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.

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