Influence of temperature on yield value of highly flowable micromortars made with sulfonate-based superplasticizers

Self-consolidating concretes (SCC) were proved to be very dependant on concreting temperature and the elapsed time. To enhance the concreting conditions of these flowable concretes, it is important to have a better knowledge of their rheological behavior, depending on the kind of superplasticizer used. The variation of the plastic viscosity and the yield value with the elapsed time and temperature must be accurately quantified. However, the methods of measuring these parameters are expensive and unsuitable with a good forecast of the material behavior due to numerous parameters that interact with each other. A simplest method to study the variation of these rheological parameters, depending on the mixture design, is proposed, using the micromortar, which derivates from the studied SCC. Moreover, to forecast the concrete behavior on the site, a simple thermodynamical approach of the cementitious matrix behavior through the study of the hydration kinetics is described.     

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 properties of highly flowable mortar containing limestone filler-effect of powder content and W/C ratio

The effect of a limestone filler addition in superplasticized cement mortar was investigated. The mixtures considered in this study are highly fluid, yet stable mortars that can be used to proportion self-consolidating concrete (SCC). All the mixtures were proportioned with a fixed unit water content of 250 l and various water-cement ratios varying from 0.35 to 0.45. A limestone filler with a specific surface area of 480 m²/kg was used at different addition percentages.This paper reports test results leading to the recommendation of suitable powder contents that can be used to proportion mortar mixtures containing a limestone filler and achieving adequate rheological properties. Test results show that the effect of limestone filler is mainly affected by the W/C and the limestone filler content in use. For a given W/C, the addition of a limestone filler within a certain range did not affect fluidity. However, beyond a critical dosage, the incorporation of some limestone filler resulted in a substantial increase of the viscosity of mortar. An accurate model that can be used to predict the viscosity of such mixtures is proposed and validated on various mixtures.     

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.     

Effect of limestone filler content and superplasticizer dosage on rheological parameters of highly flowable mortar under light pressure conditions

The influence of pressure on the yield stress and plastic viscosity of SCC mortars was investigated using an adapted Marsh cone with cylindrical shear paddles. Nine mortars proportioned with various limestone filler content and high-range water-reducing admixture (HRWRA) dosage from 0.65% to 0.85% were prepared. Test results show that when a pressure is exerted on the mortars, the material does not behave as a homogeneous fluid, i.e. having a yield stress depending only on the specific gravity of the mixture and the height of poured mortars and a constant value of plastic viscosity, but as a separative multiphasic material which consolidates, leading to exponential variations of τ₀ and lowering values of μ with pressure. The HRWRA dosage or the limestone filler content has only an effect on the initial value of the yield stress, but has more influence on the variation of plastic viscosity of the mortar with pressure.     

Flow conditions of fresh mortar and concrete in different pipes

The variation in fresh concrete flow rate over the pipe cross section was investigated on differently coloured and highly flowable concrete mixes flowing through pipes of different materials (rubber, steel, acryl). First, uncoloured (gray) concrete was poured through the pipe and the pipe blocked. Similar but coloured (black) concrete was then poured into the pipe filled with gray concrete, flowing after the gray concrete for a while before being blocked and hardened. The advance of the colouring along the pipe wall (showing boundary flow rate) was observed on the moulded concrete surface appearing after removing the pipe from the hardened concrete. The shapes of the interfaces between uncoloured and coloured concrete (showing variation of flow rate over the pipe cross section) were observed on sawn surfaces of concrete half cylinders cut along the length axes of the concrete-filled pipe. Flow profiles over the pipe cross section were clearly seen with maximum flow rates near the centre of the pipe and low flow rate at the pipe wall (typically rubber pipe with reference concrete without silica fume and/or stabilizers). More plug-shaped profiles, with long slip layers and less variation of flow rate over the cross section, were also seen (typically in smooth acrylic pipes). Flow rate, amount of concrete sticking to the wall after flow and SEM-images of pipe surface roughness were observed, illustrating the problem of testing full scale pumping.     

Influence of superplasticizers on rheological behaviour of fresh cement mortars

To assure required workability of high performance concrete (HPC), various superplasticizers are used. Only by using superplasticizers can rheological properties of HPC mix be adequately adjusted to the methods and conditions of concrete processing. Thus, the key element in efficient workability shaping is the complex knowledge how superplasticizers influence the rheological properties of fresh concrete in different technological circumstances.In the paper, the methodology and test results of an investigation into the influence of chemically different superplasticizers on the rheological properties of standard mortars are presented and discussed. The rheological parameters of mortars yield value g, and plastic viscosity h were determined using VISCOMAT PC rotational rheometer. In the research, the influence of the performance of superplasticizers was investigated taking into account following factors: chemical origin of superplasticizers (SNF/naphthalene sulfonic acid/, AP/polycarboxylate acid, PC/policarboxylate ester/), superplasticizer dosage, W/C ratio, cement type (CEM I, CEM II and CEM III), cement physical and chemical properties and temperature.The results presented in the paper show that by testing rheological parameters of mortars with rotational viscometer, it is possible to complex and precisely determine the performance of superplasticizers. On the ground of obtained results, it is possible to optimise the composition of mortars and concretes from workability point of view.     

Influence of mixing sequence and superplasticiser dosage on the rheological response of cement pastes at different temperatures

An experimental methodology to study the rheological response of superplasticised cement pastes subjected to temperatures ranging from 5 to 45 °C is presented. The content of a polycarboxylate-based superplasticiser (PC) and moment of its addition to the mix (direct or delayed) are investigated. A loop of shear rate ramps is applied to each sample in order to obtain information about apparent viscosities and yield stresses, as well as a measure of their thixotropic behaviour. Results from the experimental campaigns indicate PC saturation dosages depend only slightly on temperature. The evolution of yield stress and thixotropy with temperature shows inverted trends under some conditions.     

Correlation between L-box test and rheological parameters of a homogeneous yield stress fluid

In this paper, a theoretical analysis of the L-box test is proposed in the case where no heterogeneity is induced by the flow. It is first demonstrated that if the standard procedure is followed and the L-box gate promptly lifted, the flow is dominated by inertia effects, depending on the lifting speed of the gate and thus on the operator. When the gate is slowly lifted, the flow and flow stoppage of a homogeneous yield stress fluid in a bounded channel are first studied. The obtained theoretical shape of the sample at stoppage is successfully correlated to the experimental results in the case of limestone powder suspensions. Then, the influence of steel bars was included in both the theoretical and the experimental study. Finally, practical applications of the present work to the case of real self-compacting concrete are suggested.     

A generalized approach for the determination of yield stress by slump and slump flow

The slump test (ASTM C-143) is the most common method for assessing the flow properties of fresh concrete. Although slump provides a qualitative measure of workability, the relationship between slump and more quantitative rheological parameters is not fully understood. In this study, a dimensionless model relating slump to yield stress is further developed and generalized as a function of cone geometry. Yield stress measurements of cement paste are performed using a vane technique and compared with slump measurements using cylindrical and conical geometries. The cylindrical slump model is in excellent agreement with the experimental yield stress data obtained using the vane method. The data for the conical slump measurements fit the cylindrical model at low yield stress values, but the results deviate as the yield stress of the paste increases. Most of the other slump models available in the literature, including finite element models, predict the same yield stress for a given slump when converted to dimensionless form. The results suggest that a fundamental relationship exists between yield stress and slump that is independent of the material under investigation and largely independent of cone geometry.     

Effect of coarse particle volume fraction on the yield stress and thixotropy of cementitious materials

In order to help in modelling the yield stress of fresh concrete, we study the behavior of suspensions of coarse particles in a thixotropic cement paste. Our aim is to relate the yield stress of these mixtures to the yield stress of the suspending cement paste, to the time passed at rest, and to the coarse particle volume fraction. We present here procedures that allow for (i) studying a homogeneous and isotropic suspension, (ii) comparing the yield stress of a given cement paste to that of the same cement paste added with particles, and (iii) accounting for the thixotropy of the cement paste. We observe that the yield stress of these suspensions of cement paste with coarse particles follows the very simple Chateau-Ovarlez-Trung model [X. Chateau, G. Ovarlez, K.L. Trung, Homogenization approach to the behavior of suspensions of noncolloidal particles in yield stress fluids, J. Rheol. (2008) 52 489-506.], consistently with the experimental results of Mahaut et al. [F. Mahaut, X. Chateau, P. Coussot, G. Ovarlez, Yield stress and elastic modulus of suspensions of noncolloidal particles in yield stress fluids, J. Rheol. (2008) 52 287-313.] obtained with many different particles and suspending yield stress fluids. This consistency between the results obtained in various yield stress fluids shows that the yield stress of the suspension does not depend on the physicochemical properties of the suspending yield stress fluid; it only depends on its yield stress value. This shows that studies of suspensions in model yield stress fluids can be used as a general tool to infer the behavior of fresh concrete. Moreover, we show that the thixotropic structuration rate of the interstitial paste (its static yield stress increase rate in time) is not affected by the presence of the particles. As a consequence, it is sufficient to measure the thixotropic properties of the constitutive cement paste in order to predict the thixotropic structuration rate of a given fresh concrete. This structuration rate is predicted to have the same dependence on the coarse particle volume fraction as the yield stress.     

From mini-cone test to Abrams cone test: measurement of cement-based materials yield stress using slump tests

Stoppage tests in civil engineering consists in measuring the shape of a fresh material deposit after flow occurred. This measured geometrical value (slump, spread) is linked to the plastic yield value of these yield stress materials. The most famous example is the Abrams cone for concrete. In this work, the flow induced by a smaller cone test for cement pastes and grouts is studied. In a first part, the spread is theoretically linked to the plastic yield value. Experimental results on several cement pastes validate the obtained relation but also shows the necessity to take in account the surface tension effects for low yield stress materials. The modified relation allows the prediction of the plastic yield value from the measured spread. The proposed method is then applied to the Abrams cone and fresh concrete. It is demonstrated that it is only suitable for high slumps (>20 cm).     

Estimating time and temperature dependent yield stress of cement paste using oscillatory rheology and genetic algorithms

A controlled shear stress–shear rate rheometer was used to determine the viscoelastic behavior of cement paste incorporating various superplasticizers and subjected to prolonged mixing at high temperature. At a low applied shear stress range, the oscillatory shear strain/stress curve of cement paste was characteristic of a linear elastic solid; while the higher stress range was characteristic of a viscous liquid exhibiting a linear strain increase with increasing applied shear stress. The transition from solid-like to liquid-like behavior occurred over a very narrow stress increment. This transition stress corresponded to the yield stress parameter estimated from conventional flow curves using the Bingham model. The yield stress from oscillatory shear stress tests was estimated using the intersection between the viscous part of the oscillatory shear strain/stress curve and the oscillatory shear stress axis. In this study, equations describing the variation of shear strain versus shear stress beyond the solid–fluid transition for cement pastes incorporating various superplasticizers at different ambient temperatures and mixing times were developed using genetic algorithms (GA). The yield stress of cement pastes was subsequently predicted using the developed equations by calculating the stress corresponding to zero strain. A sensitivity analysis was performed to evaluate the effects of the mixing time, ambient temperature, and superplasticizer dosage on the calculated yield stress. It is shown that the computed yield stress values compare well with corresponding experimental data measured using oscillatory rheology.     

Temperature variation in high slump drilled shaft concrete and its effect on slump loss

Drilled shaft refers to a deep foundation system where a single large diameter pier is used to replace a whole group of piles. High slump self-compacting concrete is used in drilled shafts due to its high fluidity and less proneness to segregation. The Florida Department of Transportation (FDOT) specifications require that such concrete should have a slump between 7 and 9 in. when placed and should maintain a slump of 4 in. or more throughout the concrete placement time. Furthermore, the mix for the slump loss test should be prepared at a temperature consistent with the highest ambient or initial concrete temperature (whichever is greater) expected during actual concrete placement. It is possible that the temperature of concrete inside the drilled shaft is lower than the ambient or initial concrete temperature due to the presence of ground water. If that is the case, slump loss would be less than the loss determined at the highest ambient or the initial concrete temperature, making the FDOT requirement unrealistic. However, it should be experimentally verified. This experimental study was conducted with the objective to establish profiles of concrete temperature in time from placement to hardening along depth as well as across width of the drilled shaft. Based on the gathered data, it was found that no significant temperature differential existed along the depth and across the width of the drilled shaft during the initial setting of concrete. The temperature of concrete inside the drilled shaft was same as initial concrete temperature before placement at all locations. This finding leads to the conclusion that concrete temperature inside drilled shaft is not affected by ambient temperature and/or the underground temperature conditions.     

Prediction of fresh concrete flow behavior based on analytical model for mixture proportioning

Large number of experimental techniques and models has been developed recently in an attempt to link the parameters of Bingham equation to concrete composition. On the other hand, concrete mixture proportioning methods based on rheological approach usually do not provide direct input of a measurable rheological parameter(s) into the proportioning expression. In this study, series of concrete mixtures have been proportioned by the use of a theoretical model. The experimental results were compared with the predicted rheological quantity by the model. The evaluation of concrete flow parameters has been performed using a newly developed tube viscometer for concrete. The discussion presents a comparison between the model calculated apparent viscosity and the measured plastic viscosity of fresh mixes as function of volume fraction of solids, normalized with respect to their maximum packing values.     

The utility of a bimolecular expression to describe the heat generation and temperatures in curing Class HP concrete

For several days after concrete is poured, atmospheric conditions influence the hydration reactions of concrete's binder components and so may influence its long-term durability. Accurate concrete temperature and moisture forecasts would help engineers determine an optimal pour time. Some existing curing concrete models include complicated chemistry and/or microstructure development parameterizations or do not allow for mix design changes. A bimolecular heat generation expression that is simple but sufficiently detailed to account for mix design changes was improved for Class HP concrete. Analysis of published calorimetry data and those determined in this study indicated that a second-order formulation adequately describes the heat generation. Class HP binder has an activation energy of 35 kJ mol⁻¹. After 72 h, Class HP pastes evolved 250–280 kJ kg⁻¹. A method to account for the effect of retarders on Stage II length and Stage III hydration rates was developed. A curing concrete bridge model with the bimolecular expression predicted concrete temperatures to within 2 °C of observed temperatures and reasonable 72-h hydration fractions (0.6).     

A study on the applicability of vibration in fresh high fluidity concrete

The introduction of superplasticizers (Sp) in the production of concrete has produced highly flowable mixtures with enhanced viscosity. In cases of optimum flowability and viscosity, for example self-compacting concrete (SCC), no vibration is necessary for placement. However, such ideal conditions are not practically easy to achieve and deviations are possible. This paper reviews the results of a study to investigate the vibration of such high fluidity concrete. Two criteria were used to characterize the fresh mix, that is, slump flow and V-funnel time (V-time). Firstly, the feasibility of vibration on such mixes was studied. Then, the significance of flowability and viscosity was determined. Next, the relationship between workability and its segregation tendency was investigated. Finally, concrete mixes that missed SCC criteria were vibrated. Three different scenarios of vibration were concluded: namely, mix that accept vibration freely, mix that required controlled vibration and mix that needed prior treatment of viscosity enhancing agent (VEA) before vibration.     

Influence of clays on the rheology of cement pastes

The fresh state of concrete is becoming increasingly important in furthering the types of applications of today's construction world. Processing techniques have resulted in technologies such as self-consolidating concrete and depend on the microstructural changes that take place during and immediately after mixing and placing. These changes to the microstructure reflect the flocculation behavior between the particles in suspension. The ability to modify this behavior allows control over the balance among flowability and shape-stability of concrete. This study investigates how clay admixtures affect the microstructure of cement pastes from a rheological stand point. Shear and compressive rheology techniques are used to measure how the solids volume fraction of suspensions with different admixtures evolves with stress. Based on these relationships, the effectiveness of clays on the balance between flowability and shape-stability is measured. Results are consistent with green strength tests performed on concrete mixes derived from the cement paste mixes.     

Setting time determination of cementitious materials based on measurements of the hydraulic pressure variations

An experimental investigation was carried out to determine the setting time of cement based materials (cement paste, mortar, concrete, etc.). An original method based on measurements of both total lateral pressure and hydraulic pressure has been investigated. An original device has been engineered to measure the pressure kinetics. Just after mixing and filling of the device, a simultaneous drop and an equal value of the both hydraulic and total lateral pressures has been recorded. A definitive cessation of total lateral pressure and negative hydraulic pressures are then observed. The proposed setting time was defined as the elapsed time between the end of mixing and the time at which the hydraulic pressure becomes zero. In addition to the usual W / C parameter, the influence of the vibration and the height of the material tested on the pressure based method were studied. Comparing to other classical methods (Vicat, calorimetry, ultrasonic pulse-echo …), the presented device is efficient with major types of cement based materials (concrete, SCC …) and was able to give a simple and direct information about the mechanical state of the material.     

Effects of temperature on the hydration characteristics of free lime

The rate of heat evolution of hydrating CaO has been determined quantitatively by means of a conductive microcalorimetry, and the hydration kinetics of CaO has been studied. Some regularity of the influence of temperature on the hydration characteristics of free lime (f-CaO) has been discussed as well. Based on the present research, the mechanism of effects of temperature in the calcining process and the hydration process on the hydration of CaO has been illustrated.