Fly Ash & Entrained Air
Fly Ash & Entrained Air
(OP)
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
(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





RE: Fly Ash & Entrained Air
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.
RE: Fly Ash & Entrained Air
RE: Fly Ash & Entrained Air
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.
RE: Fly Ash & Entrained Air
RE: Fly Ash & Entrained Air
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.
RE: Fly Ash & Entrained Air
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.