Fiber-reinforced Concrete

Builders have been using fibers to reinforce clay, plaster, mortar, and concrete since ancient times. According to the fifth chapter of Exodus, Pharaoh knew the value of fiber reinforcement when he commanded the Jews to find their own straw for making bricks. Later, the Romans used animal hair to strengthen cementitious mortars that are still supporting the Coliseum and other ancient structures.

Now that hair and straw have given way to steel and polymers, fiber reinforcement can benefit concrete projects under many conditions. If you want to color or texture fiber-reinforced concrete, you’ll need to know a few tips for keeping your surface looking good.

Steel or synthetic?
Steel and synthetic fibers impart different properties to concrete. Steel fibers impart resistance to bending under load and damage from impacts. They look like straight or crinkly wires a couple of inches long, made of carbon steel or, for corrosive environments, stainless steel. A steel fiber-reinforced slab can be thinner than an unreinforced slab. Architects specify steel fiber reinforcement for projects such as factory floors that must withstand heavy traffic and impacts. Steel fibers are added to a truckload of concrete during mixing at 0.25% to 1.5% by volume (between 33 and 100 pounds per cubic yard). They are rarely used in decorative concrete.

Synthetic fibers, which look like hairs or bundles of hairs an inch or two long, help protect fresh concrete from shrinkage cracking (caused by too-rapid drying of the surface) and thermal cracking (caused by stresses as the concrete heats up during curing and then cools off). Once the concrete has hardened, they help cracked sections hang together, but they don’t increase the strength of the concrete once it has hardened. Synthetic fibers also help keep the mix homogeneous by keeping the aggregate from settling out.

Synthetic fibers may be nylon, polypropylene, or glass; exotics such as carbon fiber are sometimes used as a substitute for steel. Like steel fibers, synthetics are added to the truckload during mixing, but in lower quantities — about 0.1% by volume, or 1.5 pounds per cubic yard. Synthetic fibers are also available as mats, which can be helpful when pouring overlays.

What fibers don’t do. Although steel fibers give some impact resistance and stiffness to a slab, synthetic fibers don’t. Don’t expect fibers of either type to increase the space between control joints or to reduce freeze-thaw cracking. For the latter purpose, use air-entraining agents, which make little bubbles in the concrete into which freezing water can harmlessly expand instead of cracking the concrete.