At 04:14 PM 10/9/2005, Daron J. Wilson wrote:
> >
> > Concrete reaches in maximum strength in 28 days.
> >
>
>First, that wasn't the question. 5000# concrete generally reaches a
>strength of about 3000# or more than half its desired strength in 7 days.
>We normally wait at least a week for a self supporting base, assuming that
>has had weather conditions allowing it to dry well, 7 days should do fine.
>
>Second, it has been my understanding that concrete continues to increase in
>strength past 28 days. When being tested, cylinders are poured full of the
>exact mix and tested for compliance in 28 days. I have seen no
>documentation showing that it is at it's maximum strength in 28 days, where
>did you get that information?
Concrete keeps getting harder essentially forever.. There are several
reactions going on, some of which run fairly fast (hours, days) others
which take longer. I suspect 28 days is just a convenient marker (4
weeks), which is some compromise between incremental increase in strength
and the cost of delaying construction.
Nice explanation of portland cement and the curing process at:
http://matse1.mse.uiuc.edu/~tw/concrete/prin.html
And, from the "horse's mouth" so to speak at http://www.cement.org/ :
What does 28 -day strength mean?
Concrete hardens and gains strength as it hydrates. The hydration process
continues over a long period of time. It happens rapidly at first and slows
down as time goes by. To measure the ultimate strength of concrete would
require a wait of several years. This would be impractical, so a time
period of 28 days was selected by specification writing authorities as the
age that all concrete should be tested. At this age, a substantial
percentage of the hydration has taken place.
>Daron
>N7HQR
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