Air Entrained Concrete Slab

[NinerStruct]

I have a project where it appears that the contractor ordered the wrong concrete for a ~2000 ft^2 slab on grade and placed the slab with air entrained concrete.

The air content was 6.5% air, but the slump was only 3". The strength was already over the specified 28 day f'c at seven days. A walk-behind power trowel was used to finish, but I don't know how soon they got on the slab to trowel.

I found a few other threads that reference this type of situation, so my first question: Is this even an issue? Or at what point is air in the slab an issue? I read an article from Concrete International that listed the majority of slabs displaying issues, as having between 4.5% to 7.5%. I don't want the client to have a faulty slab, but I also don't want to be making much ado about nothing and go chicken little here.

After reading articles and the other threads, I had initially decided to have the testing agency provide a chain drag test, and depending on the results either selectively repair, or remove the slab if worse came to worst. But after speaking with his superiors, my contact at the testing agency said their opinion is that there isn't really an issue, so what would additional testing really prove.

So... I bring this up because I'm concerned that the testing agency is somewhat trying to cover their own miscue. They did not catch the air entrainment error, because their techinician believed (for some reason) that this was going to be a garage floor. I noticed it in the 7 day break test, and notified them, so now the contractor has been continuing on with work and has interior walls down. I think they're worried that if errors are found, the contractor will try to go after them for part of any additional costs, because they didn't bring this to his attention either before the slab was poured or at least before he had walls in place.

I'm still leaning towards the chain drag test and selective repair, I just wanted to get opinions of those that have no vested interest. Thanks for the help.

 

[Ron]

I don't see this as much of an issue. In my area, air entrained concrete for slabs is common, though usually in the 3 to 4% range.

Your air content is a bit high, but your slump is low, so there was not likely a lot of bleed water. Since not much bleed water, I wouldn't worry too much about blistering during finishing.

If you have a delamination (scaling) or blistering issue, it will be limited to the upper 1/4" or so of surface. Look carefully at the surface. If you don't see map cracking, then again, not much to worry about.

Watch the slab for dusting, scaling or blistering. Do the chain drag.

Have a heart-to-heart meeting with the testing lab. Yes, they should have caught the air content issue and alerted all. Tell them they won't be paid for this work. That will get their attention.

 

[cvg]

so the contractor ordered the wrong concrete and somehow this has become the testing lab's fault? how about you put the contractor on notice, not the QC sub.

 

[Ron]

There's a possibility it was just delivered incorrectly. The testing lab should know the specs...if the specs called for no air entrainment and the approved mix design showed no air entrainment, yet they tested the air content in the field...maybe they should say something!

cvg...you are correct that it could have been the contractor's error, but for the testing lab to blindly test without knowing what they should be testing is wrong. In the US, the required information to be recorded at the time of sampling includes the specified air content vs. the actual air content. If no air entrainment was specified that should be obvious.

 

[JAE]

You sometimes will see little "spider" cracking - looks like legs of a spider radiating out from a center (the center of the blister).

If you can, walk the floor and look for these. Also use a heavy spike or something metal to hand-tap on the surface - you can hear the variation and
determine pretty quickly if there are hollow sounding areas of blisters. I use a larger Buck knife and use the hilt/handle portion - a small hammer can work too.

If you find a few blisters, that can possibly justify the full chain drag.

 

[NinerStruct]

CVG, not at all. I don't blame the testing lab for having the wrong concrete. This is purely on the contractor. They ordered the wrong concrete, and even said as much. And I believe that the contractor should be paying for this test as a way to say "Yes, we didn't provide what was in the spec, but it appears to be ok." However, as Ron mentioned, the testing agent should have been familiar with the specs and what concrete should have been going in, and been on top of this faster.

I believe that, because they weren't on top of this faster, the testing agency feels as if the contractor will come after them for part of the bill if something is found to be defective(which they likely will, regardless of whether it's justified or not because that's what happens). So I do think that could influence their opinion, whether it's conscious or not. Which is why I pose the question to those who have no dog in this fight, so to speak.

I appreciate everyone's input. This project is more than a few hours away, so it's not something that I can get to easily. So I am going to have the testing agency do a visual inspection for spider/map cracking and do some tap testing. If they see or hear anything, perform the chain drag.

Thanks again.

 

[Ron]

Niner....chain dragging and point sounding are very subjective so make sure that the person who does it has experience. If you suspect a defensive bias from the testing lab, have someone else do it.

 

[cvg]

well, I would still blame this squarely on the contractor. and sure, the blame could be partly shifted to the QC tech. but the contractor is in charge of the site and ultimately is responsible. they were on site during the pour and clearly should have communicated with all that they were pouring an interior slab. the delivery ticket would have indicated that air was in the mix. it would have been at that point that the questions should have been asked.

 

[Hemifun]

Just curious but did the contractor make a concrete submittal and, if he did, did it show entrained air? Was there a clerk of the works or inspector on the job when the concrete was delivered?