BRE Concrete mix design method
A popular simplified mix design method is that from BRE.
Compressive strength is, in general, related to durability. The greater the strength the more durable the concrete. To satisfy the required compressive strength, a value for water/cement (w/c) ratio is estimated for an appropriate test age (generally 28 days) and cement type. Tables in the BRE mix design handbook are consulted relating aggregate:cement (a/c) ratio, workability and water:cement (w/c) ratio for the different aggregate particle shapes and maximum size. From these tables the a/c ratio can be selected. A desired level of workability is chosen. The ratio of sand to coarse aggregate is chosen to produce a satisfactory plastic concrete. Generally there is a minimum amount of sand necessary to fill the voids between the course aggregate particles. Increasing the percentage of sand makes for a less harsh and more easily placed mix. While this is an over simplification, it does allow an assessment of the effects that changes to any of these controlling parameters have. Other methods include: ACI (1991) and Basic Mix Method (P L Owens, C&CA 1973) British method (BRE 1988)
The method of the Department of the Environment, revised in 1988 (BRE, 1988) and cited by Neville (1995), Step 1
Determination of water/cement ratio.
* A first constraint is imposed by durability concerns (maximum value). * Then the water/cement ratio is determined by the required compressive strength, for ages ranging from 3 to 91 days. Step 2
Determination of water content.
* The principle is very similar to that adopted in ACI 211. * A further refinement is brought by paying attention to the shape of particles (crushed vs. uncrushed), which is assumed to govern the water demand, together with the MSA and required slump. Step 3
Calculation of cement content, by dividing the water content by the water/cement ratio. Step 4
Determination of total mass of aggregate.
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