Adding steel or synthetic fibers to concrete at low-volume dosage rates provides benefits not available in conventional concrete. However, care is needed during mixing, placing, and finishing to prevent the development of fiber balls, the on-site addition of water to the concrete, and the possibility of visible fibers at the surface of the concrete.
Manufacturers strongly recommend adding synthetic fibers at a reliable batch plant for at least two reasons. First, it provides an added measure of control. And second, it's very important that synthetic fibers be mixed at least three to five minutes, a goal that's not always feasible on the jobsite. To prevent the formation of steel fiber balls, at least one manufacturer uses a special adhesive to glue a number of steel fibers together; during mixing, the glue degrades, dispersing the fibers throughout the concrete.
Both synthetic- and steel-fiber-reinforced concrete can be placed using conventional equipment such as truck chutes, concrete buckets, conveyors, and pumps.
One key to finishing fiber-reinforced concrete is using external vibration. External vibration brings paste to the surface and buries fibers located at the slab surface, encapsulating them in concrete and minimizing exposed fibers. The millions of synthetic fibers in the concrete can block or delay the appearance of bleedwater at the surface. Therefore, be sure all bleedwater has evaporated before getting on the concrete. Timing the finishing of steel-fiber-reinforced concrete is the same as that for plain concrete, and finishing practices should not be significantly affected.