Under Water Concreting
When it is necessary to deposit concrete under water, the
methods, equipment, materials and proportions of the mix to be used shall be
submitted to and approved by the engineer-in-charge before the work is started.
In no case shall such concrete be considered as ‘ Design mix concrete '.
The concrete shall contain at least 10 percent more cement
than that required for the same mix placed in the dry, the quantity of extra
cement varying with conditions of placing. The volume or mass of the coarse
aggregate shall be not less than one and a half times, nor more than twice that
of the fine aggregate. The materials shall be so proportioned as to produce a
concrete having a slump of not less than 100 mm, and not more than 180 mm.
Coffer-dams or forms shall be sufficiently tight to ensure
still water if practicable, and in any case to reduce the flow of water to less
than 3 m per minute through the space into which concrete is to be deposited.
Coffer-dams or forms in still water shall be sufficiently tight to prevent loss
of mortar through the walls. Dewatering by pumping shall not be done while
concrete is being placed or until 24 hours thereafter.
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 avoided. The methods to be
used for depositing concrete under water shall be one of the following:
·
Tremie
When concrete is to be deposited
under water by means of a tremie, 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 if any. The tremie pipe shall be not less than 200
mm in diameter and shall be large enough to allow a free flow of concrete and
strong enough to withstand the external pressure of the water in which it is
suspended, even if a partial vacuum develops inside the pipe. Preferably,
flanged steel pipe of adequate strength for the job should be used. A separate
lifting device shall be provided for each tremie pipe with its hopper at the
upper end. Unless the lower end of the pipe is equipped with an approved
automatic check valve, the upper end of the pipe shall be plugged with a
wadding of the gunny sacking or other approved material before delivering the
concrete to the tremie pipe through the hopper, 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 the bottom end, thus
establishing a continuous stream of concrete. It will be necessary to raise
slowly the tremie in order to cause a uniform flow of the concrete, but the
tremie shall not be emptied so that water enters the pipe. At all times after
the placing of concrete is started and until all the concrete is placed, the
lower end of the tremie pipe shall be below the top surface of the plastic
concrete. This will cause the concrete to build up from below instead of
flowing out over the surface, and thus avoid formation of laitance layers. If
the charge in the tremie is lost while depositing, the tremie shall be raised
above the concrete surface, and unless sealed by a check valve, it shall be
replugged at the top end, as at the beginning, before refilling for depositing
concrete.
·
Drop bottom bucket
The top of the bucket shall be
covered with a canvas flap. The bottom doors shall open freely downward and
outward when tripped. The bucket shall be filled completely and lowered slowly
to avoid backwash. The bottom doors shall not be opened until the bucket rests
on the surface upon which the concrete is to be deposited and when discharged,
shall be withdrawn slowly until well above the concrete.
·
Bags
Bags of at least 0.028 m3 capacity
of jute or other coarse cloth shall be filled about two thirds full of
concrete, the spare end turned under so that bag is square ended and securely
tied. They shall be placed carefully in header and stretcher course so that the
whole mass is interlocked. Bags used for this purpose shall be free from
deleterious materials.
·
Grouting
A series of round cages made from 50
mm mesh of 6 mm steel and extending over the full height to be concreted shall
be prepared and laid vertically over the area to be concreted so that the
distance between centers of the cages and also to the faces of the concrete
shall not exceed one meter. Stone aggregate of not less than 50 mm nor more
than 200 mm size shall be deposited outside the steel cages over the full area
and height to be concreted with due care to prevent displacement of the cages.
A stable 1 : 2 cement-sand grout with
a water-cement ratio of not less than 0.6 and not more than 0.8 shall be
prepared in a mechanical mixer and sent down under pressure (about 0.2 N/mm2)
through 38 to 50 mm diameter pipes terminating into steel cages, about 50 mm
above the bottom of the concrete. As the grouting proceeds, the pipe shall be
raised gradually up to a height of not more than 600 mm above its starting
level after which it may be withdrawn and placed into the next cage for further
grouting by the same procedure.
After grouting the whole area for a
height of about 600 mm, the same operation shall be repeated, if necessary, for
the next layer of 600 mm and so on.
The amount of grout to be sent down
shall be sufficient to fill all the voids which may be either ascertained or
assumed as 55 percent of the volume to be concreted.
To minimize the formation of
laitance, great care shall be exercised not to disturb the concrete as far as
possible while it is being deposited.
Concrete in Sea-Water
Concrete in sea-water or exposed directly along the sea-coast
shall be at least M 15 Grade in the case of plain concrete and M 20 in case of
reinforced concrete. The use of slag or pozzolana cement is advantageous under
such conditions.
Special attention shall be given to the design of the mix to
obtain the densest possible concrete; slag, broken brick, soft limestone, soft
sand-stone, or other porous or weak aggregates shall not be used.
As far as possible, preference shall be given to precast
members unreinforced, well-cured and hardened, without sharp corners, and
having trowel-smooth finished surfaces free from crazing, cracks or other
defects; plastering should be avoided.
No construction joints shall be allowed within 600 mm below
low water-level or within 600 mm of the upper and lower planes of wave action.
Where unusually severe conditions or abrasion are anticipated, such parts of
the work shall be protected by bituminous or silico-fluoride coatings or stone
facing bedded with bitumen.
In reinforced concrete structures, care shall be taken to
protect the reinforcement from exposure to saline atmosphere during storage and
fabrication.
Concrete in Aggressive
Soils and Water
General
The destructive action of aggressive waters on concrete is
progressive. The rate of deterioration which varies with the alkali-resisting
property of the cement used, decreases as the concrete is made stronger and
more impermeable, and increases as the salt content of the water increases.
Where structures are only partially immersed or are in contact with aggressive
soils or waters on one side only, evaporation may cause serious concentrations
of salts with subsequent deterioration, even where the original salt content of
the soil or water is not high.
Drainage
At sites where alkali concentrations are or may become very
high, Portland cement concrete is not recommended, and if used, the ground
water shall be lowered by drainage so that it will not come into direct contact
with the concrete.
Additional protection may be obtained by the use of a
chemically resistant stone facing or a layer of plaster of paris covered with
suitable fabric, such as jute, thoroughly impregnated with bituminous material.
