Heat related concrete floor failure

[CTW]

I've got a project in a steel mill where the mezzanine floor has punched thru (by a fork truck wheel) the composite steel/concrete deck (hole is approx. 4 ft x 2 ft). The reinforcing is still intact, only slightly bent. This floor is exposed to extreme heat (2000 deg. +) from the underside. It appears that the heat weakened the concrete and steel deck and it finally crumbled under the fork truck. The steel deck sheared at the beam flanges. This is a potential serious problem as most of the deck is showing signs of heat stress. I'm at a loss as to the proper way to fix this, so I need some input.
Any advice is greatly appreciated.

 

[dik]

I would suggest that 2000 degrees is too much for the deck and that it may be considered as 'formwork'... and not much more. Prolonged 2000 degree (even fahrenheit) is too much for concrete.

From the failure, I would suspect the entire floor, where it has been exposed to this heat, is questionable and would suggest consideration of replacing it. Failure could be catastrophic.

You may want to contact the PCA or the ACI with respect to protecting concrete from the heat source. Dow Corning may have some material similar to the shuttle tiles, but for lesser temperature exposure. It may be necessary to provide 'ties' to attach refractory brick for protection.

 

[CTW]

I intend to check the entire floor for damage, and completely agree with your second paragraph DIK. But I'm curious about something. If only one span between two beams needs to be replaced, how should this be done? This is what I had in mind. Chipping the slab away back to the studs on each beam and leaving the rebar in place, forming and pouring the deck.

I'll seek help from PCI/ACI on concrete exposed to heat and search for materials that can protect the floor from high temps.

 

[ishvaaag]

2000?

Lava flows at between 900 and 1300 ºC. So even in Farenheit this ensures an status close to the destruction of any integrity and of course binding action. Furthermore mechanical strength of ordinary steels over 600 ºC is quite unreliable, hence there's something wrong about the temperature stated, at least average.

In any case, by subjecting concrete to high heat you are partly reversing the binding action that was gained through hydration (after the dehydration that made the cementitious component active). Hence it is not surprising that deterioration starts at some temperature level.

Anyway, whatever the temperature, the structural solution will be akin to any for such problem. But I find amazing that someone subjects his structure to such thermal temperature ... either the vessel has been made small, no refractory materials have been considered, heat shielding is not properly in place and refrigeration is bad ... in short, that I find it abnormal that the structure suffers such heat and not enough consideration of the thermal issue has been made in the design.

 

[CTW]

That's 2000 ºF or 1093 ºC. The material that produces the heat is slag from an electric arc furnace.

 

[ishvaaag]

I imagined some mill process was the cause of the heat. In any case to accept such levels of heat still looks to me rare, and one needs to bet that economies forbade a sounder design.

After thanking you for your explanation, and to not make just incidental this new post, is it feasible to precisely enter new variables of heat shielding that make the retrofitted building less thermally stressed?

 

[unclesyd]

I agree with Ishaaag, I see no economical solution to the current floor problem. I don’t think trying to heat- proof the steel or concrete is the way to go. It sounds like the steel/concrete is under continuous exposure. We always used gating around the furnaces, though none was ever exposed to these temperatures for any period of time. This allowed for inspection and periodic replacement. The permissible weight on the grating was far less than a fork truck.

In the interim you might be able to hang some 1800̊/2300̊ F Ceramic fiber bat insulation below the floor if there is no "poping" slag. I would contact a company like Plibrico to get more information on both the insulating material and how to install it. This is for new steel and concrete.

Is it possible to confine the slag to a trough and use a cover that can take the heat. We used a covered trough to convey the slag to an open area where it was water cooled. The hottest part of the trough 20' had a fire brick cover. The remainder 10' was open, nothing above.

As mentioned in previous posts, you have a very unsafe condition in place which should be corrected.

 

[Bbird]

The remedial measures depends on the actual structure.

I agree with many posters that the floor is unserviceable. The solution is therefore to seek a complete replacement or strengthening of the existing structure.

If for whatever reason the structure has to be kept and the columns are strong enough (or reinforced) one can cast a new and strong deck over the existing floor, which acts vitually as permanent formwork. As a long term solution, the underside of the existing floor needs to be insulated against heat but that can be done at a time when there is an outage of the slag movement underneath it.

 

[CTW]

Thanks for the replies. I do plan on implementing some form of a heat shield as this would be fairly easy to do. Providing a seperate covering over the slag area is not going to be feasible in this application. Nice idea though.

 

[dik]

I'd probably not rely on the existing rebar, either... too hot for too long...

 

[dik]

Forgot to add... might be prudent to get some cooler circulation between the 'tiles' and the concrete... else the slab can still get hot.