Constant Shear Rate Tests Research Articles

Effect of Flocculants Residue on Rheological Properties of Ultra-Fine Argillaceous Backfilling Slurry

Tailings concentration is indispensable for backfilling. Additionally, the residual flocculants in the concentration process affect the rheological properties of ultra-fine argillaceous backfilling slurry (e.g., viscosity and yield stress), resulting in a great effect on the fluidity and resistance of pipeline transportation. In this study, to explore the effect of flocculants residue on the rheological properties of the slurry, a series of rheological tests (constant shear rate test and variable shear rate test) were performed by changing the type, dosage, stirring time, temperature of flocculants addition and the amount of binder added. The results showed that the addition of flocculants increased the viscosity and yield stress of slurry. At a certain amount of flocculants additive, the flocculant network structure reached the best development state, which had a positive effect on increasing slurry viscosity and yield stress. As the stirring time increased, the scale of damage to the flocculant network structure became larger, which had a negative effect on increasing slurry viscosity and yield stress. Low temperature weakened the adsorption and bridging effect of polymeric chains, resulting in a poorly developed flocculant network structure, which had a negative effect on increasing slurry viscosity and yield stress. Caused by hydration products, the viscosity and yield stress of slurry with binder further increased. This study is significant for an in-depth study of the rheological and pipeline transport characteristics of ultra-fine argillaceous backfilling slurry, optimising the selection of flocculants for ultrafine particles, guiding backfill parameters and improving the reliability of pipeline transport.

Time-dependent fresh properties characterization of 3D printing engineered cementitious composites (3DP-ECC): On the evaluation of buildability

Failure of the structure due to poor buildability is a major concern in 3D printing of cementitious materials. Evaluation of buildability based on fresh material properties and print parameters is of significance. In this paper, the buildability of printable engineered cementitious composites was investigated and quantified at the material and the structural scale. Fresh ECC material showed excellent load capacity and deformation resistance at the material scale, therefore preventing material failure of the bottom layers, as confirmed by constant shear rate tests and incremental loading tests. To predict vertical deformation of a 3DP structure, a time-dependent strain-stress model of printable ECC was proposed and validated based on the green strength evolution of the material and the buildup rate of the designed structure. At the structural scale, the approach of predicting critical height at self-buckling failure based on stiffness evolution was validated by printing a straight wall and a cylinder structure.

Comparison between methods for indirect assessment of buildability in fresh 3D printed mortar and concrete

This paper presents a comparison between indirect methods for assessing the buildability of 3D printed cementitious materials, including constant rotational velocity (CRV) or constant shear rate (CSR) test using rotational rheometry, unconfined uniaxial compression test (UUCT), a newly proposed confined uniaxial compression test (CUCT), and fast penetration test. The experimental program was conducted on the extruded samples of six printable mortars and two printable concretes with various rheological behavior achieved by the use of different additives and admixtures. Predictions of the material failure and the stability failure in hollow cylindrical structures are provided and compared with the results of the direct printing test. Benefits and disadvantages of the employed test methods are discussed, also taking into consideration a perspective of their use for automated in-situ control of the materials' buildability.

Yield stress of fine cement-based mortars: Challenges and potentials with rotational and compressional testing methods

The present study aims to evaluate the static yield stress of cement-based mortar mixtures using the parallel-plates geometry with rotational and compressional testing set-ups. In total, six mortar mixtures with various water-to-cement (W/C) and sand-to-cement (S/C) ratios, as well as high-range water reducer (HRWR) dosages were used for investigation. Two rotational methods namely the constant-shear rate (CSR) and the steady-shear, as well as the squeeze flow test were employed to assess the yield stress of the investigated mortar mixtures. For the latter, a stress-controlled uniaxial compression machine was developed allowing a high precision to monitor and control the compressional force and the deformation. The CSR test results indicated that a 7% increase in the W/C ratio and addition of 0.5% polycarboxylate ether-based HRWR led to around 20% and 50% reduction in the yield stress, respectively. Nevertheless, the effect of W/C ratio and the PCE-based HRWR was even more pronounced in the case for the steady-shear test. Both methods were highly prone to the wall-slip phenomenon reflected by considerably lower shear stress values obtained with the smooth rotating plate. However, when a rough rotating plate was employed, the CSR method led to around 65% (in average) higher static yield stress values compared to those obtained using the steady-shear test. The uniaxial compression test results were in good agreement with the rotational tests from a comparative point of view. However, it seems to exist a threshold of the material’s consistency beyond which the compressional test set-up results are more reliable. Accordingly, mixtures with a minimum CSR static yield stress of 345 Pa were found to be adapted for testing using this set-up. The adoption of the testing methods proposed in this study led to obtaining different value ranges for the yield stress of the mortars which indicated the ability of each test to capture different transitions reflecting microscopic and macroscopic changes.

Effect of polycarboxylate ether (PCE) superplasticizer on thixotropic structural build-up of fresh cement pastes over time

Water reducing agents including polycarboxylate ether superplasticizer (PCE) have been found to have retardation effect on hydration heat and setting of cementitious materials, which is a long term effect after 8 h. However, the short term effect within 3 h of PCE on workability and workability loss, which could result in failure during SCC pumping or continuous concrete 3D printing, is not clear. This study explores the effects of PCE on flowability and structural build-up of cement pastes in fresh states using rheometer. Strength and modulus, such as the dynamic and static yield stress, and storage modulus are measured over time up to 2.5 h. They are measured by step down protocol, stress development protocol (i.e. low constant shear rate test), and small amplitude oscillatory sweep, respectively. Plastic viscosity fitted in Bingham model and critical strain using oscillatory test are also recorded. The effect of dosage of PCE addition and time are studied. It is found that in both cases where w/c are 0.3 and 0.4, low dosage of PCE addition exhibits much higher increasing rates of structural build-up than plain cement paste or at high dosage of PCE addition. It highlights attention needed on structural build-up at fresh state with low dosage of PCE addition. Possible mechanisms, especially effect of PCE polymers on colloidal flocculation and early hydrates formation, are also discussed and reasoned.

A constitutive model for thixotropic cemented tailings backfill pastes

• The reversibility time-dependent property of CTB mixtures was observed. • A novel rate equation of structural parameter was proposed. • A thixotropic model with 8 parameters for CTB mixtures was established. • The proposed model exhibited good correlations with experimental results. The thixotropic properties of fresh cemented tailing backfill (CTB) pastes applied in underground backfilling are investigated. Reversibility of thixotropy, equilibrium rheology, and stress decay curves of CTB pastes are discussed. A constitutive model that contains an evolution equation of structural parameter and a constitutive equation for shear stress is proposed. Modifications are made to incorporate the steady-state rheological model and the evolution equation of structural parameter. The proposed model has 8 parameters that are easily determined by fitting experimental data from a series of constant shear rate tests. The good agreement between the predicted and experimental shear stress responses suggests that such a thixotropic model is validated for CTB pastes. Compared to existing thixotropic models, the proposed constitutive model is simple and distinct, showing the novelty of the evolution equation of structural parameter.

The influence of water-to-cement ratio and superabsorbent polymers (SAPs) on solid-like behaviors of fresh cement pastes

Solid-like behaviors of fresh cement pastes are gaining increased attention with the emergence of 3D concrete printing. It is known that the fresh cement paste exhibits a “two-step yielding” in the solid regime. This study aims to achieve a more comprehensive understanding of “two-step yielding” behavior of the cement pastes with different water-to-cement ratios and with the addition of SAPs. The influence of water-to-cement ratio and SAPs on the solid-like behaviors of cement pastes were investigated by the constant shear rate test using a rotational rheometer. The results demonstrate that changes in the solid-like properties of the fresh cement pastes, such as static yield stress, critical strain, elastic limit, and tangent modulus, do not necessarily follow the same pattern since different aspects of solid-like properties can be attributed to the different particle interactions among cement systems. The results further emphasize that, for a better understanding of the influence of the mixture design parameter on the shape stability of 3D printed concrete, it is necessary to determine its impact on all aspects of solid-like behaviors, from pure elasticity to elasto-viscoplasticity, other than the static yield stress alone.

Effects of Volume Fraction and Surface Area of Aggregates on the Static Yield Stress and Structural Build-Up of Fresh Concrete.

With increasing interest in the use of additive manufacturing techniques in the construction industry, static rheological properties of fresh concrete have necessarily come into focus. In particular, the knowledge and control of static yield stress (SYS) and its development over time are crucial for mastering formwork-free construction, e.g., by means of layered extrusion. Furthermore, solid understanding of the influences of various concrete constituents on the initial SYS of the mixture and the structural build-up rate is required for purposeful material design. This contribution is concentrated on the effect of aggregates on these rheological parameters. The volume fraction of aggregates was varied in the range of 35% to 55% by volume under condition of constant total surface area of the particles. The total surface area per unit volume of cement paste was equal to 5.00, 7.25 and 10.00 m²/L, conditioned on the constant volume fraction of aggregates. Both variations were enabled by changing the particle size distributions of the aggregates while holding the cement paste composition constant for all concrete mixtures. To characterise the SYS and the structural build-up, constant shear rate tests with a vane-geometry rotational rheometer were performed. It was found that in the ranges under investigation the variation in volume fraction had a more pronounced effect on the static rheological properties of concrete than did the variation in surface area. An accurate mathematical description of the relationship between the initial SYS of concrete and the relative volume fraction of aggregate based on the Chateau–Ovarlez–Trung model was proposed. Challenges in deriving a similar relationship for the structural build-up rate of concrete were highlighted.

Possibilities and challenges of constant shear rate test for evaluation of structural build-up rate of cementitious materials

The rapid development of additive manufacturing technologies in the construction industry has heightened interest in the accurate estimation of the structural build-up rates of cementitious materials, especially in in-situ conditions. For this purpose the constant shear rate (CSR) test performed by rotational rheometers using the single-batch approach can be applied. The paper at hand presents possibilities in improving the methodology of experiments and data analysis in the CSR test, for particular rheometers, and discusses both the features and limitations of the test. The breaking criterion is proposed for the CSR tests to prevent excessive deformation in a single sample measurement. Evaluation of the elastic properties of cementitious materials by the CSR test is assessed; two approaches for the characterisation of structural build-up rate based on proportionality limit and flow onset points are compared. The findings are illustrated for the examples of cement paste and cementitious mortars tested using two different types of rotational rheometers.

Rheological characterization of the viscoelastic solid-like properties of fresh cement pastes with nanoclay addition

At sufficiently low applied shear deformation or shear stress, fresh cement paste exhibits viscoelastic solid-like behavior. This solid-like state is particularly important for some applications, such as self-consolidating concrete formwork pressure and 3D concrete printing. In this study, the viscoelastic solid-like behavior of fresh cement pastes is probed using a rheometer. Static yield stress, storage modulus, and cohesion are measured by creep recovery and constant shear rate test, small amplitude oscillatory sweep (SAOS) test and tack test, respectively. The effect of nanoclay addition is studied in various aspects of viscoelastic performance of fresh cement pastes. It is found that nanoclay addition not only enhances yield stress and cohesion, but also stiffness in terms of storage and tangent modulus.

Irreversible time-dependent rheological behavior of cement slurries: Constitutive model and experiments

Over the last few decades, much focus has been given to investigating the reversible rheological behavior of thixotropic materials, but the description of the rheology of materials undergoing an irreversible process is still challenging. In this work, the time-dependent rheological behavior of a cement slurry is investigated. Different rheometric experiments are performed to evaluate the structure breakdown under shear, cement gelation, and curing process. A recently proposed thixotropic elasto-viscoplastic model [de Souza Mendes, Soft Matter 7, 2471–2483 (2011)] is modified to account for irreversible effects, which can be either of a chemical or physical nature, making the current model capable of describing reversible and irreversible processes with a single structure parameter. The parameters of the model are estimated from constant shear rate tests and from the flow curve of the fresh cement slurry. The model predictions are compared to step-down and step-up in stress experiments, and the results show that the model successfully describes experimental data obtained. Interesting phenomena are observed and discussed, including (i) thixotropic behavior during the dormant period, (ii) shear banding, (iii) irreversible changes in cement slurry rheology after the hydration reactions accelerate, and (iv) the existence of a characteristic time for the transition from a thixotropic-yield-stress material to a solid during curing. The predictive capability of the new model includes bifurcation, shear banding, stress overshoots, effects of chemical reactions, and irreversible shear degradation. It is argued that the ideas employed in the present work can be used to incorporate irreversible effects into other thixotropic models, giving rise to the possibility of describing the transient rheological behavior of complex materials in an unprecedented fashion.Over the last few decades, much focus has been given to investigating the reversible rheological behavior of thixotropic materials, but the description of the rheology of materials undergoing an irreversible process is still challenging. In this work, the time-dependent rheological behavior of a cement slurry is investigated. Different rheometric experiments are performed to evaluate the structure breakdown under shear, cement gelation, and curing process. A recently proposed thixotropic elasto-viscoplastic model [de Souza Mendes, Soft Matter 7, 2471–2483 (2011)] is modified to account for irreversible effects, which can be either of a chemical or physical nature, making the current model capable of describing reversible and irreversible processes with a single structure parameter. The parameters of the model are estimated from constant shear rate tests and from the flow curve of the fresh cement slurry. The model predictions are compared to step-down and step-up in stress experiments, and the results sho...

Factors influencing the structure build-up of fresh cement asphalt emulsion paste

Fresh cement asphalt emulsion mortar (CA mortar) is required to be highly flowable for grouting when it is employed in slab track structure. Segregation and bleeding easily occur after placement. In essence, it is because fresh CA paste has a low rate of structure build-up. However, limited information is reported about the thixotropy of CA paste. A thixotropy model based on structure parameter, which is deduced from constant shear rate test, is proposed to study the structure build-up of CA paste. In the model, the initial structure parameter and the rate of structure build-up are used to describe the evolution law of the structure parameter with resting time, which can be employed to quantitatively evaluate the structure build-up process of CA paste. The thixotropy model is used to study the influence of every constituent on the structural build-up process of CA paste. Results show that asphalt emulsion and superplasticizer dosage are two main factors influencing the structure build-up of CA paste, but water-to-cement ratio and viscosity-modified agent dosage have little effect on the structure build-up of CA paste.

The thixotropic behavior of fresh cement asphalt emulsion paste

The workability of grouting cement asphalt emulsion (CA) mortar is governed by the intrinsic thixotropic behavior of CA paste, but limited information is reported about the thixotropic behavior of CA paste. A new thixotropy method of constant shear rate test is proposed to study the thixotropic behavior of CA paste with different asphalt emulsion to cement ratios (AE/C). In the model, the initial structure parameter and the structure rebuilding rate are used to quantitatively describe the structure rebuilding process, the characteristic time of de-flocculation is used to evaluate the de-flocculation process, and the equilibrium shear stress is used to study the irreversible change of CA paste in the thixotropy test. Results show that CA pastes have significantly different thixotropic behavior compared to cement paste. The initial structure parameter and structure rebuilding rate of CA pastes are much smaller than cement paste, and the structure rebuilding rate decreases significantly with AE/C. The characteristic time of de-flocculation of cement paste changes little at different resting time, however, it increases significantly at the first 5min resting time and then keep stable for CA pastes. The equilibrium shear stress of cement paste does not change in 20min, but it increases with time for CA pastes due to the demulsification of asphalt emulsion.

Thixotropic properties of normal potato starch depending on the degree of the granules pasting.

The aim of this paper was the study of the rheological instability (thixotropy and/or antithixotropy) of normal potato starch (NPS) pastes depending on their concentration (2-5%) and degree of pasting. Flow curves with hysteresis loops, apparent viscosity at constant shear rate and in-shear structural recovery tests were carried out. Granule size profiles, the pasting characteristic of corresponding starch suspensions and the transmittance of the pastes, the molecular weights and polydispersity of granular starch and its pastes prepared at 80, 95 and 121°C were also studied. The degree of pasting was dependent on the temperature and the concentration and influenced strongly the rheological behavior of the pastes. All pastes belonged to the non-Newtonian liquids thinned by shear and were rheologically unstable to the various extent. Thixotropic properties were connected to the size and the number of the starch granules in the pastes as well as depended on the measuring method used. In the 2 and 3% samples pasted at 80°C the swelling of the granules prevailed their destruction (thixotropy was observed). In the other samples the destruction predominated the swelling (antithixotropy observed).

A unified approach to model elasto-viscoplastic thixotropic yield-stress materials and apparent yield-stress fluids

A constitutive model for elasto-viscoplastic thixotropic materials is proposed. It consists of two differential equations, one for the stress and the other for the structure parameter, a scalar quantity that indicates the structuring level of the microstructure. In contrast to previous models of this kind, the structure parameter varies from zero to a positive and typically large number. The lower limit corresponds to a fully unstructured material, whereas the upper limit corresponds to a fully structured material. When the upper limit is finite, the model represents a highly shear-thinning, thixotropic, and viscoelastic liquid that possesses an apparent yield stress. When it tends to infinity, the behavior of a true yield-stress material is achieved. Predictions for rheometric flows such as constant shear rate tests, creep tests, SAOS, and large-amplitude oscillatory shear (LAOS) are presented, and it is shown that, in all cases, the trends observed experimentally are faithfully reproduced by the model. Within the framework of the model, simple explanations are given for the avalanche effect and the shear banding phenomenon. The LAOS results obtained are of particular importance because they provide a piece of information that so far is absent in the literature, namely a quantitative link between the Lissajous–Bowditch curve shapes and rheological effects such as elasticity, thixotropy, and yielding.

Effects of chemical additives on filtration and rheological characteristics of MBR sludge

The main goal of this study was to control the fouling phenomena in MBR using chemical additives. In the first phase of the study, SMP removal and bound EPS formation capacity of chemical additives were determined. Highest SMP removal (72%) was achieved by the Poly-2 additive. In the second phase of the study, short term filtration tests were conducted. Poly-1 exhibited highest performance based on membrane resistance, permeability and average TMP. According to the results obtained from constant shear rate tests in fourth phase, no significant change in viscosity with time was observed. Studies for the adaptation of rheograms to common flow models showed that chitosan and starch was not able to fit to Ostwald de Waele and Bingham models. At a shear rate of 73.4s−1 viscosities of all samples were close to each other. Chitosan and starch achieved highest viscosity values at the shear rate of 0.6s−1.

Rheological and microstructural characterization of WPI-stabilized O/W emulsions exhibiting time-dependent flow behavior

The rheological behavior of oil-in-water (O/W) emulsions stabilized by whey protein isolate (WPI) and its relationship with the microstructural changes caused by shearing was studied. O/W emulsions (50, 55, and 60 g oil/100 g) were made using ultrasound and their rheological properties were determined by: flow curve test, constant shear rate test, and hysteresis loop test. Microstructural changes were evaluated in terms of droplet size and droplet size distribution. Emulsions containing 50 and 55 g oil/100 g showed a Newtonian behavior, whereas those with 60 g oil/100 g exhibited shear-thinning behavior. Under constant deformation, the apparent viscosity of the emulsions decreased with time. The hysteresis loop test revealed that increasing oil content increased the degree of thixotropy of the emulsions. Moreover, before and after the constant deformation test droplet size distributions did not show differences, indicating that the decrease in the apparent viscosity may be promoted by breakdown and further deformation and/or reorganization of oil droplets flocs. In turn, experimental data obtained from the constant shear rate test was fitted to a structural kinetic model. The rate constant values showed no particular trend with oil content and shear rate, implying that probably wall slip occurred at high shear rates and high oil contents.

Evaluation of the Effects of Asphalt Binder on the Properties of Hot Mix Asphalt at Intermediate Temperatures

Abstract The asphalt binder is an essential component of asphaltic mixtures. The performance of an asphaltic mixture is directly related to mechanical characteristics of the binder. With the development of new material characterization methods for both asphalt binder and asphaltic mixture, there is a need to re-evaluate the relationship between the properties of binders and asphaltic mixtures such that a proper understanding and selection of an asphalt binder can be made to improve the performance of an asphaltic mixture. In this study, the effects of asphalt binder properties on asphaltic mixtures at intermediate temperatures were evaluated based on recent developments on material characterization methods for both binder and asphaltic mixture. Five asphalt binders and five asphalt mixtures containing these binders with one aggregate were tested. Four of the binders were based on a modification of a base binder with different techniques. Complex shear modulus and monotonic constant shear-rate tests were conducted on asphalt binders, whereas dynamic modulus and indirect tensile strength tests were conducted on the mixtures. The effects of modification techniques on the properties of asphalt binders and the relationships between the properties of binders and asphaltic mixtures were evaluated in this study.

Rheology and Processing of Ni Based Nano-Composite Pastes for Electronic Assembly

Lead-free solder paste printing process accounts for majority of the assembly defects in the electronic manufacturing industry. In the stencil printing process, the solder paste must be able to withstand low and high shear rates, which results in continuous structural breakdown and build-up. The study investigates the effect of nickel additional to the thixotropic behaviour of lead-free Sn/Ag/Cu solder pastes using the structural kinetic model. A hysteresis loop test and constant shear test is utilized to investigate the thixotropic behaviour of the pastes using parallel plate rheometry in temperature 30 oC. In this study, the shear rates were increased from 0.01 s-1 to 10s-1 and the second curve was a result of decreasing shear rate from 10s-1 to 0.01s-1. For the constant shear test, the samples were subjected to two constant shear rates of 0.1s-1 and 1s-1. The constant shear rate test was designed to study the structural breakdown and build-up of the paste materials. From this investigation, hysteresis loop test was shown to be an effective test method to differentiate the extent of structural recovery in the solder pastes. The results of the structural parameter values obtained from the kinetic model showed a good correlation to the breakdown and build-up of the paste structure.

A strain-hardening bi-power law for the nonlinear behaviour of biological soft tissues

Biological soft tissues exhibit a strongly nonlinear viscoelastic behaviour. Among parenchymous tissues, kidney and liver remain less studied than brain, and a first goal of this study is to report additional material properties of kidney and liver tissues in oscillatory shear and constant shear rate tests. Results show that the liver tissue is more compliant but more strain hardening than kidney. A wealth of multi-parameter mathematical models has been proposed for describing the mechanical behaviour of soft tissues. A second purpose of this work is to develop a new constitutive law capable of predicting our experimental data in the both linear and nonlinear viscoelastic regime with as few parameters as possible. We propose a nonlinear strain-hardening fractional derivative model in which six parameters allow fitting the viscoelastic behaviour of kidney and liver tissues for strains ranging from 0.01 to 1 and strain rates from 0.0151 s −1 to 0.7 s −1.