Mystery Air

[TigerGuy]

Can concrete increase entrained air content from the on site air test to hardened air content?

Equipment calibrations, cylinder weights and finishing all indicate a no air mix for this interior slab at 2-3%.

18 months later, they are witnessing delamination. Cores were taken and tested for hardened air content at 7%.

We tried a trial batch, checking the air content before and after adding water at the jobsite. No change in air.

What else can we investigate with this?

Thanks in advance

 

[TehMightyEngineer]

Increase? Not a significant amount to my knowledge (unless you count the mechanical mixing that naturally occurs when dropped in the form).

Test different from the sample out of the truck to the actual mix in the form? Absolutely.

In addition, your measured percentage for air entrained will vary depending on whether you're testing hardened or unhardened concrete. Typically +/- 2%.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[TehMightyEngineer]

I think the biggest question is how did the cores look? Was the aggregate uniformly distributed? Any signs of improper or a lack of vibratory consolidation? Was any water added during the finishing? Was finishing performed prior to evaporation of the bleed water?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[TigerGuy]

We tested during the pour and got consistent readings in the 2-3% range. Finishers had no trouble getting a hard trowel finish on the slab.

Then, when the cores were pulled (they look like properly placed concrete, uniform distribution of aggregate etc.) the hardened air is 7%. We are trying to understand how it gained 4% air being placed in a slab

 

[TehMightyEngineer]

Were any admixtures added with the water? Some admixtures can add air.

I'm also thinking this isn't the cause of your issues. Air entrainment around 7% will not cause major structural issues unless we're talking large voids and not the typical microscopic air pockets that normal air entrainment entails.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[TigerGuy]

The admixtures checked out when we ran the test batch. Adding water to the drum and sampling right after mixing it up had no effect. The problem is that it's a hard trowel finish, which doesn't work very well with too much air in the concrete. Now when hard wheeled equipment rolls across the floor and hits a delaminated area, it doesn't roll so well. Unhappy owner, contractor, supplier and testing lab that can't figure out where the extra air came from

 

[TehMightyEngineer]

Agreed, if finishing prior to the end of bleeding wasn't your issue then the hard finish trapping air is a likely candidate. As for the air, I'm out of ideas where it came from. I'd focus more on repair as this appears to be no fault of anyone involved.

Google concrete delaminations will give you some good info on the topic.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[Ron]

A few questions before I comment....

1. Was the concrete pumped? If so, how?
2. Was the concrete dropped into place?
3. Was the petrography done correctly?
4. What was the size of the air voids?

What air entraining admixture was used? Was a high range water reducing admixture used?

 

[JedClampett]

We've had bad experience with measuring air content. It seems to vary all over the place. We ask for 6%, the test comes back as 2%, but the admixture was dosed correctly. We're not going to tear out a pour for air content.
We called the batch plant and they said that air content is hard to measure and very erractic. It's possible that the air measured at 3% and was actually 7%.
My question is: What air content did you specify?

 

[Ron]

Also, was a pressure meter used for the air content? Was the technician ACI certified and was the equipment calibrated?

 

[TigerGuy]

I guess I should clarify, this is a no air mix for interior flatworm with a hard trowel finish.

Ron, yes we used a pressure meter. It's calibrations were checked shortly after testing, accurate. Tech is ACI certified.

 

[Ron]

Please post a copy of the mix design. Is there a chance the batch plant accidentally dosed an air entertaining admixture? The petrographic examination should have told you the difference between entrapped and entrained air. The void (bubble) sizes are significantly different between entrapped and entrained air. Usually it is difficult to entrap more than 1 to 1-1/2 percent air in concrete.

 

[TigerGuy]

Slab on grade, placed from truck chute to ground. So maybe w4 inch drop
Petrographic analysis says entrained air, I'll have to check report for sizes of voids when I get back to the office later next week.

The lab that did 5he hardened air has a great rrputation for quality analysis. We are stumped

 

[Ron]

In that case, look to the ready mix supplier for an inadvertent dosing of air entraining admixture.

 

[hokie66]

As others have said, 7% entrained air should not cause problems like you are encountering. 7% entrapped air might, and that would probably indicate poor compaction.

How soon after placement was the slab placed into service with the hard wheel equipment?

 

[TigerGuy]

In our area, "no air" concrete is used for interior slabs. With 7% air, the finishing crews have a hard time getting the smooth, hard trowel finish. It tends to stick on the power trowel blades and makes a mess of the floor.

The slab was placed into service about 4-5 months after placement.

I want to thank all for your replies and suggestions. We are just really confused:

1. ACI certified technician
2. Calibrated air meter
3. Cylinder weights correlate with the field air content readings (2-3%)
4. Finishing crew did not have the trouble that is typically witnessed when power troweling an interior slab with entrained air.
5. Cores examined petrographically, indicating 5.7% entrained air. From report, "numerous fine discreet, spherical and near spherical voids having sizes up to 1 mm."

My best guess is that only some of the trucks were accidentally dosed with air agent. Those that were dosed were unloaded in enough areas of the slab that the problem appeared. However, those that were dosed were few enough, than none of them were tested. This scenario seems as unlikely as all the rest.

Thanks again for all your help. I'm off to visit with the supplier

 

[TehMightyEngineer]

Wait, is it 5.7% air or 7% air? 5-6% air is on the extreme end of the difference (±2%) I've read to expect from tested samples using an air meter vs a petrographic test.

Either way, it does appear that you picked up some air beyond the natural entrainment, and possibly that it was only a few trucks which got blended into the floor and were missed during sampling.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[TigerGuy]

My apologies on that. 5.7% entrained air, 7% total air.

Having reviewed the batch tickets, there is no indication that air was accidentally added to any loads.

We may have added another mystery to the universe...

 

[TehMightyEngineer]

Just out of curiosity, what were you pouring onto? Slab on grade? Metal deck?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[Ron]

You will not get 5.7% entrained air without an addition of air entraining admixture, without regard to what the trip ticket says. Entrained air is specific in size. Entrapped air is random in size. I've done lots of petrographic examinations of concrete for air entrainment and other parameters.

Your ready mix supplier needs to answer some hard questions.

1. Why does the trip ticket not reflect the concrete mix?
2. Why was air entraining admixture added when it was not authorized?