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Would a passive and controlled amount of electricity fed through post-tension cables, heat up a slab, without ruining the dependability and health of the post-tension cables, and the concrete itself?

I am wondering because I coat concrete slabs with different types of paint, and in the winter, it is super slow to add layers of coats on top of one another because of the waiting time for drying and curing of the paint. If I could heat up the slab through the post tension, then re-grout the post tension ends, it would totally change the game for me. I am just afraid of possible snapping the cables. In the winter I can get away with only one coat for the whole day. In the summer I can put down 5 coats of paint over a 7,200 square foot area with ease.

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Nobody else applied any numbers to this so here is my take.

The smallest post tension strand I found on the web was 1/2 inch in diameter or (0.25^2*PI) about 0.2 square inches. We want to heat up a 7,200 square foot area slightly to assist in drying paint. A square that size would be about 85 feet on a side or 1020 inches.

So assume we have an iron conductor 1020 inches long with an area of 0.2 square inches.

The resistance of iron is about 4E-6 Ohm-Inch so doing the math:

ohms=(4e-6 ohm inch)*(1020 inch)/(0.2 inch^2)=0.02 ohms

Remembering P=IE and E=IR we have P=I^2R so if you put your welder on the strand and pump some current through, I get:

  • 20A 8W
  • 40A 32W
  • 60A 72W
  • 100A 200W
  • 200A 800W

Now,this is inside the concrete. If you put in 800W without being sure what is happening inside, you are ... not going to be pleased with the results. A little reading shows that if you keep the temperature of the strand below 200°C, you won't compromise the strength... of the strand but I would not want to bet on the concrete if it was expected to set at that temperature.

What I would do is connect a welder at some minimum current and feel around and feel around to see if the strand is warm before entering the slab. I expect that the heat transfer inside with contact with the concrete will be greater than outside to air so if it is not too hot to touch, you are OK. Let it sit for a while and turn up the current a little, keeping in mind that the power goes up as square of the current.

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  • $\begingroup$ Awesome reply! I mean, doing something like this would speed up the painting process by a LOT during the winter. In the east coast, resurfacing a court many times cannot even be done in the winter because its so cold. We squeegee each layer of paint, so it's quite a thick coat. $\endgroup$ Apr 21 '19 at 17:42
  • $\begingroup$ Also, by connecting the welder to the post-tension cable just for testing, wouldn't the post-tension cable be too thick to connect to the welding gun? Any idea on how to get around this? I'm guessing I can then after reaching a desired temperature on the cable I am testing (under 200°C), can then trust that such voltage I can then safely use on the post-tension cables and see how it turns out. I will let you guys know how it goes with just a piece of post-tension cable first! $\endgroup$ Apr 21 '19 at 17:50
  • $\begingroup$ Like I said, The CABLE will be fine at 200°C. But since that is a lot over the boiling point of water, I would not subject concrete to that. I would not subject newly setting concrete to any temperatures that were too hot to touch. Keep in mind that it can take a long time for the heat to reach the surface. If you get the cables to about as hot as you can keep your hand on them and let it sit overnight, the surface maybe warm enough to paint in the morning. $\endgroup$ Apr 22 '19 at 21:54
  • $\begingroup$ I have an old stick welder that has clips that look like jumper cables. If I could not clip it to the strand, I would clip it to my jumper cable and use that on the strand. The welder has a constant current output dial from 10 to 200 amps. MIG welders have constant voltage. If you are trying to warm this up with a MIG supply, be super careful you don't overheat the concrete. I would start out extra low, let it sit for 30 minutes and if you can't tell any difference, turn it up a little. Remember, slow and steady wins the race. $\endgroup$ Apr 22 '19 at 22:03
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Yes, theoretically...

But electric underfloor heating is done with thin wire to limit the current.

Concrete rebar is 1/2" or 3/4" in diameter so will need a lot of current - have you considered a source for that... Also the connections with the rebar were not designed for electricity and you may find bars that you only get to one end.

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    $\begingroup$ He's asking about post-tensioned strand. They run in conduit right through and are tensioned after the cure and before the scaffolding is removed. Which begs the question why not just run a heating wire in the conduit as well. But go ahead and hook an old welder up and see how it goes. That's how we used to thaw out frozen pipes. $\endgroup$
    – Phil Sweet
    Jan 27 '19 at 19:33
  • $\begingroup$ So diameter of the strand? Spacing for even heating? $\endgroup$
    – Solar Mike
    Jan 27 '19 at 20:11
  • $\begingroup$ I'm hoping he's just wanting to help cure a cold weather pour. If he wants to heat his house, or stop pipes from freezing, this isn't such a great idea. And it wouldn't be my first choice for bridge deicing either. $\endgroup$
    – Phil Sweet
    Jan 27 '19 at 20:33
  • $\begingroup$ Very likely to anneal or temper the cold worked cables . Also , crack the concrete. It is possible ; under CONTROLLED conditions in a steel mill , solid steel bars have been heat-treated like this . It requires more current than can be found outside an industrial setting. $\endgroup$ Jan 27 '19 at 22:20
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Based on your most recent comment, you seem to be looking for a DIY improvised way of drying your painting. I'm thinking of something, but it depends on the scale of the painting you do in the winter. I think it's way better, more efficient, and less risky to actually dry the paint from the outside given it'll take quite a lot of energy and temperature gradient to get the slab surface to temperature which increases the risk of damaging something. Why not section the slab surface and then use a heat gun to preheat each section before coating or you add the paint then shower with some hot air.

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