The truck with an empty drum was tested initially by measuring the oil pressure versus the rotation speed. This procedure, which was done only once, is shown in Fig. 4. It was not possible to measure pressure below 3.44 MPa (500 psi), therefore, preventing the measurement of more than two data points. This curve should be used to correct the measurements done with a concrete loaded truck in order to measure only the influence of the concrete on the torque measured. In this case, the curve obtained is not precise enough to be used and therefore, the values reported here are not corrected.
The concrete was mixed in the central plant mixer for 10 min and then transferred to the concrete truck mixer. For the first set of tests (C10 to C14), the truck was filled to 50 % of its maximum capacity, and then more concrete was added on top for the second set (C20 to C23) to reach 100 % of its maximum capacity. The purpose of this sequence was to determine the influence of the load on the results. Since the concrete was setting and therefore the second series was quite different from the first, it was not possible to really detect the influence of load volume on the results.
After loading, the truck was moved to the laboratory location: therefore, the first laboratory test on the concrete began 30 min after the first contact of water and cement. The truck drum turned about 100 revolutions during the transport of the concrete between the central plant and the laboratory. For each mixture, a small volume of concrete was discharged from the truck for testing with the ICAR rheometer and the slump test. These tests were conducted concurrently with the measurements from the truck. The temperature of the concrete was also recorded.
After a set of measurements, the concrete was modified by incorporating an admixture or adding water (see Table 1). The concrete was then remixed in the truck and tests were repeated. Five mixtures were prepared for the first set of tests (C10 to C14), and four test mixtures were prepared for the second set of tests (C20 to C23).
To use the truck as a rheometer, the highest speed of the drum (1.74 rad/s or 16.66 rpm) was maintained for 10 revolutions while the oil pressure was recorded. The speed of the drum was then reduced in increments of 0.21 rad/s (2 rpm). The oil pressure and speed were recorded at each increment of speed. These measurements produced the curve of oil pressure (related to the torque) vs. rotational speed used to calculate the yield stress and plastic viscosity.
Fig. 4. Flow curves obtained with the truck mixer (empty truck, C10 to C14 mixtures).