Microwaves and Rebar

[37ed]

Does anyone have any information on the effect,if any that microwave could have on rebar.I plan to design a cast in place wall,but my rebar must be close to the surface of the wall.will the microwaves be deflected,or penetrate the concrete and undermine the steel.


thanks
ed

 

[IRstuff]

microwaves from what?

Have you ever noticed that microwave antenna are often found on steel towers?

TTFN

 

[JStephen]

I can't think how microwaves would undermine the steel. I suppose at too high of an energy level, it would get hot- is that the concern?

 

[WillisV]

We often use FRP rebar in hospitals for the walls surrounding rooms containing MRI machines and the like because the manufacturer's state that the steel rebar can interfere with the machines.

 

[SperlingPE]

MRI's are large magnets.
What kind of microwaves are we talking about here?

 

[37ed]

lets imagine a microwave oven made of concrete.area would be 20 ft square. Inside our oven would be a wave generator producing microwaves in the 4700(mhz)range.
could a wave in that range penetrate the concrete about 2in and heat the rebar to the point of structural damage?

 

[JStephen]

Could the microwaves heat the steel- that sounds like an electrical engineering problem, to be asked of whoever is furnishing or designing the generator. I would think the question would more concern personnel safety in the area than safety of the rebar.

You could assume that all energy from the generator went into the rebar, then find resulting rebar temp. Then you have a heat transfer problem, instead of an electrical engineering problem. If the amount of energy was fairly low, this would be a reasonable approach to show that there was no significant effect.

Not a microwave expert- but I understood that microwaves heat up food mainly by heating water molecules. In which case, I'm not so sure you wouldn't heat up the concrete and NOT the rebar here.

 

[PEStructural]

One thing to mention is that if you heat up the conrete or heat up the rebar which in turn will heat up the conrete, you will need to add additional reinforcing due to the temperature differential. When concrete expands/contracts due to heat, it cracks. If this is the case, then care should be taken when designing the reinforcing.

Hope this helps.

 

[IRstuff]

Please provide a proper specification.

How much power are you generating?

TTFN

 

[37ed]

O.K. BOYS HERE IS HOW IT GOES.
This is a mil spec job,so my details will need to be abstract.
we have a big,big, big,microwave generator.it has an extrenal start up power source.once started it fires off microwaves in to salt water.salt water turns to steam,steam turns a tubon,turbon makes power,power feeds microwave generator,which makes more power. and you get the idea.

does a microwave cause damage to concrete,and steel.
do the walls of containment need to be reflective of the waves? will the rebar expand,or shrink?will the h2o in concrete be heated? help!

 

[JStephen]

37ed, my thought on this- If it's a dinky little microwave generator that came out of Walmart, then I can see having no information, no specs, no engineering help available, etc. But if it's a big powerful expensive piece of equipment, somebody ought to know what it is, what it does, how far it penetrates, and what effect it has on stuff without having to go to an anonymous internet site. From a liability standpoint, you need to insist that the equipment designer or supplier furnish this kind of information. From your description, it sounds like you have a major heat-transfer problem on your hands, and if you're not the heat-transfer guy, you need to insist that somebody give you the temperature variations, etc. in order for you to do your work. You won't find people designing boilers unless they have a pretty good idea of the temperatures they're designing for.

From your description above, this sounds like a perpetual motion device. If that's the case, the whole issue is kind of moot. But assuming I'm missing something somewhere, you do need reliable information from the people who are in a position to furnish it.

 

[UcfSE]

From what it sounds like, you really need to hire a professional rather than seek a forum.

I would expect the microwaves to heat the water in the concrete and possibly lead to thermal cracking and spalling if the water gets hot enough. The electric field the microwave generates could induce a current in the rebar if it is powerful enough to penetrate the concrete cover. Obviously that wouldn't be a good thing. Did you ever put metal utensils in the microwave in your kitchen? Of course, I'm no expert in microwave radiation. That's why I think you should get a professional who is.

 

[37ed]

UPDATE
GOT SOME SPEC ON THE MICROWAVE GENERATOR.WHEN I POSE Q&A TO THE DESIGNERS I AM TOLD WE'VE NEVER DO THAT BE FOR?OR THIS DEVICE WAS NEVER DESIGNED WITH THIS APP.IN MIND.THANKS GUYS I THINK I NEED TO FALL BACK AND REGROUP ON THIS AS YOU HAVE SAID

THANK ED