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Underwater concreting is a specialised job
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Special techniques are required when concrete is to be laid below ground in deep excavations, which are normally water-logged, writes Prof. A.R. Santhakumar
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Sometimes, water seeps into the foundation pits during underground construction. Normally it can be drained out before the foundation concrete is cast. Creating drains beside trenches or shallow sumps at the corners is recommended. Water collected in the trenches/sumps can then be drained using a dewatering pump or simply a bucket. However when concrete is to be laid below ground in deep excavations, which are generally water logged, special techniques are required. Concr
ete cast under water should not fall freely through the water. To ensure this one of the two courses may be adopted. Either the space may be enclosed or water excluded temporarily by a cofferdam or concrete placed directly in water using special methods. Some of these methods which are convenient to apply even in small scale construction are discussed below:
Tremie method
A tremie pipe is about 20 cm in diameter. It is a steel pipe strong enough to withstand the external pressure of water. A flanged steel pipe is used for the purpose. A funnel is fitted to the top of this pipe for easy pouring of concrete. The bottom end is closed with a cup. The pipe is lowered into the water and made to rest at the location of concreting. Then the whole length of pipe is filled with concrete. It is lifted up and given a slight jerk. This enables the concrete to get discharged in place. The bottom end of the pipe should remain inside the discharged concrete so that no water can enter the pipe through the bottom. This operation is continued sequentially till the level of concrete is above water level. In this way concrete does not get affected by water except for the last layer. The defective last layer concrete is removed and rectified.
Using bags
Old cement polypropylene bags are filled up to about two-thirds with concrete and their open ends are securely tied to make the bags square ended. These bags are deposited underwater in alternate header and stretcher courses so that all bags are inter-locked to form one solid mass. The bags should be so placed that the mouths of bags face away from the free surface. This enables a good bond between the placed bags. The bags may be held together by driving steel spikes through them after placing.
Pre-placed aggregate method
In this method, the form work placed underwater is packed with well graded stone aggregates and then grout is injected into the mass to fill the voids. This method is particularly used for repairing existing underwater structures and foundations.
Pumping
Concrete can be placed under water through direct pumping. The vertical end of the pipe lines is always inserted sufficiently deep into the previously poured concrete. The pipe should not be allowed to move sideways during pumping. The direct pumping method is similar to the tremie method except that in the former case concrete is pumped through the pipe instead of being deposited through the buckets.
Deep dump bucket method
The deep dump bucket is a specially made bucket with its bottom opening downwards and outward when it is lowered down and tipped. When the bucket touches the bed, the skirts get lowered, the bucket is opened by a mechanism and the concrete is discharged. The lowered skirts protect the concrete from the surrounding water. Instead of a bucket gunny bags filled with semi dry mixtures of cement and fine and course aggregate can be used and lowered down in water and opened out at the bottom effectively.
Precautions required during underwater concreting:
Dry ingredients should not be dumped into water nor should the concrete be allowed to fall through water from any height.
Pumping or baling out of water should not be done while the concrete is being deposited and within 24 hours of placing.
No tamping, ramming or compaction of concrete should be done until the concrete surface rises above the water level.
The concrete mix should be rich and have 10 to 30 per cent extra cement. The cement content of concrete should not be less than 380 kg/cub.m.
The aggregate should be properly graded to reduce the voids to a minimum and produce a concrete mass of excellent plasticity and strength.
Concrete should not be placed in very cold water, as this causes hardening problems. Accelerators such as sodium chloride or sodium silicates can be used in plain concrete based on requirement.
No construction joint is allowed within 600 mm below the water level.
Underwater concreting should be done in one continuous operation.
Underwater concreting is a specialised operation. It needs appropriate equipment, material and operation skills. It should be done under good supervision.
The author is emeritus Professor, Department of Civil Engineering, IIT, Madras and former Dean, Anna University.
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