Mixing, Handling, Placement, and Compaction

For ready-mixed concrete, it is essential that the class of consistency of concrete be specified. In fact, if the consistency of the concrete that reaches the construction site is stiffer than required, there wiU be a tendency to add water to the mix in order to increase workabiUty. Although it is deleterious, this is unfortunately a common practice on sites where quahty control is lacking. While it increases the workability of the fresh concrete, it also increases the w/c ratio, leading to a lowering of strength and durability. On the other hand, a plasticizing admixture could be used, to improve the workabihty without increasing the w/c ratio.  

Placing and compacting of concrete are extremely important with regard to durability. Excessive voids left after compaction, due to segregation or insufficient vibration, affect the permeability of the structure. Obviously, proper surrounding of the rebars is essential for their corrosion protection. It should be borne in mind that this regards the actual structure, but it cannot be detected by tests on cubes or cylinders made from samples of fresh concrete. 

1) (If the specification for concrete is correct, there should be no need for addition of water at the site. EN 206 forbids any addition of water and admixture at delivery, unless in special cases where, under the responsibility of the producer, it is used to bring the con- 

Surface concrete temperature t(°C) Concrete strength development (r = Scml/Scml  

The influence of temperature on the rate of strength development of concrete can be used to increase the early strength of concrete by steam curing. This process is sometimes applied to precast elements that, after initial curing at room temperature, are slowly brought to high temperature (e. g. 60 °C) in wet conditions for several hours [7]. 

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