Low concrete strength, very high entrained air 1

[maxwolf]

We have a 12" slab on grade and some small amount of foundation work (say, 18 yd3), which had a low f'c (around 2800 psi). Corrected core tests from over 56 days came in at around 3550, which is more than the spec'd f'c = 3500.
Petrographic tests/micrographs show the culprits to be very high entrained air (10 - 15% and locally higher), and a w/c ratio of 0.55, when 0.45 max was spec'd. Entrained air is spec'd at 6%(should have been 3% max for fdtn/interior).

Delamination is a possible issue. The slabs are now covered with vinyl comp tile. I'm thinking of recommending:
1. checking for delamination with a hammer/chain drag test
2. accepting the concrete (based on strength considerations)
3. fining the contractor heavily

The alternative is to require all the work to be redone.

 

[ash060]

I like the alternative. The contractor did not supply what is on the drawings, and that is what they need to supply

 

[B16A2]

With a 0.55 it probably bled like crazy but if they got on it at the right time it may be good enough not to delaminate. I would test it with chains, write an official letter to file, and move on. If its not showing delamination already, it probably never will. It's not like you have exposed concrete that durability will be an issue with the high w/c (aka high permeability)

 

[Ron]

Sounds like the use of the floor slab is for office or similar, with VCT on it. In that case, the durability is a bit less important, since not likely to experience abrasion or exposure issues. If you didn't see blistering in the finish, then the high entrained air content hasn't affected much other than strength.

What concerns me is the likelihood of high moisture in the slab that will affect the VCT adhesive. You will likely see re-emulsification of the adhesive, and sticky goo (a refined engineering term!)exuding from the joints.

Don't let the contractor get away with it though. Make him pay in some way, even if just a fine and extended warranty. Make sure that he know if his actions affect any durability or adhesion issues for the next five years, he's responsible.

 

[concretemasonry]

Probably a concrete mix with fly ash. Why such a low strength requirement? Why air entrainment for the interior work when there is normally an assumed 1 1/2% air in a non-entrained mix. I recognize that because of the size of the pour and site schedules it makes sense to use the same strength for reliability and uniformity.

The contractor is ultimately responsible for the properties of the concrete used. There is also a responsibility born by the ready-mix supplier IF he had the specifications, unless the mix was altered or souped up by the contractor on site and additions to the delivered concrete were altered by water and air entrainment to make it more workable from a short-term placement/finishing standpoint.

Considering the small amount of concrete, you will not be able to get superior analysis from a major supplier with technology that blows a local testing lab out of the water. If it is a small local concrete supplier with no truck on-site recording or documentation, the local supplier may contribute to the contractor's repair/correction.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[mbullism]

A 12" thick slab on grade covered with VCT? What are the anticipated floor loads?

 

[JAE]

mbullism - my thoughts exactly.

Why the 12" thickness...are you conservative?

 

[PenneEng]

Just relax if its covered there should be no issue. Delamination would be more of a problem if it's exposed to freeze thaw. The higher air could be due to the high water content, but the strength appear to have come up so there is no issue. Concrete is a strange material at times, use common sense with it. That's why for interior the air content is low because its not exposed to freeze thaw, but would it hurt if the air was 8%? No, it wouldn't. The strength came up and it's covered with a floor no worries.

 

[Ron]

PenneEng...air content as high as noted does not come from increased water in the mix. Water gain voids can be present in higher water cement ratios, particularly if re-tempered at the site. An air content of 1 to 2 percent can be expected from entrapped air and water gain voids. An air content in the range noted has to come from entrained air and/or a reaction of the concrete with something.

The difference between entrapped air and entrained air can be readily seen on a polished section under a microscope. Entrained air shows as very small (usually 1/32 of an inch or smaller in diameter), regular, and round. Entrapped air voids can be much, much larger (greater than 1/4 inch sometimes), are often irregularly shaped, and may be elongated, particularly with water gain voids.

 

[maxwolf]

Thanks for all the responses. Sorry for my late feedback -I came down with the flu a few days ago.

More details: The work I'm reviewing is the patching of an existing 12" slab, originally done in the 70's I think. The patching maintained the same slab depth and reinforcement as the original slab. The use of the space is storage (100 psf). We're going to have the chain drag test done per astm spec.

My lingering concern is that that carbonation will take place at an accelerated rate and so corrosion of rebar will be a good deal faster than normal; however, the reinforecement is galvanized (I have to dbl check) and there is a vapor barrier below the slab. The water table is about 8 ft below the bottom of the slab.

Thanks again for all your input.