Fly Ash & Entrained Air 1

[a3a]

Couple questions....

(Fly Ash)
Why is Fly Ash limited in the mix design. On my project, the fly ash is limited to 25% for spread footings of a building. Also, there is no fly ash allowed in the mix for the interior slab-on-grade (SOG). What are some reasons that no fly ash is permitted for interior slabs-on-grade.

(Air content)
The footings and all exterior SOG call for certain air content. I can understand this for freeze/thaw cycle. However, no air entrained air is permitted for inteior SOG. Why not? Though the interior SOG are not subject to freeze/thaw, I don't see how some entrained air would hurt. I thought that having some air made for better finishing? Please explain.

Thanks

 

[dbuzz]

In Australian practice, maximum fly ash content is limited to 40% by mass of the total mass of Portland cement. For concrete mixes with high cement content such a proportion of fly ash can lead to stickiness of the mix. High proportions of fly ash can retard the setting of concrete. These may be reasons why the fly ash content is limited to 25% in your case.

Fly ash reduces the water requirement for similar flow or sump and reduces the peak temperature and tendency to crack due to heat development, which are important for mass concrete elements such as footings but perhaps not so important for thin concrete elements such as slabs-on-ground.

I’ve never designed for concrete in a freeze/thaw location so can’t offer you any advice on the air entrainment.

 

[jheidt2543]

The limitations on fly ash and entrained air for interior slabs are primarily for finishing of the concrete. The replacing cement with fly ash in a floor slab leads to a courser mix, there is less cement paste for the finishing operation. It is somewhat similar with air entraining, the microscopic entrained air bubbles don't allow quite as dense of a surface finish. It is fine for exterior work where you trade-off some finishing charactoristics for freeze/thaw charactoristics. Interior concrete is expected to have a more dense finish, better looking finish.

 

[Laser28]

Actually, most fly ashes are as fine or finer than portland cement, and enhance the finishing properties. some specifications limit the amount of fly ash because of concerns with obtaining the 28 day strength when fly ash REPLACES cement. Where mixes are proportioned for strength, the amounts of fly ash can be increased.

Air entrainment consists of billions of microscopic air bubbles that act as a lubricant in the mix, allowing the amount of mix water to be reduced, and making finishing easier PROVIDED THE RIGHT EQUIPMENT is used. Wood tools rip and tear the surface of air-entrained concrete. The use of air entrainment has NO EFFECT on finishing or the placement of hardeners, stains or colors if done properly.

 

[Rx4slump]

I agree with laser, flyash is almost like using a filler. If used in place of cement it will reduce the strength, along with the proper cement content, it will enhance its finishing charecteristics. There is alot of paranoia regarding flyash. During cement shortages, unscrupulous concrete producers replaced percentages of cement with flyash to extend their stockpiles, leaving people with a bad attitude towards flyash. I also agree with your question regarding air entrainment, ask it certainly doesn't seem ridiculous.

 

[dirtsqueezer]

a3a,

Ah no one's given my answer yet- fresh ground. Just a heads up, our local concrete supplier recently had major problems meeting minimum air requirements for their fly ash mix. There are millions of tiny air bubbles, but for every one of those bubbles, there's a tiny particle of fly ash just waiting to pop it. Basically, what I've seen is that the more fly ash you add, the more AEA you need to add, but on top of that, the less time that AEA will last. As you turn the drum, that fly ash will eat up your air regardless of how much you've put in it. If you're going to have an inspection company on site and are planning to do this, I'd recommend placing an insignificant structure first using the fly ash mix, such as a garbage pad, just to make sure you don't have problems meeting specifications when it comes time for your pour. Glad I was able to pass that along. Best of luck.

 

[Laser28]

Why does fly ash "eat" air entrainment? Specifications for supplementary cementing materials such as fly ash limit the carbon content of the ash, because it is the carbon content that reduces the effectiveness of the air entraining admixture. Fly ash is produced in coal burning power plants, and lots of ash is produced that is unsuitable for use in concrete because of the uncontrolled carbon content. For lots more info, check out a fly ash industry website such as

http://www.circainfo.ca-factsheets.htm,

or your local ready mixed concrete association. For the best results, use a cement that is interground with fly ash or other SCM.

I didn't address this issue in my first post, because I assumed that specification fly ash was being used. As Benny Hill pointed out, when you ASSUME, you make an ASS out of U and ME.