Underwater concrete is a construction material commonly used in all types of civil engineering works. Special care must be taken whenever concrete is to be placed underwater, it should be plastic and cohesive but have good flow capacity. Many difficulties in water formation, especially in places where the depth is large, therefore proper mixing design, concrete production, placement, and quality control must be followed. During any construction where the lower part of the structures is probably to lie underwater, we can use underwater concrete  and can be used for

• Port and harbor installations
• Bridge piers in rivers
• Water industry structures
• Metro systems
• Deep shafts in unstable ground

Composition

• Aggregate
  – Use an aggregate suitable for pumped mixes
  – Fines including cement > 400 kg/m3
• Cement and Powder Additives
  – Minimum cement content 350 kg/m3
  – Limestone can be added to the fines content in the mixed design
• Admixtures
  – Superplasticiser for the reduction of free water in the mix
  – Mix stabilizer to minimize washout effect of fines and cement (especially in running water conditions)

Special requirements

The standard method is pumping a suitably modified mix through a standard concrete pump. The end of the delivery pipe must be kept deep enough in the fresh concrete. Another method of placing underwater concrete with minimum loss is the tremie process (Contractor Method). The concrete is placed directly through a 20 – 40 cm diameter 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 from going back into the pipe.

Other important considerations

• As the flow rate of water increases, more leaching can occur, it is possible to design the concrete to minimize washout at various water flow rates
• Avoid pressure differences on the pipe (such as water level differences in shafts)

Click Here To See How Concreting Is Done Underwater By Tremie Method?