How is concrete poured under the sea?

[southernlass]

I was talking to my friend the other day and got wondering about how concrete platforms are made under the ocean, and things like how the oil rig in the North Sea is anchored down? I assume it has foundations of some nature as opposed to a big metal anchor!

My real question is how does the concrete set underwater?

Thanks to anyone who answers!

[AllYourBase]

Concrete sets just as well under water as it does on land. I don't understand it myself, but all these oilrigs can't be wrong. A question was recently posted that had a good answer to a similar question.

[ian.miller]

the setting of concrete is a chemical reaction not a "drying out" due to air, the concrete when mixed generates heat through that reaction while it sets itself, there's no air invovled.

Same principle as epoxy glue, you know Araldite or whatever, that gets warm when you mix it and then it sets (which can be underwater too). mix aradite on a piece of metal and then feel the underside after 5 mins, can be (relatively) hot.

[newtron]

http://www.cement.org/basics/lessonfiveprint.htm  This site explains the hydration reaction involved.

[southernlass]

okay but what about water to cement ratio, wouldn't that change under water?

[mr. piper]

with regard to "what about the water ratio?"  the consistency of concrete is rated as a "slump" value. if it was for underwater purposes, then it would have a low slump(stiff) and very soon after mixing the water would be bonding to other chemicals and would not happily separate. this is an important characteristic of concrete, if it did lose liquid from the mix, called "grout loss". they say "three percent grout loss is nine percent loss of strength"

[Ralph]

As an aside to all the other answers, oil rigs are not bolted down into concrete foundations unless they're in very shallow water, they float just like a ship. They appear not to move up and down with the waves because the four skinny legs going into the water have a very small waterplane area. See http://www.swath.com/concept.htm for an idea on what effect this has.

[southernlass]

thanks for your answers guys!

[rja211077]

Ralph, not true - most North Sea platforms are fixed and there are very few floaters.

[Peter Pedant]

yeah there was a prog on Beeb - 2 about Romans and concrete - appatently they invented it.

The great party piece, the grand old lady - old bat - made up concrete with her spindly spindly hands, put it in a mould and lopped into the sea and yo-baby when she pulled it out, it had set!

I was amazed [at everything, old lady, mixing concrete, herself, setting under the sea and so on ]

[southernlass]

i'm amazed by the old lady mixing concrete! i don't know why she would but good on her.

[emmagleb]

The Romans did indeed invent concrete. The Pantheon in Rome (with an oculus in the domed roof) is not only made of concrete, but it was also poured constantly to avoid joins and weak points. The interior of the dome is patterned with recessed squares which lowers the overall mass of the roof. The cast for the concrete was errected and concrete poured into it, and as it cured the cast was moved to the next section. When the next section of concrete was poured, the semi-cured concrete for the previous section fused with the fresh concrete, therefore creating a continuous cast with minimal construction. The concrete in the roof of the Pantheon is actually stronger than todays concrete, and unfortunatly the Roman 'recipe' has been forgotten. To this day no one is sure of how the Romans made such strong concrete.

Gone off in a tangent from the original quesion (sorry), but I got excited when I know something about the subject! We studied the Pantheon at uni two years ago.

[Mudman]

It is true that concrete will set underwater but, generally, concrete is not used as a structural component in the North Sea. Unless it has been pre-moulded and floated/lifted to it's location.

Some of the eirlier anchors used for exploritory drilling rigs we're made of concrete poured on land. But 'dragged' badly and had to be continously re-laid.

But the best example of concrete being used in water, has to be the "Hibernia Platform" http://www.derek.mackay.btinternet.co.uk/offshore/images/rigs/HIBERNIA.jpg

This concrete giant was poured and assembled in harbour, before being towwed to the Grand Banks. It was then "sunk" and filled with Iron Ore. It had to be made of concrete as it stands in "iceburg alley".