Fly Ash in Floor Slab
Fly Ash in Floor Slab
(OP)
Hey all,
Is there any reason to not allow the use of fly ash for a slab on grade pour? Workablility issues or finishing issues are my primary concern. Mix design shows a 30% fly ash substitution for portland cement. If there are any resources out there that anyone is familiar with I would appreciate titles, links, PDFs, etc.
Thanks much!
Is there any reason to not allow the use of fly ash for a slab on grade pour? Workablility issues or finishing issues are my primary concern. Mix design shows a 30% fly ash substitution for portland cement. If there are any resources out there that anyone is familiar with I would appreciate titles, links, PDFs, etc.
Thanks much!






RE: Fly Ash in Floor Slab
RE: Fly Ash in Floor Slab
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RE: Fly Ash in Floor Slab
Class F fly ash is normally 15%-25% of cementitious materials. Class C is normally dosed at 15%-40%.
Fly ash inproves workability, and there is usually less bleeding and segregation. The amount of air-entraining admixtures is normally greater when fly ash is used. Fly ash has a lower heat of hydration than portland cement, which helps to reduce heat build-up, but this is not a problem with slabs-on-grade. Finishability is equal or better, and pumpability is generally improved.
There are two negatives: Fly ash will significantly retard the setting time, particularly Class F ash. Fly ash can also cause noticeable discoloration or mottling of the concrete surface, which is undesireable to some clients.
All that being said, I do not like fly ash in slabs-on-grade, particularly for industrial slabs.
RE: Fly Ash in Floor Slab
Most likely correct - but I wouldn't ever specify air for a hard troweled floor slab.
RE: Fly Ash in Floor Slab
When fly ash first began to gain popularity I think there were some issues with finishing, but mostly due to the fact that finishers were not yet used to the longer set times associated with fly ash concrete. Now that it is more common, I don't think it is as much of an issue.
In my experience, Class C fly ashes may delay setting as much or more than Class F.
RE: Fly Ash in Floor Slab
Why not?
Maybe the tyranny of Murphy is the penalty for hubris. - http://xkcd.com/319/
RE: Fly Ash in Floor Slab
He had other work to do and just left the GC, engineer and owner work things out. It was not a small amateur concrete contractor since they had just finished a 1,000,000 sf project. The delay and the additional requirement for fly ash raised the price and that had to we worked out.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: Fly Ash in Floor Slab
RE: Fly Ash in Floor Slab
A week or so earlier the same concrete and masonry contractor worked with the same G.C. to do the block walls (4' below grade and 20+' high) on a 100,000+ sf KMart over the Thanksgiving weekend (almost all on overtime), so there was always a constant communication there. Between 15 and 20 loads of block were delivered late Wednesday, steaks were on grill for lunch Thanksgiving Day and they were out of block on Friday morning and waiting for more. The remaining masonry was finished and braced Friday and Saturday in time for steel erection and roof beginning on Monday morning, so buried mechanicals were inplace for the slab to be poured later, obviously not with fly ash at that time of the year. Concrete finishing is very precarious in cool/cold weather and manpower has to be there and ready to start when the placed concrete is ready to be finished.
These jobs did not require the precision of a skating rink, but you have to let the concrete get ready before starting the finishing, which may require many people tied up (4 to 8 hours)waiting and scheduled. If the concrete contractor had done the smaller job, other projects would have been delayed.
The problem was the delays in the season and the last minute change to fly ash, to the small 30,000 sf slab caused construction problems and cost increases. Both the G.C. and the concrete/masonry supplier suffered problems at the wrong time of the year. The cost of the concrete was immaterial.
The problem was the last minute requirement for fly ash and unknown seasonally affected construction schedule.
If it is a small project, fly ash setting time is not a problem as long as there are good, reliable suppliers, since they have the mix designs and controls. Mass concrete (foundaions, dams, etc.) are definitely not a problem because they are not as critical and condition sensitive, but many ready-mix suppliers still carry a 10# bag of sugar in the truck for emergencies.
Dick
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.