What is Under water concreting and explain main types of methods

What is Under water concreting

Concrete is often required to be placed underwater or in a trench filled with the slurry. Special precautions can be taken whenever the concrete is to be placed under regard to the quality of concrete, certain guidelines are to be following given below.

Precautions to be observed during Under-Water Concreting are

• The concrete should be plastic and cohesive but should have good flowability
• Usually, a high slump of 150 mm to 180 mm is required.
• A richer concrete mix is generally required (than for placing under normal condition Generally eight sacks per cubic meter of concrete are required for such concretes.
• The proportions of fine and coarse aggregates should be so adjusted that the desired workability is achieved. The proportion of fine aggregates can range from 45% to 500 of the total aggregate, depending upon the grading.
• For most concreting works, the coarse aggregates should be graded up to 20 mm or 40 mm nominal size.
• The coarse aggregate should not contain loam or any other material which may cause laitance.
• The formwork should be strong enough to withstand the action of current and waves of water. Thus, they should be protective in nature so as to avoid leakage of mortar and concrete from them.
• If necessary, coffer dams are constructed to reduce the velocity of flow around the concreting zone.
• The water-cement ratio should not exceed 0-6.
• The concrete should be placed continuously until the desired quantity has been placed.

Methods of under water concreting are the following given below.

The following are the various methods which have been used for placing the concrete underwater 

1. Placing in dewatered cofferdams or caissons
2. Tremie method
3. Placing in bags
4. Prepacked concrete
5. Drop bucket method.

1. Placing in dewatered cofferdams or (Caissons)

This method of placing concrete resembles the normal practice of concreting as in places. Cofferdams, also known as caissons, are constructed around the site where concrete takes place. The water inside the cofferdam is dewatered to make the place dry. The co is the temporary structures erected in the open water to reduce the flow of water through space into which the concrete is to be placed. After the completion of concreting the cofferdams can be removed. 

2. Tremie method

The word "triemie" is derived from the French word hopper. A tremie is a water about 25 cm diameter having a funnel-shaped hopper at its upper end and a loose plug at the bottom or discharge end.

During the concreting, air and water must be excluded from the tremie by keeping the pipe full concrete all the time. For this reason, the capacity of hopper should be so designed that it equals that of a tremie pipe. 

The valve at the discharge end is used to de-water the tremie and control the distribution of the concrete. The tremie and control the distribution of the concrete. The tremie pipe is lowered into the position with the help of a crane or powered hoist. 

As the hopper is filled, the pressure of fresh concrete (having a high slump of 150 mm to 200 m forces the plug down the pipe and the water in the tremie is displaced by concrete. Since the end blocked, no water will enter the pipe. 

For concrete, the tremie pipe is lowered into position and the discharge end is kept as deeply out merged beneath the surface of freshly placed concrete as the head of concrete in tremie permits. As the concreting proceeds, the pipe is raised slightly, with the help of a crane, and the concrete flows outward 

The tremie should never be moved laterally through freshly placed concrete. It should he always lifted vertically above the surface of the concrete. 

When a large quantity of concrete is to be placed continuously then it is preferable to use a series of tremie with spacing between them not exceeding 3.5 m to 5 m and the end tremies be 2.5 m from the formwork.

3. Placing in bags

This method consists of partially (usually about 2/3 rd) of cloth or gunny bags with concrete and tying them in such a way that they can be accommodated in a profile of the surface on which they are placed. These tied up bags are lowered into the water and placed carefully in a header and stretcher manner (as it is done in brick masonry). The placement can be done with the help of divers. 

Advantages - The main advantages of this method are the following given below.

1. Mostly no formwork is necessary to accommodate the concrete.
2. Lean mixes may be used provided sufficient plasticity is retained.

Disadvantages - The disadvantages of this method are the following given below.

1. The accurate positioning of bags can be done with the help of divers, therefore it is slow and laborious.
2. Intermediate voids between bags are difficult to fill.
3. There is little bonding between adjacent bags except that achieved by mechanical interlocking.
4. The method is suitable for shallow waters only because the bags and labour required are expensive.

4. Prepacked Concrete (Grouting)

This technique of placing concrete underwater is also termed as Grouted concrete. It consists of only placing the coarse aggregate in the forms and thoroughly compacting it to from a prepacked mass. 

Then this mass is grouted with cement mortar of the required proportions. The aggregates should be wetted before they are placed in position. The grout, under pressure, enters the prepacked aggregates and displaces the water to fill the voids. 

This technique is very much suitable for underwater construction and repair work of mass concrete structures, such as spillways, dams, etc. This type of concrete shows lower drying shrinkage and higher durability, especially freezing and thawing resistance when compared with the ordinary mix for the same proportions. Some important points to be kept in mind are the following given below.

1. The aggregate should be well-graded to produce a dense and compact concrete
2. The size of aggregates may go up to 80 mm for use in prepacked concrete.
3. Only shutter vibrators can be used to help in compaction of coarse aggregates
4. The freefall of aggregates from a height of 4 m may be allowed without causing segregation
5. The air-entraining agent is also added to the mortar, to entrain about 4% of air.

Composition of grouting mortar 

The mortar consists of fine sand, pozzolanic filler material, and a chemical agent. The chemical agent serves the following purposes : 

1. Helps in penetration
2. It helps in gaining an early setting of cement.
3. It helps in the dispersion of the particles.
4. Increases the fluidity of the mortar.

Methods for Grouting of prepacked aggregates

The grouting of prepacked aggregates can be done in any of the following three ways 

1. The mold can be filled partially with grout, and the coarse aggregate can be then deposited in the grout.
2. Pumping the grout into the aggregate mass from the bottom at the carefully designed position through a network of pipes. The formwork should be constructed at the top of the coarse aggregate in this method.
3. Pouring the grout on the top surface of the aggregates and allowed to penetrate to the bottom. This method is best suitable for grouting thin sections.

The quantity of grout required for aggregates is dependent upon the void ratio. The grout pressure applied should be of the order 0-2 MPa - 0-3 MPa (or N/mm).

5. Drop bucket method

This method of concreting underwater is suitable for depositing small quantities of concrete underwater. It has the advantage that the concreting can be carried out at considerable depths. 

The drop bottom buckets are usually fitted with drop bottom or bottom-roller gates which open freely outward when tripped as shown in Fig.

The concrete is filled completely in the bucket and is covered with a thick canvas cloth or a gunny bag to prevent the disturbance of concrete as the bucket is lowered into the water. 

The bucket is lowered into the water by a hoist or crane up to the bottom surface of concrete and then opened either by divers or by a suitable arrangement from the top. It is to make sure that the concrete is discharged directly against the surface on which it is to be deposited. 

The main advantage of the drop bottom bucket method is the difficulty in keeping them to surface of the placed concrete reasonably level.

Secondly, this method permits the use of slightly stiffer concrete than used in the tremie method.

For more information watch this video