[TowerTalk] thanks for the hardline answers and a concretequestion

[Jim Lux]

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