Entrained air in interior SOG & slab on metal deck 2

[WWTEng]

As far as I know, there should be no entrained air in interior SOG & concrete on metal deck, because none shall be exposed to freeze-thaw. And of either of these floors are to be trowel finished then the entrapped air is to be no more then 3%.

Is my understanding correct or is it too simplistic?

Thanks.

 

[dik]

Generally spec 0% to 3% to accommodate entrapped air...

Dik

 

[henri2]

Air-entrainment serves purposes other than increasing freeze-thaw resistance. For structural lightweight concrete over metal deck, use of air-entrainment is common practice. It is incorporated in the concrete mixture in order to reduce the density in the hardened state.

 

[hokie66]

henri2,
Although air entraining does reduce the density slightly, I don't think that is a key reason for its use in light weight structural concrete. The aggregates are the main controlling factor for concrete density. Air entraining is used to improve durability and also to enhance workability during placement.

 

[henri2]

Hokie66

I agree that workability and pumpability of the mix are also enhanced but I do also believe unit weight is also a main consideration when expanded shale coarse aggregate and normalweight sand is used. Even with the air-entrainment, it is still a challenge to produce structural lighweight concrete (using expanded shale) with an air-dry unit weight not exceeding 115 pcf - the value typically specified in the US. What is the typical value in AU specifications?

By the way, how does air-entrained structural LW concrete perform from a durability standpoint when subjected to repeated freeze-thaw cycles in a cold weather environment?

 

[hokie66]

Structural lightweight concrete, to my knowledge, is not currently produced in Australia.

 

[henri2]

Thanks for your response Hokie66.

 

[JAE]

I would NEVER specify any air entrainment in LW concrete for slabs that will be hard troweled. Big mistake.

The troweling will densify the top 1/4" or so of the slab and air pockets will form directly under these lenses of concrete - thus - blisters.

There is a natural amount of air entrainment in concrete - perhaps 1 to 2 percent without any additives but specifying air to help lighten the concrete (you would only get about a 0.5% change in density) is not a good idea.

 

[WWTEng]

I think I am not going to specify any AE in both of these items, and I'll limit the entraped air to 3% max.

 

[hokie66]

Just educate me...how do you limit entrapped air to 3%?

 

[JAE]

I think they mean simply not adding any entrainment admixture and relying on the fact that natural concrete will only have limited air content.

 

[henri2]

JAE, I have never seen a mix design for structural lightweight concrete (containing expanded shale or pumice) for use in composite floor construction that did not contain an ASTM C260 admixture.

"Virtually all structural lightweight concrete is air-entrained" - [NRMCA CIP 36]. The article states that precautions should be take when hard troweled finish is required in order to minimize blisters and delaminations

http://www.nrmca.org/aboutconcrete/cips/36p.pdf

When you specify that you want no air-entrainment, do ever have any resistance from the concrete supplier?

Thanks

 

[JAE]

henri2,

I think you are essentially correct. I just had some resistance from a contractor some time ago after he had to grind off 1 1/2" of concrete on a 5" slab over 100,000 s.f. due to blisters all across the floor from air entrainment included in a floor that was hard troweled. The soundings showed blisters everywhere and their were "spider" cracking as well. The finishing obviously wasn't done correctly. I've seen references to the need to use a magnesium blade on the final troweling equipment.

Here are a few links I dug up:

Air Entrainment and Delaminations

Concrete Surface Blistering - Causes and Cures

ASCC Article (refers to normal weight concrete)

This last article does imply that most LW concrete is air entrained so I think you are correct, henri2. The problem, as I see it, is that you run the risk of getting a subcontractor that doesn't understand the sensitive timing of the floating/floating/troweling stages relative to slower bleed water, etc.
ESCSI Article

 

[henri2]

JAE,

Thanks for the links. They certainly contain some nuggets of useful information.

Perhaps a way to minimize problems associated with finishing of air-entrained concrete slabs is to specify in (Div 3 of Specicification and Structural Notes)acceptable finishing methods, require the work be done by ACI certified concrete flat work finishers, and require post-concrete placement inspection of finishing. The current IBC already requires verification that curing is performed, so adding observation of finishing should not be too much of a problem.

 

[hokie66]

henri2,
Specifying a procedure does not insure that it will get done...and if it doesn't, the engineer will be involved in the mess whether it is his fault or not.

I can't think of a reason to specify a hard troweled finish for an air entrained slab.