Why casting concrete underwater? Well, it is not the most common operation in the world, but sometimes, it is necessary to cast a foundation, a consolidation, a ramp or things like those….normal concrete? Forget about that! It will be immediately washed-out, which means pollution (clearly visible because the water will become immediately filled with cement particles) and, of course, irreversible damage to the concrete itself.
That´s why a specific family of anti-washout concrete is available. This mix-design is studied to provide with superior viscosity and cohesion by means of specific admixtures, which reduce mobility of water. These admixtures brings in some side effects, as reduction of compression strength due to the increased necessity of water and the presence of air (just to summarize and keep in line with this blog philosophy) which leads to the necessity of more cement, both to keep the w/c ratio to the desired level and to compensate the little part that unavoidably will be washed-out. The fillers are good allied for this specific family of mixes, both mineral and pozzolanic.
The most common way to cast concrete underwater, is the so called “tremie system” that implies the use of vertical hard pipes kept 40-50 cm. higher than the ground level (but always immersed in concrete) equipped with a funnel out of the water.
Concrete workability (see my post “Workability of concrete” and “Measuring concrete workability”) is still important even when casting underwater.
The more fluid the concrete, the more distant it will flow and, of course, it is preferable to use more fluid concrete if thin structures should be casted. If concrete is casted quite stiff, it will come out of the pipe keeping close to it, which means that a scuba should move it away manually or the pipe will be soon obstruct.
Absolutely more practical is to cast with the help of a concrete pump. The pump´s arm can be easily immersed in the water and maneuvered to the casting spots.
Bur don't forget this important thing: underwater concrete fears exposure. That means watch out for layering. It is pretty easy to have cold joints and total lack of adhesion between concrete layers when the casting is performed underwater. In order to avoid that issue the easiest thing to do is "immerge" the pump hose (or the pipe used for the pouring) and never lift it over the concrete level if not at the very end of the casting.
Compaction equipment are normally not used while pouring concrete underwater so it should be taken into consideration the possibility to prefer SCC if a good compaction grade is required (read the posts "Concrete compaction" and "Self Compacting Concrete). About mix requirements can be also remembered that the anti washout admixtures commonly used (AWA) normally tend to extend the concrete setting time and that could be dangerous because the concrete mass is ironically left at risk of washout for a longer period. It is advisable to implement the mix with accelerating admixture.
Must be remembered that when the casting is done there still is a weak zone, and this is the surface, which is exposed to the currents and therefore to probable washout effect. A good practice is to cover the surface with a water-tight cover so to minimize this issue.
Some example of practical use of concrete for underwater casting
Foundations and structures under the water table
Submerged structures in lake and marsh basins
Submerged structures in the river bed (piles of bridges, etc.)
Consolidation of land in water or protection from erosion
Maintenance of canalization and hydroelectric works
Restoration of sewers and underground pipes
Construction and restoration of harbor works, piers, etc.
Coating and consolidation of boulders for cliff foundations
Sealing plugs in wells