Low concrete strength, very high entrained air
Low concrete strength, very high entrained air
(OP)
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.
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.






RE: Low concrete strength, very high entrained air
RE: Low concrete strength, very high entrained air
RE: Low concrete strength, very high entrained air
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.
RE: Low concrete strength, very high entrained air
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.
RE: Low concrete strength, very high entrained air
RE: Low concrete strength, very high entrained air
Why the 12" thickness...are you conservative?
RE: Low concrete strength, very high entrained air
RE: Low concrete strength, very high entrained air
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.
RE: Low concrete strength, very high entrained air
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.