# Is there permanent size loss in metals from contraction?

• Gary Weller
In summary: Thermodynamics. Your explanation was very easy to understand. Thank you!In summary, the conversation revolved around the issue of gaps appearing between sections of anodized aluminum tracks used for automatic pool covers. It was suggested that the gaps may be caused by the expansion and contraction of the concrete decks they are anchored to, which can be affected by temperature changes. The principle of metal shrinking and the equation for calculating the change in length of aluminum due to temperature were also discussed. It was concluded that leaving a small gap between track sections may prevent any potential buckling or distortion.
Gary Weller
I install automatic pool covers for a living. The tracks that our cover rides on are composed of extruded anodized aluminum. These tracks come in 22' lengths. Usually there is more than one section of track, set end to end, on a given side of the pool. We drill and anchor these sections to concrete decks with NO gap between.

Several years down the road, we notice that there is now a 1/8" - 1/2" gap between the sections. These systems are installed in the heat of the Summer and we usually come back to the jobs in the same heat. If that's the case, does this mean that there is some sort of loss happening during the expansion/contraction process that happens between the seasons and hot days/cold nights?

I know that there's potential for deck heaving and expansion and contracting of the concrete, but this has been seen across the board and there's no way that so many different stones and concrete mixes have the same expansion/contraction ratio.

I guess my end-all questions are "Is there Net size-loss throughout the expansion/contraction process? If the aluminum is 10 units long at 100º, then 9 units long at 50º, but only returns to 9.9 units long when brought back to 100º, where did that .1 unit of length go? Is it gas loss that allows the aluminum atoms to get closer together?"

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Gary Weller said:
I install automatic pool covers for a living. The tracks that our cover rides on are composed of extruded anodized aluminum. These tracks come in 22' lengths. Usually there is more than one section of track, set end to end, on a given side of the pool. We drill and anchor these sections to concrete decks with NO gap between.

Several years down the road, we notice that there is now a 1/8" - 1/2" gap between the sections. These systems are installed in the heat of the Summer and we usually come back to the jobs in the same heat. If that's the case, does this mean that there is some sort of loss happening during the expansion/contraction process that happens between the seasons and hot days/cold nights?

I know that there's potential for deck heaving and expansion and contracting of the concrete, but this has been seen across the board and there's no way that so many different stones and concrete mixes have the same expansion/contraction ratio.

I guess my end-all questions are "Is there Net size-loss throughout the expansion/contraction process? If the aluminum is 10 units long at 100º, then 9 units long at 50º, but only returns to 9.9 units long when brought back to 100º, where did that .1 unit of length go? Is it gas loss that allows the aluminum atoms to get closer together?"
Welcome to the PF.

Very interesting question! My initial guess is that the concrete is expanding over time, but others may have other ideas. Are these all new pools? if you install the cover hardware on a new pool, it seems plausible that the concrete structure around the pool can expand over time. But if this happens even on old pool retrofits, that would shoot that theory down...

Gary Weller
If a piece of metal is heated and cooled uniformly and unconstrained, it will return to the exact original dimensions when cooled. However if the ends are constrained when heated, the metal will become shorter and fatter when cooled. The metal has yielded, as if squeezed in a press, but the volume is unchanged.

Gary Weller, Rx7man and berkeman
Aerospaceenginer said:
If a piece of metal is heated and cooled uniformly and unconstrained, it will return to the exact original dimensions when cooled. However if the ends are constrained when heated, the metal will become shorter and fatter when cooled. The metal has yielded, as if squeezed in a press, but the volume is unchanged.
I yield to the better idea!

I think that's the right conclusion.. since there was no gap between them, and the strength of the concrete is so much higher than the aluminum, it is in fact shrinking the aluminum.. I would venture a guess that if you installed them with 1/8th gap between them, you'd probably still have a 1/8th gap later.

Gary Weller and berkeman
This is the principle behind "flame bending". If you heat one side of a pipe, the colder side will be a constraint. When cooled, the pipe will be bent towards the heated side. This is also called metal shrinking, but in reality a local area gets thicker and shorter. The volume is unchanged.

Gary Weller
You can calculate how much a length of aluminium might expand and contract as it heats up..

dl = L0 α (thot - tcold)

where
dl is the change in length (meters)
L0 is the initial length (meters)
α is the coefficient of expansion for aluminium (22.2 * 10-6 m/m.k)
(thot - tcold) is the change in temperature (C/K)

So if you installed a 22ft/7m length when the temperature was say 20C and it was heated by the sun to say 50C (?) then the change in length works out at about 4.5mm (bit over 1/8th inch).

Do the gaps cause any problems?

If you leave an 1/8th inch gap between lengths you might stop any potential buckling/distortion of the track.

NascentOxygen, Vanadium 50, Gary Weller and 1 other person
CWatters said:
You can calculate how much a length of aluminium might expand and contract as it heats up..

dl = L0 α (thot - tcold)

where
dl is the change in length (meters)
L0 is the initial length (meters)
α is the coefficient of expansion for aluminium (22.2 * 10-6 m/m.k)
(thot - tcold) is the change in temperature (C/K)

So if you installed a 22ft/7m length when the temperature was say 20C and it was heated by the sun to say 50C (?) then the change in length works out at about 4.5mm (bit over 1/8th inch).

Do the gaps cause any problems?

If you leave an 1/8th inch gap between lengths you might stop any potential buckling/distortion of the track.
That is amazing. I'm pretty proficient in Math and Astronomy, but Physics is still somewhat new to me. This is such a fun conversation for the guys in my office right now. I've always theorized a loss in length, but my boss was skeptical. This puts Science behind it, which I always knew was there; I just never took the time to ask the right people. Thank you so much. This is great.

Vanadium 50, Greg Bernhardt and berkeman

## 1. Does contraction cause permanent size loss in metals?

Yes, contraction can cause permanent size loss in metals. When a metal is exposed to low temperatures, it contracts and its atoms move closer together. This can lead to a decrease in the overall size of the metal.

## 2. How does contraction lead to size loss in metals?

Contraction occurs when the temperature of a metal decreases, causing the atoms to move closer together. This results in a decrease in the interatomic spacing, leading to a decrease in the overall size of the metal.

## 3. Is the size loss in metals reversible?

No, the size loss in metals due to contraction is not reversible. Once a metal has contracted, it will maintain its smaller size even when the temperature increases again.

## 4. Are all metals affected by size loss from contraction?

Yes, all metals are affected by size loss from contraction. However, the degree of contraction may vary depending on the type of metal and its specific properties.

## 5. Can size loss from contraction be prevented?

Yes, size loss from contraction can be prevented by using alloys or materials with lower coefficients of thermal expansion. These materials are less susceptible to contraction at low temperatures, reducing the potential for permanent size loss in metals.

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