Underwater Concreting | Methods Of Underwater Concreting | Process | Principal Techniques

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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