There are several fiber physical properties that affect both slump and workability:
- Fiber Type and Configuration
- Fiber Quantity and Proportions of the Plain Mix
- Fiber Length
- Admixtures or Additives incorporated in the Mix
2. Fiber Quantity and Proportions of the Plain Mix - The quantity of fiber in a mix will definitely affect the slump and workability. The key factor is the surface area of the fiber, since the mortar (sand and cement) in the mix must coat both the coarse aggregate and the fibers. If the mortar is insufficient to coat both the coarse aggregate and fibers, 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. If the mix will be pumped, an increase in mortar is a must. Synthetic fibers when used at 0.5 to 3.0 pcy typically require no changes to the mix proportions. At 3.0 pcy and above a review of the mix proportions is recommended. If there is any doubt, a trial mix is recommended to evaluate the proportions of conventional ingredients and the effect of the fibers on the workability. In general, synthetic fibers at 0.5 to 1.5 pcy will reduce the slump approximately 1.0 to 2.0 inches in a well-proportioned mix. At this dosage level there should be no effect on workability. Synthetic fibers at dosage levels of 3.0 pcy or steel fibers at 40.0 pcy and above, modifications to the mix design and the addition of a mid-range or high-range water reducer are recommended. 3. Fiber Length - Longer fibers 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” fiber of the same type. 4. Admixtures and Additives Incorporated in the Mix- 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. We recommend a trial mix.
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 - how readily the concrete can be mixed, placed, consolidated and finished.
Mixing - Synthetic 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 bags of synthetic fiber should not be introduced at the same time as the cement since the cement may coat the bags thus retarding the breakdown of the bags. To ensure optimum distribution when mixing steel fibers, it is imperative that the fins in the mixing system be in good condition. 2. Placing - Fiber reinforced concrete is easy to place, either directly from the truck or pumped. Mixes containing fibers will have a lower pump pressure than the same mix with no fibers. Why? In a non-fiber reinforced mix, it is easy to move the mortar portion of the mix through the pipe. It is not so easy to move the coarse aggregate. The mortar moves through the center of the pipe while the coarse aggregate is pushed to the walls of the pipe. The coarse aggregate, pushed against the pipe wall creates 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, so there is less 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 both the coarse aggregate and fiber. 3. Consolidation - Consolidation of the fiber reinforced concrete requires the proper balance of ingredients.
Fibers are part of the mortar, thus over consolidation (or vibration) of the concrete will separate the mortar and the fibers from the coarse aggregate. Some view this separation as an indication of poor distribution of the fibers. It is actually a result of poor consolidation practices by the contractor. We recommend fabrication of trial batches of the designed mix to ensure there is adequate mortar to coat the coarse aggregate and the fiber. We also recommend the use of a vibrating screed or laser screed, see Item 4 below. 4. Finishing - 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 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 fibers at the surface of the slab within the mortar. In summary, there are a number of fiber properties that may affect the slump as well as the workability. The only effect on both 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.