Tremie Pipe – A Underwater Concreting Method
Underwater Concrete is often required to be placed underwater or in a trench filled with bentonite slurry. In such cases, the use of a bottom-dump bucket or tremie pipe is made use of. In the bottom-dump bucket, the concrete is taken through the water in a water-tight box or bucket and on reaching the final place of deposition the bottom bucket mechanism gets open automatically and the whole concrete is dumped slowly.
This method will not give a satisfactory result as a certain amount of washing away of cement is bound to occur.
In some situations, dry or semi-dry mixture of cement, fine and coarse aggregate are filled in cement bags and such bagged concrete is deposited on the bed below the water. This method also does not give satisfactory concrete, as the concrete mass will be full of voids interspersed with the putrescible gunny bags. Tremie pipe method of concrete placing underwater is a satisfactory method and widely used all around the world.
The word “tremie” is derived from the French word hopper.
Underwater Concrete Mixes: Structural concrete
- Coarse Aggregate: Gravel of 3/4” max. Size. The total aggregate by weight is about 50-55 %.
- Fine Aggregate: Sand is added about 45-50% of the total weight of aggregate.
- Cement: ASTM (moderate heat of hydration) Type II, 600 lbs/yd3.
- Pozzolana: ASTM 616 Type N or F, 100 lbs/ yd3.
- Water/Cement Ratio: 0.42 (0.45 Maximum).
- Use of water-reducing admixture (preferably it is also plasticizer): Do not use superplasticizers.
- Air-Entrainment Admixtures: To give 6% total air.
- Retarding Admixture to increase the setting time from 4 to 24 hours.
- Slump: 6 1/2 in. ± 1 in.
- This concrete mix gives a compressive strength of 5,600 – 7,000 psi at 28 days.
- It will flow out easily on the slope of 6:1 to 8:1 horizontal vertical and, if properly placed, should give nominal segregation and laitance.
Tremie Pipe Concreting Method:
A tremie pipe is a pipe having a diameter of about 20 cm capable of easy coupling for an increase or decrease of length. A wide funnel is fitted at the top of the tremie pipe which allows the pouring of concrete quickly.
The bottom end is closed with a plug or thick polyethylene sheet cm such o material and taken below. The water is kept clam at the point where the concrete is going to be placed. Since the end is blocked, no water will have entered the pipe. The concrete must have a high slump of about 15 to 20 cm is poured into the funnel. When the whole length of the pipe is filled up with the concrete, the tremie pipe is lifted up and a slight jerk gave by a winch and pulley arrangement.
When the pipe is raised and given a jerk, due to the weight of concrete, the bottom plug falls and the concrete gets discharged. Particular care must be taken at this stage to see that the end of the. Tremie pipe remains inside the concrete so that no water enters the pipe from-the-bottom. In other words, the tremie pipe remains plugged at the lower end of the concrete.
Again concrete is poured over the funnel and when the whole length of the tremie pipe filled with concrete, the pipe is again slightly lifted and given a slight jerk. Care is taken all the time to keep the lower end of the tremie pipe well embedded in wet concrete. The concrete in the time tremie pipe gets discharged. Such a sequence is continued until the concrete level comes above the water level.
This method if executed properly has the advantage that the concrete does not get affected by water except the top layer. The top layer is scrubbed or cut off to remove the affected concrete at the end of the whole operation.
During the course of concreting, no pumping of water should be permitted. the cement particles if simultaneous pumping of water is done. Underwater concreting need not be compacted, as concrete gets automatically compacted by the hydrostatic pressure of water.
Secondly, the concrete is of such consistency that it does not normally require compaction. One of the disadvantages of underwater concreting in this method is that a high water/cement ratio is required for high consistency which reduces the strength of concrete. But at present, with the use of superplasticizer, it is not a constraint. A concrete with as low as w/c ratio as 0.3 or even less can be placed by tremie method.
Another method, not so commonly employed to place concrete below water is the grouting process of pre-packed aggregate. Coarse aggregate is dumped to assume the full dimension of the concrete mass. The prepared Cement mortar grout is injected through the pipe, which extends up to the bottom of the aggregate bed. The pipes are slowly withdrawn, as the grouting progresses. The grout forces the water out from the interstices and occupies the space. For a good foundation plugging this method is often adopted.
Concrete also can be placed underwater by the use of pipes and concrete pumps. The pipeline is plugged at one end and lowered until it rests at the bottom. Pumping is then started. When the pipe is completely filled, the plug is forced out; the concrete surrounding the lower end of the pipe seals the pipe.
The pumping is done against the pressure of the plug at the lower end. When the pumping effort required is too great to overcome the pressure, the pipe is withdrawn and the operation is repeated. This process is repeated until the concrete reaches the level above water.