# Automated Control of the Fresh State of Industrial Concrete Behaviour by Rheometer Test Adjustment
## 5 Results
### Rheological Measurement Adjustment  
#### 5.2.1 Selection of the Pre-Shear Period  
The figures demonstrate that the pre-shear time cannot be shortened to 5 s, and that 90 s is required for the complete breakdown of the mixture. Figure 8 shows that, regardless of the impeller type or the velocity value, with a pre-shear period of 90 s, the torque reaches the equilibrium at the end of the breakdown time, that is, it remains constant overtime at the end of the constant speed period. However, when the pre-shear period is 5 s, the equilibrium is not reached (torque goes on decreasing at the end of the constant speed period).  
Figure 8: Time–torque evolution.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Compressive Strength (MPa)** & \multicolumn{3}{c}{**Slump Test (cm)**} \\ \cline{2-5}  & **28 Days** & **10 min** & **15 min** & **20 min** & **30 min** \\ \hline Mean & 77.27 & 19.57 & 19.00 & 19.04 & 18.79 \\ Std.dev & 6.21 & 1.29 & 1.02 & 1.64 & 2.17 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Compressive strength and slump test results.  
Figure 9 presents the flow curves 15 min after the addition of water to the mixer. The two parameters that varied were the velocity, which was 10 or 30 rpm, and the pre-shear period, which was established at 5 or 90 s.  
When a 90 s breakdown period was used, for both velocities and both types of impellers, the Bingham model (solid line) provided a good flow curve adjustment with R\({}^{2}\) around 0.99 (Figure 9). However, with the 5 s breakdown period, the flow curves (dotted line) present a non-linear behaviour that suggests shear thickening performance and would require the use of the Hershel-Bulkley model. As was seen in Figure 8, for the 5 s breakdown period, the torque values are not constant, which indicates that the non-linearity of the curve that is due to thixotropy has not been eliminated, and that the testing protocol is not suitable as it may lead to erroneous conclusions [30; 31].  
The study of the torque reduction as a function of the duration of the breakdown period (Figure 10) indicates that these reductions are always lower in tests with a breakdown period of 5 s. In this case, the differences between the types of impeller and speed applied were small. The reductions were greater with a breakdown period of 90 s, and those achieved in the 10 \(\times\) 90 test with impeller B were particularly noteworthy. The low torque reduction values indicate that the mixture did not break down effectively.  
The short pre-shear period of 5 s may be apparently insufficient to ensure the complete breakdown of the mixture. Therefore, the flow curve for these tests may be incorrectly measured, as the first pairs of torque speed included part of the remaining thixotropy of the concrete and incorrectly indicated an apparent non-linear behaviour [31]. This was confirmed by the shape of the flow curves (Figure 8), which were clearly non-linear. In these cases, the equilibrium of the torque at each rotational speed step in the flow curve measurement should be verified. When the pre-shear period was 90 s, a complete breakdown of the sample was achieved, and all the flow curves presented the behaviour of a Bingham linear model.  
Based on this result, the FCT using the pre-shear period of 5 s should be disregarded. This indicates that the total time of the FCT adds up to 120 s, where 90 s correspond to the  
Figure 10: Torque reduction.  
Figure 9: Flow curves at 15 min.  
pre-shear period and 30 s correspond to the testing of the descending ramp. This is not an excessive time for testing, although it presents a drawback that should be considered.