Aggregate segregation, which is also referred to as sedimentation, is controlled by the viscosity and yield stress of the mixture, the binder density, aggregate size, aggregate density, as well as the content of fines. This implies that the stability of SCC (of low yield stress) can be enhanced by increasing both the viscosity and density of the matrix and by decreasing the maximum size and density of the aggregate. It follows, that higher w:c ratio and/or SP:c ratio increase the susceptibility  to segregation and vise-versa, lower w:c ratio and SP:c ratio increase stability and therefore robustness. Similarly, greater fines content increases robustness either because it increases the viscosity or increases the density of the matrix. Silica fume is an example of a viscosity modifier, and slag and limestone are examples of density modifiers.

 

Aggregate particles in SCC may be regarded as discrete inclusions in a homogeneous matrix. Consequently, the tendency of the aggregate to segregate depends on the properties of both the aggregate and the homogeneous matrix. Large aggregate size and high density decreases stability and vise-versa. However, within common ranges of SCC mixtures and densities of aggregate, Bonen and Shah[i] argued that the most important factor that governs the rate of sedimentation is the aggregate size.

 

In addition to w:cm ratio and VMA concentration, the stability of SCC depends on the total content of fines in the mixture. Khayat et al. reported that SCC can exhibit greater resistance to surface settlement when the content of total fines in the mixture (smaller than 80 µm) increases for mixtures with similar aggregate packing densities. This was especially the case for SCC made with medium to low content of binder.

 

It should be noted that the resistance to segregation of the mixture during placing into the forms and after placing might not be the same, because the forces acting on the aggregate under these two conditions are not the same. Once the concrete has been placed in the forms and it is in a static state, the forces acting on the aggregate can be calculated from Stokes’ Law. However, during placing, and in particular during horizontal flow, an aggregate particle is subjected to additional forces; the mixture drag and vertical drag that help to keep the particle suspended in the mixture. The mixture drag is proportional to the square of velocity of the mixture and the square of the particle diameter, whereas, the vertical drag is proportioned to the velocity of the mixture and the aggregate shape. Consequently, as the velocity of the mixture is increased, the mixture stability is also increased. Based on this realization that the dynamic stability is less severe than the static stability, Bonen and Shah pointed out that that visual evaluation of segregation during slump flow is an inadequate measure for predicting the static stability.

 

As noted, the sedimentation velocity of aggregate in a static mode in the formwork is proportional to the radius square of the aggregate, the differences in the specific densities of the aggregate and matrix, and inversely related to the viscosity of the matrix. Because the viscosity of the mixture cannot be too high (otherwise the mixture will not flow), the ability to control the sedimentation rate by increasing the viscosity is limited to certain ranges. Therefore, robustness can be achieved either by reducing the aggregate size or increasing the matrix density or a combination thereof.

 

Figure 4 shows a plot of equal sedimentation rates of a 12.7 mm spherical aggregate with a density of 2.7 g/cm as a function of the matrix density and viscosity. Because the slopes of the sedimentation rates are highly negative, within the normal ranges of concrete densities, the density of the matrix has a greater effect on the sedimentation rate than the change in viscosity. Second, as the density is increased, the effect of viscosity becomes more prominent.

 

 

Since incorporation of fines affects the density of the matrix, Fig. 4 also indicates that the sedimentation rate can be reduced by increasing the content of the fines with high specific density. Consequently, robustness increases by incorporation of density modifiers, and with regards to fines, the best density modifiers is slag, followed in decreasing order by ground dolomite, ground limestone, and ground quartz. By contrast, neither silica fume nor most types of fly ash can be considered as density modifiers. The density of fly ash varies over a large range, commonly from about 2 to 2.5 g/cm^³ and that of silica fume is about 2.24 g/cm^³. Thereby, in most cases, addition of fly ash and silica fume does not affect the matrix density.

 

To reiterate, the resistance to segregation should not be based on visual inspection of the slump flow. For example, Ye, et al. showed that high superplasticized SCC mixtures that did not show segregation during slump flow test were prone to high segregation, and addition of VMA was instrumental for controlling it (Fig. 5). Nevertheless, even at relatively high dose of VMA of 0.08%, sedimentation was not completely eliminated. This observation is in agreement with similar results reported by Khayat and Guizani.[ii]