Air Entrained Concrete and Trowel Finishing 3

[pbc825]

I'm getting into training for slab on grade design, and I had a question for my instructor that he wasn't able to answer.

I recall an instance where the engineer for the structural floor in our new office building (part office, part shop) specified 5-8% air entrainment. To the best of my understanding, the air entrainment was specified per CSA A23.1-09 requirements as we had specified the shop may not always be heated (we're in Canada). The floor was steel trowel finished and a chain drag test sounds delaminations over 20 to 30% of the floor area in the shop and office. Air entrainment tests showed just over 6%. For the office we had planned on polishing the concrete and leaving it, but we decided to tile the floor as we feared surface defects would "pop up" in the future. The shop was left as is and two years later we don't have visible surface defects (it's been heated continuously).

I understand naturally air entrained concrete tests around 2-3%, and steel troweled finishes on naturally air entrained don't typically make for delaminations. Some confirmation or further guidance is welcome here.

I also understand that air entrained concrete (4-8% per Table 4 of CSA A23.1-09) is required for concrete subject to freezing and thawing. So, my questions are as follows:

1) Was the engineer for the floor unlucky or is that a clear error to specify high air entrained concrete and a steel trowel finish? (the GC sure didn't think much of it)
2) What sort of surface finish should be specified for an interior slab that "could" be exposed to freeze thaw?
3) What sort of surface finish should be specified for an exterior slab that will clearly be exposed to freeze thaw?
4) If it needs to be a steel finish (for an interior slab) then is the lesser of the two evils to specify less air entrainment or natural air entrainment?
5) Is there a standard of practice that defines answers to these questions?

 

[Canuck67]

pbc825,

Excerpt from A23.1 "7.5.4.3.2 Exterior or air-entrained concrete
One or more passes of a magnesium float or concrete broom shall be made at suitable time intervals to
obtain a level finish free of ridges. A steel trowel finish shall not be applied to air-entrained concrete."

I can only guess, but perhaps they trowelled too soon and bleed water was trapped resulting in blistering or scaling.

 

[JAE]

1) Was the engineer for the floor unlucky or is that a clear error to specify high air entrained concrete and a steel trowel finish? (the GC sure didn't think much of it)
In my opinion it is an error to specify air entrainment wtih a steel troweled floor. There may be ways to get it done but most finishers don't have the skill. The troweling action densifies the top 1/8" to 1/4" of the floor and tends to trap the rising bleed water as well as provide longer "vibration" via the troweling energy which adds to air rising and creating void pockets with the water just below the surface. Thus you get spider cracking and delaminated areas.

2) What sort of surface finish should be specified for an interior slab that "could" be exposed to freeze thaw?
If the floor is to be kept dry without moisture exposure there is not as much water to freeze and thaw. However, I would specify a very light broom finish after the float - perhaps allowing a very light initial trowel only. But the air amount - without the excess water exposure, may not be needed as high as outside concrete.

3) What sort of surface finish should be specified for an exterior slab that will clearly be exposed to freeze thaw?
Light broom finish.

4) If it needs to be a steel finish (for an interior slab) then is the lesser of the two evils to specify less air entrainment or natural air entrainment?
Less air in my opinion - or no air.

5) Is there a standard of practice that defines answers to these questions?
No standard of practice - but Google "air entrainment troweling" and see what you get.

Check these links out for similar information:

http://www.concreteconstruction.net...ir-Entrained Concrete Floors_tcm45-343501.pdf

http://www.nrmca.org/aboutconcrete/cips/20p.pdf

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[TXStructural]

From ACI 301-10, in reference to industrial floor slabs:
11.2.2.1 Air content—Concrete for slabs to receive a
hard trowel finish shall not contain an air-entraining admixture
or have a total air content greater than 3%.

 

[Ron]

Although somewhat unnecessary in my area (no freeze thaw cycles), ready mix concrete here is routinely air entrained, typically from 4 to 6 %.

Steel troweling is almost always done on warehouse floors and blistering/scaling is rarely an issue unless other aberrations in the mix or placement have occurred. Proper finishing techniques will usually control the occurrance of blistering.

A lot of that has to do with the coarse aggregate. If the absorption of the coarse aggregate is high and the aggregate is batched at less than a saturated surface dry condition, bleeding is minimal. If the opposite occurs, bleed water is common and contributes to blistering. Another technique common in finishing is to use dry cement sprinkled on the surface during finishing to take up the bleed water.....terrible practice and leads to scaling and blistering.

In my opinion the engineer did not erroneously specify the concrete; however, if appropriate batching, delivery, placement, finishing and curing controls were not specified, enforced or employed, that is wrong.

 

[pbc825]

Much appreciated gentlemen.

Is anyone able to confirm natural air entrainment at 2-3%?

 

[jguer005]

Based on my own field experience, entrapped (natural) air can vary from < 1% to 3-4%.

EIT

 

[Ron]

Entrapped air is usually 1 to 2%, depending on consolidation technique used.

 

[pbc825]

Very good. Thanks again.