Concrete Double Tee Roof Spalling only bottom face. 2

[joshmaughan]

I have been asked to look at some spalling that is occuring along the bottoms of some precast/prestressed concrete roof double tees in an industrail building. T

he span is very short (22') and the tees are 18" deep. The only load is dead load (self wt.) and a 40 psf snow load, so the loading is minimal for this span and type of construction. The building has been in place for roughly 40 years.

The spalling is occuring along the bottoms of the slab portions of the tees, and along the panel joints. None on the stems. Most of the spalls are about 2" X 2" and 1" deep or so. Some are larger, some are smaller. Most of the spalling appear to be recent, as chuncks of concrete are still on the floor in several places.

It is very stange. No loading changes, chemical exposure, or extreme temperature changes have occured as far as we can determine. The room that this is occuring is around 80 degrees F constantly, with a ceiling temperature around 100 degrees or so.

Has anyone seen anything like this? Any idea what could cause it at this point in time? Thanks! See attached pic.

 

[Ron]

Look at the apex of the spalling and view the exposed aggegate. Check for a powdery or gelatinous substance.

This looks like "pop-outs" caused by minor aggregate reactivity. It is unusual for the manifestation to take so long, but could also be a function of the minor nature of the reactivity taking a long time to build enough expansion stress to cause a localized failure.

 

[joshmaughan]

Thanks Ron. I will look for that. FYI, the spalls have cracked "through" the aggregate, and not around it if that means anything. So the aggregate is fractured along the spalling.

 

[NS4U]

Cracks thru the aggregate are indicive of reactivity

 

[NS4U]

Indicative*

 

[Ron]

josh...that's an indication of good paste bond and higher stress. That's consistent with a long-term stress build-up from reactivity.

Check the area in which the double-tees were cast and see if there is any history of minor aggregate reactivity. If I'm right, this is consistent with the presence of chalcedony chert, rather than an alkali-silica reaction that creates a large ettringite formation.

If ettringite formation, you'll likely see a crumbly, formerly gelatinous residue. If chalcedony chert, you'll see a powdery white to pale green residue in the middle of a small aggregate particle remaining at the apex of the popout.

 

[joshmaughan]

Thanks guys! Any thoughts on why there would be no spalling on the stems, only the bottom of the slab portion? Thinner concrete (2-1/2" slab) maybe? Stems are about 6" thick. Maybe the steel mesh in the thinner slab portion is adding to the stresses, or helping provide failure planes.

 

[Teguci]

Star for the detective.

Ron,
How is this different from ASR? And, I guess the big question, will these DTs continue to fail regardless of treatment?

As for why the flange only, I am guessing that has to do with the prestressing and exposure to humidity.

 

[PMR06]

New roof recently? This looks like fasteners shot from above spalling out the slab. They are somewhat evenly spaced, concentrated at the opening.

 

[JAE]

I think PMR06 is onto something,

 

[joshmaughan]

Thanks again guys. No recent new roofing. It's been several years. Still, could be from old fasteners. Also, it has not been a particularly cold winter here. Past years have been much worse. I'm having a hard time finding anything that has "changed" this year. That's why it's so strange.

 

[ImminentCollapse]

With all due respect to Ron, it just doesn’t “smell” to me like a time-dependent materials reactivity issue. Popouts don’t seem random enough and I would expect to see more pattern cracking throughout and on the stem too. Looking for the residues he mentions will be definitive though.

What is at the very bottom center of the picture transverse to the T-beams? It appears to have the pop-outs too but doesn’t look precast.

What occurs in this facility?

 

[Ron]

Agree PMR06 brings up a good point. Could be fasteners starting to corrode and expand as well.

 

[joshmaughan]

Thanks. Imminent - those are through bolts with plate washers. No spalling on the stems. This is the ceiling above a pump/motor room. The are ammonia refridgeration lines in the area, but no reported leaks. I'm going to attempt to make a determination on the fasteners.

 

[Ron]

IC...it would be unusual, as you indicate, for a reactive aggregate to manifest so long after construction....it just has a similar appearance and is easy to rule out with visual assessment on site.

As we've all surmised, it could be just about anything...past hangers for piping where the fasters were just pull out with the piping....fasteners shot from the top (though not commonly done), fastener corrosion...though you'd expect to see staining and some "pre-popping" cracks with stains....probably half a dozen other things.

Teguci...it is a form of ASR, it just doesn't form the classic ettringite gel...it initially expands volumetrically, causing the popout but prior to the gel formation.