Acetylene torch & concrete

[lejam]

Bars extended outside concrete needs to be cut using acetylene instead of saw which can take long time. What is the behavior of concrete when it is torched with acetylene?

 

[kingnero]

Might be dangerous: I've heard several stories of expanding (even exploding) concrete when welding on steel embedded in concrete structures.
Your's is a different situation, probably other members might give you their experiences. I'd be hesitant though, unless it was proven otherwise.

 

[Ron]

From personal experience....moisture inside concrete will expand and spall the concrete, sometimes causing injury. (I was cutting angle on a concrete floor).

For cutting rebar, damage will probably be localized but should check for microcracking in concrete as result. Not a good practice if you can avoid it.

 

[lejam]

Dear Ron, When you use acetylene on a rebar let's say of size 1 inch. What is the temperature at every 5mm increment away from the torch center? And do you know how and what value the rebar expand for every say 100 Celsius in temperature given grade 60? Some formulas?

 

[Ron]

lejam...physics. Conduction, thermal gradient and thermal expansion.

 

[lejam]

it is said that the coefficienty of concrete and bars are the same so they should experience similar expansion. Now, in a bar inside a cement. When you acetylene torch the bar and it expands, can the concrete take the expansion? Maybe it can since limit hasn't yet reach. It's like the wieght is not from upside and inside. So how do you caculate it's braking point, do you actually belive the concrete would just crushes but not load but from fire load?

 

[Archie264]

For what it's worth, I'm told lightweight concrete has less of a propensity for spalling or exploding when heated, due to the extra voids wherein the expanding steam can evacuate. Not something I'd want to have to depend on, though.

 

[miningman]

Forget the theoretical BS. Concrete can and will explode when heat is applied. Many years ago ,I was using a propane tiger torch (lower temperature than oxy acetlyne) on a 20 year old concrete floor (probably not much water still entrapped) in an attempt to soften up and recover thick grease (a 20 year accumulation) that was heavily impregnated with gold dust. My overall efforts were successful and financially extremely rewarding (just burnt off the grease) but I was sure glad I was wearing safety glasses the first time the concrete floor exploded with adequate violence to spray my face with hot molten grease.

 

[Archie264]

lejam,

It's not the expanding steel that causes the explosion, it's the expanding steam resulting from the residual moisture in the concrete.

 

[cvg]

concrete will absorb water, it is not just "residual moisture". in addition, aggregates also contain moisture

expanding steel causes the concrete to crack due to tensile stress. steel is a good conductor of heat while concrete is not. the steel will heat up far more than the concrete and expand to a much greater degree and very quickly. This could cause cracking or reinforcing bar debonding within the concrete mass.

 

[lejam]

If the steel thermal expansion is beyond the concrete crushing strain of 0.0035, then the concrete will break, but if the expansion is less than that, then it won't break. Do you agree with this? Since the temperature of the acetylene in the outside is more.. then breakage will form more from the outside to the inside depending on the bar expansion and concrete straining negotiated. Is the theory correct?

Of course this is ignoring moisture which can burst the concrete so I won't do it but interested to know the above to explain to co-workers.

 

[MikeHalloran]

You can't ignore the moisture; it's guaranteed to be there.

Just use/specify an abrasive cutoff wheel to cut the bar.
A big one is only slightly slower than oxyacetylene cutting.

Do use/specify goggles and a face shield; abrasive cutoff wheels don't fail gracefully.





Mike Halloran
Pembroke Pines, FL, USA

 

[lejam]

For those who believe the failure mode of heating the bar with acetylene is tensile stress (in addition to moisture mediated evaporation and explosion). Imagine you have a cylinder of concrete with a bar in the middle. You heat the bar with acetylene, the bar will expand. Isn't it that this will cause compression of the bar outwards. If there is nothing to restrain the cylinder, the cylinder would get bigger from poisson's like effect? Or the tensile stress cracking is due to the concrete expansion causing tension between the fibers as it expands outwards? Please elaborate on the mechanisms you think in addition or in correction of the above.

 

[MikeHalloran]

You've never worked with steam, have you?


Mike Halloran
Pembroke Pines, FL, USA

 

[cvg]

the concrete will fail in tension due to the expansion of the bar

 

[miningman]

Lejam , in addition to Mike Hallorans comment , go back and reread your original post. The answer is "It explodes"