HOw does concrete set under water? I never understand how they build bridges or tunnels!
by 
tata30
ANSWERS: 2
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Concrete requires water to set -- the water triggers a chemical reaction in the cement that makes it hard. It's not like glues, which harden when the solvent evaporates. There are even boats made of "ferrocement" -- i.e. concrete over a steel wire frame. If the hull leaks, you mix up some cement and go below decks, and mash it into the crack :)
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Most of the time, when not using a precast foundation or column, a "Cofferdam" will be used. This is a form that is lowered into the water and the water is then pumped out. They will then auger down into the soil to bedrock. To keep this hole from collapsing they will pour a "slurry into the hole. then a steel cage will be lowered into the hole. As they pump the concrete into the hole it will displace the slurry which will rise to the top and be reused.
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