Most underwater concrete is mixed on the surface in the conventional manner and then placed underwater by one of a variety of methods.
Properly mixed concrete is a stable material with a density of more than twice that of water. Once in position it will remain unaffected by the water in which it is immersed unless it is subjected to agitation or movement while it is setting. Concrete hardens as quickly underwater as it does in the dry.
However, vibration and levelling of concrete underwater can be difficult or impractical because of the risk of mixing water into the concrete. Conventional concrete can be disrupted by the movement of either the concrete through the water or water against the concrete, causing washout of cementitious materials and fine aggregate. Because of these factors, special ´anti washout´ admixtures and placing techniques are used.