DESIGN OF A CONCRETE RHEOMETER
2.3. DESIGN OF CONCRETE RHEOMETER 1. Conceptual Design
2.3.2. Actual Design and Construction
As a first step, preliminary minimum design requirements were developed for the rheometer. These requirements were based on the expected operating range of the rheometer and the operating characteristics of existing rheometers. These requirements are listed in Table 2.1.
Fig 2.1 Principle of available parallel plate rheometers (Velocity profiles are all at the surface)
The development of present rheometer consisted of three major aspects: the selection of motor and gear box, development of control system and impeller. The motor should be able to provide sufficient torque to turn an impeller in concrete. Motors that are able to provide necessary torque may have very large rpm that may not be within the limit for concrete rheology. Size of high hp motor is also prohibitive in a rheometer. Shear rate setting on a motor within desirable torque range is difficult that is necessary for a flow curve measurement. In the present case, 1 HP motor with different rated revolutions
were used with different impeller sizes as discussed in detail in next paragraph for selection of motor.
Table 2.1 Preliminary design requirements
Particulars Design Requirements Maximum expected torque 25 N-m
Maximum shear strain rate 40 per sec
Size As compact as possible
Control Operations to be controlled by electrical appliances Impeller As small as possible while still generating representative
flow and minimizing segregation.
The impeller size should be such that gap sizes are a proper multiple of maximum aggregate size and that increment of torque generated at each higher speed increment can be measured accurately. Still, torque generated should not be too large in order to avoid unreasonably large size motor. In order to select impeller size, it was necessary to test experimentally impellers of several diameters. It is also useful to consider total torque generated after structural breakdown. If the torque measured by the impeller is too low, the resolution in torque measurements may be insufficient for accurate flow curve measurement. Further, low torque may be an indication that only a small portion of material is flowing. If the torque is too high, the range of concrete workability that can be measured will be limited. Based on series of tests of 75 mm, 125 mm, 150 mm and 175 mm impellers, the vane plate having diameter 150 mm was selected for use in the present case. Fig 2.2(a) & (b) shows the photograph of impeller and cylindrical container of presently designed and fabricated rheometer. The amount of torque generated by the impeller was acceptable for providing adequate torque resolution while not exceeding maximum torque capacity of rheometer during structural breakdown stage. Small size impellers can easily shear stiff mixes but the sensitivity is less for high slump concrete.
Tests were conducted with 1 HP-1425 rpm motor and 150 mm diameter vane plate to arrive at the gear ratio of gear box for optimum speed to be used to measure yield stress.
If the speed is too low, structure of concrete may reform due to flocculation before the yield stress is reached and there may be possibility of plug flow during rheological measurements. If the speed is too high, there may be possibility of particle migration. For the purpose of optimum speed, several gear boxes were fabricated with gear ratios 5, 7and 10. Gear ratio 7 was found suitable and finally selected keeping in mind the design requirements listed in Table 2.1.
A schematic diagram of the proposed parallel plate rheometer is shown in Fig 2.3 to describe its components and working principle. It consists of a 150 mm diameter flat circular vane plate driven by an induction motor through a gear box. The gear box reduces the rpm and at the same time it increases the torque. The thickness of the vane plate is 20 mm and it is mounted coaxially with a cylindrical container of effective diameter 270 mm (total diameter being 310 mm) with sleeve and bearing arrangement to ensure accurate alignment. The torque and speed of rotation of the motor and hence the vane plate is controlled manually by varying input voltage with a 10 ampere AC variac.
Variac is a type of auto-transformer. An auto-transformer is a one-winding transformer.
The same winding acts as the primary and a part of it as the secondary. The winding is tapped at a suitable point to obtain the desired output voltage across the secondary. In an auto-transformer, the smaller the ratio of primary to secondary voltage, the smaller is the amount of power transformed and hence larger is the amount of power conducted from primary to the load. A variac has a winding wound around on a toroidal core. A thick carbon brush makes contact with the winding in the desired position. The output voltage can be varied from zero to 120% of the input voltage. Variacs are mainly used for making available variable AC voltage from fixed supply voltage.
The number of revolution of the vane plate is measured automatically with a non-contact infrared digital tachometer, by focusing at the retro-reflective tape glued to the spindle or shaft. The contact type tachometer cannot be used in the present case because at low voltages, spindle cannot rotate once the tachometer is pressed against the shaft. Thus, one cannot measure the shear rate at low voltages with contact type tachometer.
The cylindrical container is provided with vertical ribs of 20 mm projection at a pitch of 60 mm along the circumference. A circular vane plate of diameter 310 mm and thickness 20 mm is also welded to the bottom of the cylinder. The effective gap between the bottom and the shearing surface is 75 mm. The effective concrete height above the vane plate is also 75 mm. The no-slip condition of flow at top of the cylinder is achieved by providing 20 mm high mesh of blades. The mesh can be detached for convenient cleaning as and when necessary. The spindle has a pulley welded to it at its mid height that is used for calibration purpose only. The photograph of the built up rheometer used in the present study to measure rheological parameters of HPC has been shown in Fig 2.4.