# Determination of the Optimum Amount of Superplasticizer Additive for Self-Compacting Concrete
## Materials and Methods
### Concrete Flowability  
Once the saturation point in the Marsh cone test had been defined at 2%, different mixes were made with the previously indicated mix proportions, modifying the amount of the superplasticizer SPA used: 1%, 1.5%, 2% and 3%. Workability tests were carried out on these mixes to verify whether the saturation point in the Marsh cone test corresponded to the optimum amount of SPA in the concrete.  
The diameter of the slump flow is useful to evaluate the deformation capacity of SCC, while the t\({}_{500}\) is used to characterize its viscosity and cohesion.  
Figure 9 shows the values of the slump flow diameter and the t\({}_{500}\). It can be seen how both the diameter and the t\({}_{500}\) show an important variation when passing from SPA/C = 1.5% to 2%, agreeing with the saturation point obtained with the Marsh cone test. However, between 2% and 3%, there is no variation.  
Figure 9 does not show data for the mix with 1% SPA/C as no conclusive data are available, since it is a concrete with plastic consistency, as shown in Figure 10, where it can be seen that its fluidity is almost zero.  
Figure 10 shows the appearance of the different mixes after the slump flow test. Based on this result of the mixing with SPA/C = 1% in the slump flow test, the L-box and V-funnel tests will not be carried out for this mix, given that it does not make sense as it is not a SCC. In the case of 1.5%, it can be seen that it is a concrete with good workability that does not present a water/paste/mortar ring beyond the coarse aggregate; nevertheless, it can also be seen that the thickness of the flow spread is excessive, so it can be expected that when increasing the quantity of SPA, it will continue to maintain its cohesion without presenting segregation. In the case of 2%, it can be seen how the concrete has spread uniformly without presenting a water/paste/mortar ring beyond the coarse aggregate, reaching a higher diameter value. Finally, it can be seen that for the 3% admixture mix, there is a clear water/paste ring beyond the coarse aggregate that penalizes SPA contents higher than 2%.  
Figure 9: Slump flow test results.  
Figure 11 shows the results obtained with the L-box. When this saturation point value is exceeded, it can be seen that, although the value of H2 remains approximately constant, the value of H1 increases. This is because this amount of SPA causes the thixotropy of the mixture to become inappropriate, producing a degree of segregation of the coarse aggregate, which stops moving together with the mortar and accumulates in the initial part of the L-shaped box.  
On the other hand, the blocking coefficient (CB = H2/H1), a good indicator of the ability to pass through narrow openings, which includes the capacity for deformation and resistance to blocking, presents a maximum also coinciding with the saturation point.  
The results obtained in the V-funnel test are shown in Figure 12. For this test, the results obtained decreased to 3%, with no minimum at the saturation point. Greater fluidity can be seen in this figure when the SPA/C ratio is increased, but it has been proven in the previous test that SPA/C contents above 2% cause segregation, so the paste and the aggregate do not flow together.  
Figure 11: L-box tests results.  
Figure 10: Detail of slump flow test.  
The method proves to be efficient for evaluating the ability of concrete to flow in restricted areas in the vertical direction; however, the test does not allow a good qualification of the tendency towards segregation and blockage between the reinforcements. Moreover, the result does not reflect the ability to flow in the horizontal or anti-gravity direction. Therefore, the flow time obtained in this test is not enough to quantify the properties necessary to guarantee the performance of the concrete as an SCC and must be complemented by other tests.