Adding steel fibers to fresh concrete makes it possible to increase the interval between joints or eliminate them altogether. This doesn't mean, however, that you can reduce the quality of the concrete. Steel-fiber reinforcement along with a good mix design will provide good cracking resistance.
Nick Mitchell, executive vice president of Fibercon International, Evans City, Pa., says that steel fibers come in a variety of sizes and shapes. Sizes typically run ¾ inch to 2 inches (depending on how the fibers are made). He says they are used primarily for industrial floors and bridge decks where joints and cracks present the greatest maintenance problems. Steel fibers reduce the potential for micro-cracks that form in the curing concrete to join together to make visible cracks. As a result, shrinkage forces end up widely distributed.
In terms of recommended dosage, Mitchell says it's usual to place up to 50 pounds of fiber per cubic yard of concrete for industrial floors and up to 80 pounds for bridge deck overlays. “Only 40% of fibers are aligned in
the right direction in the concrete. Up-and-down oriented fibers don't help you,” he says. In the 1980s, in a freeze/thaw climate, he installed a stamped concrete driveway 300 feet long and 16 feet wide for his father and decided on the 80-pound dosage. As an added precaution he cut control joints every 20 feet. Twenty years later, there are no cracks.
Doug Bannister, owner of The Stamp Store, Oklahoma City, installed 4000 square feet of decorative pavement as a part of his new warehouse project, the largest single placement being 40x50 feet—2000 square feet. “We placed the slab last September with no control joints, and so far there are no cracks,” he says. “Trucks making deliveries use the slab to reach the loading dock.” He ordered 3-inch-slump concrete specified at 3500-psi compressive strength and placed 25 pounds of 1¼-inch-long steel fibers per cubic yard.
There are some risks with steel fibers though. Stamping concrete with them in the mix can result in fiber ends sticking above the surface, injuring people walking with bare feet on slabs, so don't consider an installation intended for this use. Bob Annett, product specialist for Propex Concrete Systems (formerly SI Concrete Systems), Chattanooga, Tenn., says that you can't guarantee that a fiber won't be exposed, though coloring with dust-on color hardeners helps.
The installation of steel-fiber-stamped concrete involves the same steps as with stamped impressions without fiber, with a few exceptions:
- When concrete gets too stiff, fibers act like springs, pushing through the surface as one end of a fiber is pressed down by the pattern blade of a stamp. Therefore, you should start the stamping process earlier than you normally would.
- Steel-fiber concrete is more difficult to strike-off, so consider adding a superplasticizer or midrange water reducer. As a result, the mix can have a lower w/c.
- Steel-fiber-reinforced concrete shrinks like any other concrete. So the first step should be the installation of a good subbase, compacted to leave a smooth surface. This results in minimal friction between the concrete and the base material as it shrinks.
- When placing concrete, both Mitchell and Bannister recommend striking it off with a vibratory screed. Mitchell further suggests passing a “rollerbug” over the top immediately afterwards to push fibers further below the surface before bullfloating.
Bannister recommends stamping the concrete 15 to 30 minutes earlier than you normally would. It's the best guarantee that fibers will remain below the surface and won't spring up. The time-honored way to tell when a slab is ready to start stamping is to push your finger into the surface of the concrete with moderate pressure. If the indent is 1/8 to ¼ inch, begin the stamping process. Bannister says you can also touch the concrete with light pressure. If sand sticks to your finger, wait longer. But if only color sticks to your finger, it's time apply the release agent and begin stamping.
According to Annett, “macro synthetic fibers”—plastic fibers about 2 inches long and the diameter of a toothbrush bristle—can also provide cracking resistance, added to concrete at rates from 5 to 7½ pounds per cubic yard.