reproportioning of steel fibre reinforced concrete mixes and their impact resistance

In this experimental investigation, a practical rapid method of proportioning steel fibre reinforced concrete (SFRC) mixes is developed and validated. The basis for developing this is to use the reproportioning method, which has already been developed for proportioning normal density cement concrete mixes, for SFRC mixes. Based on the results of the trial mix, two SFRC mixes having 28 day target strength of 30 and 50 MPa are designed using this technique and examined regarding its validation. In addition, the impact resistance of these reproportioned Plain Concrete (PC) and SFRC is studied at 7 and 28 days. It is observed that the SFRC has developed significant impact resistance even for a small addition of steel fibres. Pulse velocity test is conducted at different ages to assess the quality of concrete. It is found that all concrete specimens could be classified under good quality.     

Effect of compound mineral powders on workability and rheological property of HPC

The compounding effect of silica fume (SF) with phosphorus slag (PS) or limestone (LS) powder is investigated in this paper. It is found that such a compounding can achieve various rheological behavior of HPC. The compound powders of PS with SF lower plastic viscosity and yield stress of fresh concrete, but increase the slump and promote continuous flowability of concrete greatly. However, the compounding of LS with SF increases the yield stress, but decreases both slump and slump flow of concrete, although the viscosity remains broadly unchanged compared with the concrete containing LS only. It is demonstrated that rheological property can be highly correlated with the surface characteristic of each component of the compound powders. Based on the experimental work, the appropriate fractions of the components in these compounds, especially the optimum content of SF, have been suggested for improving rheological property of HPC.     

风机叶片用环氧树脂体系流变性能研究

本文对风机叶片用环氧树脂体系的流变性能进行了研究,在粘度实验的基础上,依据双阿累尼乌斯方程建立了与实验数据较为吻合的流变模型。结果表明,两种树脂体系的粘度随温度变化情况基本一致,在23～50℃范围内,其粘度都低于300mPa.s,且低粘度保持时间大于30min,符合风机叶片真空成型对树脂低粘度的要求。所建立的粘度模型可有效预测和模拟树脂体系在不同工艺条件下的粘度行为,揭示树脂体系的优化工艺参数和低粘度平台工艺窗口,为合理拟订工艺参数和保证产品质量提供必要的科学依据。     

Multilayer rheology: A comparison of experimental data with modeling of multilayer shear flow

The rheological behavior of multilayer polyjeric structures has been investigated Measurements of the bulk viscoelastic properties via small-amplitude oscilatory rheometry indicated that the shear viscosity is independent of both the numbe of layers (83 vs. 165) and the composition (30/70, 50/50, and 70/30 PC/PMMA by weight) within the limits of the data obtained. It is also apparent tha tthe shear viscosity is influenced strongly by the skin layer material. In additon, a model has been developed tha tcan be used to proedict the shear viscosity and shear stress ofa multilayer structure experiencing shear flow. The model predicts tha thte shear viscosity of a multilayer structure should be independent of the number of layers and strongly dependent on the material in the skin layer. These predictions are in agreement with experimental data.     

Evaluation of dynamic viscosity and rheological properties of ceramic materials during liquid phase sintering by numerical‐experimental procedure

The effective shear and bulk viscosity, as well as dynamic viscosity, describe the rheological properties of the ceramic body during the liquid phase sintering process. The rheological parameters depend on the physical and thermo‐mechanical characteristics of the material such as relative density, temperature, grain size, diffusion coefficient, and activation energy. In this paper, the numerical‐experimental method has been developed to study both viscous and rheological behavior of hard porcelain ceramic body during liquid phase sintering. The other aim is to acquire a complete understanding of the response of an incompressible viscose material during sintering such as stress‐strain relations, sintering, and hydrostatic stress. Densification results confirmed that the bulk viscosity was well‐defined with relative density. The stress analysis proved that the sintering stress is more than the hydrostatic stress during the entire sintering time so, the sintering process occurs completely. Deflection results showed that the shear viscosity was a fair estimation of real ones. Dilatometry, SEM, XRD investigations as well as bulk viscosity simulation results confirmed that the “mullitisation plateau” was presented as a very little extraordinary expansion at the final sintering stage.     

Elongational,hysteresis, and oscillatory flows of complex fluids

The simple model proposed earlier to describe te rheological properties of complex fluids is used to calculate (a) the extensional viscosity, (b) the hysteresis loops, and (c) the complex viscosity. It has been found that the rheological properties predicted by the model agree with experimental observations. It is shown that for some viscoelastic fluids the extensional viscosity is always finite and for some other fluids the extensional viscosity tends t infinity at finite extensional rate. In the latter case, steady flow is not attainable. The shape of the hysteresis loops depends on the maximum shear-rate. It also depends on the material properties. In a small amplitude oscillatory flow, our model reduces t a linear viscoelastic fluid.     

Coupled effect of time and temperature on variations of plastic viscosity of highly flowable mortar

Self-consolidating concrete (SCC) is being increasingly used as construction material for its workability. However, the rheological properties of such concrete, which is made with significant concentration of high-range water-reducing admixture (HRWRA), depend in most cases on the casting temperature of the material. The study presented herein aimed at evaluating the coupled influence of time and temperature on the variations of plastic viscosity (µ) of micro mortar made with polymelamine (PMS), polynaphtalene (PNS) and polycarboxylate (PCP) polymer. In total, seven micro mortar mixtures proportioned with various binder compositions and water-to-binder ratios of 0.42 and 0.53 were prepared at 10 to 33 °C. Test results show that the plastic viscosity varies linearly with the coupled effect of time and temperature for mixtures made with PNS or PMS HRWRA. However, for mixtures made with PCP-HRWRA, both temperature and mixture proportioning have influence on the variation of viscosity with time.     

Rheological modeling and finite element simulation of epoxy adhesive creep in FRP-strengthened RC beams

We have shown that a significant creep occurs at the concrete–fiber reinforced polymer (FRP) interface based on double shear long-term test. The primary test parameters were the shear stress to ultimate shear strength ratio, the epoxy curing time before loading as well as the epoxy thickness. The test results showed that when the epoxy curing time before loading was earlier than seven days the shear stress level significantly affected the long-term behavior of epoxy at the interfaces, and in particular the combined effect of high shear stress and thick epoxy adhesive can result in interfacial failure if subjected to high-sustained stresses. In this paper, based on the previous experimental observations, an improved rheological model was developed to simulate the long-term behavior of epoxy adhesive at the concrete–FRP interfaces. Furthermore, the newly developed rheological creep model was incorporated in finite element (FE) modeling of a reinforced concrete (RC) beam strengthened with FRP sheets. The use of rheological model in FE setting provides the opportunity to conduct a parametric investigation on the behavior of RC beams strengthened with FRP. It is demonstrated that creep of epoxy at the concrete–FRP interfaces increases the beam deflection. It is also shown that consideration of creep of epoxy is essential if part or the entire load supported by FRP is to be sustained.     

Flow of molten mesophase pitch

The rheological behavior of molten mesophase pitch has become a subject of major scientific curiosity as well as of technological importance. With the exception of a few recent studies, the early experimental techniques were not entirely adequate for precise rheological characterization of such a complex fluid as molten mesophase pitch. A reasonably extensive literature, therefore, does not seem to exist on mesophase pitch rheology. The factors which could contribute to the difficulties of accurately measuring the apparent viscosity of carbonaceous anisotropic materials are discussed herein. A Couette-flow instrument has been modified for measuring the apparent viscosity of mesophase pitch, circumventing most of the experimental difficulties. The reliability of this apparatus has been tested by cross checking the apparent viscosity measurements with the values measured from two other rheometers, Instron Capillary and Seiscor/Han, for a number of fluids. Changes in rheological properties that occur during the transformation of a commercial pitch into a totally anisotropic pitch have now been studied by an improved technique on the modified Couette apparatus. The rheology of two mesophase pitches, made from model compounds, have also been investigated for comparison.     

Effects of rheological properties and processing parameters on ABS thermoforming

Understanding effects of material and processing parameters on the thermoforming process is critical to the optimization of processing conditions and the development of better materials for high quality products. In this study we investigated the influence of both rheological properties and processing parameters on the part thickness distribution of a vacuum snap‐back forming process. Rheological properties included uniaxial and biaxial elongational viscosity and strain hardening and/or softening while processing parameters included friction coefficient, heat transfer coefficient, and sheet and mold temperatures. The Wagner two parameter nonlinear viscoelastic constitutive model was used to describe rheological behavior and was fit to shear and elongational experimental data. The linear viscoelastic properties along with the Wagner model were utilized for numerical simulation of the thermoforming operation. Simulations of pre‐stretched vacuum thermoforming with a relatively complex mold for a commercial refrigerator liner were conducted. The effects of nonlinear rheological behavior were determined by arbitrarily changing model parameters. This allows determination of which rheological features (i.e., elongational mode, viscosity, and strain hardening and/or softening) are most critical to the vacuum snap‐back thermoforming operation. We found that rheological and friction properties showed a predominant role over other processing parameters for uniform thickness distribution.     

A generalized model for predicting non-Newtonian viscosity of waxy crudes as a function of temperature and precipitated wax

Precipitated wax, shear and thermal history have pronounced effects on viscosity and rheological behavior of waxy crudes. On the basis of mechanism of waxy crude rheology, a shear-rate-dependent viscosity model has been developed by applying theory of suspension rheology. This model is characterized by its capability to predict viscosities of crude oils with various thermal and shear history and beneficiated with pour-point-depressants (PPD). Once viscosities at only two temperatures above the wax appearance temperature and apparent viscosities at one temperature in the non-Newtonian regime are known, viscosities or apparent viscosities at any temperatures above the gel point can be predicted by using the model together with the concentration of precipitated wax at that specified temperature. Verification by using 3458 viscosity data points ranging from 5 to 2900 mPa s from 33 virgin crudes and 14 PPD-beneficiated crudes with various thermal and shear history shows that the model predicts viscosities with an absolute average deviation of 7.43%. Furthermore parameters of rheological models such as the consistency coefficient K and the flow behavior index n of the power law model may be obtained by regressing predicted viscosity data and corresponding shear-rates.     

Rheology of ordinary and low-impact environmental concretes

This study deals with the rheological properties of concrete mixtures incorporating various types of mineral additions as a partial replacement of cement in order to produce a low-impact environmental concrete. The control mixture contained only Portland cement as the binder, while the remaining mixtures incorporated binary cementitious blends of slag, limestone filler, and fly ash with different rates of replacement. After mixing, the plastic viscosity and yield stress of the concretes were evaluated at different slump values using a rheometer apparatus. The results showed that the type of mineral additions and the rate of substitution affect the rheological parameters of low-impact environmental concrete. Indeed, increasing the degree of substitution leads to an increase in the plastic viscosity of the concrete made with different types of additions used in this study.     

Modeling rheological behavior of bentonite suspensions as Casson and Robertson-Stiff fluids using Newtonian and true shear rates in Couette viscometry

The Casson model and the Robertson-Stiff model have been used to determine whether they can describe the rheology of aqueous bentonite suspensions. The assessment utilized a total of twelve sets of experimental viscometric data from literature and from this work. Equations have been presented which allowed the determination of the true shear rates experienced by the fluids within the gap of the rotational viscometer for both rheological models. Non-linear regression has been applied to determine the two rheological parameters for the Casson model and the three rheological parameters for the Robertson-Stiff model using true shear rates and Newtonian shear rates, which are used most often in the analysis of rheometric data. The results showed that both models describe well the experimental data of these bentonite suspensions with good statistical indicators. Furthermore, analysis showed that true shear rates are always higher than Newtonian shear rates for both models. The differences depend on the particular suspension and are larger at low shear rates while they become smaller at higher shear rates indicating that the fluid behavior approaches Newtonian behavior at higher shear rates. The shapes of the rheograms remained essentially unchanged indicating that the rheological parameters determined with the use of true shear rates are very similar to the rheological parameters determined with the use of Newtonian shear rates. This was further confirmed with the computation of the rheological parameters for both models and both approaches. For the Casson model differences in the yield value computed with true shear rates were at most at 7% while for the plastic viscosity at 3%. For the Robertson-Stiff model, differences of the order of 2 to 5% were observed for the K-values, of 7% for γ˙₀-values while no differences were observed for the n-values. These small differences, however, do not justify use of Newtonian shear rates when analytical solutions exist which allow use of true shear rates without any compromise.     

A rheological study of the low molecular weight butyl polymer

A rheological study has been performed to characterize the low molecular weight butyl polymers using a couette coni-cylindrical viscometer. The bulk viscosity was determined as a function of temperature, weight-average molecular weight, viscosity-average molecular weight, and shear rate. The temperature dependence of the viscosity, while adequately represented by the Williams, Landel, and Ferry equation, is best described by an Arrhenius equation for the temperature range investigated. The viscosity is shown to vary with the 3.5th power of the weight-average molecular weight above a critical molecular weight and to the 1st power below this molecular weight. Although the ratio of the weight-average molecular weight to the number-average molecular weight usually affects the flow properties of polymers, this was not true for the polymers investigated. The bulk viscosity was found to be independent of the molecular weight distribution for the temperature and shear rate range studied. It has been shown that a definite relationship exists between the bulk viscosity and the viscosity-average molecular weight as determined by dilute solution viscosity. A mathematical model has been developed to relate these two parameters as a function of temperature and shear rate.     

Flow behavior of partially crosslinked low-density polyethylene

The viscosity of low-density crosslinked polyethylene was studied as a function of gel content and shear rate (3.75–33 sec^?1). A simple model relating viscosity with gel content is suggested. It is shown that sol viscosity decreases with cross-linking propagation. The rheological parameters of the sol fraction are changed as a result, and it is natural that this effect should be utilized for flow calculations. Experimental data indicate a high degree of interaction between the sol molecules and the gel, and an experimental technique is presented for measuring it. The rheological parameters of crosslinked polyethylene are closely dependent on the gel content, viscosity and pseudoplasticity increasing with the latter.     

The effect of molecular weight and weight distribution upon polymer melt rheology

A major objective in polymer rheology is to predict a fluid's response to a general deformation from molecular information. A method has been developed which allows one to predict the viscoelastic properties of polymer melts from a limited amount of rheological and molecular data for the polymer. The input parameters are: (a) zero-shear viscosity; (b) molecular weight distribution; (c) temperature and density; and (d) constants relating Graessley's relaxation time to the Rouse relaxation time. The technique then “simulates” a discrete relaxation spectrum using G′ and G″ data from the Rouse theory and finally requires that a continuum model of polymer viscoelasticity be fit to shear viscosity data predicted by Graessley's theory. Examples of the utility of the procedure are given to illustrate the role of molecular weight and weight distribution in determining rheological behavior.     

发泡浆体流变特性对轻质混凝土匀质性的影响

为改善轻质混凝土匀质性,提升轻质混凝土力学性能,本文通过建立Herschel-Bulkley(H-B)塑性黏度计算模型,探究了发泡浆体流变特性对泡沫沉降和浆体中陶粒分布的影响。结果表明：随增稠剂掺量的增加,轻质混凝土发泡浆体的屈服应力和基于非线性H-B模型η_H-B=0.016 67K(80ⁿ-20ⁿ)(η_H-B为H-B模型得出的发泡浆体塑性黏度;K为一致性系数;n为流动指数)得到的塑性黏度增大,导致发泡浆体沉降距下降,陶粒在浆体中的分形维数提高;当发泡浆体塑性黏度大于1.74 Pa·s、屈服应力大于92.87 Pa时,可制备抗压强度大于10 MPa的轻质混凝土。由此可见,优化发泡浆体流变特性是克服陶粒在发泡浆体中易上浮问题的关键,可有效改善轻质混凝土匀质性,提升轻质混凝土力学性能。     

Concentration by Membrane Ultrafiltration of a Shear-Thinning Fluid

Abstract

Many commercial membrane processes involve fluids whose rheological properties are non-Newtonian. However, very little has been published on ultrafiltration of non-Newtonian fluids. The aim of this work is to show some experimental results concerning the concentration by membrane ultrafiltration of fluids whose viscosity is high and shear-thinning. Experiments were performed with xanthan solutions as model shear-thinning fluids. The variations of permeate flux with respect to the operating parameters show the unusual effects of some of these parameters. It is shown that when the feed solution in an ultrafiltration process has shear-thinning properties, those properties have an enormous influence in determining the operation efficiency.