What is the purpose of keeping tremie pipe’s tip immersed in freshly-placed concrete for about 1m in underwater concreting?

  What is the purpose of keeping tremie pipe’s tip immersed in freshly-placed concrete for about 1m in underwater concreting? The size of tremie pipe is about 300mm with sections having flange couplings fitted with gasket to prevent water leakage. The tremie pipe should be closed initially to prevent water from entering the pipe. It should be designed with sufficient thickness and weight so that it would not be buoyant when empty inside water. The placement of tremie concrete is commenced by putting the closed pipe underwater to the location for concreting, followed by partial filling of tremie pipe with concrete. In order to have tremie concrete flowed out of the pipe, it is necessary to fill the pipe with concrete of sufficient height to overcome the water pressure and frictional head. After that the tremie pipe is raised about 150mm to allow concrete to flow out. To enhance sufficient bonding, each succeeding layer of concrete should be placed before the preceding layer has reached
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What is the difference between carbonation and carbon dioxide attack?

What is the difference between carbonation and carbon dioxide attack?

For carbon dioxide attack, carbon dioxide dissolves in water to form a weak acid called carbonic acid. It would dissolve the cement matrix. However, the amount of carbon dioxide from the atmosphere is usually not sufficient to cause harm to concrete structures until additional source of carbon dioxide is available (e.g. decaying vegetable matter).

Carbonation is the process of converting alkaline hydroxides in concrete to carbonates by reaction with carbon dioxide. The significance of carbonation lies in the reduction of pH of pore water in concrete structure from 12-13 to 8-9 so that it drops the protection to steel reinforcement. The process takes place at concrete surface and spreads inwards. The passive nature plays an important role in steel corrosion as it prevents corrosion even in the presence of water and oxygen. This passive nature is derived from a stable and thin layer of iron oxide formed at the surface of steel reinforcement. However, if the pH of concrete is dropped, this passive oxide layer becomes unstable and corrosion may start once water and oxygen supply is available.

Stress would be generated owing to the occurrence of shrinkage. For instance, concrete at surface tends to dry more rapidly than the interior, leading to differential shrinkage. This results in the formation of internal balancing forces in which there is tension on concrete surface with compression in the interior concrete. About 90% of ultimate concrete strain occurs in the first year.

When the water is allowed to absorb water, only part of shrinkage is reversible. Water loss from gel pore with C-S-H and capillary pres can be readily replenished. However, water loss from the space between C-S-H layers cannot be easily replaced with water. Once water is lost from the space between C-S-H layers at the first place, the bond between the layers becomes stronger and the layers would get closer together, thus making it difficult for water to re-enter into the reduced space.

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