Underwater Concreting

underwater concreting

Underwater concreting requires a huge amount of planning to ensure that the job is executed properly. This process involves many parties and coordination is key to avoid any unforeseen problems.

The dry or semidry mix concrete is taken in water-tight box or bucket and once it reaches the place of deposition the bottom of it is opened by some mechanism. This method does not give satisfactory results.

Tremie Pipe

Tremie pipe is one of the most common methods that are used for concreting underwater. It is a long steel pipe that extends from above water level to the desired location of concreting. Concrete is poured over the funnel-shaped end of the tremie pipe. Once the concrete has covered the entire funnel, the tremie is slightly boosted and given a slight jerk to release the concrete. The process is repeated until the concrete reaches above the water surface.

Pumping of water is not permitted during the concreting process, as it can displace cement particles and cause blockage. Using a foam rubber plug called a pig can solve this problem.

Suction Caissons

Suction caissons are a common foundation type for deep offshore structures, including tension leg platforms and taut and catenary mooring systems. They are designed to resist axial tensile loads and allow for lower material costs than traditional driven piles.

The technique works by placing dry or semi-dry concrete mix into a watertight bucket which is then moved to the desired location under the water. Once the bucket is at its place, a release mechanism is used to open the bottom and concrete is dumped on the seabed.

A cofferdam is created around the area and pumping is done to remove the water for a safe working space. This process can be time consuming and it’s important to coordinate all the parties involved. The final product is a strong, stable foundation which can withstand any load. HR Wallingford has conducted experimental work to investigate the capacity of suction caissons under a variety of loading conditions, including both wave and current-induced scour and seabed liquefaction.

Skips

Skips are large open waste containers that are used for rubbish removal from construction sites and also for commercial and domestic clear out jobs. They are available in a range of sizes and are ideal for general waste items such as wood, metal and builders waste. WEEE waste (computer equipment, telecoms equipment and set top boxes) and household waste such as bricks, tiles, concrete and soil can also be disposed of.

Generally, the effective placement of conventional concrete mixture underwater depends on two main factors, which are: mix design and placing method during concreting [1, 2]. The addition of anti-washout admixture and viscosity-modifying admixtures such as silica fume, ground granulated blast furnace slag and fly ash increases the concrete resistance against washout during the underwater placement.

One of the most commonly used techniques for underwater concreting is to use a tremie pipe which is a steel tube with a hopper at its upper end above water level and a bottom opening for pouring concrete. However, pumping methods and preplaced aggregate concrete can also be used.

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Underwater concreting involves casting concrete in water which must be carefully managed. Unlike concrete placed on dry land concrete that is cast in water keeps its moisture much better. This helps the concrete cure properly and achieve high strength.

Conventional concrete used in underwater constructions needs to be formulated with addition of anti-washout admixture for minimizing the dispersion of cement and fine materials upon casting in water. Besides, mineral admixtures such as silica fume and ground granulated blast furnace slag (GGBS) are necessary to increase the concrete resistance against washout.

This research focused on investigating the setting time, washout effect, compressive strength, and chemical composition analysis of alkali-activated fly ash (AAFA) concrete through underwater placement in seawater and fresh water (river and lake water). The concrete mix is composed of class C fly ash as source material and sodium silicate as a set-activator. The AAFA concrete was tested using a portable batching system, field and laboratory testing methods such as water loss test, permeability test, compressive test, and X-ray fluorescence.



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