MIX DESIGN OF SELF COMPACTING CONCRETE (SCC)
Self Compacting Concrete has to be designed and tested before it is produced and used for construction. During SCC evaluation, the local materials and equipments have to be tested to find out the right concrete mix proportions and mixing times suitable for the element to be cast. Various kinds of fillers can result in different strength, shrinkage and creep but shrinkage and creep will usually not be higher than for traditional vibrated concrete.
A flow-chart describing the procedure for design of SCC mix is shown in Figure below,
General Requirements in the Mix Design of SCC
1. A high volume of paste:
The friction between the aggregate limits the spreading and the filling ability of SCC. This is the why SCC contains a high volume of paste (cement + additions + efficient water + air), typically 330 to 400 l/m³, the role of which is to maintain aggregate separation.
2. A high volume of the particles (<80µm):
In order to ensure sufficient workability while limiting the risk of segregation or bleeding, SCC contains a large amount of fine particles (around 500 kg/m³).Nevertheless, in order to avoid excessive heat generation, the Portland cement is generally partially replaced by mineral admixtures like fly ash (cement should not be used as a filler). The nature and the amount of filler added are chosen in order to comply with the strength & durability requirements.
3. A high dosage of super plasticizer:
Super plasticizers are introduced in SCC to obtain the fluidity. Nevertheless a high dosage near the saturation amount can increases the proneness of the concrete to segregate.
4. The possible use of viscosity agent (water retainer):
These products are generally cellulose derivatives, polysaccharides or colloidal suspensions. These products have the same role as the fine particles, minimizing bleeding and coarse aggregate segregation by thickening the paste and retaining the water in the skeleton. The introduction of such products in SCC seems to be justified in the case of SCC with the high water to binder ratio (for e.g. residential building). On the other hand, they may be less useful for high performance SCC (strength higher than 50 MPa) with low water to binder ratio. For intermediate SCC, the introduction of viscosity agent has to be studied for each case. Viscosity agents are assumed to make SCC less sensitive to water variations in water content of aggregates occurring in concrete plants. Because of he small quantities of viscosity agents required, however it may be difficult to achieve the accuracy of dosage.
5. A low volume of coarse aggregate:
It is possible to use natural rounded, semi crushed or crushed aggregate to produce SCC. Nevertheless, as the coarse aggregate plays an important role on the passing ability of SCC in congested areas, the volume has to be limited. On the other hand the use of coarse aggregate allows optimizing the packing density of the skeleton of the concrete & reduction of the paste volume needed for the target workability. Generally speaking, the maximum aggregate size (Dmax) is between 10mm &20mm. the passing ability decreases when Dmax increases, which leads to decrease of the coarse aggregate content. The choice of a higher Dmax is thus possible but is only justified with low reinforcement content.
Admixtures added to SCC can have a retarding effect on the strength and the temperature development in the fresh concrete, & this will have to be borne in mind in the construction process. Suppliers of admixture can produce various admixtures suitable for different weather conditions & temperatures.