There are several methods to carry out underwater concreting such as tremie method, pumping methods, preplaced aggregate concrete etc. which are described.
The underwater concreting techniques designed mostly to prevent cement washout. These methods did not obtain the full purpose of avoiding cement wash out at early stages of using under water concreting apart from cases where large masses of concreting were employed.
However, more recent techniques could obtain the objective of preventing washing out of concrete. In this article, various methods will be explored.
Methods of Underwater Concreting
Following are the methods of underwater concreting:
- Tremie method
- Pumping technique
- Hydro valve method
- Pneumatic valve method
- Skip method
- Tilting pallet barge method
- Preplaced aggregate concrete
- Toggle bags method
- Bagged concrete method
1. Tremie Method of Underwater Concreting
Underwater concreting using tremie method is convenient for pouring large amount of high flowable concrete. The concrete is moved to the hopper by either pumping, belt conveyer or skips.
Tremie pipe, which upper end connected to a hopper and lower end continuously submerged in fresh concrete, is used to place concrete at the exact location from a hopper at the surface. The reason to immerse the tremie pipe lower end is to prevent intermixing of both concrete and water. Tremie pipe typical arrangement is shown in Figure-1.
Figure-1: Typical Arrangement of Tremie Method of Underwater Concreting
Process of Underwater Concreting using Tremie Method
There number of factors that should be considered during Tremie pipe technique of underwater concreting:
The tremie pipe might be configured in three different ways such as constant length that is raised during concreting, pipe with different sections which dismantled during concreting and telescope pipe.
An aluminum alloy pipe can adversely affect the concrete due to chemical reactions between them therefore it should be avoided. The pipe should have an adequate diameter to prevent blockage because of aggregate size.
The usual diameter is between 200- 300 mm and occasionally 150 mm and 450 mm could be used but aggregate size should be considered for example 19 mm and 40 mm aggregate size is lower limit for 150 mm 200 mm pipe diameter respectively.
To avoid intermixing water and concrete in the pipe, a wooden plug of plat is used to seal the end of the pipe. This prevents entering water in to the pipe and keeps it dry.
After the pipe reach the intended position the concrete is poured and break the seal. Then concrete flow out of the pipe and creating a seal by accumulating around the lower end of the pipe
Placing the concrete
As soon as concreting began the pipe mouth should be submerged up to 1- 1.5 m into fresh concrete to prevent water entering the pipe. The concrete flow rate is controlled by lowering and raising the pipe and either decrease or increase in concrete discharge indicates the loss of the seal, therefore flow of concrete should be continuous and carefully monitored.
Two types of flow pattern are recognized namely, layered and bulging. The bulging flow is desired because it displaced the concrete uniformly which leads to lesser laitance deformation and flatter slopes.
2. Underwater Concreting using Pumping Technique
Underwater concreting using pumping technique is a developed version of Tremie pipe and it is quicker method for concreting in areas that is difficult to access such as under piers.
Pumping provide several advantages that Tremie pipe is lacking for example, pouring concrete from mixer to formworks directly, solve blockages in the pipe because concreting is through pumping instead of using gravitational force, and risk of segregation is decreased. Figure 2 show typical pipeline configuration.
Figure-2: Typical Configuration of Underwater Concreting Pump Line
3. Hydro Valve Method of Underwater Concreting
This method of underwater concreting is developed and employed by the Dutch in 1969. A flexible hose which hydrostatically compressed is employed to pour concrete.
As soon as concrete placed in the upper of the pipe, both friction inside the pipe and hydrostatic pressure is overcame by concrete weight. This leads to move concrete slowly in the pipe and avoid segregation. A rigid tubular section is used to seal the end of the hose. This method is not costly and quite simple. Figure 3 shows typical hydro valve arrangement.
Figure-3: Hydro Valve Apparatus for Underwater Concreting
4. Underwater Concreting using Pneumatic Valves
Pneumatic valves are joined to the end of the pipe line of concrete. There are different types of valves which are employed for underwater concreting such as Abetong-Sabema and Shimizu. These two valves are alike apart from a sensor that attached to the latter; its function is to close the valve when concrete reach determined thickness.
Another type of valve is available which can be used to pour concrete at a depth of 52m without immersing end of the pipe. The function of the valves is to permit, restrict, stop the discharge of concrete and this method is the useful technique. Figure 4 show Abetong-Sabema valve.
Figure-4: Abetong-Sabema Pneumatic Valve
5. Underwater Concreting using the Skips Method
The equipment that is used for conveying concrete is a bucket with double door opening at the bottom and overlapping canvas flaps which is fitted at the top to prevent concrete washing. The skip is lowered down through water slowly as soon as it filled with concrete and when it reaches the location the doors are opened either automatically or manually.
The Skip technique of underwater concreting is suitable for cases where a large mass of concrete is required for stabilizing foundations and small amount of concrete is needed for different locations. Shows opened and closed skips.
Figure-5: Skips for Underwater Concreting (a) Closed and (b) Opened
6. Underwater Concreting using Tilting Pallet Barge
This technique is useful for shallow water and the concrete is poured in thin layers. Along the deck of the barge a tilting pallet is constructed upon which concrete is spread uniformly and then fell into the water freely.
7. Underwater Concreting using Preplaced Aggregate Concrete
Preplaced aggregate concrete method is quite good for cases where pouring ordinary concrete is difficult or improbable. It includes placing aggregate in the forms then injecting concrete into the bottom and filled the forms to the top.
To prevent trapping water and air, concreting is beginning from the bottom. That is why it is necessary to position the tubes in the forms before placing the aggregate.
Concrete strength about 70 to 100 percent of conventional concrete can be obtained in this technique. The pipes are distributed with the maximum distance of 1.5 m and their diameters ranges from 19 – 35 mm.
Figure-6: Preplaced Aggregate Concrete with Injecting Tubes
8. Toggle Bags Method
Toggle Bags method is useful when small amount of concrete is required. A reusable canvas bag is sealed at the top with chain and secured with toggles is filled with concrete and dropped carefully into the determined location then through opening at the bottom of the bag the concrete is discharged.
9. Bagged Concrete Method
Bagged concrete method used for renew ballast or to seal holes temporarily. The bags are produced from considerably strong fabric with capacity of 10 -20 liters and it carried by divers to the selected position.
The concrete slump is between 19- 50 mm and 40 mm is the maximum aggregate size that can be used. The installation of the bags is similar to bricks in order to create bonds.