Underwater concreting is same as conventional concreting except for one difference. It is done underwater.
There are mainly four methods for placing concrete underwater which are listed below-
- Tremie method
- Pump method
- Hydrovalve method
- Skip Method
- Preplaced-Aggregate Method
Among these five methods, Tremie method is the most common method used for underwater concrete placement. This article only covers the tremie method however there will be tutorials on other methods soon.
Tremie method is a way of placing concrete using gravity as the main driving force. Tremie method mainly consists of an impermeable, rigid pipe (also known as tremie) which is suspended vertically through the water.
Tremie method has been a reliable method for engineers of placing high-quality concrete. Its main advantage is that concrete can be done in a flow with very little disturbance or turbulence.
• A Tremie is placed up to a point where concreting is required.
• Tremie pipe generally is 250mm in diameter.
• Funnel-shaped hopper at its upper end so that concrete can be poured with ease.
• A loose plug or seal is present at the bottom which prevents concrete to move out before pipe is full. It also prevents water to enter from the bottom. After pipe becomes full with concrete a slight jerk is
given to break the seal to let concrete out.
The area where tremie placement is needed dictates us to use certain scheme for placement. There are two general schemes used for Tremie placement.
- Continuous concrete scheme
- Advance Slope scheme
Continuous concrete method:
In the scheme, concrete is fed into several tremie pipes at the same time. The benefit of doing so that concrete rise everywhere at approximately the same rate.
This placement technique is very useful in small areas but for large concrete placement, it becomes almost impossible to meet concrete demand. Furthermore, cold joints can also come into play. To counter this negative aspect it is advisable to divide a large area into several smaller areas.
Advancing Slope Method:
In advancing slope scheme, Concrete placement starts at one location and then with time it is moved to cover the entire area.
When tremie location reaches a particular elevation, with an adjacent tremie is immersed in concrete by about 1m. The tremie placement will proceed to the adjacent tremie.
Tremie concrete flows with an advancing slope and it ranges from 1:5 to about 1:40 which depends on concrete slump (flowability) and also placement rate. The main advantage of this scheme is that it demands less concrete production as compared to the first scheme. Secondly, this scheme eliminates the potential of cold joints between adjacent concrete pours.
Slump Range Used for tremie method is on higher size which is understandable as it needs to flow down and settle without and disturbance. A self-compacting concrete system is a very good option.
Cement Washout and Laitance Problem
cement washout problem is always a concern in underwater concrete placement. In order to minimize it, placement operation should cause very little disturbance to concrete underwater. Sources of disturbance can be
- Starting and restarting of placement.
- Loss of seal before time.
- Dragging the tremie pipe horizontally while embedded in concrete. It is advisable that tremie pipe should be embedded to a minimum depth of about 2 ft in fresh concrete.
Cold Joint Problem:
Cold Joints can form as described earlier between two adjacent layers of the pour. Cold joints as we all know generally decrease the quality of concrete so it becomes important to continuously produce and supply concrete at required placement rate.
It is also essential that necessary material quantities are supplied to the batch plant at the required rate. There should be an alternative concrete source so that in case if concrete supply is stopped from one source, concreting goes on from another source.
1) Under Ground Concrete – Mix Design And Construction Practices by Sam X.Yao, Ben C. Gerwick
2) Underwater concrete by Dr Jagadish.R.
3) Foundation engineering by R. B. Peck, W. E Hanson, T.H.Thornburn.