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Q: How do synthetic and steel fibers affect slump? More accurate question: how do the fibers affect the workability of the concrete?

A: A frequently asked question focuses on the effect of the synthetic or steel fibers on the slump of the concrete. The companion question and more accurate question is how will the fibers affect the workability of the concrete?

First, the reader must understand that slump and workability are not interchangeable terms. The slump is the means of measuring the consistency of the concrete from one truckload to the next. Workability is not indicated by the slump measurement.

That said there are a number of physical fiber properties that will affect both slump and workability:

Type of Fiber/ Configuration of the Fiber

Quantity of Fiber

Length of Fiber

Proportions of the Plain Mix

Admixtures or Additives Incorporated in the Mix

Let us look at each of these factors.

The type of fiber can include steel or microsynthetic or macrosynthetic fibers. Within the microsynthetic fiber category, we have monofilament and fibrillated polypropylene or monofilament nylon fibers. Macrosynthetic fibers have no standard configuration.  Steel fibers are either circular, rectangular or shaved circular in cross-section. The standard configurations are continuously deformed, end deformed or hooked end. Steel fibers being rigid can have a major influence on both slump and workability, particularly when other parameters are factored in.

One concern with steel fibers is orientation. For example, steel fiber reinforced concrete compression cylinders cannot be internally rodded. The reason being the steel rod used to consolidate the specimen will reorient those steel fibers hit by the rod to a vertical or near vertical direction.

The current ACI document that talks about this subject recommend that all Fiber Reinforced Concrete test specimens be externally vibrated, which would include all synthetic fibers.

Fibrillated Tape Polypropylene fibers can reduce the slump and workability more than monofilament polypropylene and nylon fibers.

2. The quantity of fiber in a mix will definitely affect the slump and consistency. The factor to consider here is the surface area of the fiber. In addition to the coarse aggregate, the mortar (sand and cement) must also coat the fibers. If the mortar fraction is insufficient, then the effect on the slump and workability will be greater. Therefore, it is mandatory to consider the quantity of fibers when determining the proportions of the conventional ingredients in a fiber reinforced concrete mix. More fibers require more mortar. Synthetic fibers when used at 0.5 to 3.0 lbs/cy typically require no changes to the mix proportions. At 3.0 lbs/cy and above one must consider a review of the mix proportions. If there are any questions, a trial mix should be requested to evaluate the proportions of conventional ingredients and the effect of the fibers. In general, synthetic fibers at 0.5 to 1.5 lbs/cy will reduce the slump 1 to   2 inches in a well-proportioned mix. At this dosage level, there should be no effect on workability. At dosage levels of 3.0 lbs/cy and above of synthetic fibers a mid range or high range water reducer is recommended. For steel fibers, the breakpoint is approximately 40 lbs/cy when considering a water reducer and modifications to the mix design.
3. It has been found that longer fibers will reduce the slump to a greater degree than shorter fibers. Thus a 1 ½” fiber at the same dosage level will reduce the slump more than a 3/4 inch fiber of the same type.
4. As noted above the consistency or proportions of the plain concrete mix will have an effect on how great the slump reduction may be when fibers are added. This is why when 3.0 lbs/cy or more of synthetic fibers are added to a mix or 40 lbs/cy of steel fiber or more, it is wise to put together a trial mix. Another factor will be whether the mix will be pumped. If this is the case the increase in mortar is probably a must.
5. As we note above, admixtures and/or additives like plasticizers and fly ash can be used to increase the workability of a fiber reinforced concrete mix.

When introducing synthetic and/or steel fibers, a change in the slump/workability can be created by one or more of the items discussed above. This is why we suggest putting together a trial mix anytime there is a question.

Let’s expand on our discussion of workability.

Above we have reviewed those factors related to the use of fibers in concrete that affect both the slump and workability of the concrete. Now we will look more closely at fiber’s effect on workability. Workability, as defined in ACI 116, is the measure of how readily the concrete can be mixed, placed, consolidated and finished. There is no mention of the slump in this definition. There is no direct or indirect correlation between slump and workability of plain or fiber reinforced concrete.

The four properties of the plastic concrete found in the definition of workability are ease of mixing, placing, consolidation, and finishing. At best slump only reflects on one of these properties ‘placing’. The slump will not provide any indication of mixing, consolidation or the finishing properties of a given concrete mix.

How do fibers effect mixing? They do not effect mixing. If properly introduced into the mixing system, the fibers will homogeneously distribute within 3-4 minutes in a truck mixer. The introduction of the steel fibers does require special attention (See special FAQ on this subject). The single requirement to ensure optimum distribution is that the fins in the mixing system are in good condition. One additional point: The bags of synthetic fiber should not be introduced at the same time as the cement. It has been found the cement may coat the bags thus retarding the breakdown of the bags.

Fiber reinforced concrete is easy to place, either directly from the truck or pumped to the point where required. It has been reported that mixes containing fibers will have a lower pump pressure than the same mix with no fibers. I believe this is readily understood if one visualizes what happens within the pipe. It is easy to move the mortar portion of the mix through the pipe. It is not easy to move the coarse aggregate. Thus what happens is the mortar moves through the center of the pipe while the coarse aggregate is pushed to the walls of the pipe. The coarse aggregate and pipe create a frictional resistance, which retards the movement of the concrete. When fibers are added to the matrix, the coarse aggregate remains suspended in the matrix, which reduces the friction at the wall of the pipe. For this theory to work it is important that the mortar fraction of the mix be sufficient to coat the coarse aggregate and fiber.

Consolidation of the fiber reinforced concrete is another property, which requires the proper balance of ingredients. Here, again, we emphasize the need to fabricate trial batches of the design mix to ensure there is adequate mortar to coat the coarse aggregate and the fiber. It is important to note that the fibers are part of the mortar, thus over consolidation (read vibration) of the concrete will separate the mortar (and the fibers when used) 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.

The final workability property to consider is finishing. With plain concrete and fiber reinforced concrete it is known that a vibrating screed, a roller screed or a laser screed really enhances the finishing of the concrete. The action of the screed pushes the coarse aggregate down and brings the mortar up. In a fiber reinforced concrete the screed actually helps encapsulate those fibers at the surface of the slab within the mortar.

In summation, there are a number of fiber properties that may affect the slump as well as the workability. I would emphasize that the single common thread between slump and workability is the ease of placement. The slump will not show how well the fibers will distribute in the mix or how the fiber-reinforced mix consolidates or finishes. Thus there is technically no way to make a blanket statement that slump is a measure of workability for plain or fiber reinforced concrete.

-R.C. Zellers, PE/PLS, Director, Engineering Services