Concrete slab under fire 2

[Gtoyo]

Does anyone has suggestions and checkpoints for concrete slab and rebar under fire.
I have participate in assesment of this slab for warehouse which damaged by fire( more than 4 hrs). This warehouse use for chemical raw material stocking. After fire the superstructure was demolished, but we trying to use or repair this slab.
existing slab was designed for 600 psf live load on pile which space 12 ft by 12 ft(flat slab). 8" thickness with no.4@8" top&bottom, bothways.By inspection concrete cover around 1" with concrete hardener surface. 5% of surface : concrete surface spall to top bars, 20% with minor spall off to depth around half inch(max).( total 60000 sft)
Verifying point;
- remaing capacity of this slab and long time service.
- What extent of damage to this concrete and rebars.

Thanks in advance.

 

[Ron]

The first thing to check for is surface damage to the concrete. This will be in the form of cracking and spalling. Often, the spalling is a result of the quick cooling by water when the fire is extinguished, but can also be caused by expansion of interstitial moisture during the fire (moisture vapor expands, "blows" the concrete out).

Check the cracking pattern and draw a crack pattern map. See if they follow typical shrinkage patterns. If more than expected for typical shrinkage of the slab, then do some coring to determine the age of the cracks (did they occur before or during the fire). Microscopic examination might be required.

Check the depth of carbonation of the concrete. Fire damage results in exacerbated carbonation (to a depth greater than would be expected in normal exposure application).

Run compressive strength tests and consistency tests on the concrete.

Concrete is a good insulator, so I would not expect damage to the rebar. If the rebar were damaged, you would also see resulting localized damage in the concrete.

 

[Rjeffery]

As usual, Ron is exactly correct. I would only add that Petrogragic studies will detect microcracks and other damage associated with this type of damage.

If the concrete is locally damaged but the damage extends over a large area, Hydroblasting would leave the re-steel intact and undisturbed (generally) and allow for placement of concrete repairs sooner than other methods. this method us used extensively in virginia to replace 1 to 2 lanes on bridge decks.

 

[rapt]

Gtoyo,

I will disagree with Ron on the damage to rebar aspect of his reply. Depending on the temperature of the fire, the bottom rebar especially could have been badly weakened.

The temperature at which this occurs and the amount of reduction in strength of the rebar varies deopending on the rebar type, the member type and the amount of cover reinforcement.

You should be able to find out from you steel ,supplier how his steel will perform but some sample figures on strength loss would be

steel temp = 400 degrees C strength loss = 15%
steel temp = 500 degrees C strength loss = 40%
steel temp = 600 degrees C strength loss = 80%

The temperature of the steel will depend on the fire intensity, the period and the amount of concrete cover to the rebar. I thought I had some tables defining temperature gradients for different fire periods but cannot find them. Someone else may have figures on this for a 4 hour period fire.

 

[Gtoyo]

Thank Ron, Rjeffery,Rapt.
These are more informarion;
- This warehouse is just single story, slab that I mentioned is slab at grade rest on piling support.
- This warehouse was fired more than 4 hrs. Somebody said that all night long. The Steel roof truss was totally collapsed. Some Concrete columns have severe damaged but some especially at perimeters have liitle damage.
What Rapt says is in my mind we are planning to cut top bars inside the slab to test and also concrete coring. Our focus is Can and what extent this slab sustain load.
Our assumption is that concrete may loss some strength under fire as well as its reinforcement.
What we are under considering is the depth of coring. At first we going to coring through slab thickness but as concrete in difference level may have diference effect. Concrete sample may failed at upper part during test results in lower strength.
Does anyone have suggestions.