Fly Ash in Floor Slab 5

ACI 301 limits fly ash to between 15% and 25%. See tables 4.2.2.9 and 4.2.2.1 for requirements.

When proportioned properly, fly ash in concrete can increase workability, easing pumping and finishing. It also increases strength and durability by increasing density. Fly ash content can also guard against sulfate attack, alkali-silica reactivity and efflorescence. It might slow down the cure time and strength gain will be more gradual.

The best resource is "Design and Control of Concrete Mixtures" by PCA.

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.

The amount of air-entraining admixtures is normally greater when fly ash is used.

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Most likely correct - but I wouldn't ever specify air for a hard troweled floor slab.

Workability shold not be an issue. Fly ash concrete generally has a higher slump due to the spherical particals in the FA.

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.

"All that being said, I do not like fly ash in slabs-on-grade, particularly for industrial slabs."

Why not?


Maybe the tyranny of Murphy is the penalty for hubris. -

In some instances, fly ash can actually raise the cost of a job. I recall a project of about 30,000 sf of slab in the building. Steel columns erected and roofed over with sides open. The project was delayed about 2 months with winter coming. When it was ready, the concrete contractor was informed that flyash had to be in the concrete plus it had to be done inne or two days and he refused to do the entire large flatwork because it was now colder than anticipated and slow setting was unpredictable and he would have to have too many masons sitting and waiting for the proper time to start finishing and prevented him from doing other work for the entire crew. Enclosing and heating the base and entire space (25' height) was not feasible and limited access for materials and equipment.

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.

I would say the poor management and lack of communication between the GC, Owner, Engineer and the subcontractor was the culprit in that case. The flyash didn't cause the problem.

The fly ash did cause the problem since it raised the cost and was imposed late in the flow of things and the 2 month delay and weather compounded the situation.

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.