The ViscoCorder is a single-point device used in Germany to measure the consistency of fresh mortar. Banfill (1990) modified the test to measure both the yield stress and plastic viscosity of mortar.
The device consists of a metal cylinder mounted on a rotating turntable. A paddle inserted in the cylinder is connected to a calibrated spring that measures the torque on the paddle. As the cylinder is rotated, the mortar applies a torque to the paddle. Traditionally, the device was operated at only one rotation speed. Banfill (1990) modified the device to measure torque at multiple rotation speeds. To obtain a plot of torque versus speed, the speed of the cylinder is changed in steps from zero to a maximum speed and back to zero. Unlike other rheometers, the device is not automated to change rotation speed and continuously record torque versus time. The device can be calibrated to correlate values for torque and speed to yield stress and plastic viscosity.
Banfill (1990) found that the ViscoCorder works well for fluid mortars; however, stiff mortars slip on the wall of the container, resulting in torque readings that are not an accurate representation of rheology. The container does not include any protrusions to prevent slip.
The setting time of concrete, mortar, or paste can be measured as an indication of workability (Ferraris 1999). One of the most common tests is the Vicat needle test for testing cement paste (ASTM C191). The Vicat needle test is also used in ASTM C953 for grout for pre-placed aggregate concrete.
The Vicat needle apparatus consists of a 300 g moveable rod with a 1 mm diameter needle at one end. The rod slides through a frame, where an indicator on the rod moves over a scale mounted to the frame. A specimen of fresh cement paste prepared in a certain proscribed manner is placed in a conical ring below the frame. After thirty minutes the needle is placed on the cement paste specimen and allowed to settle under its own weight. The depth of penetration is recorded from the scale. The test is repeated every 15 minutes (10 minutes for Type III cement) until a penetration depth of less than 25 mm is obtained. Each subsequent reading is taken at a different location on the paste specimen.
Similarly, the penetration test method described in ASTM C403 is used to determine the setting time of concrete by measuring the penetration resistance of mortar specimens sieved from concrete samples. Unlike the Vicat needle test, the apparatus used in ASTM C403 measures the force required to cause penetration, not the depth of penetration.
The mini-flow test (Zhor and Bremner 1998) is a variation of the mini-slump test described in the above subsection. The plexiglass sheet used in the modified version of the mini-slump test is mounted to a standard flow table, as described in ASTM C230. After the mini-slump cone is lifted from the sample of cement paste, the table is dropped 15 times in 15 seconds. The rest of the test procedure is unchanged. The mass of the cement paste is measured in order to determine air content. The results of the mini-flow test reflect the addition of energy to the cement paste. The mini-flow test is more appropriate than the mini-slump test for stiff mixes.
The mini-slump test, which was originally developed by Kantro (1980) and later modified by Zhor and Bremner (1998), measures the consistency of cement paste.
The mini-slump cone is simply a small version of the slump cone. The mini-slump cone has a bottom diameter of 38 mm, a top diameter of 19 mm, and a height of 57 mm. The cone is placed in the center of a square piece of glass on which the diagonals and medians are traced. The cone is lifted and after one minute, the average spread of the paste, as measured along the two diagonals and two medians, is recorded.
Zhor and Bremner (1998) modified the device in order to measure more effectively the air entraining and plasticizing effects of admixtures on cement pastes. A clear plexiglass sheet, which is used instead of glass, is set on a balance. After the cone is removed, the mass of the concrete is measured and used to determine the air content of the paste in accordance with ASTM C185. The paste is left to harden on the plexiglass for two days. A planimeter is then used to measure the area of the hardened paste on the plexiglass sheet. Like the conventional slump test, the results of the mini-slump test should be related to yield stress. Research conducted by Ferraris, Obla, and Hill (2001) into the influence of mineral admixtures on the rheology of cement paste showed poor correlation between the results of the mini-slump test and yield stress, as measured with a sophisticated, laboratory grade parallel plate rheometer.
The turning tube viscometer (Hopkins and Cabrera 1985; Ferraris 1999) is based on the same principle as the moving sphere viscometerânamely, Stokeâs Lawâbut is only considered appropriate for testing mortar.
An 800 mm long, 60 mm diameter tube is attached to a rotating arm, which allows the tube to be rotated in the vertical plane. A metal ball is allowed to fall through the fresh mortar in the tube.
A magnet can be placed on the specially milled end caps to ensure that the ball starts in the center of the tube. Inductance coils wrapped around the tube at two locations detect when the ball passes in order to determine the time for the ball to fall a known distance.
The test is conducted with different ball diameters and the results of the test are plotted on a graph of the inverse of the ball diameter squared versus time. The apparent viscosity of the concrete can be calculated based on Stokeâs law. Since the assumption in Stokeâs law that the ball is moving slowly through a fluid of infinite size is not valid for the test apparatus, correction factors are applied to provide a more accurate result.
The dimensions of the device are not large enough to permit the turning tube viscometer to be used for concrete. The ball diameter should be significantly greater than the maximum aggregate size so that the fluid can be considered a uniform medium. Further, the diameter of the tube should be sufficiently large to avoid interlocking of aggregate particles, which could interfere with the ballâs descent.