Agree - and I once had a contractor add air to a lightweight mix - pumped up three stories for a topping pour. The hard troweling on it caused 100,000 s.f. of delamination and blisters....so be careful with it.
If the mix design has air entrainment in it, it is common for field QC agents for the concrete company will add air on-site for pumped mixes to account for air loss due to damage of the air bubbles in the pumping process. also there can be air content loss if a porous aggregate (for lightweight concrete) is used that was not properly saturated and the pumping process causes a slump loss.
also, many pumped mixes will go to elevated decks where the weight of the concrete requires a lighter mix so concrete supply companies will entrain air for this reason.
i am not aware of air being added to mixes to make them perform better with the pump. Nor can i think of a way it would make the mix more pumpable.
If the air entrainment is an important item and the concrete is being pumped, the concrete QC should be checking for air loss. A parking deck would be a good example of a project where AE is important.
Air can really screw up a concrete mix...including a pump mix.
It is beneficial if designed properly and used properly in the mix, but the results can be disastrous as well. I've done enough failure investigation and petrography on concrete with too much entrained air that bares this out.
Adding supplements (admixtures) at the site is much less controlled than at the batch plant. Air entraining admixtures are measured in very small quantities (ounces) as compared to large quantities of the other constituents. I don't know how long you've been in the business, but when plasticizer admixtures were first made popular in the late 70's and early 80's, they were often added at the site with such widely varying results as to be unpredictable. Further, when such admixtures are added at the site, they are generally mixed with water to gain adequate dispersion in the mix. This increases the water-cement ratio as well.
Bottom line....I still don't recommend adding admixtures of any type or even water at the site. Design and produce the mix for its intended purpose and don't let the concrete foreman, the superintendent or the truck driver screw it up.
...and no...I give the producers hell about mix design and production as well....the list is long.
Whooaaa....What do the concrete foreman, super, and truck driver have to do with this??
We're talking about AIR not WATER. I will bet my house that I never see any of those people run a test and adjust the air.
Unless on-site addition is clearly prohibited by spec, the concrete company owns the concrete until it hits the hopper in the pump and can adjust out-of-spec materials to meet spec.
When air content is under spec and the QC adds an air-pack to the truck, I've never seen it bump the air up more than 1 to 1.5% above the previously low test. For forensic work where high air contents are at fault, I would expect problems to have occurred at the batching and not from voluntary field adjustment. And if it were due to field adjusting, I would expect that it was done without testing.
I gave an example of a situation where air could be adjusted by QC agents. To clarify for all, i'm talking about the plant QC rep, not the testing company or even a testing company working for the contractor. The testing company should only be testing and reporting. Pump configuration can slaughter the air content and neither the desk engineer or the batch plant will know how this will play out during design review or during the batching.
My personal preference is to pick a mix and stick with knowing there will be some variation including air loss below tolerance, but many design professionals and GCs have different takes on it. Some interesting examples that have happened in just the last 2 years on jobs i've staffed:
1- Air content outside of 4%-7% for a portion of the tested trucks caused the design team to require the steel deck to be sprayed with fireproofing at contractor cost because of the UL Design reference concrete properties.
2- The fresh unit weight was a little high on a portion of tested trucks. The GC withheld significant money from the concrete sub who withheld on the producer. The "damage" was the steel framed slabs were "out of some degree of level" and the other subs (walls for one) were backcharging for materials. The GC connected the fresh unit weight to this.
3- A post-tensioned parking deck is constructed by pumping and by crane bucket at the same time with the same mix. Additionally, every truck has to be tested for slump and air at the point of discharge by spec. Air contents on ~5%-15% of the trucks proved out of spec and triggered freeze-thaw and petrographic testing at contractor cost
If i were a QC agent for the concrete supplier, i would feel comfortable enough in my testing and knowledge of concrete to add air on-site... but would only do it for projects with drama that would merit the time loss.
DSG....you raise some very valid points and I agree with each of them with the exception that I just don't like air added at the site. I agree that if it is done, testing should be done both before and after addition.
Yes, specification interpretation and response to issues get a bit nutty at times.
I agree wholeheartedly that the mix belongs to the supplier and he has the right to bring it into spec tolerance; however, if the mix cannot be delivered within spec consistently, then the supplier needs to look at its operation.
Bottom line....I still don't recommend adding admixtures of any type or even water at the site. Design and produce the mix for its intended purpose and don't let the concrete foreman, the superintendent or the truck driver screw it up.
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