Model fluids substituting fresh UHPC mixtures flow behaviour

Abstract

Carboxymethylcellulose (CMC) and ultrasound gel (USG) seem to be suitable clear model fluids to substitute real ultra-high-performance concretes (UHPCs), enabling experimental monitoring of events occurring within these mixtures at similar flow behaviour. USG is eligible for UHPC mixtures with significant yield stress. CMC having a suitable concentration is better for UHPCs exhibiting indistinct yield stress. The rheological behaviour of real UHPC mixtures as well as substituting model fluids was measured using a non-standard system consisting of a vessel stirred by an anchor or a screw agitator. In addition, a standard rotational rheometer system with concentric cylinder geometry was used for optimization of the above-mentioned method of measurement and also for measuring the rheological parameters of the substituting model fluids. This article deals with the methodology for measuring the rheological properties of these substances and presents the results of the measurements using flow curves and rheological models.

CFD Analysis of Ultra-High-Performance Concrete Rheological Tests

“…It is well-documented that UHPC can be differentiated into two groups according to the level of yield stress. The first group is characterised by Newtonian or pseudoplastic characteristics with an indistinct yield stress, and the second group is represented by mixtures exhibiting nonlinear characteristics with significant yield stress [3]. Hence, models that can describe such behaviour, such as the Bingham (1) [4][5][6] or Herschel-Bulkley (HB) (2) model [4,6,7], are often used.…”

“…Hence, models that can describe such behaviour, such as the Bingham (1) [4][5][6] or Herschel-Bulkley (HB) (2) model [4,6,7], are often used. However, the three-parameter HB model is often too complex, as the yield stress can be insignificantly small; hence, a simpler Power Law (PL) model can be used instead (3) [3]. However, such a simplification may not be suitable for the slow low-shear-stress flows that are often seen in construction.…”

“…Moreover, the traditional Narrow-Gap Couette geometry defined by DIN EN ISO 3219 [12] is unsuitable for suspensions with large or dense particles due to sedimentation, wall slip, and small differences between the diameter of the particle and the size of the gap. To overcome these problems, a measurement method that utilises the power input characteristics in the creep region of a known agitator with the Metzner-Otto relation [13] between the effective shear rate and rotation speed for non-Newtonian fluids was suggested by Dostál et al [3]. A similar methodology is often used to measure the viscosity of more complex systems utilising screw agitators [3], helical ribbons [14,15], or anchor agitators [16,17].…”

“…To overcome these problems, a measurement method that utilises the power input characteristics in the creep region of a known agitator with the Metzner-Otto relation [13] between the effective shear rate and rotation speed for non-Newtonian fluids was suggested by Dostál et al [3]. A similar methodology is often used to measure the viscosity of more complex systems utilising screw agitators [3], helical ribbons [14,15], or anchor agitators [16,17]. The weakness of these methods is hidden in the imprecise nature of the agitator constants, which may vary between agitators of the same type due to imperfections during manufacture, measurement errors, and their intertwined nature.…”

“…This methodology is still applicable for suspensions with a very small particle size (substantially smaller than the measurement geometry), but it fails when used for suspensions with large aggregates such as concrete [23]. To allow for larger aggregates, a different geometry with a wider gap must be adopted, which is, in principal, similar to the method presented by Dostal et al [3]. Such designs, however, are rarely adopted in industry, where a simple slump test is still preferred [23].…”