The mix ratio for self-compacting concrete (SCC) is typically defined by specific proportions of its constituent materials and key ratios like the water-to-powder ratio, rather than a single fixed numerical ratio like conventional concrete. Achieving self-compaction requires careful balance of aggregates, binder (powder), water, and superplasticizers.
Based on common guidelines for SCC mix design:
Key Mix Proportions for SCC
The relative amounts of aggregates and mortar are crucial for ensuring flowability and preventing segregation. According to some references:
- Coarse Aggregate: Content is generally fixed at 50 percent of the total solid volume.
- Fine Aggregate: Content is fixed at 40 percent of the mortar volume.
These proportions help create a mix with sufficient paste volume to allow the aggregates to flow freely past each other without external vibration.
Water to Powder Ratio
A critical parameter influencing the flowability and strength of SCC is the water-to-powder ratio. The 'powder' typically includes cement and fine fillers like fly ash, ground granulated blast furnace slag, limestone powder, or silica fume.
- The water/powder ratio is often assumed to be 0.9-1.0 by volume. This ratio depends on the properties of the specific powder materials used and the dosage of superplasticizer.
This relatively high water/powder ratio (compared to the water/cement ratio in conventional concrete which is often lower for strength) contributes significantly to the high fluidity of SCC. However, the overall water content must be carefully balanced with the powder content and superplasticizer dosage to achieve the desired flow without segregation or excessive bleeding.
Summarized Proportions & Ratio
Here is a summary of the typical proportions and the specific ratio mentioned:
| Component | Proportion / Ratio | Basis |
|---|---|---|
| Coarse Aggregate | 50 percent of total solid volume | Solid Volume |
| Fine Aggregate | 40 percent of mortar volume | Mortar Volume |
| Water / Powder | 0.9-1.0 by volume | Volume (Water/Powder) |
Understanding SCC Mix Design Principles
Designing SCC is different from conventional concrete. The focus is on achieving high flowability (measured by slump flow), passing ability (flowing through reinforcement), and segregation resistance.
- Powder Content: Higher powder content (binder + fillers) compared to conventional concrete is common. The powder provides the necessary paste volume to suspend aggregates and ensure cohesion.
- Superplasticizers: High-range water reducers (superplasticizers) are essential. They significantly increase fluidity without adding excessive water, allowing for the low water/powder ratios needed for adequate strength while maintaining flow.
- Aggregate Grading: Careful selection of aggregate size and grading helps minimize voids and improve flow characteristics. The maximum aggregate size is often smaller than in conventional concrete.
Achieving the optimal mix ratio or proportions involves balancing these components through trial mixes to meet specific performance criteria for the application.
