Though it is never specified, workability is one of the most critical characteristics of concrete. Workability refers to the ease of handling, placing and finishing of fresh or “plastic” concrete. Slump is the general indicator of workability, yet different concretes can have greatly different levels of workability with the same slump measurement. Use of fly ash in concrete can greatly enhance workability.

Blemishes in concrete, typically called rock pockets/honey combing, are indicative of concrete without suitable workability, even though the slump may be judged to be acceptable. Rock pockets indicate a separation of the paste from the coarse aggregate in the concrete mix. Concrete must be cohesive even at high slumps to maintain its homogeneous character and avoid segregation and costly rock pockets. Fly ash offers this feature without extra cost.

The main reasons for this increased workability are:

1. Reduced Water of Convenience: Approximately 25 pounds (three gallons) of water are normally required to hydrate 100 pounds of cement. A normal concrete mix will generally contain twice the required amount of water to hydrate the cement – enough to facilitate handling and placing of the concrete. This additional water, called “water of convenience”, increases slump but at the cost of decreased cohesiveness.

Fly ash reduces the amount of water needed to produce a given slump. The spherical shape of fly ash particles and its dispersive ability provide water-reducing characteristics similar to a water reducing admixture. Typically, water demand of a concrete mix with fly ash is reduced by 2% to 10%, depending on a number of factors including the amount used and class of fly ash.

Great benefits can be obtained when using more completely consolidating fly ash concrete in areas of difficult placement where rock pockets and other placing defects often occur. Engineers understand the effectiveness of using fly ash concrete in tall thin walls, such as those used in water tanks. They know they have a better chance of getting the dense, void free concrete they have specified when fly ash is included in the mix.

2. Paste Volume Increases: Fly ash produces more cementitious paste. It has a lower unit weight, which means that on a pound for pound basis, fly ash contributes roughly 30% more volume of cementitious material per pound versus cement. The greater the percentage of fly ash “ball bearings” in the paste, the better lubricated the aggregates are and the better concrete flows.

Proportioning performance concrete with fly ash virtually guarantees a greater solid volume of cementitious materials, which in itself helps promote cohesiveness and workability.

Proportioning concrete mixtures with only water-reducing admixtures results in a greatly diminished volume of cementitious fines. In effect, this amounts to taking cement out of the mix and replacing it with sand and gravel. The strengths may be acceptable,but the workability may not be. Cementitious fines are very important to the contractor who finishes flatwork. These fines are necessary to allow proper leveling, sealing, and densification of the surface. Fly ash spheres help ease the contractor’s job by lubricating the surface, making it much easier and faster to finish the job.

In lean mixes, or where aggregates are deficient in fines, an increase in the volume of paste and an improvement in consistency will be advantageous for workability and may also increase strength by allowing more complete compaction.

3. Fly ash reduces the amount of sand needed in the mix to produce workability: Because fly ash creates more paste, and by its shape and dispersive action makes the paste more “slippery”, the amount of sand proportioned into the mix can be reduced. Since sand has a much greater surface area than larger aggregates and therefore requires more paste, reducing the sand means the paste available can more efficiently coat the surface area of the aggregates that remain.

Pound for pound, no other solid material improves the workability, strength, and other properties of a concrete mix like fly ash can, resulting in the most economical of mixtures.

Placing and finishing concrete becomes easier because of the improved workability from the spherically shaped fly ash particles. Lower slump concrete can be placed more easily (and at lower water content) because of the plasticity provided by fly ash spheres. Segregation and bleeding are reduced because of to the increased cohesiveness of fly ash concrete, so form finish and sharpness of detail are enhanced. And coarse, clean sands can be used in concretes utilizing fly ash and still have good workability. 

Fly ash concrete is actually more workable than plain cement concrete at equivalent slump. The VEBE test measures the time and energy necessary for consolidation of concrete under vibration. Figure  below shows the remarkable difference in time and energy required for consolidation of plain and fly ash concretes:

 

Typical VEBE Time vs slump

Evidence of the contribution fly ash makes to workability includes:

• Lightweight concrete including fly ash is much easier to pump.
• Finishers notice the “creamier” texture when working. They also see reduced “bug holes” and segregation when stripping forms. Slip form pavers eliminate rock pockets and voids in an otherwise harsh, no-slump paving mix.