When fibers are added to a concrete mix design, slump may be affected. But a properly proportioned mix design using fibers should not result in a less workable mix.
When fibers are added to a concrete mix design, slump may be affected. But a properly proportioned mix design using fibers should not result in a less workable mix.

Q: We will be supplying ready mixed concrete for a commercial floor project. The engineer of record has added fibers to the project specification. The concrete contractor is now concerned that with this job change, the fibers may slow his placement procedures and has submitted a change order request to place and finish the fiber concrete mix.

Does adding fibers affect the workability of a concrete mix? Do synthetic and steel fibers affect slump?

A: Introducing synthetic and/or steel fibers can change workability and slump. Any time producers consider adding fibers to a standard mix design, they should prepare a trial mix for the concrete contractor's approval.

According to Bobby Zellers, past president of the Fiber Reinforced Concrete Association and now director, engineering services for ABC Polymer, the physical properties of fibers may affect fresh concrete's slump and/or workability. Zellers identifies three main properties to consider.

The type of fiber is important. For example, Zellers notes that fibrillated polypropylene fibers can reduce the slump and workability more than monofilament polypropylene and nylon fibers. And since steel fibers are rigid, mixes can often have less slump, thus affecting workability.

The quantity of fibers in a mix will definitely affect the slump and consistency. Zellers explains the influencing factor is the increase of surface area from the fibers. In addition to the coarse aggregate, the mortar must also coat the fibers. If the mix's mortar fraction is insufficient, then the effect on the slump and workability will be greater.

Producers must monitor fiber quantity when determining the proportioning ingredients in a fiber-reinforced concrete mix. More fibers require more mortar. Synthetic fibers, when used at 0.5 to 3.0 pounds per cubic yard, typically do not require changes to the mix proportions. In general, synthetic fibers at 0.5 to 1.5 pounds per cubic yard will reduce the slump 1 to 2 inches in a well-proportioned mix. At this dosage level there should be no effect on workability.

But when the fiber dosage reaches 3.0 pounds per cubic yard and above, producers should review the mix proportions. It's best to batch a trial mix to evaluate the proportions of conventional ingredients and the effect of the fibers. At dosage levels of 3.0 or more pounds of synthetic fibers per cubic yard, Zellers suggests including a mid-range or high-range water-reducing admixture. For steel fibers, He suggests that when the dosage rate reaches about 40 pounds per cubic yard, producers should consider adding a water-reducing admixture and modifying the mix design.

The length of fibers affects slump. Researchers have reported that longer fibers will reduce the slump to a greater degree than shorter fibers. Zellers says that a 1 ½-inch-long fiber will reduce the slump more than a ¾ -inch-long fiber of the same type, at the same dosage level.

Zellers cautions that a reduction of slump does not also result in a less workable mix. His experience with fiber-reinforced concrete suggests that with a properly proportioned mix design, fibers will have little or no effect on the workability. He backs this conclusion by referencing the effect fibers have on the four properties of the plastic concrete found in the ACI 116 definition of workability: ease of mixing, placing, consolidation, and finishing.

If properly introduced into a properly maintained mixing system, synthetic fibers will homogeneously distribute within three to four minutes in a truck mixer. Zellers says the key is to avoid adding bags of fibers at the same time as the cement. The mixing of steel fibers does require special attention.

Fiber-reinforced concrete is as easy to place as a mix without fibers. According to Zellers, researchers have reported that mixes containing fibers will have a lower pump pressure than the same mix with no fibers. When the mortar fraction of the mix is sufficient to coat the coarse aggregate and fiber, coarse aggregate remains suspended, thus reducing friction at the pipe's wall.

In a properly designed concrete mix, consolidation of the fiber-reinforced concrete is no different than concrete without fibers. However, when the contractor over-consolidates the mix, the fibers can quickly separate from the coarse aggregate. Some would view this as poor distribution of the fibers where, in fact, it is a result of poor consolidation practices by the contractor.

When finishing a fiber-reinforced concrete slab, Zellers says contractors should not experience any significant delays or quality concerns. Zellers observes that a vibrating or laser screed actually helps encapsulate fibers at the surface of the slab within the mortar, creating a more durable surface.