Concrete cover for fire resistance for RC beams and slabs

[Ingenuity]

With regards fire rating/resistance for an existing reinforced concrete slab and beam construction, is it implied that the slab and beam system only need to comply with minimum concrete cover requirements for FIRE to the bottom surfaces (ie slab soffits, beam bottom and sides etc) and that the top of slab and beam surfaces do not have to meet the fire-related concrete cover requirements?

I ask this in relation to strengthening an existing concrete beam and slab structure, where top and bottom surfaces require of flexural strengthening.

Thank you.

 

[haynewp]

I don't know about the top not having to meet any requirements. Check the UL 90 for the construction needed to meet your fire rating requirement.

 

[rapt]

Ingenuity,

Where have you been?

Fire cover requirements apply to surfaces exposed to heating from a fire. The top of a slab does not get heated by a fire to the same extent as the bottom surface to the floor above the fire. The temperature has to reach about 500C to cause a reduction in the strength and the top surface does not increase by this much.

So in general it is only the bottom of slabs and the bottom and sides of beams that need to be checked for fire cover requirements.

 

[Ingenuity]

Hi rapt,

Been a long time. Happy New Year. I travel back and forth between HNL and SYDNEY at present. I will send you an email to catch up.

Thanks for the reply. Your reply is consistent with AS 3600 (table 5.5.3 from 1994 code) but I am trying to convince an AHJ (Authority Having Juridiction) in the US that this is the case and whilst ACI and its back up references sort of imply this, they do not but state it directly in these documents. AHJ is requiring that the top cover concrete be removed, FRP be installed, then concrete cover be reinstated. For thin RC slabs (4.5"), where the top cover is removed and FRP is installed, the fire insulation now provided by reduced concrete thickness (less effective thickness) to the top recessed FRP are now reduced at these locations for bottom-fire exposure. Does not make sense to me. Thinking of undertaking a ACI 'rational analysis' based upon time-temperature etc.

haynewp,

Thanks for the reply. I am familiar with UL, but what is UL 90? I did not think there was UL rating for reinforced concrete, as such.

 

[rapt]

Ingenuity,

That 500C number I mentioned is actually significantly less for PT strand, but it is still not a problem for the top surface as long as sufficient exposure cover has been provided.

 

[haynewp]

Sorry, I have had my mind on steel roof deck lately.

Regarding the UL and other references for concrete:

http://www.cement.org/tech/faq_fire.asp

There are UL assemblies for FRP on beams at the bottom face if I remember correctly. The FRP won't have a UL rated assembly for the top case you describe. If the concrete has a floor covering, there is a spray-on intumescent paint fireproofing you can get from the FRP manufacturers that may can go under the flooring. That would keep from cutting into the existing concrete thickness through using this paint applied to the FRP at the top surface of the existing slab. You can ask the FRP mfr for test data on their fireproofing to show the AHJ.

It doesn't take a lot of heat to reach the glass transition temperature of the FRP epoxy.

 

[rapt]

Ingenuity,

So are they worried about the temperature/strength of the top reinforcement due to fire below?

Insulation requireemnts are simply to limit the temperature of the top surface and therefore the temperature of the top reinforcement due to a fire from below. Basically in Eurocode they are trying to limit the temperature of the top surface to 180C at any point and 140C as an average over the floor surface. Presumably this is sufficient to limit the temperature of anything above the floor to below its flamable point and also to limit the temperature of the top reinforcement so that it can be assumed to not lose any strength during the fire. Top surface PT strand will lose some as its strength starts droppiong at 100C while reinforcing steel starts losing tensile sttrength at 400C. Not sure about FRP but I assume it is relatively low!

4.5" according to the latest AS3600 and EC codes would only give a 90min fire rating (actually about 110, but they only rise in 30's in the tables!).