# Automated Control of the Fresh State of Industrial Concrete Behaviour by Rheometer Test Adjustment
## 5 Results
### Rheological Measurement Adjustment
#### 5.2.2 Selection of Speed and Impeller  
We first analysed the torque at the end of the pre-shear period to select the speed. If this torque was too low, a limiting effect related to the measurement range would exist, which may be too small, making it impossible to adjust the linear flow curve and provide precise slope values. The torque results at the end of the pre-shear period are shown in Figure 11. A speed of 30 rpm provided the highest values, where the impeller type was not a significant parameter. The values obtained at a speed of 10 rpm were lower than those obtained at 30 rpm, where impeller A provided higher values than impeller B. Although a speed of 30 rpm appears to be the best option, using high velocities presents certain segregation risks. If the speed is too high and the mixing energy is excessive, part of the volume mix may not move, leading to segregation. This also produces imprecise values for the yield stress and plastic viscosity. A visual inspection was performed after each rheological test for both impellers, A and B, and for all the testing times. It is important to ensure the absence of heterogeneous areas after the shear tests of the concrete. These can manifest as the separation of the coarse fraction or, more likely, as the segregation of the liquid paste on top of the mix. Segregation was detected during the FCT with impeller A, which combined a higher velocity of 30 rpm and the longest pre-shear period of 90 s. As shown in Figure 12, the coarse aggregate and mortar are separated, which is analogous to the inner cylinder of a coaxial cylinder rheometer [10]. There was no sign of segregation under the slower velocity of 10 rpm and with the longer pre-shear time of 90 s. None of the other parameter combinations produced any segregation. For impeller B, there was no sign of segregation in any of the tests. Figure 11: Torque at the end of the pre-shear period. Figure 12: (**Left**): visual observation of FCT-30 \(\times\) 90-A-30. (**Right**): visual observation of FCT-10 \(\times\) 90-A-30. According to this result, the speed of 30 rpm combined with impeller A must be disregarded. Finally, the flow curves and relative parameters obtained were used to select the best procedure. The values of the relative yield stress and relative plastic viscosity can be obtained from the flow curve as a relative parameter related to the yield stress and plastic viscosity. Figure 13 presents the flow curves of the different tests with a pre-shear period of 90 s. The flow curves at both speeds, 10 rpm and 30 rpm, tend to be clearly linear and present a similar slope. Regarding the impellers, according to the values of the torque after the pre-shear period, the differences between impellers A and B were insignificant when the speed was high (30 rpm). However, when the speed was 10 rpm, the differences were more notable, where the flow curve of impeller A was always above that of impeller B. Using the flow curves shown in Figure 13, the values of the relative yield stress and relative viscosity were obtained, as shown in Figure 14.  
The analysis of the viscosity results confirms the segregation in test 30 \(\times\) 90 with impeller A. In this case, the relative viscosity over time demonstrates a decreasing trend from 15 min onwards, while it increases in all the other tests, indicating that the concrete becomes more viscous over time. The segregation detected by visual inspection is responsible for this behaviour. The test procedure of 30 \(\times\) 90 with impeller A should be disregarded. According to the viscosity values, all other tests appear to be suitable and can accurately predict a slight increase in the concrete viscosity over time. Figure 14: Relative yield stress and relative plastic viscosity. Figure 13: Flow curves of the mixes. The relative yield stress results were always negative when impeller B was used. Note, this concrete was highly fluid, thus a low yield stress was expected.