Self Compacting Concrete (SCC) mix design must involve key practices to achieve the requirements of quality and durable mix. The mix designed as per the requirements are tested before using it in construction.
The basic properties to be achieved by an SCC mix is flowability, passing ability, strength, and segregation resistance. The requirements to attain these properties are explained below.
SCC Mix Design Requirements
1. High Volume of Paste
As SCC concrete undergoes self-compaction by its own weight, it has to attain adequate filling ability so that the mix reaches every area. Friction between the aggregates restricts this spreading and hence the filling ability. This issue is solved by increasing the paste content in SCC mix design in a range of 300 to 400 l/m³. The volume of paste implies the combination of cement, water, additions, and air. This increase in paste helps separation of the aggregates and easy movement of the mix.
2. High Volume of Fines (<80µm)
SCC must be designed for sufficient workability to show the property of self-compaction. This workability must not bring segregation and bleeding issues. To limit these risks, SCC is designed to have a large number of fines in a range of 500 kg/ meter cube.
Excessive fines in the form of cement alone bring chances of an increased heat of hydration. For this, a part of fines is replaced by pozzolans or mineral admixtures like silica fume or fly ash.
The strength and durability requirements of the SCC concrete governs the volume of filler fines added to the mix.
3. High Dosage of Superplasticisers
Superplasticizers are introduced in SCC to obtain the fluidity and workability. Nevertheless, a high dosage near the saturation amount can increase the proneness of the concrete to segregate.The increase of workability by using superplasticisers won’t leave segregation or bleeding issues.
4. Use of Viscosity Modifying Agent
The viscosity modifying agent in SCC mix design has the same objective as that of fine particles. These help to attain flowability property for concrete without segregation and bleeding issues. These hold the mix by thickening the paste and holding the water with the skeleton created by these agents.Viscosity modifying agents are cellulose derivatives, polysaccharides or colloidal suspensions.
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.
Viscosity agents make SCC less sensitive to water variations in water content of aggregates occurring in concrete plants.
5. Less Coarse Aggregate
To increase the passing ability of SCC, the volume of coarse aggregate added is less. The coarse aggregate used can be either naturally rounded, crushed or semi-crushed aggregate.
The coarse aggregate has a role in increasing the packing density of the SCC. So the volume of the coarse aggregate must not be too high nor too low. The size of coarse aggregate used can be between 10mm and 20mm. With the increase in the size of the coarse aggregate, the passing ability decreases. The choice of a higher aggregate size is thus possible but is only justified with low reinforcement content.
6. Addition of Admixtures
Admixtures added to SCC can have a retarding effect on the strength and the temperature development in the fresh concrete, & this has to be kept in mind in the construction process. Suppliers of admixture can produce various admixtures suitable for different weather conditions & temperatures. The additions have to be performed based on the guidelines provided by the admixtures.