Exterior Slab on Grade

[SteelPE]

I have a somewhat large exterior apron slab in front of an existing building that needs to be replaced. The slab is in an area of the country where there is winter (ie freeze thaw conditions). A question has come up with regards to air entrainment for this slab. I am going to tell the client that the slab needs to be air entrained with a broom finish. I know there are concerns with regards to air entrainment and slabs, but I believe this is mostly related to trowel finished slabs, which we do not have.

So, will we have an issue with an exterior air entrained slab with a broom finish? I say no.

 

[phamENG]

I don't think so. Pretty common in specs where I am and we don't get as cold as you do.

 

[GC_Hopi]

PCA lists two reasons for not trowel finishing exterior slabs. 1) loss of entrained air caused by overworking the surface. Related to this the difficulty of trowel finishing air entrained slabs, the air rising and you get voids on the surface. 2) troweled surfaces are slippery when wet.

Side note: You could use a float finish if the client does not like the look of a broom finish.

 

[jayrod12]

Super common here, central prairies in Canada. But definitely no steel trowel finish. But a broom finish is ideal.

 

[Enable]

The issue with a steel trowel finish is you can (not necessarily will) densify the surface such that the air entrainment is entirely pushed out of the upper layer of concrete. This causes massive scaling problems in freeze thaw environments. See ACI 302.1 sec 5.7.1. ASCC position statement here.

This is not an issue with any finish that does not significantly densify the surface. So broom finish or swirl finish or pretty much anything else is totally fine. It should also be noted though that you CAN power trowel air entrained concrete but you need to be very careful and typically one adds plastic float shoes to the trowel. I would only trust someone who has done it for quite some time to attempt it.

 

[dik]

There's nothing like a hard trowelled slab surface outside with a little rain on it... like catsh*t on linoleum...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik