FAQs

Workability, as defined in ACI 116, is the measure of how readily the concrete can be mixed, placed, consolidated and finished. Slump is not a measure of workability. There is no direct or indirect correlation between slump and workability of plain or fiber reinforced concrete. (See FAQ on slump.)

The four properties of the plastic concrete to look at when considering workability are:

1. Ease of mixing
   
2. Placing
   
3. Consolidation
   
4. Finishing
   

(At best, slump only reflects one of these properties: placing. Slump will not provide any indication of mixing, consolidation or the finishing properties of a given concrete mix.)

1. Ease of Mixing. Fibers do not affect mixing. If properly introduced into the mixing system the fibers will homogeneously distribute within 3-4 minutes in a truck mixer. 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 fiber should not be introduced at the same time as the cement. It has been found that the cement may coat the bags thus retarding the breakdown of the bags.

2. Placing. 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. This is readily understood if one visualizes what happens within a 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 outside. 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 thus reducing the friction at the wall of the pipe. For this theory to work it is important that the mortar fraction (quantity) of the mix be sufficient to coat the coarse aggregate and fiber.

3. Consolidation. Consolidation of the fiber reinforced concrete is another property, which requires the proper balance of ingredients. Here again, the need to fabricate trial batches of the design mix to ensure there is adequate mortar to coat the coarse aggregate and the fiber. Typically there is no need for trial mixes when 1.5 lbs. per yard (1.0 kg per cubic meter) or less of fiber is used. At 3.0 lbs. (1.8 kgs) and above, a trial mix may be necessary. Remember, the fibers are part of the mortar, thus over consolidation of the concrete will separate the mortar (and the fibers when used) from the coarse aggregate.

4. Finishing. The final workability property to consider is finishing. Slump will not indicate the finishing properties of concrete. 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.