Dec 2019 UNDERWATER CONCRETE
The most widely used material in all types of construction; concrete can be placed in the most obscure places and even under water! From laying concrete foundations to supporting beams and lintels, airfield runways, freeways and expansive reinforced dam walls, one of the most interesting places to pour concrete has to be in under water applications.
With a surprisingly large number of instances where civil and structural engineers have to work with placing concrete under water, the most common underwater concrete work takes place in coastal regions, out at sea, as well as areas that experience a high volume of rainfall which results in a greater occurrence of ground water, rivers and runoff.
While concrete gains strength when hydrated, not shying away in its presence of water, it’s the pouring or the placement of mixed concrete on sea and river bed floors, dams or even in trenches filled with just an inch of water that tests engineers to find effective solutions to placing concrete submerged in water.
Notwithstanding all other normal precautions to place concrete underwater the design procedure is a little bit different from other normal concrete designs in that no compaction or consolidation can be done to the concrete once placed. In some instances pouring concrete under water is done by making use of bottom dump buckets and other under water concrete solutions make use of bagged concrete mix solutions similar to sand bagging, but the most common and effective means to place concrete under water is by way of tremie pipe installations.
- Bottom Dump BucketsUsing a Bottom Dump Bucket to place concrete under water, mixed concrete is “sunken” into place using a watertight box or bucket-type transporter. Once positioned into the desired location, the base of the concrete bucket opens and “dumps” the wet concrete into place. When placing concrete under water using this process, engineers expect to experience a certain degree of cement to be washed away which will ultimately influence the overall strength of the hardened concrete. As with most or all underwater placement of concrete, concrete mix designers use some ant-washout admixtures from reputable admixture suppliers
- Cement SandbaggingFilling “sandbag” type sacks with semi dry or dry cement, blended with the required ratios of aggregate and additives, bagged concrete mix is stacked beneath the surface of the water, positioned in desired locations. While this solution reduces the washing away of cement typically encountered when depositing cement under water using a dump bucket method, it too comes with its own challenge whereby concrete bags more commonly encounter air pockets and bubbles as well as bond between different layers of bags with the effect of the same limitations as a brick wall, which also have an adverse effect on the overall strength of cured concrete.
- Tremie Pipe InstallationsThe most effective and versatile means to placing concrete under large volumes of water is employing the help of a Tremie pipe set up to cast and mould wet concrete into position. Designed for easy coupling and made from 20cm diameter piping, Tremie pipes can be customised to measure the desired length of a concrete installation. Where the base of the pipe is sealed with a polyethylene plug, the top of the pipe is fitted with a funnel to help guide wet concrete mix into the length of the pipe. Submerged under water into its desired locations; the Tremie pipe is filled with cement and once the final length has been reached, the pipe is then carefully removed leaving the plug as the base of the installation. In theory the concrete must never free fall through water, therefore the tremie pipe is lifted just enough to allow concrete to flow into the place and keep the pipe full of concrete at all times.
Whether installing a boardwalk or pier, out to sea rigs, harbour or dam walls, over water bridges and roads, while pouring concrete submerged in water can be a complicated process, under water placement solutions are constantly being re-engineered with new innovations, technology and techniques.