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Fresh concrete can exhibit different rheological behavior when at rest than when flowing. This difference is due to thixotropy, which can have important consequences for formwork pressure, multi-lift casting, slip-form paving, pumping, and segregation resistance. This TechNote defines thixotropy and distinguishes it from other changes in rheologica...
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... loop test method-The hysteresis loop test (Ferron et al. 2007) is conducted by using a rheometer to apply a range of shear rates to concrete initially at rest. To perform the test, concrete is placed in a rheometer, and left at rest for a defined period of time to allow structural build-up to occur. Rest time, typically on the order of minutes, could change from test to test depending on the materials, equipment, and application considered. The shear rate in the rheometer is increased from zero to a predetermined maximum level and then decreased back to zero. Typically, the time period over which the shear rate is increased (up- curve) is equal to the time period over which it is decreased (down- curve). The response in terms of material shear stress is recorded and plotted against the shear rate, resulting in flow curves as shown in Fig. 2. Any difference between the up-and down-curves is termed ...
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... the material is sheared at an increasing rate, the structure of the material gradually becomes more and more broken. When the material is then sheared with a decreasing rate, the gradual building up of the structure begins again; however, the rate of the structural build-up is slower than the rate of the breakdown that has occurred during the up-curve. Thus, a hysteresis loop is formed by the up- curve and the down-curve as shown in Fig. ...
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This experimental work investigates into the effect of a change in the compressive strength of concrete along the length and height of monolithic deep beams made of high performance self-compacting concrete (HPSCC). In the tests, three different HPSCC mixes were used in which the amount of silica fume differed (0, 5 and 10% by mass of cement). The...
Citations
... In the absence of formwork, 45 extruded layers must withstand the gravitational loads of self-weight and vertical hydrostatic 46 pressure [3] exerted by the layers placed ever upward, one by one. These requirements are often 47 met by increasing the structuration rate of the material at rest by a) controlling hydration and/or 48 setting kinetics through the addition of chemical admixtures [4][5][6] and b) controlling the 49 flocculation rate and, consequently, structural build-up by adding supplementary cementitious 50 materials and/or other 'fine' constituents such as nano-clay [7,8]. the fundamental difference in their rheological behaviour at the time of placement [9]. ...
As a first step toward characterising structural build-up of high-strength, printable concrete mixes, the structural build-up of cement pastes of varying compositions and rheological properties are investigated. It is demonstrated that applying low shear rates over short measurement periods does not always result in the achievement of flow-onset in stiffer cementitious materials commonly used in digital construction. For such materials, a characteristic delay exists before the effective shear rate reaches the applied shear rate. This leads to effective strains in the materials tested and consequently to the erroneous characterisation of structural build-up. A strain-based approach is suggested here as a more appropriate method for characterising structural build-up in the case of stiff materials. Maintaining a low and constant-shear-rate among various measurements is not necessary if total effective strain is kept constant. Investigations on pastes with different compositions show that pastes in which a portion of the cement was replaced with micro-silica and fly ash exhibit the high structuration rate. The use of secondary cementitious materials (SCM) appears to be similarly appropriate measure to make cementitious materials ‘printable’ when compared to using set-accelerators in the absence of inline mixing of accelerators in the printhead.
This paper investigated the effect of adding lignosulfonate (LS) superplasticiser at the different stages of mixing on the workability and rheological behaviour of sodium silicate activated slag (SSAS) in order to find a practically feasible approach to tackling the incompatibility issue between superplasticiser and alkaline activator. In addition to rheology and minislump tests, adsorption, zeta potential and environmental scanning electron microscopy tests were also undertaken to understand the interactions between the lignosulfonate and the fresh SSAS in order to reveal the mechanisms behind the observation. The results show that adding the LS and the activator separately at the different stages of mixing can significantly improve both the initial minislump and 60-minute minislump retention due to the increased adsorption of LS and the improved dispersion of slag particles, with the prior addition of LS better than the delayed addition. However, a nonlinear rheological behaviour of SSAS was observed in the LS-superplasticised SSAS under separate addition and, consequently, modified Bingham model was found to be more suitable for describing this kind of rheological behaviour.
Rheology is an effective and efficient indicator to characterize the flow properties of fresh concrete. Through rheology measurement, the influence of variations in constituents and mixing proportions on the fluidity of fresh concrete can be effectively evaluated. In the case of self-compacting concrete and 3D printed concrete, the user can predict and optimize the flowability of fluid concrete by controlling yield stress and viscosity. The rheology models have been proven to be an efficient tool to predict the fresh properties of cementitious composites. In most cases, fresh concrete follows Bingham’s model. However, the special concrete combined with nano-materials requires certain modifications in the existing models. This paper presents a critical review of applicable rheological models based on flow curves as well as the viscosity equations of dense suspensions employed to characterize the flowability of cementitious materials in general. In addition to the governing variables included in the model formula, this review paper also highlights the effect of cement hydration, temperature, mixing methods, mixing sequence and time, vibration, pressure and air entrainment on rheology. Their respective effects can be potentially reflected and quantitatively evaluated by determining the values of the governing variables accordingly. The summary and discussions presented in this paper provide relevant information and knowledge on the rheological properties of the cementitious matrix with and without nano-modification and promote further research.
