# Investigations of Mixing Technique on the Rheological Properties of Self-Compacting Concrete
## 3 Results and Discussion
### Effect of Mixing Time on Rheological Properties of SCC  
Figure 5 shows the slump-flow of SCC at different mixing time period. It is obvious that an increase of the mixing time leads to a higher slump-flow. However, once the mixing time reaches 4 min, a further increase of the mixing time will not increase the slump-flow of the SCC, i.e., attaining the plateau after 4 min. Figure 6 presents the changes of the rheological parameters with mixing time. In a general trend, both the yield stress and the plastic viscosity decrease with a longer mixing time. This is acceptable in view of the high content of SP and low water-powder ratio of SCC. A short mixing time is not favorable to the dispersion of the components, especially to the dispersion of the SP, resulting in a smaller slump-flow of the SCC mixtures [18]. With the prolongation of mixing time, the components in the mixtures is more sufficiently mixed. The SP can then be better dispersed and gradually achieve the best water reducing effect, and a maximum slump-flow of the mixture is eventually reached. From this moment onward, further extending the mixing time will lead to the state of \"over mixing,\" and the slump-flow of the SCC mixtures can be slightly decreased afterward. This is a result primarily from the friction and extrusion of the coarse aggregates, whereby the particle size decreases, the surface roughness increases, and the water demand increases [40; 41].  
Figure 4: Effect of charging sequence on the shear thickening behaviour of SCC.  
Figure 5: Effect of mixing time on the slump-flow of SCC.  
On basis of the modified Bingham model, the rheological curves of SCC with different mixing time were determined. The results are presented in Figure 7. An obvious observation points to the shear thickening behavior in all the SCC mixtures, a finding similar to that as drawn from Figure 4. Figure 7 presents the relationship between \(c/\mu\) and mixing time. The value of \(c/\mu\) shows a decreasing trend with the increase of mixing time. It is a direct evidence supporting that a longer mixing time can weaken the shear thickening behavior of SCC. In this regard two aspects are responsible. First, with a very short mixing time, the particles present in a high heterogeneity inside the mixture and are in a disordered distribution state. Under the action of shear stress, the granular system changes gradually towards to the ordered distribution state and correspondingly the interference between particles decreases. Second, the SP disperses unevenly in a short mixing time and it is more prone to intertwine under shear stress. The degree of disorder will therefore increase, resulting in a further enhancement of the shear thickening behavior of the mixtures.  
In order to speculate the critical mixing time at which the transition of shear thickening to shear thinning could occur, the values of \(c/\mu\) are plotted as a function of the mixing time, as shown in Figure 7. On curve-fitting basis, an expression given in Equation (4) can be obtained. In case the \(c/\mu\) is set as zero, the mixing time equates at around 5.9 min, which is defined as the critical mixing time for the transition of shear thickening to shear thinning.  
\[c/\mu=1.405-0.155t-0.014t^{2}, \tag{4}\]  
where \(t\) [min] is the mixing time.  
Figure 6: Effect of mixing time on the rheological parameters of SCC.  
Figure 7: Effect of mixing time on shear thickening behaviour of SCC.