Originally posted by JML
I remember hearing something about the hoover damn not being hardnend for a like another 100 years or something like that..
Concrete is "hard" in a very few hours. Normal specification times are 7 days and 28 days for hardening to design strength. On a typical residential foundation the material is "hard" enough that you can remove forms in under 24 hours and have a usable foundation. But there is a specific strength to be met at the 7 day point, and another at 28 days, both of which relate to the design strength of the mixture. Design strength is what the mix is supposed to yield and is considered adequate to do the job intended. Once at that point any further increase in strength is irrelevant to the use of the project.
Ultimate strength is a different thing though. And that is where concrete is considered to continue to "harden" for years. Not all the cement particles are thoroughly wet during the mixing process. They will continue to react with ambient moisture and raise the strength of the concrete although that increase is normally a tiny fraction of, and in addition to, the design strength.
But there is no "liquid" concrete in a poured section within a very few hours of concrete placement, irrespective of the size of the pour.
One of hte main concerns with really big mass pours is the heat given off by the cement/water reaction (heat of hydration). That heat causes the mass to swell slightly while hte concrete is still in a plastic state. As the mass cools the thermal shrinkage can cause cracking due to the lesser strength of concrete in tension. For that reason it is common to mix crushed ice into the concrete in palce of normal mix water to cool the mix as far as possibe prior to placement. Some huge pours incorporate piping within the pour that allows the circulation of chilled water to remove the heat. The pipes are abandoned in place after the curing period.
