Trowel finish for Air-Entrained Flatwork 1

[HeavyCivil]

This problem comes up for me a lot. Most clients don't want a broom or float finish on their exposed flatwork. But when the building is subject to freeze-thaw (Unheated garage, barn, etc.) or when the extra R-Value of AE Concrete is desired, there aren't many other options, as far as I know.

Does anyone have a good solution for finishing AE Concrete where no floor coverings will be applied?

 

[JAE]

If you air entrain concrete and then try to smooth trowel the surface, you would probably end up with lots of blistering and spalling - the affect of troweling the top surface tends to harden and densify the concrete in the top 1/4" or so and the vibration from the troweling works the air and water up to the surface where it meets the densified paste and can't get out. You then end up with lenses of voids just underneath the top surface of the concrete. Air entrainment also inhibits the migration of water to the surface.

Attached is an article on this subject.

 

[JAE]

Here's a follow up letter to the editor on that article and the author's response.

 

[Ron]

In my area, air entrained concrete is used more often than non-air entrained. Further, most slabs get hard troweled finishes. The article JAE provided is a good one and points out some of the pitfalls of the blistering issues that can occur.

One key to controlling the blistering is controlling the bleed rate relative to the finishing process. While there is an environmental component to this, it can be influence by the placement crew / finishers in several ways.....don't add water at the site, maintain a low water cement ratio, delay the finishing until bleed water is gone.

Most blistering is caused when bleed water gets entrapped below the troweling process. If the finishing crew is "drying" the surface and creating a dense surface (as desired) while there is still a lot of bleed water rising in the mix, then problems will result.

 

[HeavyCivil]

JAE - the issue is certainly well known which is why I'm looking for alternatives. Thats a great article though - thanks.

Ron- I agree- The real question here is whether under controlled circumstances if a 4.5% Air (+/-) concrete mix can be trowels without compromising the integrity of the surface.

 

[JAE]

VTEIT - one of the things that happened to us on a project which had blistering was that, with air, the bleed water comes up a lot slower. As a result, the finishers many times get the timing wrong and get on it too fast. It takes a very skilled and experienced finisher to know when the water has come up enough to start finishing.

 

[Ron]

VTEIT....4.5% air is not excessive and should be able to be finished without blistering.

JAE....exactly!