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View Full Version : Concrete hardens with age - needs water to do so?



Stephen Tashiro
04-08-2010, 12:19 PM
I've read that concrete gradually hardens with age for up to 40 or 50 years. Does it require moisture (in addition to the water that was initially mixed with it) to do this? Would concrete that was kept moist ( like that on the face of a dam) harden quicker than concrete in a dry environment?

Dave Wagner
04-08-2010, 12:45 PM
Yes...to a certain extent.

http://en.wikipedia.org/wiki/Concrete_curing

Al Wasser
04-08-2010, 1:52 PM
I'm not a concrete engineer by any means but I do know from experience and reading that if you can keep it moist so it cures more slowly it has more strength and cracks less. For flat work like a floor if you cover it with plastic sheeting for a week + it will slow the curing down. Wet the ground under the concrete before the pour so that dry soil does not pull the moisture out.

Stephen Tashiro
04-08-2010, 2:09 PM
There are two pieces of concrete wisdom that seem contradictory. On the one hand, concrete that is mixed fairly dry is strongest (but hard to work with). On the other hand, after concrete initially sets, it should be kept damp. I didn't find the explanation for this in the wikipedia article.

I agree that both pieces of wisdom are true. I know from experience that concrete that is too wet turns out to be weak. I don't understand whether too much water interferes with the chemical process or whether it merely causes all the aggregate to sink down to the bottom giving you a inhomogeneous cast.

I know that concrete should be kept damp once it sets because my father said so!

Chris Damm
04-08-2010, 6:34 PM
Concrete cures to 95% strength in 28 days and takes 28 years to gain the last 5%. Then it starts to deteriorate. Moisture does slow down the initial surface cure and makes it stronger.

Russ Filtz
04-08-2010, 7:07 PM
This seems to be a good synopsis.

http://matse1.mse.uiuc.edu/concrete/prin.html

You can see too much water ends up with a weaker product. By the same token when curing the concrete, you want it moist usually, but not totally flooded. Can depend on the specific mix though, weather conditions, etc. I've seen burlap bags that are wetted periodically used for this.

http://matse1.mse.uiuc.edu/concrete/8.gif

Stephen Tashiro
04-09-2010, 12:20 AM
That's an understandable explanation - if you put more water in concrete, when you pour it into a form of a given volume you have less actual cement in there than when you use less water.

Now, I want to understand why replacing water with old latex paint would make concrete stronger. (This is being investigated as a method of recycling old latex paint and some patching concrete is called "latex concrete".) I haven't read that entire article yet so I don't know if it answers this question.

Harlan Coverdale
04-09-2010, 12:29 AM
That's an understandable explanation - if you put more water in concrete, when you pour it into a form of a given volume you have less actual cement in there than when you use less water.

Now, I want to understand why replacing water with old latex paint would make concrete stronger. (This is being investigated as a method of recycling old latex paint and some patching concrete is called "latex concrete".) I haven't read that entire article yet so I don't know if it answers this question.

The latex becomes a bonding agent, as I understand it.

Regarding the OP, yes concrete continues to gain strength the longer it's kept moist, although the biggest gain in strength is over the first week, then a bit slower gain for the next month or so, then things really start to level off. I used to have a buddy who did demolition work, and he told me some of the hardest concrete he'd had to break up was the floor of a car wash.

Rob Wright
04-09-2010, 9:52 PM
In my Mechanics of Masonry Materials Course in college, our mix designs and samples were always water cured, full submerged in a water bath after the initial setting of the concrete. Even now when I take test cylinders during pours on roads or foundations, routinely the cylinders are removed from the form and submerged in a water bath to cure for XX days until they are ready to be "broke" in the testing press....