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Sunday, February 22, 2009

Method of underwater concreting - Tremie method


This is a method on how to place concrete underwater, this method place a big role in offshore concreting, since cement looses its strength and fade away under water, Tremie method is to be used. Tremie Concrete is done by using a formwork/pipe which will have one end of the formwork/pipe above water and other bottom end immersed under the water and with the help of gravity. 

A tremie is a watertight pipe, generally 250mm in diameter, having a funnel shape hopper at its upper end and a loose plug at the bottom or discharge end. The valve 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 3.5m section.

During the concreting, air and water must be exclude from the tremie by keeping the pipe full of concrete all the time; and for this reason the capacity of the hopper should be at least equal to that of the tremie pipe. In charging the tremie a plug formed of paper is first inserted into the pipe as the hopper is filled the pressure of fresh concrete forces the plug down the pipe, and the water in the tremie I displaced by concrete.

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 placed concrete as the head of concrete in tremie permits. As concreting proceeds the pipe is raised slightly and the concrete flows outwards. Care should be taken to maintain continuity of concreting without breaking the seal provided by the concrete cover over the discharge end. Should this seal is broken, the tremie should be lift and plugged before concreting is recommended. The tremie should never be moved laterally though freshly placed concrete. It should be lifted vertically above the surface of concrete and shifted to its new position.

When large quantities of concrete are to be placed continuously, it is preferable to place concrete simultaneously and uniformly through a battery of tremies, rather than shift a single tremies from point to point. It has been recommended that the spacing of tremies be between 3.5 and 5m and that the end tremies should be about 2.5m from the formwork. The risk of segregation and non-uniform stiffening can be minimized by maintaining the surface of concrete in the forms as level as possible and by providing a continuous and rapid flow of concrete.




How is the Underwater Concrete Mixes?


For Structural concrete following must be considered 

  • Coarse Aggregate: Gravel of 3/4” (20mm) max. size.  Use 50-55 % of the total aggregate by weight.
  • Sand, 45-50% of the total aggregate by weight.
  • Cement: Type II ASTM (moderate heat of hydration), 600 lbs/yd3
  • Pozzolans: ASTM 616 Type N or F, 100 lbs/ yd3
  • Water/Cement Ratio: 0.42 (0.45 Maximum).
  • ƒWater-Reducing Admixture (preferably it is also plasticizer):  Do not use superplasticizers.
  • ƒAir-Entrainment Admixtures:  To give 6% total air.
  • ƒRetarding Admixture:  To increase setting time to 4-24 hours, as required.
  • ƒSlump:  6 1/2" ± 1"
  • ƒThis mix will develop compressive strength in the range of 5,600 – 7,000 psi at 28 days.



Method of underwater concreting - Tremie method (inside view)
Basic principle behind Method of underwater concreting - Tremie method


Maldives bridge work using this method

3 comments:

  1. Do we use any coffer dam like structures here for segregating the sea water? If not, then, why the first layer of concrete, had not washed out?

    ReplyDelete

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