Measuring Cell for Mortars
Both quality and quantity of rheological tests are mainly determined by the available range of measuring cells. In this field, unmet needs exist especially with respect to building materials testing. This article describes one of these cells that are suitable for highly flowable mortars. This so-called basket cell enables fluid-in-fluid shear, and thus suppresses wall shear effects. The basket cell has been designed as a double annular gap cell ensuring high performance at low yield stresses.
Working principle
Under the assumption that solids of revolution can be used to create an appropriate shear area (see DIN 53018 or 53019), it must be observed, as a restrictive condition, that aqueous fluid may segregate at the interfaces when subjected to compression and/or shear. From a processing point of view, this is often a useful characteristic of mortars. However, this behaviour results in the well-documented wall shear in the measuring cells referred to above, and thus leads to a distortion of measurement results. This effect may be limited by providing the measuring cells with a special surface texture, such as a ribbed pattern. Another possible solution could be to replace the adhesive fluid-wall bond with a cohesive bond. When pursuing this idea further, at least the critical cell walls would have to be replaced with fluid walls.
Following a large number of various experiments to create such cell walls, i.e. a “fluid-in-fluid shear” using external fields of force and the correspondingly high effort, the experience gained has been used to identify an easy-to-implement compromise solution. The setup comes very close to fluid-in-fluid shear if the intended fluid wall is established by means of a filigree framework. If a cylindrical framework is chosen – which would inevitably create an inner and outer fluid wall leaf, it would appear logical to also look for a solution similar to a double annular gap cell.
Measuring cell design
The double annular gap geometry is a favourable design not only on the basis of the considerations outlined above. It is also a suitable option because of the anticipated low limit yield stresses that result from the stability criterion [1] for self compacting mortars (SCM). Co-axial double gap measuring cells are not a new invention. For example, DIN 54453 recommends the use of such a cell to determine the dynamic viscosity of anaerobic adhesives. While the cell’s basic design is retained, its inner cylinder forming the double gap is replaced with the framework referred to above. As a result, the new measuring cell consists of an annular gap vessel – as shown in Fig. 1 – and the basket-like framework forming the sensor (Fig. 2). The framework is enclosed by a disc-shaped keyseat at the top and stiffened by an annular insert at the bottom. Following the infill of mortar into the annular gap vessel, the sensor will be moved coaxially, at low speed, to its measurement position. The fluid displaced during immersion can run off through the levelling openings of the sensor to fill the overflow receptacle of the annular gap vessel. The effective shear height L is determined by the immersion depth of the basket into the “fluid bath” in the annular gap space.
This cell setup has been subjected to numerous tests, in particular with respect to determining relative gap widths and framework designs. In order to ensure a sufficiently stable bottom flow in the gap against the impact of inertial forces, an instrument drive similar to the Hatschek device was used. On this basis, diamond-shaped framework mesh structures were found to be particularly well suited. These also allow for particle incorporation while ensuring a strong bond between the inner and outer fluid interface of the mortar attached on both sides. For mortars with a maximum particle size (mesh aperture) of 2 mm and a permissible oversize of up to 2.5 mm, tests involving various wall distances in the range 32≤ (4Δ ≈ D4 – D1) ≤8dK,max demonstrated extreme measuring fluctuations, up to a total standstill of the cell, only at the lowest distances. This effect was caused by a more or less significant wedging of the natural particles used. The wall distance ultimately chosen to determine the main dimensions equals Δ ≈ 3dK,max, and is usable only in conjunction with the framework design shown in Fig. 2.
Further design features were determined on the basis of a rather pragmatic approach. These include the position and design of the overflow receptacle and the minimization of the total weight of the annular gap vessel. These features also include the sensor immersion depth, which was determined at T [mNm] =τ [Pa] on a preliminary basis to enable a quick overview in the course of individual measurements. Prior to its use, the measuring cell must be calibrated. It is always advisable to calibrate the cell together with the other instruments available.
Findings and outlook
According to the experimental results obtained to date, the basket cell is suitable for all highly flowable fluids that are characterised by several phases, such as self-compacting mortars, grouting mortars, float finishes and fillers, as well as cementitious binder glues, ceramic slips, slurries of all types, and paints. Solutions can also be subjected to testing. As a prerequisite to any rheological materials testing, the object to be tested must be stable. This is also required for the setup described in this article.
The experience gained in the course of development, testing and application enables continuous improvement of the basket cell depending on the specific requirements. This includes both the main dimensions of the measuring cell determined by varying maximum particle sizes and the adjustment of the framework structure to special modes of application.
Prof. Dr.-Ing. habil. Ruprecht Vogel
Malerstieg 6 99425 Weimar / Germany
r.vogel@vogel-labor.de
References and further information
[1] Vogel, R. Ein Stabilitätskriterium für Selbstverdichtenden Beton, BFT Betonwerk+Fertigteil-Technik 12 (2005), S.42 bis 49 .
[2] Vogel, R.; Riedel, M. Untersuchungen mit dem Rotationsviskosimeter RHEOTEST 2 zur Abgrenzung des Einsatzbereiches ,Silikattechnik 41 (1990), Heft 2, S.59…64
[3] Vogel, R. Stabilität und Fließverhalten von Selbstverdichtendem Beton, Vortrag Ibausil Weimar Sept. 2006, Druck Bd.2, S.1047…1058
[4] Vogel, R. Fließen von Selbstverdichtenden Beton – Das Fließgesetz; www.vogel-labor.de (Mitteilung 04/6)