Why is it that concrete produces heat as it dries? I guess it is some kind of chemical reaction??? Is this true with all types of concretes, asphalt, grout, etc.?
The chemical reaction by which concrete sets is exothermic, that is it produces heat. Asphalt, on the other hand, is endothermic, it sheds heat as it hardens.
I dunno about grout, but I suspect it’s exothermic.
Concrete is cement with sand or gravel mixed in. Cement doesn’t dry, it sets. It is made (or at least it used to be made) by baking all the water out of limestone, which caused the stone to get all crumbly. Re-hydrating the stuff causes a chemical reaction and reforms the bonds. Asphalt doesn’t set, it is an aggregate held together with a thick petroleum based tar. I believe grout would produce heat upon setting, I know that plaster does, as they are (IIRC) both gypsum-based cement mixes.
Dr. Fidelius, Charlatan
Associate Curator Anomalous Paleontology, Miskatonic University
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DrFidelius…ok, Asphalt doesn’t dry or set, it is just held together. So what would the process be called that occurs when the asphalt is hot and melted when it is being pored to the time when it is cool and hard? This would also mean the asphalt could be returned to a some ‘liquid’ state by applying high heat. True?
So when a new pillar is being built for a large bridge, there must be some system to help with heat disipation when it is being poured.?
On a day which is 100º+, pools of liquid asphalt can be seen. Ever walk on blacktop in bare feet in August?
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[[ So what would the process be called that occurs when the asphalt is hot and melted when it ]] [[ is being pored to the time when it is cool and hard? This would also mean the asphalt could ]] [[ be returned to a some ‘liquid’ state by applying high heat. True? ]]
I guess it would be called “setting” at best, “cooling” at worst. Asphalt is a viscous liquid. As asphalt cools, the viscosity increases to the point at which the whole mess can bear the weight of heavy vehicles without flowing (much). Over a period of months to years, additional hardening occurs as excess volatiles are released.
Hot asphalt is known to flow, and young asphalt deforms plastically if its bearing strength is exceeded.
[[ So when a new pillar is being built for a large bridge, there must be some system to help ]] [[ with heat disipation when it is being poured.? ]]
Pillars are not too much of a problem. Though they have less surface area per unit volume than, say, a poured wall, pillars contain iron rebar cages for reinforcement purposes that does a pretty good job of conducting excess heat from the interior.
Too much heat does weaken the bonds and make the concrete “crumbly,” which is a problem if you’re pouring a large structure such as a dam. I believe it was during the construction of Hoover Dam that engineers figured out that they could dissipate heat by setting metal pipes into the structure horizontally.
jrepka sed: Too much heat does weaken the bonds and make the concrete “crumbly,” which is a problem if you’re pouring a large structure such as a dam. I believe it was during the construction of Hoover Dam that engineers figured out that they could dissipate heat by setting metal pipes into the structure horizontally
AWB: They run water through the metal pipes to absorb the heat and remove it from the dam structure.
On a TLC or Discovery Channel show about Hoover Dam, they said that it’s still drying sixty-some years later.
The problem is that people keep using the term “drying” to describe the process of concrete chaning from a liquid to a solid. The correct term is “curing”, and this is an on-going process. Concrete is typically considered completely cured after 28 days, but it never entirely stops.
Concrete can also cure when wet. During extremely hot periods, uncured concrete can be sprayed with water to prevent it from drying out too quickly.
Even better, it can cure while underwater. Swimming pools are filled with water while they are curing.
I know that plaster of paris gives off a heat as it sets. I was warned by a sculpture teacher never to cast a body part in plaster as he’d had a student who’d tried to cast her hands and forearms by putting them in a bucket of plaster while it set. Of course, it wasn’t a really bright idea to begin with (how would she get her hands out once it had set?), but when the exothermic reaction set in, she sustained second degree burns and broke a couple of fingers pulling her hands free.
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28 days is the standard curing time for testing. Usually when the strength of concrete is given, it is the strength after 28 days.
Typically concrete has only reached 75-80% of its theoretical maximum strength after 28 days.
Concrete never reaches its maximum strength, it is usually damaged by use and weather.
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