Design production and placement of self consolidating concrete
Higher aggregate content increases the yield stress and viscosity, so does aggregate with high aspect ratio.[xii] Similarly Assaad and Khayat[xiii] showed that an increase in the coarse-to-fine aggregate ratio and an increase of size of aggregate bring about a significant increase in the rate of stiffening. The effects of the superplasticizer:binder ratio (by wt.) on slump flow of concrete at w:b ratio of 0.39 (Bonen and Shah) Bonen and Shah reported on the effects of the superplasticizer content, coarse aggregate-to-cement ratio, and fine aggregate-to-coarse aggregate (c:f) ratio on the flow properties of concrete. Forced pan mixers have higher mixing efficiency than drum or mortar mixers (Deshpande and Olek,[iv] and Takada et al.). performed laboratory investigation of the effect of mixer type on fresh concrete properties of SCC and concluded that for the same composition and mixing sequence, tilting drum mixer increases the V-funnel flow times of SCC as compared to SCC mixed in pan mixer, and to achieve the same slump flow (650±30 mm), smaller amounts of superplasticizer (SP) were needed in the tilting drum mixer. It was reported that for the same water-to-powder ratio by volume (Vw/Vp), longer mixing times for 7.5 and 3.5 minutes in gravity mixtures required lower SP dosages and resulted in higher slump flow values and low V-funnel flow times as compared to shorter mixing times of 5.5 and 2.5 minutes.
As an example, a typical yield stress of SCC is about one order of magnitude smaller than the corresponding yield strength of regular concrete.[xvii] These low yield stresses and plastic viscosity values inherently compromises the segregation resistance and countermeasures must be taken as discussed below. The authors concluded that although the SCC mixtures generally require longer mixing times than conventional mixtures, their mixing time can be reduced by increasing the fine particle content, (with a constant w:c ratio), increasing the total water amount, and replacing part of the cement by silica fume. Another approach is to measure the electrical conductivity along a vertical section as a function of time.[iii] This method is sensitive to bleeding, rather than settling of aggregate.
Similarly, the robustness of the flow is proportioned to the agg:b volume and SP:b wt%. It was observed that for the same mixing sequence and mixing time, the mixtures produced using the pan mixer had higher viscosity as compared to the mixtures mixed in the mortar mixer. "Influence of Mixing Efficiency on the Mixture Proportion of General Purpose Self-Compacting Concrete " International Symposium on High-Performance and Reactive Powder Cements Sherbrooke, Canada, 19-39. While such results might not be representative, interpretation of the results suggests that it is not the mixing time, but rather the sheer energy and sheer rate that count.