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Concrete: how does the reaction process work? Why can't you...

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Question by kel
Submitted on 11/8/2003
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Concrete: how does the reaction process work? Why can't you pour concrete in sub freezing temperatures, or can you? what happens to concrete if you pour it and it and the temperature drops below freezing before the concrete has dried? During the curing process, concrete give off heat, then what does it matter if you pour it below freezing? Why is it that the hoover dam had to have an elaborate cooling to ensure the concrete didn't cure too fast?


Answer by Trev
Submitted on 11/23/2003
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Concrete is a polymer. When dry it consists of short molecular chains. When mixed with water and given a small amount of heat (i.e. summer temperatures) the short chains start to react with each other and form long chains which is where concrete gets its strength. The heat is given off as a natural by-product of the chemical reaction (long chain polymers are more stable than short so the reaction is heat producing). If the ambient temperature is too cold there is insufficient heat to start the reaction (you always need some heat to start the ball rolling! - Basic Chemistry) If it freezes during curing you will end up with seriously weakened concrete as there will be an area on the freeze boundary where no long chains formed. The Hoover Dam needed the cooling system to ensure that the concrete didn't crack from the massive heat being generated. (heat makes things expand!!)

 

Answer by chiochio
Submitted on 9/21/2004
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You pour cement, concrete is the by product. To pour cement in cold temperatures add a percentage of calcium chloride to produce the heat needed to cure the concrete.

 

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