Under Water Concreting

The supervision requirements which are specified in Section on Reinforced concrete, in  general will apply here also with some obvious exceptions. The supervisor shall examine the  method, equipment, materials and proportions of the mix to be  used, before the work  is started. In no case shall such concrete be considered as "Design Mix Concrete". 

Supervision Checklist 

a)  Coffer dams or forms shall he sufficiently tight to ensure  still water, if practicable, and in  any case 10 reduce the flow of water to less than 3 metre per minute through the space into which concrete is to be deposited. Cofferdams  forms in still water shall be sufficiently tight to prevent  loss of mortar  through the walls. Deteriorating by pumping shall not be done while concrete is being placed or until 24 hours therefore. 

b)  The concrete shall contain at least 10 percent more cement than that  required for the same mix placed  in the day conditions. The quantity of extra cement will  vary  with conditions of placing. 

c)  The volume or mass of coarse aggregate shall be not less than one and a half times nor more than twice that of fine aggregate. The materials shall be so propositioned so as to produce a concrete having a  slump of not  less than 100 mm and not more than  180 mm. 

d) Concrete shall be deposited continuously until it is brought to the required height. While depositing, the top surface shall  be kept as  nearly level as possible and the formation of seams should be avoided. To minimise the 

formation of laitance, great care shall be exercised not to disturb the concrete as far as possible while it is being deposited. 

e) While  several  methods can he used  for deporting concrete like tremie, drop bottom bucket, bags  and grouting; the tremie method is generally preferred. 

f)  Tremie Method 

1. In case the top section of the tremie shall be  a hopper large enough to hold one entire batch of the mix or the entire contents of the transporting 'bucket. 
2. The tremie pipe shall not be less than 200 mm in diameter and shall be large enough to allow free flow of concrete. It shall be strong enough to withstand the external pressure of water in which it is suspended, even if a partial  vacuum develops inside the pipe. 
3. Unless  the lower end of the pipe is equipped with an approved automatic check valves the upper end of pipe shall be plugged  with a wadding of gunny sacking or other approved material before delivering the concrete to the tremie pipe through the hopper. This is done so that  when  the concrete is forced down from the hopper to the pipe, it will force the plug and along with it any water in the pipe, down the pipe and out of bottom end, thus establishing a continuous stream of concrete. 
4. Ensure that  at all times after the placing of concrete is  started and till  all concrete is placed, the lower end of the tremie pipe shall be below the top surface of plastic concrete. This prevents formation of laitance layers.