UNDERWATER CONCRETING
Special precautions need to be taken whenever the concrete
is to be placed underwater. In regard to the quality of concrete, the
recommendations of the Portland Cement Association are as follows.
“The concrete should
be plastic and cohesive but should have good flowability. This requires a fairly
high slump, usually 150 to 180 mm. A
richer mix than generally used for placing under normal conditions is required;
usually, the cement requirement is not less than eight sacks per cubic metre of
concrete. The proportions of fine and coarse aggregates should be adjusted to
produce the desired workability with a somewhat higher proportion of fine
aggregate than used for normal conditions. The fine aggregate proportion can
often be from 45 to 50 per cent of the total aggregate, depending on the grading. It is also important that the
aggregate contain sufficient fine material passing the 300 and 150-micron
sieves to produce a plastic and cohesive mixture. ASTM standard specifications
for concrete aggregate require that not less than 10 per cent of fine aggregate
pass the 300-micron sieve and not less than 2 per cent pass the 150-micron sieve. The fine
aggregate should meet the minimum requirements and a somewhat higher percentage of
fines would be better in many cases. For most works, coarse aggregate should be
graded up to 20 mm or 40 mm.”
In addition, the coarse aggregate should not contain loam or any other material which may cause laitance while being worked.
The demands on the formwork are usually higher than in
normal concreting un- der dry conditions. The formwork not only has to impart
the required shape to the structure or its elements, it must also protect the
concrete mix during placing until it matures from the direct action of current
and waves. Thus, the formwork also serves as a temporary protective casing
which during concreting prevents possible washing out of cement and the leakage
of cement mortar from the concrete mix. After completion of concrete, it
will protect the soft concrete from the impact and abrasive action of the water
currents. If necessary, coffer dams are to be constructed to reduce the
velocity of flow through the construction zone.
CONCRETING METHODS
Following
are the principal techniques that have been used for placing concrete
underwater:
1. Placing in de-watered caissons or cofferdams
2. Tremie method
3. Bucket placing
4. Placing in bags
5. Prepacked concrete
1.
The placing in de-watered caissons or cofferdams
follows the normal in- the-dry practice.
2.
Tremie method: A tremie is a watertight pipe, generally
250 mm in diameter, having a funnel-shaped hopper at its upper end and a loose
plug at the bottom or discharge end as shown in Fig. 1. The valve at the
discharge end is used to de-water the tremie and control the distribution of
the concrete. The tremie is supported on a working platform above water level,
and to facilitate the placing it is built up in 1 to 3.5 m section
Fig. 1 A typical arrangement for a
tremie pipe
During the concreting, air
and water must be excluded from the tremie by keeping the pipe full of concrete
all the time; and for this reason, the capacity of the hopper should be at least
equal to that of the tremie pipe. In charging the tremie a plug formed of paper
is first inserted into the top of the pipe. As the hopper is filled the pressure
of fresh concrete forces the plug down the pipe, and the water in the tremie is
displaced by concrete.
For concrete, the tremie
pipe is lowered into position and the discharge end is kept as deeply submerged
beneath the surface of freshly placed concrete as the head of concrete in
tremie permits. As concreting proceeds, the pipe is raised slightly and the
concrete flows outward. Care should be taken to maintain continuity of concreting
without breaking the seal provided by the concrete cover over the discharge
end. Should this seal be broken, the tremie should be lifted and plugged before
concreting is recommended. The tremie should never be moved laterally through
freshly placed concrete. It should be lifted vertically above the surface of
concrete and shifted to its new position.
For placing concrete
underwater a tremie should be set up as shown in Fig. 2(a). This will prevent
the larger size aggregate from being washed out of the concrete mix as shown in Fig.
2 (b). The tremie is gradually pulled up as the pipe gets filled with concrete.
The mix for underwater application should contain a much larger amount of cement,
i.e., the mix should be richer. The following procedure can be adopted for
placing the concrete in water-filled forms:
(a) Components of tremie pipe: (i) A 900-mm tall tremie
section, (ii) Spreader bar and (iii) Super-chute tremie and funnel support
(shown over a manhole)
(b) Placing the concrete on a water-filled formwork
Fig. 2 Components and arrangement of
tremie pipe for underwater concerting
(a) The
formwork is generally a hollow steel piling driven to a depth 'h’ meter below
the bed, i.e., the level of the concrete. This additional depth 'h’ depends upon the depth of concreting level.
(b) An
auger can be used to remove filled material from inside the piling to a depth of
concreting or bed.
(c) As
the filled material is removed, the subsurface water will fill the piling.
(d) The
reinforcing steel skeleton is placed in position.
(e) The
tremie is lowered into the piling to the bed.
(f) As
the tremie is open-ended, it will get filled up with water.
(g) A
soccer ball or a paper plug is placed on the top of the tremie.
(h) The
concrete is pumped into the tremie.
(i) The
descending ball will prevent the concrete from mixing with the water.
(j) The
ball will exit the bottom of the tremie and shoot to the surface.
(k) As
concrete exits the tremie, the piling will start to be filled up with concrete.
(l) Water
displaced by the concrete will gush out of the top of the piling.
(m) The
tremie is slowly raised so that the lower end of the tremie always stays in the
concrete mass.
When large quantities of
concrete are to be placed continuously, it is preferable to place concrete
simultaneously and uniformly through a battery of tremies, rather than shift a
single tremie from point to point. It has been recommended that the spacing
of tremies be between 3.5 and 5 m and that the end tremies should be about 2.5
m from the formwork.
The risk of segregation and
non-uniform stiffening can be minimized by maintaining the surface of the concrete in the forms as level as possible and by providing a continuous and
rapid flow of concrete.
3. Dump bucket placing This method has the advantage that
concrete can be carried out at considerable depths. The dump buckets are
usually fitted with drop-bottom or bottom-roller gates that open freely outward
when tripped as shown in Fig. 3 The bucket is completely filled with concrete
and it's top covered with a canvas cloth or a gunny sack to prevent the
disturbance of concrete as the bucket is lowered into the water. Some buckets are
provided with a special base that limits the agitation of the concrete during
discharge and also while the empty bucket is hoisted away from the fresh
concrete. The bucket is lowered by a crane up to the bottom surface of concrete
and then opened either by divers or by a suitable arrangement from the top. It
is essential that the concrete be discharged directly against the surface on
which it is to be deposited. Early discharge of bucket, which permits the fresh
concrete to drop through the water, must be avoided. The main disadvantage of the
bucket method is the difficulty in keeping the top surface of the placed
concrete reasonably level. The method permits the use of slightly stiffer
concrete than does tremie method.
4. Placing
in bags The method consists in partially (usually about
two-third) filling of cloth or gunny sacks with concrete, and tying them in such
a way
Fig. 3 Typical arrangement for a bottom
opening dump bucket
that they can readily be
accommodated in a profile of the surface on which they are placed. The properly
filled bags are lowered into the water and placed carefully in a
header-and-stretcher fashion as in brick masonry construction with the help of
divers.
The method has advantages
in that, in many cases, no formwork is necessary and comparatively lean mixes
may be used provided sufficient plasticity is retained. On the other hand, as
the accurate positioning of the bags in place can be only accomplished by the
divers, the work is consequently slow and laborious. Voids between adjacent
bags are difficult to fill, there is little bonding other than that achieved by
mechanical interlock between bags. The bags and labour necessary to fill and tie
them are relatively expensive, and the method is only suited for placing the
concrete in rather shallow water.
5.
Prepacked concrete This technique, also called grouted
concrete, consists of placing the coarse aggregate only in the forms and
thoroughly compacting it to form a prepacked mass. This mass is then grouted
with the cement mortar of the required proportions. The aggregate should be
wetted before being placed in position. The mortar that grouts the concrete
displaces water and fills the voids.
The aggregate should be
well-graded to produce dense and compact concrete. Aggregates up to a maximum
size of 80 mm can be conveniently used. Only shutter vibrators can be used
for compacting the coarse aggregate. The coarse aggregate may also be allowed
to fall from heights of up to 4 meters, without causing any appreciable
segregation.
The mortar consists of fine sand, pozzolanic filler material and a chemical agent, which serves
(i) to help the penetration,
(ii) to inhibit the early setting of cement,
(iii) to aid the dispersion of the particles, and
(iv) to increase the fluidity of mortar.
An air-entraining agent is also added to the
mortar to entrain about four per cent of air. A small variation of the
procedure of preparation of the cement mortar for grouting leads to a process
called concrete. In this process, the mortar grout is prepared in a special
high-speed mixer. No admixtures are used in this process. The high-speed
mixing produces a very fluid grout that is immiscible with water. The maximum
size of sand used is 5 mm and the sand should be well graded. The mix ratio
ranges from 1:1.5 to 1:4 with a water-cement ratio of about 0.45. Rich cement
mortar is used for underwater construction and grouting of prestressing cables
in post-tensioned bonded construction.
The
grouting of prepacked aggregates can be done in any of the following methods:
(a) The
mould can be filled partially with grout, and the coarse aggregate can then be
deposited in the grout.
(b) The
grout can be poured on the top surface of aggregate and allowed to penetrate
to the bottom. The method is particularly useful for grouting thin sections.
(c) Pumping
the grout into the aggregate mass from the bottom at carefully designed positions
through a network of pipes. The formwork should be constructed at the top of
the coarse aggregate in this method.
The quantity of grout in
any of these methods should be estimated from the void contents of the coarse
aggregates. The grout pressure employed will be of the order of 0.2 to 0.3 MPa.
This technique is very much
suited for underwater construction and repair work of mass concrete structures,
such as dams, spillways, etc. The prepacked concrete is known to exhibit lower
drying shrinkage and higher durability, especially the freezing and thawing
resistance compared to ordinary concrete of the same proportions. The rate of
development of strength is comparatively slow for the first two months and the
eventual strengths are about the same as for normal concrete. In USA and USSR,
the tremie method is most commonly used. In Holland, where large volumes of
concrete have been placed underwater, the usual method is that of placing by the bucket. The bag method is nowadays seldom used for important works overseas but has found some applications in the building up of permanent underwater
forms.
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