Finishing air entrained concrete 3

[ajk1]

For a new parking garage floor with specified 5% to 8% air entrained concrete, and a urethane waterproofing membrane 60 mils thick, a wood float finsh is too rough. The concrete requires a steel trowel finish according to the membrane manufacturers. CSA Standard S413 says that a "single pass" of the steel trowl is ok on the air entrained concrete. I know that a higly bernished finish on air entrained concrete is problematic, but is a single pass of the steel trowel on air entrained concrete a problem? We have specified that on a number of garages over the last 20 years and I am not aware of a problem, but I don't know what the floor finisher really did. I know a couple of people who say that air entrained concrete should not be steel trowel finished.

 

[JAE]

The main problem with troweling an AE concrete is that the troweling activity vibrates the concrete, raising air bubbles and water upward towards the surface while at the same time densifying the top 1/4" or so of the slab. This then results in those air bubbles and water getting trapped just below the surface and blisters occur where the top 1/8" to 1/4" of concrete is suspended over a void.

The trick with all slabs is to not "get on it" until as late as possible, letting as much bleed water come up to, and get off, the slab surface.
So waiting until the right time and minimizing the troweling activity would seem to be the two things that should occur in your slab.

Limiting the troweling activity can be specified to a degree - i.e. x number of passes - or specify somehow the desired smoothness allowed.

However, the timing of such troweling is hard to specify and I think more an art than a science.

 

[dik]

In these environs, there are contractors that specialise in finishing concrete slabs.

Dik

 

[Archie264]

Does the above assumed mechanical troweling or will all troweling do that? If not, is it practical to specific manual troweling using a bull float, or something similar? Would that make a difference and, if so, is it practical?

 

[dik]

It's best to let the bleed water escape, first... and to minimise the re-work of the surface... doesn't matter what kind of trowelling.

Dik

 

[ajk1]

Thanks all for this information.


Yes I can check with a couple of the floor specialist floor finishers. The thing is that they will give me their most conservative opinion, but I will check with them.

I wonder if anyone has done any scientific research as to how harmful a single pass of the steel trowel is.

Also, can a magnesium float give a smooth finish similar to a steel trowel?

JAE - thanks for the explanation about air entrained concrete. If air bubbles and water are raised towards the surface, wouldn't that make the concrete less dense? Can you help me to understand how the process densifies the surface.

For an experienced finisher who knows what he is doing, is a single pass of the steel trowel on air entrained concrete ok? Or is even a single pass significantly detrimental?

 

[Ron]

ajk1...dik is correct. Let the bleed water dissipate and most of the potential problem with "blistering" that occurs with hard troweling the surface will go away. As JAE noted, the issue is that water and air will go to the surface. If the troweling is done before that water and air escapes, then the top 1/8" or so will be sealed by the compaction of the troweling, thus trapping the water and air below this densified surface. This sometimes causes delamination at the bottom of the finish layer that presents itself in the form of "blisters" and scaling at the surface.

Timing of the finish operation is key as is the technique of closing the surface without air entrapment. Good finishers can do this. Finishers in a hurry will screw it up.

We routinely hard trowel air entrained concrete in this area.

 

[JAE]

akj1,

My understanding of this is that the densification occurs in the top 1/4" layer or so - the troweling will slowly bring the Portland cement/sand mix up to the surface and somewhat drive down the larger aggregates resulting in a dense, hard material.
The air and water moving upward will, as you suggest, diminish the density of competence of the concrete but this stops just under the upper densified material - thus you get buried blisters just under a hard, dense surface.

 

[hope9010]

Ron and JAE -

Thank you both very much for your clear responses. I understand things better now. Can I take it from your remarks that there is nothing wrong with a single pass of the steel trowel on an air entrained concrete slab provided that the finisher is an experienced finisher who knows what he is doing? I don't want to put words in your mouth, but I just want to be clear in my own mind.

Incidentally do either of you live in the Toronto area? Just curious.

 

[hope9010]

..by the way, hope 9010 and ajk1 are both me...just depends whether I send from home or the office.

 

[JAE]

I think we are both very far from Toronto.

 

[cliff234]

For what it's worth, I was told that the only problem with finishing air entrained slabs is that air entrainment inhibits the ability of bleed water to rise to the surface. When finishers start troweling an air entrained slab after the same amount of time that they trowel a non-air entrained slab, that's where the problem is - the bleed water has not finished rising to the surface - and the bleed water gets sealed in under a hard crust of concrete on the surface of the slab. So I suppose a single pass of the trowel won't do harm because you won't be sealing the surface as would happen if you hard trowel the surface.

 

[Ron]

hope9010/ajk1....I don't see an issue with a single pass provided a majority of the bleed water is out.

Yes...neither JAE nor I are anywhere near Toronto! He's probably a bit closer than I. Haven't been there since 1988 when my association with Trow ended.

 

[dik]

I usually spec a light broomed surface for parkades... even those with a traffic coating.

Bringing the fines and cement to the top provides a surface that is prone to delamination and there are many examples of the top layer scaling off for this reason.

Dik

 

[ajk1]

dik:

Is that done with a broom that is swept across the concrete by workers, similar to a municipal sidewalk, or is there a machine that puts the broom finish on?

On a parking garage for say 1500 vehicles, say somehwere around 500,000 square feet, it seems to be a lot of manual brooming to be done.

- Any feel for roughly what that might cost per square foot?

- Would you have any concern that the ridges of the broom finish are weaker points that may eventually break off when vehicles brake or turn?

- Have you found that the waterproofing membrane manufacturers/installers are agreeable to this type of finish?

- Have you done bond tests of the membrane on this type of finish to determine if there is any change in the bond strength as compared to the membrane on a properly finished shotblast steel trowel (one pass only) floor? We find that we can consistently get 2 MPa (290 psi) bond strength with a high quality urethane membane on a steel trowel finish floor that has been shotblast.

Sorry to ask ask so many questions. I very much appreciate your response and am just wanting to learn more, not trying to be negative.

Thanks

 

[Ron]

ajk1....initial floating is done by hand. Brooming can be just the next adjunct to that. Probably less time consuming than power troweling.