Corrosion within Fireproofing

[drewtheengineer]

Let's say we have a residential building comprised of a parking garage with a steel bar joist system above. The joists had significant surface rust however no reported penetration/deterioration into the cross section of the members.

Without any further preperation (i.e. sand-blasting, priming) fireproofing was sprayed onto the members.

The question is, will the bar joists continue to deteriorate? Would they continue to deteriorate without the fireproofing installed?

 

[SperlingPE]

Yes, but probably at a slower rate.

 

[LPPE]

The spray-on fireproofing may actually absorb moisture from the environment and act like a continuous wet sponge wrapped around the joists.

 

[Rich2001]

However, if the fireproofing get wet from a leaking roof ....

 

[mshimko]

From a corrosion engineering perspective, I have two concerns:

1. If the fire insulation ever gets wet, or even really moist, it will likely stay moist and hence increase the rate of corrosion of the steel.

2. Is there anything in the fire insulation that could possibly cause an increase in the corrosion rate of the steel?

The only real benefit you have in your favor, is that the steel bar joists were likely overdesigned initially, with a large allowance built-in for corrosion and possibly other unknowns.

 

[drewtheengineer]

I appreciate all of your input.

I should clarify that there is a residential building over the parking garage and that there shouldn't be any reason for the joists getting wet (barring a pipe leaking). It is located in NJ and not near the coast so I don't think the humidity is so bad that it would moisten through the fireproofing.

Mshimko, I think that's an excellent point about the material in the fireproofing possibly accelerating the steel corrosion (similar to pressure treated wood). I would trust that the material itself would not be detrimental to the steel since that is what it is specifically designed for. I disagree with the overdesign due to corrosion. Bar joists, if corroded, have no forgiveness since there is no extra "meat" in the design.

Look at it this way... If you put an uncoated steel beam in your garage in NJ and left the garage door open, would it rust eventually? How many years?