low air and no air question? 1

[healing]

Had my farm shop slab on grade just completed in 2 different pours. I saw on the first slab the reciept from the truck said low air 4000 psi. On the second pour the ticket said no air 4000 psi. Can someone tell me the difference between no air and low air? Will the low air withstand freeze thaw cycles better than the no air? Or is the air just for finishers preferance? Thanks

 

[BigH]

First - call the ready mix plant and ask them what they mean by "low air" and "no air". It might have to do with air entraining agent or not. After you get that information, please advise us.

 

[dirtsqueezer]

It's possible they mean the same thing. Any load of ready-mix concrete will contain entrained air just from the turning of the drum, usually on the order of just below 2%. In engineering terms, this isn't considered helpful for freeze/thaw. Is the building over your slab enclosed? I believe the verbage given on the subject is something like, "concrete exposed to weather". You might not experience extreme conditions if your slab is indoors.

 

[gatorn10ac]

It sounds like they mean the same. (according to the concrete company) As stated above the concrete has ETRAPPED air of 2 to 3.5 percent. The concrete companies add a liquid that increases the air for freeze/ thaw conditions. If your slab is not exposed to extreme freeze/thaw cycles it should be ok. (As stated above)

 

[RDK]

Air due to mixing action is called entrapped air. As mentioned above it has no benefit for the concrete. Entrapped air can be in large bubbles. Air in the concrete due to the adding of a liquid air entraining agent is called entrained air. Entrained air will have very small bubbles.

After the concrete has hardened, air content can be estimated by examining a core sample in a testing lab. ($$)

While entrained air is usually considered to be of benefit for freeze thaw only, it also gives the benefits of improved workability, decreased bleeding, increased resistance to scaling from chemicals, improved resistance to sulphate attack and increased water tightness.

CSA A23.1 (Canadian Standards Association concrete practice code) only gives one recommended mix with less than 3-6% air. The only exception is structural concrete with non-exposure to freezing or thawing or chlorides. Normal practice is to use some air even in these applications for workability reasons.

For an interior garage slab I would assume that there is potential for chloride attack (salt from the vehicles if nothing else) other chemical attack from other vehicle liquids (glycol, gasoline, oils etc) and possibility some threat from sulphides in the ground.

I would have specified type 50 (sulphate resistant) for ALL concrete in contact with the ground and 5-8% air with maximum 20 mm aggregate. (Might have gone to 4-7% air for areas where freeze thaw is not a major concern, but since you asked about it I am assuming that it is an issue.)

I cannot see any respectable concrete batch plant selling no air concrete unless specifically instructed to do so.

What did you order?

Did you order it yourself or did the contractor order it?

Did an engineer design your building, if so what was specified?

If no engineered design, what is the local building code requirements for this slab?


Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[healing]

The Ready mixed plant said the difference in low air 2% and no air concrete is mainly for workability ie low air is easier to finish. The contractor made the order, and the building is erected. However the building is not heated as of yet and the inside temperature will be in the mid 20's F in the mornings and warming to above freezing during the day depending on the warmth of the day. I am not sure if this falls under "extreme freeze thaw cycles" or not?

The ready mix guy said that as long as the slab is covered by the building it would be ok.

Also the concrete was placed over insulation so it is not in contact with the ground.

I guess only time will tell if it'll hold up.
Thanks to all replies

 

[Ron]

In order to provide true freeze-thaw durability, the air content should be up in the 4 to 6 percent range, with 3 to 5 percent of that air as entrained air.

Freeze-thaw cycling is an exterior exposure phenomenon, that requires surface interstitial moisture presence to cause problems. Since your application is generally covered, it is not likely to be a significant issue.

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