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When does concrete dry?

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Old 05-09-2005, 03:20 PM   #1
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When does concrete dry?

I was watching an episode of Monster House the other day (LA Sheriff's Office - Compton dept.) when they were going at a hunk of concrete that was 40 years old. Steve Watson steps in and says that concrete continues to bake and bake over the years and gets harder and harder.

I was wondering - is this true? When does it stop hardening? It's got to stop sometime, right?
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Old 05-09-2005, 03:22 PM   #2
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it takes a while to completely cure, but it should be dry well before 40 years time..... most of the time in dry conditions its dry to walk on in 2 days
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Old 05-09-2005, 03:25 PM   #3
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But it supposedly continues to set and get harder? That's what seems to be the case on several of these shows...?
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Old 05-09-2005, 03:38 PM   #4
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since you dont like my answer, go find your own then
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Old 05-09-2005, 03:40 PM   #5
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You.. you...MEANIE!

Actually, what I meant was that I acknowledge that the concrete is dry within a few days, but does it really continue to harden? Is concrete that is 20 years old really tougher than concrete that is 1 year old?
Why is that?
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Old 05-09-2005, 03:42 PM   #6
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There is concreat inside the Hover Dam that is still drying according to my Engineering 211 (statics) instructor. The thicker it is, the longer it takes. The Hover Dam is the tickest chunk of concreat on the planett and will likely take hundreds of years to cure compleetly.
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Old 05-09-2005, 03:45 PM   #7
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Concrete does continue to harden over time. In the hoover dam, the inner concrete is still not cured It will take a few decades more before it cures. Concrete cures in a matter of 1 or 2 days (thin layers, 4"-6"), but you can put accellorators in it to quicken that process. It's really a fascinating product. The hardening process of concrete is actually an implosion of the materials that are mixed together which causes it to harden.
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Old 05-09-2005, 04:11 PM   #8
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Despite what was mentioned earlier in the thread, concrete curing has nothing to do with it "drying" or how dry the conditions are. It's an exothermic chemical process, not dependant upon evaporation. That's why with the right mix, it can set while under water. Water does escape from the mix and the humidity can be used to partially throttle the speed of the cure, but it's not what actually does the hardening.

Read more here;

http://matse1.mse.uiuc.edu/~tw/concrete/ware.html
http://www.olemiss.edu/courses/engr3...roduction.html
http://www.etown.edu/Public.aspx?topic=Press%20releases-Nov.%202004-Hager's%20research
http://www.cement.org/basics/concretebasics_faqs.asp
http://www.cement.ca/cement.nsf/0/20...D?OpenDocument
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Old 05-09-2005, 04:11 PM   #9
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sidewalks aren't completely dry as well......a normal sidewalk, if you were to cut it up and look at it, the very middle will still be soft. it takes a sidewalk 100 years to completely cure. lol, just a tidbit i learned this winter working in the concrete business building bridges! and i thought i would never get to use that fact!
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Old 05-09-2005, 04:19 PM   #10
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And regarding how long concrete can take to set, that depends on the mix and the thickness...

Quote:
RE: Hoover Damn; Bureau of Reclamation engineers had calculated that if all 6.6 million tons of concrete were poured in one continuous operation, it would take 125 years for the dam to harden. In addition, the heat generated by the setting process would cause the concrete to crack and weaken. They developed an innovative approach: The concrete for the dam would be poured in individual blocks no more than five feet deep. Those blocks would be threaded with 650 miles of one-inch pipe carrying refrigerated water to cool the concrete and prevent cracking.
And some fun facts

http://www.chooseconcrete.com/choose...)?ID=fastFacts
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Old 05-09-2005, 06:38 PM   #11
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Quote:
Originally Posted by bike4miles
There is concreat inside the Hover Dam that is still drying according to my Engineering 211 (statics) instructor. The thicker it is, the longer it takes. The Hover Dam is the tickest chunk of concreat on the planett and will likely take hundreds of years to cure compleetly.
The Hoover Dam was poured in hundreds (thousands?) of small, interlocked, "pour" sections about 3-4 ft. deep, I believe, and not in one big pour. Anotehr bit of Hoover trivia, the base of the dam is about as wide as the dam is tall. There is an awesome Discovery Channel show about the Hoover Dam that makes the rounds every few months or so. Quite a fascinating bit of engineering, and done pre-computer era.

Edit: I guess someone beat me to the trivia. Someday I'll remember to read all the posts befre replying.

Last edited by DealMaker; 05-09-2005 at 06:42 PM.
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Old 05-09-2005, 07:04 PM   #12
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Yeah but is some BODIES in the dam or not?
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Old 05-09-2005, 08:23 PM   #13
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Quote:
Originally Posted by DealMaker
Anotehr bit of Hoover trivia, the base of the dam is about as wide as the dam is tall.

Quite a fascinating bit of engineering, and done pre-computer era.
As wide as it is high, is essentially a the same idea as a pyramid. Not a whole lot of engineering to that.
The Egyptians have been building those for a while .

Just pulling legs
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Old 05-09-2005, 08:30 PM   #14
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Quote:
Originally Posted by RED 85
Yeah but is some BODIES in the dam or not?
According to the show, there are no bodies buried in the concrete. However, the loss of life during the construction was huge. No OSHA, etc.
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Old 05-09-2005, 08:44 PM   #15
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generally i thought when doing something on that big of a scale, they don't even bother stopping the pour if somebody falls in. reasoning being that they're already gone...
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Old 05-09-2005, 09:34 PM   #16
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Quote:
Originally Posted by kyle_22r
generally i thought when doing something on that big of a scale, they don't even bother stopping the pour if somebody falls in. reasoning being that they're already gone...

Wouldnt that suck to get knocked out and in,

talk about a horrible way to go.
wade
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Old 05-09-2005, 10:53 PM   #17
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Quote:
Originally Posted by bike4miles
There is concreat inside the Hover Dam that is still drying according to my Engineering 211 (statics) instructor. The thicker it is, the longer it takes. The Hover Dam is the tickest chunk of concreat on the planett and will likely take hundreds of years to cure compleetly.
Yes, drying was not the corect term. I should have said cured. BTW I heard from a civil engineering student that concreat cures faster and deeper under water.
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Old 05-10-2005, 05:08 AM   #18
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I busted an old concrete staircase on the side of my house and I busted up a relatively new sidewalk that connected too it....the staircase was probably 60-70 years old then sidewalk was 4....since the sidewalk used a more modern mix it was alot more difficult to to bust up compared to the stairs....and the sidewalk interior center was just as hard as the extirior surface....

Oh I saw a show on the Hoover damn, they said a body in the conrete would comprimise the strength of that section so they said that was a major concern and there is no known bodies in it.....I dunno though.....
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Old 05-10-2005, 06:42 AM   #19
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Quote:
Originally Posted by bike4miles
Yes, drying was not the corect term. I should have said cured. BTW I heard from a civil engineering student that concreat cures faster and deeper under water.
Concrete cures better with water present which is why you always need to keep it wet while it is going through the initial curing process (ie if you pour a slab you need to keep hosing it down every day). Therefore if it is underwater, it should cure more thoroughly (I don't think it would cure faster, but not sure on that) and you should end up with a stronger product due to fewer crack flaws. Many slabs are actually flooded after setting to ensure a strong final product.

http://www.concretenetwork.com/concr...abs/curing.htm



"After concrete is placed, a satisfactory moisture content and temperature (between 50F and 75F) must be maintained, a process called curing. Adequate curing is vital to quality concrete.

Curing has a strong influence on the properties of hardened concrete such as durability, strength, watertightness, abrasion resistance, volume stability, and resistance to freezing and thawing and deicer salts. Exposed slab surfaces are especially sensitive to curing. Surface strength development can be reduced significantly when curing is defective.

Curing the concrete aids the chemical reaction called hydration. Most freshly mixed concrete contains considerably more water than is required for complete hydration of the cement; however, any appreciable loss of water by evaporation or otherwise will delay or prevent hydration. If temperatures are favorable, hydration is relatively rapid the first few days after concrete is placed; retaining water during this period is important. Good curing means evaporation should be prevented or reduced."


BTW I highly doubt a normal sidwalk is still soft inside after a 1 year curing period....I think someone's leg was getting pulled there. Remember that typical concrete needs to have a compressive strength of 3000-4000 psi and if it is soft in the middle there is no way to get there on a small piece (the larger the pour, like the Hoover Dam, the more tolerance there is to uncured sections beause of load distribution). A large bridge also might be a different story, but would depend on if it were poured in place or made from prebuilt components and what volume of cement was poured. Also keep in mind the higher strength the concrete is (ie lower porosity) the longer it will take to cure.

Here's a bit more on hydration too:
http://matse1.mse.uiuc.edu/~tw/concrete/prin.html

Last edited by MTL_4runner; 05-10-2005 at 09:31 AM.
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Old 05-10-2005, 07:13 AM   #20
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It is capable of walking on it the matter of a couple of hours really.....



As you are finding out, It doesn't dry, it cures. The other are giving you a pretty good explanation here. Chloride can be added to concrete for the initial set-up, generally if you are expecting cold or freezing tempuratures with the first 28 days. These are the most critical really. Curing the concrete correctly in those 28 day is critical also. Too much heat or wind will suck the moisture out of the concrete.

As concrete ages, it will get harder. Another thing that would make an old sidewalk "easier" to bust up would be salts and chemicals in the enviroment. Part of the reason that shop floors need to be sealed is to keep the chemicals out of it.
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