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Fire resistance duration of concrete wall panels with flexural cracking

Fire resistance duration of concrete wall panels with flexural cracking

Fire resistance duration of concrete wall panels with flexural cracking

(OP)
Does anyone have experience with fire resistance of concrete tilt-up wall panels (that are intended to serve as a 3 hour duration separation wall) with FLEXURAL cracking?

40'H x 12'W panel with 18'H x 8'W door opening, central about bottom of panel - so a inverted U-panel.

The EoR has stated that the 'cracking is cosmetic and not structurally significant'. However, the architect is rejecting the GC's panel due to compromising of fire resistance due to the local flexural cracking.

I found a few technical papers on the subject, although most are in relation to post-earthquake damaged concrete elements that are then subjected to fire. The flexural cracking for our project is due to erection/handling stresses.

I did find this ASCE Journal of Materials paper that seems appropriate:

"Thermal Propagation through Tensile Cracks in Reinforced Concrete." by Ervine, A., Gillie, M., Stratford, T., and Pankaj, P. (2012).

Quote (ASCE paper)

"Consequently, it is concluded that the effects of tensile cracking on the thermal propagation through concrete can be ignored in structural analyses."

We are assisting the GC in repair options, however, I am interested if other engineers have encountered the argument above put forward by the architect and how you resolved it.

RE: Fire resistance duration of concrete wall panels with flexural cracking

(OP)
I forgot to state that the cracks are well distributed, at about 8" centers, over a 3' region. Typically the width of crack is 'fine' hair-line cracks, however, a few are wider with a max crack width of approx 0.040" (1 mm).

RE: Fire resistance duration of concrete wall panels with flexural cracking

My understanding it concrete provides fire protection through thermal mass and insulating properties; I would think that flexural cracks would not impact this signifcantly. Your ASCE paper seems to agree.

Something tells me though that if the architect wants to reject these panels I'm sure he or she will find a way. That much flexural cracking doesn't seem appropriate for a new structure.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com

RE: Fire resistance duration of concrete wall panels with flexural cracking

I agree with TME. The architect probably doesn't like the look, and has arrived at this reason for rejection off the top of his head...or perhaps from some other part of his anatomy.

RE: Fire resistance duration of concrete wall panels with flexural cracking

(OP)
Thanks TME for the reply.

The flexural cracking was caused by erection/handling of the panel - the lift points were incorrectly located on site so the center of lift was out of alignment to CG. Cracking is very localized at the re-entrant corner.

GC intends to epoxy inject the cracks via pressure injection, BUT, the architect then said that the epoxy-resin will not meet the fire resistance required. We have assumed he is referring to the glass transition temp (Tg) of epoxy being only about 150°F. Damn architects!



RE: Fire resistance duration of concrete wall panels with flexural cracking

I think you're in a pretty typical range of crack sizes there. Your big cracks are getting into the atypical range.

ACI 224R-01 says that the the allowable loading in ACI 318-95 corresponded to a probable crack width of 0.41mm for interior exposures and that the co-efficient of variation is about 40% with isolated cracks sometimes in excess of twice the computed value occurring on occasion.

I suspect you won't find any real fire testing on cracks in the 1mm+ range. I suspect it probably doesn't seriously affect the fire-resistance but couldn't prove it.

Could you toss them a bone on the big cracks and use ACI 224R to show that the small cracks are industry typical? Do a patch job on the larger ones.

RE: Fire resistance duration of concrete wall panels with flexural cracking

(OP)
Yep hokie66, I think you and TME are right - architects wants it replaced and will use any/all avenues to insist upon it.

The facility is a storage warehouse, and the wall gets cement washed then painted anyway. Seems wrong to reject a panel due to such a relatively small/localized defect.

RE: Fire resistance duration of concrete wall panels with flexural cracking

(OP)

Quote (TLHS)

I suspect you won't find any real fire testing on cracks in the 1mm+ range. I suspect it probably doesn't seriously affect the fire-resistance but couldn't prove it.

I found a PhD dissertation that tested beams with upto 0.7mm crack widths - with similar conclusions to the ASCE paper I quoted above.

See my comments above that the GC intends to inject the cracks except the architect is rejecting that proposal as the epoxy will not meet the fire endurance.

RE: Fire resistance duration of concrete wall panels with flexural cracking

https://www.era.lib.ed.ac.uk/handle/1842/6229

There's a pdf download link on this page for the thesis. His 'major damage' crack width was in the order of 5mm +/- 0.7mm and showed no difference in temperature inside the concrete. There could be convective effects that weren't captured, but I think this probably should show that minor cracks aren't significant risks to fire protection.

RE: Fire resistance duration of concrete wall panels with flexural cracking

Also, I found the guy on linkedin with a ten second google. People are generally pretty happy to have answer a quick email about their research.

RE: Fire resistance duration of concrete wall panels with flexural cracking

Yeah, I'd take this research to the architect and say "I believe these panels are good after repair. These sources backup my claim." If the architect still wants to fight you on it then I'd advise the contractor you will have to bill him for hours spent on this. Let the contractor decide which is cheaper; replacing panels or paying you to fight it. This way you're not the bad guy, architect will get new panels, contractor will probably pressure architect/owner to pay for the panels and if they drag you into the fight you'll get paid for it. Regardless, I still suspect that the architect will just come up with some other reason to reject even if you shoot down his fire protection reason. Most specifications I've seen for wall and floor panels will have some QA/QC rejection clause related to cracking in them.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com

RE: Fire resistance duration of concrete wall panels with flexural cracking

(OP)

Quote (TLHS)

https://www.era.lib.ed.ac.uk/handle/1842/6229

There's a pdf download link on this page for the thesis. His 'major damage' crack width was in the order of 5mm +/- 0.7mm and showed no difference in temperature inside the concrete. There could be convective effects that weren't captured, but I think this probably should show that minor cracks aren't significant risks to fire protection.

Yep, that's the one...and he is one of the co-authors of the ASCE paper too. I misread the 'major damage' crack width of 5 mm +/- 0.7mm - I read it as 0.7 mm total crack width in error.

5mm is a BIG crack width:

Quote (Adam Ervine, PhD Dissertation: "Damaged Reinforced Concrete Structures in Fire")




Thanks TLHS.

RE: Fire resistance duration of concrete wall panels with flexural cracking

No worries. I'd be curious to hear how your discussions work out!

RE: Fire resistance duration of concrete wall panels with flexural cracking

(OP)
I shall update this thread when I get further info.

RE: Fire resistance duration of concrete wall panels with flexural cracking

(OP)
===>UPDATE: 11-16-2016:

We were informed by the GC that they attempted to re-erect the cracked tilt-up panel (with some 'strong-backs' installed) and the panel cracked further, so they decided to recast/replace the panel.

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