Air entrainment 4

[lsmfse]

We have a project where the specifications call for 4%-5% air entrainment for an underground concrete water tank. Testing during the pour showed air content greater than 12% for some of the batches. The site isn't subject to freeze/thaw. Based on my research, other than the concrete looses about 5% of strength per 1% of air, what other issues might be of concern?

 

[conceng]

Depending on the size of aggregate you've got, I'd expect there to be some serious strength issues at that air content. From what I've experienced, the 5% strength loss per 1% air is only valid up to about 7% air. Beyond that, the strength loss seems to increase exponentially.

With a high air content, there is also a concern about abrasion resistance. All those little air voids just don't have any strength.

Please don't let this scare you away from using air entrainment for this application. Entrained air is very effective as a consolidation aid to reduce honeycombing and larger entrapped voids.

Greg

 

[SlideRuleEra]

Interaction of the air entraining additive with other additives is possible.

http://www.fhwa.dot.gov/infrastructure/materialsgrp/airentr.htm

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[lsmfse]

Greg and SlideRule, thanks for your input. They will be doing 3 day break tomorrow. Greg, I do like air entrainment for the reasons you mentioned, I have never seen 12-13%.

 

[Ron]

As Greg noted, strength loss goes up very quickly with high air contents. It significantly affects the durability of the concrete at that level and because the air voids can agglomerate, the concrete is more porous and will allow more water migration, thus affecting corrosion of rebar.

The concrete also shrinks more and is more susceptible to greater volume change from wetting and drying.

A little air is a good thing...a lot of air, not so good.

 

[lsmfse]

5 day break came back @ 1650 psi

 

[Ron]

lsmfe...what was the specified strength of the concrete? 1650 psi at 5 days is slightly low for conventional 3000 psi concrete, as long as the mix did not contain flyash or slag cement, both of which will retard the strength gain. If the specified strenght was more than 3000 psi, the result is likely low and I would carefully watch the 28 day results. Make sure they test two specimens at 28 days, not just one specimen.

 

[lsmfse]

Ron, the specified f'c was 4000 psi.

 

[lsmfse]

Attached is the mix design.

 

[Ron]

lmsfe...the strength is low at 5 days. The flyash will retard the strength gain. For 4000 psi concrete, I would expect 55 to 60 percent of f'c at 5 days and 70 percent at 7days. Even with the flyash, I would expect around 2000 psi at 5 days, considering the design mix's w/c of 0.39.

Excessive air content is a result of improper dosing of the AE agent. You can't do anything in the delivery or placement of the concrete to increase the air content to those levels.

 

[conceng]

Was there a high-range water reducer (superplasticizer) used? Some water reducers significantly enhance the effectiveness of air entrainers, or entrain some air on their own.

Greg

 

[lsmfse]

Greg, yes there was Plastol 341 added as well as Eucon NW water reducer. See batch data above.

 

[conceng]

lsmfse,

Sorry I missed looking at the mix design you posted. I've never worked with the Eucon NW, but I have worked with the Plastol 341 and AEA-92. Based on the dosage rate using our materials here, I would expect high air. The AEA-92 is one of the more effective air entrainers I've worked with, and with the Plastol 341, I will usually cut the air dose at least in 1/2 because of the synergy of the two in entraining air.

Greg

 

[MultiVar]

SO... does this mean his water tank has to be demolished and re-poured, the mix supplier penalized, etc.??

We CAD geeks are stuck in the lab, wondering what happens when the numbers don't add up in the field...?

 

[conceng]

I've been involved with a couple of projects where the owner has demanded remove and replace in this type of situation. Often times it may just be deduct.

Greg

 

[beton1]

You may want to consider saving a cylinder for 56 days in the event that 28 day strength does not come up to design f'c. Agree with posts that there is likely a AEA and SuperP interaction. You or design (structural) engineer may have to review design based on a lower f'c. Last resort, in-situ tests to confirm f'c.

 

[concretemasonry]

Ismfse -

Where is the project located? Materials and fly ash can vary widely (especially for short term by-product fly ash results), with short term results. Since it seems strength is your real concern and not just a air test.

Did you start with a mix design that was very similar (same properties, materials and admix additives) to your specifications. This provides a history and shifts the performance to the supplier. Most good ready mix suppliers do not just switch admixtures and and dosage since many admixture suppliers actually own the dosing equipment and the supplier has a good record of the individual batches.

The job site tech conducting sampling and tests and preparing samples can have a bigger effect than you think on the tests results.

How did the cylinders test for strength since freeze/thaw was not a problems but just "specification boiler plate".

Banging around a cylinder for a 5 break day test leads to many variations including site curing, sample preparation and handling. the 3 Day breaks can be even more misleading unless you have some information on the strength gain curve.

 

[darthsoilsguy2]

28-day breaks in?

 

[conceng]

The latest "Concrete Construction" magazine has a very good article regarding the use of polycarboxylate based water reducers. Here is the link:

http://www.concreteconstruction.net/industry-news.asp?sectionID=714&articleID=1140574

Greg

 

[lsmfse]

Update: The 56 day breaks came in just under 3000 psi. (we have one additional cylinder) Permeability tests came back "low" permeability. Cores taken in the area in question tested above 4000 psi. Response to MultiVar: The concrete that was out of spec represented about 20% of the total concrete placed. The structural demand requires less than 3000 psi concrete.