Dive summary:

• Contractors, including Forta Corp., are finding that mixing as much as 7½ pounds of polypropylene macrofibers into concrete is reducing shrinkage by 40% and preventing curling at joints.
• The fibers – greater than 0.012 inches in diameter – also keep aggregates more evenly distributed, and Forta Vice President Dan Biddle said he believes they also keep mix water more evenly distributed in the mix rather than letting it rise to the surface.
• The fibers also appear to keep cracks in the micro category.

From the article:

They can be placed in lengths up to 2¼ inches, can be evenly blended in concrete at much higher dosage rates (micro fibers tend to “ball up” in higher concentrations) and they change some concrete properties over the life of a concrete slab. ...