bars cut by acetylene torth

[2204z]

The worker use acetylene to cut excess rebars protruding from a footing. When a rebar is heated by acetylene until it is cut. How much is the thermal expansion of the bars inside the concrete? What is the strain usually? Would it be more than the strain the concrete can handle and would the concrete crack from the inside from the thermal expansion of the bars?

 

[Ron]

Not a big deal. Heating is very localized as would be any effect on concrete. Not enough heat transfer to affect the concrete more than a couple of inches away.

 

[2204z]

Not a big deal. Heating is very localized as would be any effect on concrete. Not enough heat transfer to affect the concrete more than a couple of inches away.

If you hold a rebar and acetylene torch the front of it a foot away. Are you saying the heat of the torch won't even travel 1 foot? Isn't it iron a great heat conductor?

 

[MikeHalloran]

Slight correction: Acetylene provides enough heat to get the steel up to orange heat. The cutting starts when the oxygen is turned on. At that point, acetylene is superfluous; iron is the fuel.

As to the other points, yes, the steel conducts heat fairly well, but the concrete is a pretty good heat sink, and effectively limits the temperature excursion.

Should you elect to conduct an experiment, do be careful; concrete spalls violently when heated directly by oxyacetylene.




Mike Halloran
Pembroke Pines, FL, USA

 

[kingnero]

The previous, last sentence is very important from a safety related point of view. Welding (or cutting) of steel on/adjacent to/partly embedded in concrete can have disastrous consequences due to the moisture content of (cured) concrete.

 

[CANPRO]

will there now be rebar exposed to soil where it was cut off? or is there more footing to be poured? Sounds like the fact you may now have zero concrete cover on the rebar is the bigger issue.

 

[oldestguy]

Campro: Does it really matter? Steel stress there is pretty skimpy. How far in will corrosion go? Probably not far.

 

[racookpe1978]

Corrosion will go as far as any and all of the metal left inside the concrete can touch both air (oxygen) and the metal, and goes faster when ANY water or humidity near the metal can't evaporate because it is trapped under the concrete.

Sure, it will be a little bit slower deep inside the concrete because air flow is limited down there, but it will never stop corroding. Completely covered rebar will NOT corrode.

Cut off rebar (by definition!) will be left with the ends exposed, and is likely to be further exposed by the concrete spalling from incidental heating at the concrete surface.

To the original question. This is a bad idea, but not because of the thermal expansion of the rebar. The distance between "cold" rebar and the very hot tip is very, very short once the rebar goes into the concrete and doesn't get hit with the flames. thus, there very very little linear thermal expansion. Right at the surface, the rebar is constrained against movement INTO the concrete, and has no restraint against expanding into the air outside the concrete, so it will tend to expand away from the surface, not axially nor radially into the concrete. Spalling is a danger.

 

[BAretired]

Cutting with a grinder is preferable. After cutting, apply a little tar over the cut ends.

BA

 

[hokie66]

racookpe,
Cutting it off at the surface will not lead to corrosion deep within the footing unless the footing is in a very corrosive environment. The alkalinity of the concrete protects the reinforcing. There is always oxygen within concrete, as it is a porous material, but corrosion does not occur as long as the pH is high.

 

[racookpe1978]

I was looking at 'worse case" - near ocean, concrete piers and railings with constant exposed conditions (winter salt + summer heat/humidity). 25+ years and the foundations and rebar and exposed steel bolts (embed anchor bolts) in concrete were "gone" underneath the towers and bents I've looked at.

 

[oldestguy]

What about floors with mesh that may not all be held up during concrete placement. I've never run into a problem there.

 

[2204z]

racookpe1978

[To the original question. This is a bad idea, but not because of the thermal expansion of the rebar. The distance between "cold" rebar and the very hot tip is very, very short once the rebar goes into the concrete and doesn't get hit with the flames. thus, there very very little linear thermal expansion. Right at the surface, the rebar is constrained against movement INTO the concrete, and has no restraint against expanding into the air outside the concrete, so it will tend to expand away from the surface, not axially nor radially into the concrete. Spalling is a danger. ]

Were you describing about concrete or bars spalling (breaking away into pieces)?

What is the evidence concrete can act as heat sink of the bars inside it.. concrete are not metal and doesn't conduct heat so easily. It is said concrete conducts heat only about 1/500 the rate of silver.

Not that I'd do it again. The workers did it without my knowledge. I won't let them do it again, but want to know the principles. Ty

 

[MikeHalloran]

Thermal inertance of a heat sink has more to do with mass than with conductivity. Trust me on this; I live in a concrete block house; it takes a long time to react to any temperature change at its surface.



Mike Halloran
Pembroke Pines, FL, USA