A.Bill Palmer, former editor of Concrete Construction magazine and president of Complete Construction Consultants in Lyons, Colo., responds: For the last 30 years or so, synthetic fibers made from nylon, polypropylene, and other plastics have been used in low dosage rates of around 1.5 to 3.0 pounds per cubic yard to help reduce plastic shrinkage cracks (the ones that occur immediately following placement) in concrete. Plastic fibers can also increase a slab’s impact resistance.

However, they’re ineffective at controlling crack width and location; that’s the main purpose of wire mesh, welded wire fabric (WWF), and regular reinforcing steel. But even these metal reinforcements aren’t very effective if they’re not chaired up properly and end up in the bottom of the slab rather than in the upper half.

Research into polymer fibers and improvements in concrete admixtures have recently led to the development of concrete mixed with much higher dosages of “structural” fibers. Sometimes called macro polymeric fibers, these are much longer (about 2 1/4 inches in length) and thicker than regular fiber additives, and are added at dosage rates of as high as 30 pounds per cubic yard. (A more typical rate is 7 1/2 pounds per cubic yard.) The resulting “high-volume synthetic fiber” concrete has yet to gain widespread acceptance — perhaps because of concerns about workability — but studies show that it resists cracking better than regular steel-reinforced concrete. So it’s possible that structural fibers may eventually replace WWF and other traditional crack-control reinforcements.