thermal expansion

[Fullmetal Chocobo]

I know that water volume increases when it freezes. And they have the breaks in between concrete slabs to allow room for thermal expansion. However, no one I ask seems to know if concrete gets expands with heat or cold.
I also know that as a gas expands, it gets colder (IE CO2 coming out of a nozzle), and compressed items get heated up. But I still don't know about the concrete. Anyone have any information on this?
--Tas

 

[Matthias99]

Concrete's a little funny, because there's a fair amount of water trapped in it. Interestingly, if the temperature of it gets high enough, the trapped water will boil and cause the concrete to shatter explosively (but that's a different phenomenon).

Since it's mostly rock, though, I would suppose that it has a positive coefficient of thermal expansion (that is, it gets bigger as its temperature increases). A quick Google search for "concrete thermal expansion" turned up this. It says "concrete" (which is pretty vague) has a CTE of (7.4-13 * 10^(-6) / degree C).

 

[Fullmetal Chocobo]

Okay, so it has a very low thermal expansion rate as per the positive coeffecient. But water has a negative coefficient. Do all solids have a positive coeffecient?
--Tas

 

[brentkiosk]

The huge majority of solids expand when heated. There must be a few exceptions, but there are very few.

 

[f95toli]

Most solids have positive coefficients, plutonium is the only exception that I know of among the elements.
AFAIK a "simple" calculation will tell you that ALL solids should expand, so solids that shrink are somwhow "odd".
But I suspect that there might be alloys that have a negative coefficients, simply because the properties of an alloy is often dominated by the effects of grain boundaries etc and does not need to follow any "simple rules", however I can't think of an example at the moment.

 

[Fullmetal Chocobo]

Excellent.... Thanks for the info...
--Tas.

 

[Witling]

I assume that you know that water contracts until about 4 degrrees centigrade and then it expands.

I' a little curious about expanding and compressing gasses. Does the expanding gas get colder? I know that the container that it came out of gets colder. Going the other way, I think I know that compressed gas gets warmer. But I wonder if these phenomona have more to do with the rate at which the gas is expanded or compresssed?

 

[CycloWizard]

Originally posted by: Witling
I assume that you know that water contracts until about 4 degrrees centigrade and then it expands.

I' a little curious about expanding and compressing gasses. Does the expanding gas get colder? I know that the container that it came out of gets colder. Going the other way, I think I know that compressed gas gets warmer. But I wonder if these phenomona have more to do with the rate at which the gas is expanded or compresssed?

Gases do cool down as they expand, and the effect is rate-dependent. It's called the Joule-Thompson effect, IIRC.

One solid material that has a negative CTE is carbon fiber. This is one of the reasons it's so useful in aerospace applications where a large temperature range must be considered. It also makes it a real pain to design for, since the matrix you embed it in will have a positive CTE and microcracking can and usually does occur. With the right combination, you can get a near-zero CTE, which is the general goal for temperature-cycling applications like planes and spacecraft.

Water density decreases during freezing because it becomes highly oriented. When in the liquid state, its polarity causes the molecules to pack together very tightly. When it freezes, it forms highly ordered geometric shapes (hexagons maybe? It's been a while). I'm guessing this should appear in most freshman-level college chemistry books.

 

[DrPizza]

Originally posted by: tasburrfoot78362
I know that water volume increases when it freezes. And they have the breaks in between concrete slabs to allow room for thermal expansion. However, no one I ask seems to know if concrete gets expands with heat or cold.
I also know that as a gas expands, it gets colder (IE CO2 coming out of a nozzle), and compressed items get heated up. But I still don't know about the concrete. Anyone have any information on this?
--Tas

We were just discussing the expansion of gas on a physics teachers listserv. No one could think of a practical way to expand a gas, keeping it at a constant pressure.

On the contrary - if the volume of gas is increased and the pressure is held constant, the temperature will increase Yet, doing this is not a practical matter. When CO2 comes out of a nozzle, not only is it expanding, but more importantly, the pressure is decreasing. It is the decrease in pressure that causes the decrease in temperature. Nonetheless, there are three variables functioning here: pressure, volume, and temperature.

 

[Armitage]

Some carbon fibers actually have a negative CTE. Combine that with matrix materials which have a positive CTE, and with the right layup, you can tailor the CTE of the material, even make it zero. Of course, the internal stresses can get very ugly.

They did this for some structural components on hubble. They contracted for several times the number of of pieces they actually needed, and then tested them to find the ones that had the lowest actual CTE. My grad school advisor was trying to get some of the pieces that didn't make the cut.

It's a big deal on satellites with optical components, because the thermal environment is so severe, and the alignment is so critical. Tungsten ias a very low CTE also, but its damn heavy - heavier then lead IIRC.