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  #1  
Old 05-25-2004, 10:48 PM
Earl Snake-Hips Tucker Earl Snake-Hips Tucker is offline
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Is the Hoover Dam concrete still curing?

I remember seeing a documetary on the Hoover Dam on the Discovery Channel a few years back, and they mentioned this factoid (my paraphrase):

The concrete in the Hoover Dam would take 125 years to cure. However, taking that long would cause instability in the structure, and a way had to be devised to make the concrete cure in a shorter amount of time.

A few weeks ago, I was at a family get-together and one of the members mentioned his trip out west, which included a tour of the dam. He mentioned that “the concrete is till curing,” according to some reference (guide or something) at the dam.

I’ve seen cites on the net both ways. Some mentioning that a special procedure had to be done to expedite the curing process, and other mentioning that “the concrete will still be curing for another 50 years.”

What’s the straight dope?
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  #2  
Old 05-25-2004, 11:09 PM
Duckster Duckster is offline
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Quote:
As the dam began to rise to fill the canyon, it grew it fits and starts. Rather than being a single block of concrete, the dam was built as a series of individual columns. Trapezoidal in shape, the columns rose in five foot lifts. The reason that the dam was built in this fashion was to allow the tremendous heat produced by the curing concrete to dissipate. Bureau of Reclamation engineers calculated that if the dam were built in a single continuous pour, the concrete would have gotten so hot that it would have taken 125 years for the concrete to cool to ambient temperatures. The resulting stresses would have caused the dam to crack and crumble away.

It was not enough to place small quantities of concrete in individual columns. Each form also contained cooling coils of 1" thin-walled steel pipe. When the concrete was first poured, river water was circulated through these pipes. Once the concrete had received a first initial cooling, chilled water from a refrigeration plant on the lower cofferdam was circulated through the coils to finish the cooling. As each block was cooled, the pipes of the cooling coils were cut off and pressure grouted at 300 psi by pneumatic grout guns.
Source: http://www.usbr.gov/lc/hooverdam/His...s/concrete.htm



Quote:
How was chemical heat caused by setting cement in the dam dissipated?

By embedding more than 582 miles of 1-inch steel pipe in the concrete and circulating ice water through it from a refrigeration plant could produce 1,000 tons of ice in 24 hours. Cooling was completed in March 1935.
Source: http://www.usbr.gov/lc/hooverdam/History/damfaqs.html

Dam FAQs dam straight!

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  #3  
Old 05-25-2004, 11:21 PM
SandWriter SandWriter is offline
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Quote:
Originally Posted by Earl Snake-Hips Tucker
Is the Hoover Dam concrete still curing?
Isn't concrete amazing? You take some powdery stuff, add some water, and after a while...BAM, rock hard substance! I think what you are really asking is: How does concrete work.

I too wondered how it actually worked and found this site...

http://www.cement.org/basics/concretebasics_faqs.asp

...where it makes very clear that concrete is a mixture of stuff and water and cement, Portland cement. No, it don't come from portland, it's just the common name for the mixture of limestone, etc. You can read the site for more details.

This page on the site explains how concrete gets hard, etc.

http://www.cement.org/basics/concret...retebasics.asp

Curing concrete gives off heat, that is why, during the Hoover Damn construction they used refrigderation to cool the concrete. The above page has a great description of hydration, or the process by which the concrete cures to hardness. The hydration continues for many years after the initial pour, causing the concrete to get harder as it ages. However, the Hoover Damn was built over 65 years ago and has hardened considerably.

So, to answer your original question, not so you'd notice.
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Old 05-25-2004, 11:42 PM
samclem samclem is offline
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Hopefully, one of our geek engineer members will show up and offer the facts.

To me, It's rather hard to believe that it would have taken 125 years for the concrete to cure, if it had been poured the "wrong" way.

That just smacks of someone repeating what they heard at the time.
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  #5  
Old 05-25-2004, 11:59 PM
Duckster Duckster is offline
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Quote:
Originally Posted by samclem
Hopefully, one of our geek engineer members will show up and offer the facts.

To me, It's rather hard to believe that it would have taken 125 years for the concrete to cure, if it had been poured the "wrong" way.

That just smacks of someone repeating what they heard at the time.
Well, "Bureau of Reclamation engineers calculated that if the dam were built in a single continuous pour, the concrete would have gotten so hot that it would have taken 125 years for the concrete to cool to ambient temperatures. The resulting stresses would have caused the dam to crack and crumble away," from the official source I quoted above. Seems to me the OP was quoting incorrectly the Bureau of Reclamation engineers calculations.

BTW, ...

Quote:
Although the use of cements (both hydraulic and non-hydraulic) goes back many thousands of years (to ancient Egyptian times at least), the first occurrence of "portland cement" came about in the 19th century. In 1824, Joseph Aspdin, a Leeds mason took out a patent on a hydraulic cement that he coined "Portland" cement (Mindess and Young, 1981). He named the cement because it produced a concrete that resembled the color of the natural limestone quarried on the Isle of Portland, a peninsula in the English Channel. Since then, the name "portland cement" has stuck and is written in all lower case because it is now recognized as a trade name for a type of material and not a specific reference to Portland, England.
Source: http://hotmix.ce.washington.edu/wsdo.../03-4_body.htm
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  #6  
Old 05-26-2004, 01:27 AM
rjk rjk is online now
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Good links!

I recall an article I read many years ago (in Scientific American, I think) describing the physical structure of concrete and how it forms as it cures. As mentioned in the article SandWriter linked above, the minerals used are fully hydrated and insoluble. They're roasted to drive off the water (making them soluble) and then ground fine enough to stay suspended in the mix. As molecules dissolve from the cement grains, they diffuse outward and become hydrated enough to precipitate out of the water, forming a spherical shell. Water diffuses through the walls, causing outward flow from any flaws in the shell. Hydrated molecules keep precipitating along the sides of the flow, making tubes radiating out from the shell. When a tube runs into another object, the precipitating cement sticks, so we get a tangled lattice of fine tubes holding the structure rigid.

(I thought the article was so interesting that I clipped it and reread it quite a few times before I finally tossed it out. Too bad - I think it had pictures.)
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  #7  
Old 05-26-2004, 08:41 AM
Chairman Pow Chairman Pow is offline
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Quote:
Originally Posted by Duckster's Cite
Mindess and Young, 1981
Tell me, I'm not the only one who read: Mindless and Young.
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  #8  
Old 05-26-2004, 08:53 AM
Kamino Neko Kamino Neko is offline
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Quote:
Originally Posted by Chairman Pow
Tell me, I'm not the only one who read: Mindless and Young.
Nope...in fact I had to reread the original 4 times after you mentioned that before I knew what it really said.
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  #9  
Old 05-26-2004, 09:06 PM
samclem samclem is offline
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Here's a 1999 thread which says much the same http://boards.straightdope.com/sdmb/...oover+concrete
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  #10  
Old 05-26-2004, 09:15 PM
Mr. Blue Sky Mr. Blue Sky is offline
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  #11  
Old 05-26-2004, 09:32 PM
aruvqan aruvqan is offline
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lol, mrAru's grandfather was a highscaler* on the dam [and he always grumped that Hoover had *nothing* to do with that dam =)]

His least favorite question was if there really were bodies of workers sealed in the pour...he was amazed that nobody ever believed him when he told them that they always fished the bodies out because the cavities eft after they rotted away would be a *serious* problem later on.

*highscaler is the guy that shinnys down a rope, lights the fuse on some explosives and shinnies right the hell back up .. real fast. They are the guys you see in pictures eating lunch halfway up the rock face held up with ropes=)
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