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 Fly Ash???Advantages Of Fly Ash

WHAT IS FLY ASH?? 



Fly ash is a by-product of the combustion of pulverized coal in thermal power plants. The dust collection system removes the fly ash, as a fine particulate residue, from the combustion gases before they are discharged into the atmosphere.

Fly ashes exhibit pozzolanic activity. The American Society for Testing and Materials (ASTM) defines a pozzolan as “a siliceous or siliceous and aluminous material which in itself possesses little or no cementitious value but which will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties.”

Advantage of using Flyash

1) Improvement in durability.

2) Less heat of hydration.

3) Higher strength at later age.

4) Improvement in workability and cohesion.

5) Environment-friendly material.

6) Economy.

Fly ash is tested as per IS 3812 & Requirements of IS code are as follows

Physical requirements.

For Siliceous flyash

1) Fineness= 320 sq.m/ kg. ( Min. )

2) Particles retained on 45 micron ( Wet sieving )= 34 ( Max )

3) Lime reactivity = 4.5 N /sq.mm Min.

4) Compressive strength in 28 days = not less than 80% of the strength of corresponding plain cement mortar cubes

5) Soundness= 0.8

6) Loss on Ignition = 5 % Max.

Chemical requirements

Sio2+Al2O3+Fe2O3 = Min 70

Sio2 by mass = Min 35

Reactive Silica = Min 20

MgO= Max 5.0

So3= Max 3.0

Na2)= Max 1.5

Total Chlorides= Max 0.005

LOI= Max 5.0

Typical result of Flyash



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