Mystery Air
Mystery Air
(OP)
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
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





RE: Mystery Air
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
www.americanconcrete.com
RE: Mystery Air
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Mystery Air
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
RE: Mystery 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
www.americanconcrete.com
RE: Mystery Air
RE: Mystery Air
Google concrete delaminations will give you some good info on the topic.
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Mystery Air
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?
RE: Mystery Air
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?
RE: Mystery Air
RE: Mystery Air
Ron, yes we used a pressure meter. It's calibrations were checked shortly after testing, accurate. Tech is ACI certified.
RE: Mystery Air
RE: Mystery Air
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
RE: Mystery Air
RE: Mystery Air
How soon after placement was the slab placed into service with the hard wheel equipment?
RE: Mystery Air
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
RE: Mystery Air
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
www.americanconcrete.com
RE: Mystery 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...
RE: Mystery Air
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Mystery Air
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?
RE: Mystery Air
Ron - I appreciate your point. And the supplier knows they have a big check to write. However, as the testing lab, we are being questioned as to why our tests did not indicate the air content. That's the mystery. "Why pay for testing if the testing agency doesn't catch the problems in the concrete?" - I'm sure nobody has ever heard this before.
I have the individual load tickets now, and they loaded the correct mix. The tickets indicate that the air agent wasn't added.
RE: Mystery Air
As for why you did not show air in your tests, that could be because you likely didn't test every truck and the petrographic samples came from a truck that was screwed up. At this point, the only thing you can do to verify is several more petrographic examinations for air content.
RE: Mystery Air
I have 20+ years testing concrete also. I agree that air had to be introduced at some point, to some trucks but not others. I'm just working with available information, based on the assumption that everybody is being honest. (Yes, I know what happens when we assume)
The mix includes Polyheed 997 (mid-range water reducer) and Delvo (retarder). These have not demonstrated a tendency to add air to a mix, in our experience.
Each batch ticket shows no air agent added to the load. This is verified on the loads that we tested, but contradicted by the cores. I'm not ready to make the accusation that somebody manually added air agent to trucks that were not getting tested. Even if that is what appears to have happened, why would you do that? Based on the areas of delamination, it seems several trucks were screwed up. There are small spots all over a 12,000 sq ft slab. I can offer explanations for the delamination that don't address the air content (finishing too early, chemicals spilled on relatively new slab, etc.), but nothing explains the air other than somebody added it outside of everybody's knowledge.
Thus the client, and supplier, are hinting that our results are no good. You know as well as I do, that's a losing battle no matter what you respond with.
I really appreciate your thoughts on this. Have a great weekend, I intend to