# Investigations of Mixing Technique on the Rheological Properties of Self-Compacting Concrete
## 3 Results and Discussion  
### Effect of Charging Sequence on Rheological Properties of SCC  
It has been recognized that the fluidity of ordinary cement pastes can be improved by retarding the addition of SP. Early addition of SP can easily result in part of the SP to be wrapped by the early cement hydration products, reducing the effective content of SP. This disadvantage can be avoided when the SP is added in the later mixing stage [18]. Figure 1 shows the slump-flow of SCC prepared under five different charging sequences. As can be seen, the slump-flow of SCC by charging sequence No. A is obviously higher than that by charging sequence No. B. It seems that the conclusions mentioned above are not entirely applicable in the case of SCC. This may be due to the fact that the mixing time is shorter after the addition of SP, and the dispersant of SP is not uniform. In charging sequence No. C, by first mixing the sand and 3/4 of the water content for 30 s and then adding other raw materials, adsorption of a large amount of free water to the sand will be promoted, resulting in a decrease of the slump-flow of SCC. By first mixing the sand, coarse aggregate and 3/4 of the water content for 30 s and then adding other raw materials, charging sequence No. D will lead to an extra increase in the water adsorption by aggregate and hence further decrease the fluidity of the SCC. The charging sequence No. E, where the cementitious materials and water were mixed first and the other raw materials were added thereafter, avoids direct contact between aggregates and water. As a consequence, adsorption of the free water to aggregate surface is hindered significantly and the fluidity of the SCC is improved. On the other hand, due to the short mixing time after adding SP, the slump-flow of the SCC by charging sequence No. E shows a slight decrease, compared to that by charging sequence No. A.  
Figure 1: Effect of charging sequence on the slump-flow of self-compacting concrete (SCC). Figure 2 shows the effect of the charging sequence on the rheological parameters of the SCC. Combinations of Figures 1 and 2 reveal that a larger slump-flow corresponds to a smaller yield stress. An approximately linear relationship has been found between the two parameters, as shown in Figure 3. The plastic viscosity is relatively larger when the SCC was prepared by the charging sequence No. B (SP subsequently added method) than by the No. A. This is a result of the shorter mixing time of SP, whereby the dispersing effect of SP was weakened and the volume of free water was reduced. A further increase of the plastic viscosity is observed when the SCC was obtained by the charging sequence No. C. As a matter of fact, a direct contact of the sand with the water can result in a considerable reduction in free water content due to the impact of the water adsorption by the sand. Of particular interest to note is that the plastic viscosity by charging sequence No. D is apparently lower than that by charging sequence No. A. This probably results from the friction and collision between aggregate particles in the mixing process, reflected by a decrease of the flow resistance as well as a decrease of the plastic viscosity. It should be mentioned that charging sequence No. E results in the smallest plastic viscosity. This is reasonable against the background of interface physics. By first mixing a large amount of water with cementitious materials, a paste with low water-powder ratio will be produced and, as a consequence, the water adsorption of aggregate after its addition is greatly reduced and thus the free water content is increased, leading to a higher fluidity and a lower plastic viscosity. Figure 4a shows the rheological curves of SCC versus different charging sequence obtained by fitting with the modified Bingham model (Equation (3)). As can be seen, these rheological curves are all concave in shape. As such, the shear thickening behavior has taken place for these SCC mixtures. Similar tendencies of the torque-speed pattern can be observed in all cases.