Tremie Concreting – On Site Precautions
During tremie concreting the following precautions are to be taken while doing the work:
- The concrete should be coherent, rich in cement (not less than 370 kg/m3) and of slump between 150 and 180 mm.
- When concreting is carried out under water, a temporary casing should be installed to the full depth of the bore hole or 2 m into non collapsible stratum so that fragments of ground cannot drop from sides of the hole into concrete as it is placed. The temporary casing may not be required except near the top when concreting under drilling mud (Bentonite slurry.)
- The hopper and the tremie should be a closed system embedded in the placed concrete, through which water cannot pass.
- The tremie should be large enough with due regard to size of the aggregates. For 20 mm sized aggregate the tremie pipe should be of diameter not less than 200 mm. Aggregates more than 20 mm in size shall not be used.
- The first charge of concrete should be placed with a sliding plug pushed down the tube ahead of it or with a steel plate of adequate charge to prevent mixing of concrete and water. However plug should not be left in the concrete as a lump.
- The tremie pipe should always penetrate well into the concrete with an adequate margin of safety against accidental withdrawal of the pipe surging to discharge the concrete.
- The pile should be concreted wholly by tremie and the method of deposition should not be changed part way up the pile, to prevent laitance from being entrapped within the pile.
- All tremie tubes should be scrupulously cleaned after use.
- Normally concreting of the piles should be uninterrupted. In an exceptional case of interruption of concreting, but which can be resumed within 1 or 2 hours, the tremie shall not be taken out of the concrete.
Instead, it shall be raised and lowered slowly, from time to tome to prevent the concrete around the tremie from setting. Concreting should be resumed by introducing a little richer concrete with a slump of about 200 mm for easy displacement of the partly set concrete. If the concreting cannot be resumed before final setting of concrete already placed the pile so cast may be rejected or accepted with modifications.
- In case of withdrawal of tremie out of the concrete either accidentally or to remove a choke in the tremie, the tremie may be reintroduced in the following manner to prevent impregnation of laitance or scum lying on top of the concrete deposited in the bore.
The tremie shall be gently lowered on to the old concrete with very little penetration initially. A vermiculite plug should be introduced in the tremie. Fresh concrete of slump between 150 mm and 175 mm should be filled in the tremie which will push the plug forward and will emerge out of the tremie displacing the laitance/scum. The tremie will be pushed further in steps making fresh concrete sweep away the laitance/scum in its way. When tremie is buried by about 60 to 100 cm, concreting may be resumed.
- In case of concreting through tremie or such tubes which are subsequently withdrawn, the concrete shall be placed in sufficient quantity to ensure that during withdrawal of the tube a sufficient head of concrete is maintained to prevent the in-flow of soil and water or bentonite slurry.
- The top of the concrete in a pile shall be brought above the cut-off level to permit removal of all laitance and weak concrete before capping and to ensure good concrete at the cut off level for proper embedment into the pile cap.