Denver Airport's Renovator Uncovers Potential Snag 2

[JAE]

So in 1995 Denver, Colorado, built a large airport. The main floor was designed for 250 psf live load - equivalent to that required for heavy storage loads and provided for equipment necessary to erect the large 150 ft. span "mountain" tent roof.

They now are in the midst of a large renovation project ($1.8 billion I think) and recently they hit a snag in that some concrete cores on the floor came up lower than the original 1995 specified concrete compressive strength.

I searched for information on the numbers here but didn't find anything.

I am a little suspicious of the lower strengths in that concrete coring doesn't always reflect the full strength of the concrete...i.e. ACI even allows you to compare your cores to 85% of the specified strength. I'm sure that the experts on board know that so the lower strengths may indeed be lower than 0.85f'c.

If there is lower strength then obviously this is a material supplier "failure" as opposed to an engineering failure. Hopefully the lower strengths aren't endemic to the whole structure.

https://www.enr.com/articles/46407-denver-airports-renovator-uncovers-potential-snag

https://www.denverpost.com/2019/02/18/denver-airport-construction-delay/

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[Settingsun]

You'd think there was testing during construction given the project size and slab load rating. Low insitu strength can also be due to the site work practices such as inadequate compaction. I've seen a couple of concreters who could produce a first class finish but who didn't vibrate the concrete enough.

 

[BUGGAR]

They seemed a little vague in saying that the petro tests precluded ASR.

 

[VEBill]

Excuse the naive question...

Wouldn't a concrete floor's load capacity be more of a large scale structural characteristic, ultimately limited by the design, and by the tensile strength (of the reinforcement) on the lower side? Not so much by the psi crush test of a core sample?

Pulling a core sample, presumably crushing it to confirm kilo-psi, would seem to be a parameter that could conceivably have minimal impact on the load capacity of the floor.

Concrete might be (for example) 5,000 psi (720,000 psf), so it's not directly related to the floor's 250 psf load.

Acknowledge in advance this isn't my area. Just curious how core samples (I'm presuming crush test) would relate to floor's large scale load performance.

 

[JAE]

VE1BLL yes you are correct for the flexural capacity. But for shear capacity the concrete strength is very important.

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[epoxybot]

There was a lot of cold weather concreting done on the Denver Airport, including the use of an admixture that allowed work down to 20F.

 

[BUGGAR]

I've experienced problems with steam cured precast concrete panels. Even after years in service, the chemicals in the concrete can commence to destroy the concrete. Our situation was delayed ettringrite formation(?) or DEF. Ask the chemists.
There was a conspiracy theory that this method of curing was going to be used in the wall so it would self destruct in a couple of years.