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Principles of mix design

There have been many methods developed from the simple volumetric batching to prescribed rules to the highly complicated using computer simulations. In all cases, some or all of the following parameters need to be specified from the outset to enable a concrete to be designed for a particular purpose; maximum water-cement (w/c) ratio, minimum cement content, air content, slump, maximum size of aggregate, and strength requirement. Estimating the required batch weights for the concrete involves a sequence of logical straightforward steps whether based on a series of trial mixes, computer simulations, sound rule of thumb advice or a combination of all three.

An essential part of mix design is to minimise voids in order to produce a closed structure. It is assumed that any voids (micro- not entrapped air) within the concrete will be filled with water. By minimising these a lower water content and, for a given w/c ratio, cement content are needed. However, the concrete must contain enough fine material (less than say 63 micron) so that the voids become filled with hydration products from the cement, additions, admixture and water combination. The pore structure resulting determines the concrete’s resilience to carbonation, chloride ingress etc. Inadequate fines will lead to harsh concrete that has a tendency to entrap air. In comparison, high levels of fines can lead to cohesive mixes which can entrap air.

The volume of fines needed for a closed structure increases with the size of the coarse aggregate. Fines are derived from the cements and additions, the smaller sand fraction and crushed rocks etc.

Acknowledgement: The Concrete Society