# Determination of the Optimum Amount of Superplasticizer Additive for Self-Compacting Concrete
## Materials and Methods
### Concrete Mixer Method  
Based on the rheometer concept, there is a correlation between the concrete's flowability and the electrical power that the mixer consumes to make its blades rotate. Since SPAs modify the fluidity of the concrete, it is suggested that the electrical power consumed by the concrete mixer will also be modified. During the test, the concrete mixer's engine works with lower power consumption than its capacity. In this situation, the mixer's blades rotate at the same velocity.  
To validate the hypothesis that there is a correlation between the power consumption and the workability, during the mixing process, controlled amounts of SPA are added at programmed times,  
Figure 4: Assembly for L-box test.  
Figure 5: (**a**) Assembly for V-funnel test; (**b**) V-funnel dimensions.  
modifying the flowability of fresh concrete. The method proposes the use of a conventional vertical axis concrete mixer, as shown in Figure 6. The method requires the measurement of the flowability in an indirect way through the power consumption of the electrical engine, coming from a three-phase electrical source. To do this, the voltage and current of each of the phases were measured, in order to monitor the power consumed by the mixer during the mixing process, using a DIEM DZ100 VS. The mixer has a vertical axle with an orbital mixing mechanism, a capacity of 100 l and a 700 mm diameter. The concrete mixer was fitted with a Gossen Metrawatt MAVOWATT 40 three-phase power analyzer. During the entire mixing process, the voltage and intensity in each of the three electrical phases were recorded at a frequency of one item per second.  
Firstly, the aggregates were introduced into the concrete mixer, from the largest to the smallest. Then, the cement and the adjusted amount of water were added; at this point, the concrete mixer was started. The mixer was run for 3 min without adding any SPA to allow the components to be mixed properly. After this time, the SPA was added every 3 min. In the first phase, until 1% of SPA/C was reached, 0.2% of SPA/C was added in each step. In the second phase, until 3.4% of SPA/C was reached, 0.4% of SPA/C was added in each step. This increase in the quantity of SPA/C per step was because the effect of SPA is greater for a small SPA/C. Table 4 shows the entire process of mixing concrete with the SPA. Where t\({}_{0}\) is the initial time for each step, t is the duration of each mixing step and SPA a is the accumulated quantity of SPA in each of the steps.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Step** & **t0 (min)** & **t (min)** & **SPA (mL)** & **SPAa (mL)** & **SPA/C (\%)** \\ \hline
0 & 0 & 3 & 0 & 0 & 0 \\
1 & 3 & 3 & 50 & 50 & 0.2 \\
2 & 6 & 3 & 50 & 100 & 0.4 \\
3 & 9 & 3 & 50 & 150 & 0.6 \\
4 & 12 & 3 & 50 & 200 & 0.8 \\
5 & 15 & 3 & 50 & 250 & 1.0 \\
6 & 18 & 3 & 100 & 350 & 1.4 \\
7 & 21 & 3 & 100 & 450 & 1.8 \\
8 & 24 & 3 & 100 & 550 & 2.2 \\
9 & 27 & 3 & 100 & 650 & 2.6 \\
10 & 30 & 3 & 100 & 750 & 3.0 \\
11 & 33 & 3 & 100 & 850 & 3.4 \\ \hline \hline \end  
As can be seen in the graphs, the saturation point is easier to determine as the volume of paste analyzed increases. On the other hand, there is hardly any difference between the two w/c ratios. For w/c = 0.4, the saturation point is clearly defined for 2% SPA. However, for w/c = 0.38, although the definition is not so clear, it is verified that above 2%, the fluidity does not improve, so this value is also considered as the saturation point in the Marsh cone test, as increasing the SPA implies a cost increase. If more tests were done, the measurements results would be influenced by the time of SP addition.