We are very acquainted with the Concrete that is put in the rain, so what if the concrete has to be positioned in water, it includes several alerts such as cement washout, isolation, laitance, cold joints, water trap, etc and the location that is completely invisible to us, and no other opinion without relying on highly trained divers. The concrete will have certain remarkable and unique properties, it will naturally be compacted underwater, and it must be compacted by its weight (named SCC- self-compacted concrete) along with strong workability, self-leveling strength (self-leveling concrete).

Significant recent advances by the engineers in the building of locks and dams underwater involve the in-the-wet construction process. Underwater Concreting utilizes concrete precast structures as the in situ process in which concrete is specifically mounted underwater. The underwater concrete is designed to interact with the precast concrete in a composite shape. In-the-wet building has been shown in comprehensive studies to result in significant expense and construction time reductions, in addition to reducing the effect on river flow during development and removing the effects on shipping during reconstruction.

There are also excellent instances of high-quality concrete built underwater. However, there have been a small number of deficiencies that have culminated in unnecessary overruns and delays in the prices. Such accidents may have been avoided due in large part to unsuitable concrete mixtures and inadequate positioning methods if state-of-the-art underwater construction technologies had become more widely disseminated by engineers and contractors. This study reflects on the essential performance specifications of concrete products underwater, with focusing on realistic design concerns for navigation systems in-the-wet building. Later parts of the paper discuss concrete building practices underwater.

Concreting is classified as Underwater Concreting and is performed in the mud. The differences in properties are not so specific, the issue associated with concrete washout raises the need to incorporate admixtures to that Under Water Concrete’s washout products. Placing concrete underwater and even in very deep bases, such as caissons, involves specific techniques than normally used to position it in the sand. It is typically impossible to restore concrete underwater, involving specialist equipment and systems and the expertise of highly skilled and trained practitioners.

Uses of  Underwater Concreting

• Allow construction and excavation in a poor environment
• User-friendly.
• Reduction in shrinkage & creep.
• The heat of hydration of green concrete is significantly lower than traditional concrete.
• provides a safe environment to work.
• Sheet piles can easily be installed and removed.
• materials can typically be reused on other projects.

Performance requirements for UWC(Underwater Concreting)

• Workability & self compaction
• Cohesion against washout, segregation & laitance
  formation
• Low bleeding
• Low heat of hydration
• Controlled set time
• Compressive strength
• Adequate bond

Problems faced

• Segregation of fine aggregates from coarse aggregates
• Water pollution
• Increased w/c ratio
• washout

Materials used for Underwater Concreting

• Cement
• Coarse Aggregates
• Fine Aggregates
• Admixtures

Types of Cement :

Types of cement used in Underwater Concreting:

• Ordinary Portland Cement (OPC)
• Sulphate Resisting -Portland Cement ( SRPC )
• Low Heat – Portland Cement ( LHPC)

Aggregates:

Coarse Aggregates :

• The coarse aggregate for intruded in concrete shall conform to the following gradation:
• Maximum Size – 1.5-2 inch (100% shall pass a 75mm sieve )
• Minimum Size – material passing a 19 mm sieve shall not exceed 5% by mass of the coarse aggregate.

Fine Aggregates :

The sand for the intruded grout shall be well graded, preferably of round grains and shall conform to the following gradation:

• Passing 1.18 mm sieve 95 – 100%
• Passing 600 um sieve 60 – 85%
• Passing 300 um sieve 20 – 45%
• Passing 150 um sieve 15 – 30%
• Passing 75 um sieve 0 – 10%

Use of Different Admixtures

Anti-washout admixtures

• Hydrocem
• Mellose
• Rheomac UW-450

Viscosity modified admixtures

• Naphthalene Formaldehyde
• Sulfonated Melamine Formaldehyde

Methods of Underwater Concreting

• Tremie method.
• Toggle bags method.
• Bagged concrete method.
• Pumping technique.
• Hydro valve method.
• Tilting pallet barge method.
• Preplaced aggregate concrete.
• Pneumatic valve method.
• Skip method.

1. Tremie Method

Underwater concreting by the tremie process is ideal for pouring vast amounts of strong flowable concrete. The concrete is delivered to the hopper by either injection, belt conveyor or skips. Tremie tubing, which is attached to the upper end of a hopper and constantly immersed in fresh concrete, is used to bring concrete from a hopper to the surface at the same spot. The explanation for plunging the lower end of the tremie pipe is to avoid the intermingling of concrete and water.

Process of Underwater Concreting using Tremie Method

There are a variety of considerations that should be found underwater concreting during Tremie pipe technique:

Tremie Equipment: The tremie pipe can be designed in three separate forms, such as constant duration elevated during concreting, pipe with specific parts that are removed during concreting and telescope pipe. Thus, owing to chemical reactions within them, an aluminum alloy pipe may negatively influence the concrete. Owing to the overall scale the pipe would have an appropriate diameter to avoid blockage. The normal diameter is about 200-300 mm and often 150 mm and 450 mm may be used, but for example, 19 mm aggregate size and 40 mm aggregate size is the lower limit for pipe diameter of 150 mm 200 mm, respectively.

Tremie seal: A wooden frame insert is used to secure the end of the pipe to prevent intermingling with water and asphalt in the drain. This prohibits water from reaching the tubing and leaves it dry. The aggregate is pumped out until the tubing enters the expected location and the seal falls. Then water spills out of the pipe and produces a barrier by accumulating along the lower end of the pipe

Placing the concrete: The pipe mouth will be immersed up to 1-1.5 m in fresh concrete as soon as concreting begins to avoid water from reaching the shaft. The concrete flow rate is regulated by lowering and increasing the pipe and any decrease or increase in concrete discharge implies seal failure, hence concrete flow should be tracked constantly and carefully.

Flow pattern: Two forms of flow-form, respectively layered and bulging, are known. The bulging flow is preferred as it evenly dispersed the concrete contributing to less deformation of the laitance and flatter slopes.

2. Underwater Concreting using Pumping Technique

Underwater concreting using pumping technique is a refined variant of the Tremie pipe and a quicker approach to concrete in places that are challenging to reach, such as under piers. For starters, pumping has some benefits that Tremie pipe lacks, pouring concrete directly from the mixer to formworks, overcoming blockages in the pipe as concreting is by pumping rather than through gravitational power, which eliminates the chance of segregation.

3. Hydro Valve Method of Underwater Concreting

In 1969 the Dutch invented and employed this process of underwater concreting. A lightweight hydrostatically compressed hose is used to pour concrete. As long as the water is put in the upper section of the container, water weight overcomes all the pressure within the container and the hydrostatic seal. This helps in the slow-motion of concrete in the pipe and avoids segregation. The end of the hose is sealed with a tight tubular portion. It is not an expensive and fairly easy process.

4. Underwater Concreting using Pneumatic Valves

At the edge of the concrete pipeline are connected pneumatic valves. There are various forms of valves, such as Ableton-Sabema and Shimizu, used for underwater concreting. Apart from a sensor that is connected to the other, these two valves are alike; their purpose is to close the valve when concrete exceeds defined thickness. There is another form of valve available which can be used to pour concrete at a depth of 52 m without the pipe end being submerged. Valves have the purpose of enabling, limiting, preventing concrete discharge and this approach is a useful technique.

5. Underwater Concreting using the Skips Method

A tank with double door opening at the bottom and overlapping fabric flaps that are installed at the top to avoid concrete washing is the machinery used for conveying water. The skip is gradually lowered down by the water as long as it is loaded with concrete and the doors are either opened automatically or manually until it hits the spot. The Skip technique of underwater concreting is ideal for situations where the stabilization of foundations involves a large mass of concrete, and a limited volume of concrete is required for specific places. Shows are free and skips locked.

6. Underwater Concreting using Tilting Pallet Barge

In deeper water, this method is effective because the aggregate is poured into thin layers. A tilting pallet is built around the barge’s hull, in which concrete is poured thinly and then falls freely into the sea.

7. Underwater Concreting using Preplaced Aggregate Concrete

Preplaced concrete aggregate system is relatively perfect for situations where it is impossible or unlikely to pour ordinary concrete. It involves pouring cement in the shapes then inserting concrete into the bottom and filling the shapes upwards.

To keep water and air from being stuck, concreting begins from the bottom. This is why the tubes need to be installed in the forms before the aggregate is mounted. In this method, concrete strength can be achieved from about 70 to 100 percent of standard concrete. With an average distance of 1.5 m, the pipes are spaced and their diameters vary from 19-35 mm.

8. Toggle Bags Method

Toggle Bags system is effective when providing a minimal volume of concrete. A reusable canvas bag is sealed with the chain at the top and secured with toggles is loaded with concrete and carefully lowered to the specified spot, then the concrete is discharged by opening at the bottom of the container.

9. Bagged Concrete Method

Concrete bagged process used to refill ballast or partially close cracks. The bags are constructed from a moderately strong cloth with a volume of 10 -20 liters and are transported to the chosen location by divers. The concrete slump is the highest aggregate scale that can be used, between 19-50 mm and 40 mm. To build bonds the construction of the bags is identical to bricks.

Conclusion

For in-the-wet construction of navigation structures, underwater concrete construction is a critical component. Underwater concrete construction can be accomplished with the same degree of reliability as water construction.

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