What is a grading design zone?
A grading design zone — in Dutch ontwerpgebied — is a bounded field in the particle size distribution diagram for concrete: the band within which the composition of the aggregate (sand and gravel) should lie. It is a tool for controlling the water demand and the workability when designing a concrete mix.
Concrete is made from particles of many different sizes. The ratio between fine and coarse material is plotted as a grading curve: for each sieve size it shows what proportion passes through. That curve has to stay within a particular field, and that field is the design zone.
Why it matters
Fine particles together have a far greater surface area than the same mass of coarse particles. All of that surface has to be wetted. A finer mix therefore needs more water to reach the same consistency.
- Design zone I is coarser and contains relatively less fine material; the water demand is lower.
- Design zone II is finer and contains relatively more fine material; the water demand is higher.
| Design zone I | Coarser, relatively less fine material, lower water demand |
|---|---|
| Design zone II | Finer, relatively more fine material, higher water demand |
More water at the same cement content means a higher water-cement ratio, and therefore lower strength and poorer durability. If you want to keep the strength, you have to add cement — which makes the mix more expensive and more prone to shrinkage and heat development.
Applications
- Mix design — the batching plant selects the design zone that suits the strength class, consistency and application.
- A finer mix for congested reinforcement — where there is a lot of steel or the formwork is narrow, you want finer material so the concrete reaches everywhere; you accept the higher water demand.
- A coarser mix for mass elements — for slabs and foundations with no obstructions you prefer coarse, with less water and less cement.
- Quality control — sieve analyses of delivered aggregate are checked against the limits of the design zone.
Related terms
- Grading
- Fresh concrete
- Consistency class
- Segregation
- Water-cement ratio
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