Concreting Under Water
There are often situations, such as basements, sewerage, and marine works, in which concrete is to be placed underwater. The demand on the formwork is usually higher than in normal concreting under dry conditions. The formwork not only has to impart the required shape to the structure or its elements but it must also protect the concrete mix during placing until it matures from the direct action of current and waves. Thus, the formwork also serves as a temporary protective casing which during concreting prevents possible washing out of cement and the leakage of cement mortar from the concrete mix. After completion of concreting, it will protect the soft concrete from the impact and abrasive action of the water currents.If necessary, cofferdams are to be constructed to reduce the velocity of flow through the construction zone.
Underwater Construction method
Tremie method
A Tremie is a water light pipe, generally, 200-250 mm in diameter having a funnel-shaped hopper at its upper end and a loose plug at the bottom for discharge end. The value at the discharge end is used to de-water the tremie and control the distribution of the concrete. The tremie is supported on a working platform above water level and to facilitate the placing it is built up in 1 to 3.5 m sections.For concreting, the tremie pipe is lowered into position and the discharge end is kept as deeply submerged beneath the surface of freshly placed concrete as the head of concrete in tremie permits. As concreting proceeds, the pipe is raised slightly and the concrete flows outward. Care should be taken to maintain continuity of concreting without breaking the seal provided by the concrete cover over the discharge end. If this seal is broken, the tremie should be lifted and plugged before concreting. The tremie should never be moved laterally through freshly placed concrete. It should be lifted vertically above the surface of concrete and shifted to its new position. The risk of segregation and non-uniform stiffening can be minimized by maintaining the surface of the concrete in the forms as level as possible and by providing a continuous and rapid flow of concrete.
Grouting Method
A series of round gages made of 50 mm mesh of 6mm steel and extending over the full hight o be concreted are prepared and laid vertically over the area to be concrete so that the distances between the casters of the cages and also to the faces of the concrete do not exceed 1 m. The stone aggregate of size between 50 mm and 200 mm is carefully deposited outside the steel cages over the full area over the height to be concreted. This is done to prevent the displacement of cages.A 1:2 cement-sand grout with a water-cement ratio between 0.6 and 0.8 is prepared using a mechanical mixer. This is sent down under pressure through a38-50s mm diameter pipe terminating into steel cages. As the grouting proceeds. the pipe is raised gradually up to a height of more than 60 cm above its starting level, after which it is withdrawn and placed into the next cages for further grounting by the same procedure. After grounting the whole area for a height of about 60 cm, the same operation is repeated, if necessary, for the next layer of 60 cm and so on.
Using bags
Old cement polypropylene bags are filled up to about two-thirds with concrete and their open ends are securely tied or sewn to make the bags square ended. These bags are deposited underwater in alternate header and stretcher courses so that all the bags are interlocked to form one solid mass. the placement of bags should be such that the mouths of the bags face away from the free surface. This enables a good bond between the placed bags. The course of bags may be held together by driving steel spiker through them after placing. In deep water, the service of a diver is necessary for a satisfactory job.Pumping
Concrete can be placed underwater through direct pumping. The vertical end of the pipelines is always inserted sufficiently deep into the previously poured concrete. The pipe should not move sideways except that in the former case concrete is pumped through the pipe instead of depositing through buckets.Bucket placing
This method has the advantage that concreting can be carried out at considerable depths. The buckets are usually fitted with drop-bottom or bottom-roller gates which open freely outward when tripped as shown in the figure.The bucket is completely filled with a canvas cloth or a gunny sack to prevent the disturbance of concrete as the bucket is lowered into the water. The bucket is lowered by a crane up to the bottom surface of concrete and then opened either by divers or by a suitable arrangement form the top. It is essential that the concrete is discharged directly against the surface on which it is to be deposited.
The main disadvantage of the bucket method is the difficulty in keeping the top surface of the placed concrete reasonably level. The method permits the use of slightly stiffer concrete than does the tremie method.
Pre-placed Aggregate Method
In this method, the formwork placed underwater is packed with well-graded stone aggregate and then grout is injected into the mass to fill the voids. This method is used particularly for repainting existing underwater structures such as jetties wharfs, spillway, and piers.Precautions during Underwater Concreting
- Dry ingredients should not be dumped into the water.
- Concrete should not be allowed to fall through water from any height.
- Water should not be pumped within 24 hours of placing concrete.
- Compaction of concrete should not be done until the concrete surfaces rise above water level.
- Concrete mix should be rich by 10% to 30% of extra cement.
- concrete should not be placed in cold water.
- Underwater concreting should be done in one continuous operation.
- The aggregate should be properly graded.