Air-Entrained Concrete 1

[EngineerRam]

Hi Guys,

This has been a subject of debate in the office. I live in a country in the tropics (temperature is constantly in the 80-90 degree range), so no freeze/thaw concerns. Currently, the specifications for a project indicate to air entrain the concrete. I believe that these specifications are in place as a result of being copied over from US specifications, with little attention paid to the specifics of where the project is located. The engineer who wrote the specifications states that the air entrainment is required for workability of the concrete. This seems to be an issue of means and methods. Also, there are much more economical alternatives to improve workability where we are located. As this is not normally done, air-entraining concrete is vert costly. Are there any other reasons why one would air entrain concrete somewhere where there are no freeze/thaw concerns?

Thanks.

 

[Ron]

As the engineer who wrote the specifications correctly observed, air entrainment has benefits other than freeze-thaw resistance. I live and practice in Florida, in the US. Freeze-thaw resistance is not an issue for us; however, most of the ready mix concrete produced has air entrainment. It is a mix enhancement that increases workability without increasing the water-cement ratio. It is not a means and methods issue, it is a practical design consideration for workability and durability.

 

[JAE]

I believe that air entrainment also creates concrete with less permeability - i.e. for water tightness improvement.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[DamsInc]

I have been told that ready mix companies like to air entrain the concrete even when not required because the chemicals to do so are very cheap and it reduces the weight of the trucks (saving gas, wear and tear etc).

 

[jayrod12]

I practice in an area where air-entrained concrete is prevalent (freeze-thaw capital of the world I feel some days), but I have never had anyone ask to put air in concrete that hasn't required it. I guess I'm trying to say that I don't feel the workability argument really holds any merit, and I even go so far as to let the contractor choose the slump. Queue the angy mob.

 

[BUGGAR]

What happened to the problems claimed when steel trowel finishing air entrained concrete?

 

[JAE]

BUGGAR is correct that you don't want to steel trowel finish a slab with significant air in it.

I also have had contractors demand air in concrete when they have to pump it up several stories so the air does affect pumpability (i.e. workability)

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[hokie66]

DamsInc,
"Reduces the weight of the trucks"? I had a chuckle about that one.

 

[Ron]

Sorry guys, but we use steel trowel finishing on air entrained concrete routinely. Part of the success with that in our area is the type of coarse aggregate we typically use. It doesn't "bleed" as much as less porous aggregates, thus there is less potential to trap the bleed water under the trowel-densified surface, which causes blistering.

Blistering is a finishing issue that can be properly handled by appropriate finishing techniques, even with air entrainment. The finishers often get on the concrete too quickly and seal the surface before the bleed water escapes. There are other impacts on blistering potential as well, but this is a big one!

 

[dik]

jayrod... the addition of air does improve the workability and flowability of the mix.

Missed the sarcasm...

Dik

 

[jayrod12]

Dik,

No sarcasm intended. I've never had a contractor ask me to add air to improve workability. More often than not, I've got contractors asking if the air is really required in the mix.

Air may help with workability, but I feel like there are other equally efficient and economical ways to improve the workability of the concrete.

My "Queue the angry mob" comment was about how I rarely specify slump. I let the contractor choose the slump but their desired slump must be on the sealed mix designs and truck tickets so I know they aren't screwing with the actual performance of the concrete.

 

[DamsInc]

DamsInc,
"Reduces the weight of the trucks"? I had a chuckle about that one.

Makes sense though no? For most companies the benefit would be negligible, but if you have a fleet of a few hundred trucks then why wouldn't you reduce the loads they carry?

 

[JAE]

Ron - good point - I wonder, though, as a humble engineer working with lots of different contractors all with different skill set levels - I would worry about specifying air in troweled finished concrete no knowing whether the concrete sub we end up with would do it right.

Joe Bubba in his pick'em up truck would just do it wrong and we'd have blisters a-go-go all over the floor.
(Queue Mr. Bubba's lawyers for JAE defamation suit)





Check out Eng-Tips Forum's Policies here:
faq731-376

 

[hawkaz]

In my part of the USA, there is no freeze-thaw, and we don't call out any air entrainment. The mix designs we receive do normally have 1% air however

 

[TehMightyEngineer]

hawkaz, that's the naturally entrained air in concrete or due to some of the chemicals added which add small amounts of air.

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

http://www.americanconcrete.com

 

[Ron]

JAE...your point is even better! We see more problems with placement and finishing than we ever see with the mix designs.

When we are doing construction consulting, we require a "Pre-placement Meeting" before any concrete is placed. Mix design, slump control, air content, vibration, grade control under slabs, finishing, curing, etc.....are some of the things we lay out. Another is that control joints must be sawn the same day as placement....they hate that one.

The "payload" of a transit mixture is essentially reduced by the air content......5% air>>>>>5% less weight. In my opinion, not enough to worry about. I concur with hokie66's assessment