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

 

[TigerGuy]

TME - slab on grade

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.

 

[Ron]

TigerGuy....I spent over 20 years in a testing lab environment and I teach construction materials at a regional university. I repeat....there cannot be 5.7% entrained air without the addition of an air entraining admixture. Were other admixtures used (high range water reducer, set enhancement, etc)? I think your ready mix supplier made a mistake.

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.

 

[TigerGuy]

Ron,

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