Underwater Concrete

Underwater Concrete

As the name suggests, underwater concrete is installed below the water line, e.g. for

·    Port and harbour installations

·    Bridge piers in rivers

·    Water industry structures

·    Metro systems

·    Deep shafts in unstable ground, where an internal fall in the water level could lead to hydraulic ground heave, etc.

Composition

·     Aggregate

-       Use an aggregate suitable for pumped mixes

-       –Fines including cement > 400 kg/m³

·     Cement

-       Min. cement content 350 kg/m³

·     Special requirements

A reliable method of placing underwater concrete with minimum loss is the tremie process (Contractor method). The concrete is placed directly through a 20–40 cm pipe into and through the concrete already installed. The pipe is raised continuously, but the bottom end must always remain sufficiently submerged in the concrete to prevent the water going back into the pipe.

Another method also used today is pumping a suitably modified mix through a standard concrete pump. Here again, the end of the delivery pipe must be kept deep enough in the fresh concrete.

·     Other important considerations:

-       As the flow rate of water increases, more leaching can occur. Mini-mum flow conditions are best

-       Avoid pressure differences on the pipe (such as water level differ-ences in shafts)

·    Special underwater concrete

Previously installed rough stone bags or “gabions” can be in filled later with modified cement slurries (the bag method).