HomeCONSTRUCTION TIPSTHINGS CIVIL ENGINEER SHOULD KNOWUnderwater Concreting | Methods Of Underwater Concreting | Process | Principal Techniques
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Special precautions need to be taken whenever the concreteis to be placed underwater. In regard to the quality of concrete, therecommendations of the Portland Cement Association are as follows.
“The concrete shouldbe plastic and cohesive but should have good ﬂowability. This requires a fairlyhigh slump, usually 150 to 180 mm. Aricher mix than generally used for placing under normal conditions is required;usually, the cement requirement is not less than eight sacks per cubic metre ofconcrete. The proportions of ﬁne and coarse aggregates should be adjusted toproduce the desired workability with a somewhat higher proportion of ﬁneaggregate than used for normal conditions. The ﬁne aggregate proportion canoften be from 45 to 50 per cent of the total aggregate, depending on the grading. It is also important that theaggregate contain sufﬁcient ﬁne material passing the 300 and 150-micronsieves to produce a plastic and cohesive mixture. ASTM standard speciﬁcationsfor concrete aggregate require that not less than 10 per cent of ﬁne aggregatepass the 300-micron sieve and not less than 2 per cent pass the 150-micron sieve. The ﬁneaggregate should meet the minimum requirements and a somewhat higher percentage ofﬁnes would be better in many cases. For most works, coarse aggregate should begraded up to 20 mm or 40 mm.”
In addition, the coarse aggregate should not contain loamor any other material which may cause laitance while being worked.
The demands on the formwork are usually higher than innormal concreting un- der dry conditions. The formwork not only has to impartthe required shape to the structure or its elements, it must also protect theconcrete mix during placing until it matures from the direct action of currentand waves. Thus, the formwork also serves as a temporary protective casingwhich during concreting prevents possible washing out of cement and the leakageof cement mortar from the concrete mix. After completion of concrete, itwill protect the soft concrete from the impact and abrasive action of the watercurrents. If necessary, coffer dams are to be constructed to reduce thevelocity of ﬂow through the construction zone.
Followingare the principal techniques that have been used for placing concreteunderwater:
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 cofferdamsfollows the normal in- the-dry practice.
2.Tremie method: A tremie is a watertight pipe, generally250 mm in diameter, having a funnel-shaped hopper at its upper end and a looseplug at the bottom or discharge end as shown in Fig. 1. The valve at thedischarge end is used to de-water the tremie and control the distribution ofthe 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 atremie pipe
During the concreting, airand water must be excluded from the tremie by keeping the pipe full of concreteall the time; and for this reason, the capacity of the hopper should be at leastequal to that of the tremie pipe. In charging the tremie a plug formed of paperis ﬁrst inserted into the top of the pipe. As the hopper is ﬁlled the pressureof fresh concrete forces the plug down the pipe, and the water in the tremie isdisplaced by concrete.
For concrete, the tremiepipe is lowered into position and the discharge end is kept as deeply submergedbeneath the surface of freshly placed concrete as the head of concrete intremie permits. As concreting proceeds, the pipe is raised slightly and theconcrete ﬂows outward. Care should be taken to maintain continuity of concretingwithout breaking the seal provided by the concrete cover over the dischargeend. Should this seal be broken, the tremie should be lifted and plugged beforeconcreting is recommended. The tremie should never be moved laterally throughfreshly placed concrete. It should be lifted vertically above the surface ofconcrete and shifted to its new position.
For placing concreteunderwater a tremie should be set up as shown in Fig. 2(a). This will preventthe 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 forplacing the concrete in water-ﬁlled forms:
(a) Components of tremie pipe: (i) A 900-mm tall tremiesection, (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 oftremie pipe for underwater concerting
(a) Theformwork is generally a hollow steel piling driven to a depth 'h’ meter belowthe bed, i.e., the level of the concrete. This additional depth 'h’ depends upon the depth of concreting level.
(b) Anauger can be used to remove ﬁlled material from inside the piling to a depth ofconcreting or bed.
(c) Asthe ﬁlled material is removed, the subsurface water will ﬁll the piling.
(d) Thereinforcing steel skeleton is placed in position.
(e) Thetremie is lowered into the piling to the bed.
(f) Asthe tremie is open-ended, it will get ﬁlled up with water.
(g) Asoccer ball or a paper plug is placed on the top of the tremie.
(h) Theconcrete is pumped into the tremie.
(i) Thedescending ball will prevent the concrete from mixing with the water.
(j) Theball will exit the bottom of the tremie and shoot to the surface.
(k) Asconcrete exits the tremie, the piling will start to be ﬁlled up with concrete.
(l) Waterdisplaced by the concrete will gush out of the top of the piling.
(m) Thetremie is slowly raised so that the lower end of the tremie always stays in theconcrete mass.
When large quantities ofconcrete are to be placed continuously, it is preferable to place concretesimultaneously and uniformly through a battery of tremies, rather than shift asingle tremie from point to point. It has been recommended that the spacingof tremies be between 3.5 and 5 m and that the end tremies should be about 2.5m from the formwork.
The risk of segregation andnon-uniform stiffening can be minimized by maintaining the surface of the concrete in the forms as level as possible and by providing a continuous andrapid ﬂow of concrete.
3. Dump bucket placing This method has the advantage thatconcrete can be carried out at considerable depths. The dump buckets areusually ﬁtted with drop-bottom or bottom-roller gates that open freely outwardwhen tripped as shown in Fig. 3 The bucket is completely ﬁlled with concreteand it's top covered with a canvas cloth or a gunny sack to prevent thedisturbance of concrete as the bucket is lowered into the water. Some buckets areprovided with a special base that limits the agitation of the concrete duringdischarge and also while the empty bucket is hoisted away from the freshconcrete. The bucket is lowered by a crane up to the bottom surface of concreteand then opened either by divers or by a suitable arrangement from the top. Itis essential that the concrete be discharged directly against the surface onwhich it is to be deposited. Early discharge of bucket, which permits the freshconcrete to drop through the water, must be avoided. The main disadvantage of thebucket method is the difﬁculty in keeping the top surface of the placedconcrete reasonably level. The method permits the use of slightly stifferconcrete than does tremie method.
4. Placingin bags The method consists in partially (usually abouttwo-third) ﬁlling of cloth or gunny sacks with concrete, and tying them in sucha way
Fig. 3 Typical arrangement for a bottomopening dump bucket
that they can readily beaccommodated in a proﬁle of the surface on which they are placed. The properlyﬁlled bags are lowered into the water and placed carefully in aheader-and-stretcher fashion as in brick masonry construction with the help ofdivers.
The method has advantagesin that, in many cases, no formwork is necessary and comparatively lean mixesmay be used provided sufﬁcient plasticity is retained. On the other hand, asthe accurate positioning of the bags in place can be only accomplished by thedivers, the work is consequently slow and laborious. Voids between adjacentbags are difﬁcult to ﬁll, there is little bonding other than that achieved bymechanical interlock between bags. The bags and labour necessary to ﬁll and tiethem are relatively expensive, and the method is only suited for placing theconcrete in rather shallow water.
5.Prepacked concrete This technique, also called groutedconcrete, consists of placing the coarse aggregate only in the forms andthoroughly compacting it to form a prepacked mass. This mass is then groutedwith the cement mortar of the required proportions. The aggregate should bewetted before being placed in position. The mortar that grouts the concretedisplaces water and ﬁlls the voids.
The aggregate should bewell-graded to produce dense and compact concrete. Aggregates up to a maximumsize of 80 mm can be conveniently used. Only shutter vibrators can be usedfor compacting the coarse aggregate. The coarse aggregate may also be allowedto fall from heights of up to 4 meters, without causing any appreciablesegregation.
The mortar consists of ﬁne sand, pozzolanic ﬁller material and a chemicalagent, which serves
(i)to help the penetration,
(ii) to inhibit the early settingof 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 themortar to entrain about four per cent of air. A small variation of theprocedure of preparation of the cement mortar for grouting leads to a processcalled concrete. In this process, the mortar grout is prepared in a specialhigh-speed mixer. No admixtures are used in this process. The high-speedmixing produces a very ﬂuid grout that is immiscible with water. The maximumsize of sand used is 5 mm and the sand should be well graded. The mix ratioranges from 1:1.5 to 1:4 with a water-cement ratio of about 0.45. Rich cementmortar is used for underwater construction and grouting of prestressing cablesin post-tensioned bonded construction.
Thegrouting of prepacked aggregates can be done in any of the following methods:
(a) Themould can be ﬁlled partially with grout, and the coarse aggregate can then bedeposited in the grout.
(b) Thegrout can be poured on the top surface of aggregate and allowed to penetrateto the bottom. The method is particularly useful for grouting thin sections.
(c) Pumpingthe grout into the aggregate mass from the bottom at carefully designed positionsthrough a network of pipes. The formwork should be constructed at the top ofthe coarse aggregate in this method.
The quantity of grout inany of these methods should be estimated from the void contents of the coarseaggregates. The grout pressure employed will be of the order of 0.2 to 0.3 MPa.
This technique is very muchsuited for underwater construction and repair work of mass concrete structures,such as dams, spillways, etc. The prepacked concrete is known to exhibit lowerdrying shrinkage and higher durability, especially the freezing and thawingresistance compared to ordinary concrete of the same proportions. The rate ofdevelopment of strength is comparatively slow for the ﬁrst two months and theeventual 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 ofconcrete 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 underwaterforms.
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