Tremie

Vibration and jerking of the pipe may be applied to encourage slumping and levelling of the upper surface of the pour, and the tremie may need to be raised occasionally during the pour so that the bottom end is not too deeply embedded, but the pipe must not be moved sufficiently to break clear of the mound and expose the bottom opening to the water, as this would allow washout of cement.

Various types of foot valve may be used to shut off flow while moving the pipe when pouring small volumes in disjoint areas, where it is impracticable to maintain immersion of the nozzle in the fresh concrete, as in repair work.

[6] The tremie can be inclined to control flow rate when working in small or shallow volumes, where it may be impossible to keep the nozzle adequately immersed.

This can be measured by finding the level of the concrete surface below the top of the pipe with a weighted tape and subtracting from the known length of the tremie.

[2] If it is necessary to move the tremie laterally, it is better to lift it out vertically, plug it and start a new pour at the new position than to drag it sideways through freshly placed concrete.

The risk of segregation and uneven setting can be minimised by providing a continuous flow of concrete through all the tremies to maintain a moderately even top surface.

[7] Concrete for tremie placement should be fluid but resistant to segregation, with a very high slump of about 150 to 200 mm (6 to 8 in),[8] typically achieved by adding superplasticizers.