Mix Design

Summary

Mix Design - should aim to create a homogeneous and moderately cohesive concrete that will not segregate when being compacted and finished.

Technical Information

Excessively cohesive concrete can be difficult to place, compact and finish. Excessive bleeding should be avoided but some limited bleed water is required to assist with the formation of a surface mortar layer that can be levelled and closed by the power-finishing process. Where dry-shake toppings are used, significant water is required at the surface for the wetting of the dry material and hydration of the cement component, as well as allowing the air to escape.

Aggregate content should be maximised by using an overall aggregate grading that provides the optimum packaging and the minimum effective surface area. In practice, there may be limitations on the aggregate grading available. However, it is important to have consistent grading.

After batching, the designed consistence can reduce as a result of absorption by the aggregates and by the influence of warm weather will both increase the effects as hydration accelerates. A practical way of dealing with this is for the concrete producer and contractor to make provision for the consistence to be adjusted under controlled conditions on site.

Water additions should be supervised by a competent technician and should be limited to that required to increase the consistence to that originally specified. The procedure should ensure that the maximum specified water / cement ratio required for the specified strength, whichever is the controlling value, is not exceeded. When water is added on site, the concrete should be adequately remixed. Site records of water additions and final consistence should be kept.