Used as secondary reinforcement, polypropylene fibers help reduce shrinkage and control cracking. To use these fibers, concrete mix design does not have to be altered, and no special equipment or slump modifications are required, even for pumping or shotcreting. Only two things must be determined: how much fiber to add and what length of fiber to use. Polypropylene fibers are manufactured in small bundles. During the mixing operation, the movement of aggregate shears these bundles into smaller bundles and individual fibers. If the jobsite is more than a 30-minute drive, the fibers should be added at the site.
In fiber-reinforced concrete, cracks can open only if the tensile stresses in the concrete exceed the tensile strength or the pull-out strength of the fibers. The longer the fibers are, the stronger the bond between fibers and paste is and thus the greater the fiber pull-out strength is. If fibers are too long, uniform distribution of the fibers becomes difficult. Longer fibers can be used when larger aggregates are present to shear the bundles of fiber apart. Short fibers are used with small or lightweight aggregate.
Polypropylene fibers tend to hold the concrete mix together. This slows the settlement of coarse aggregate and thus reduces the rate of bleeding. A slower rate of bleeding means a slower rate of drying and thus less plastic shrinkage cracking. In hardened concrete, polypropylene fibers act as crack arresters. Like any secondary reinforcement, the fibers tend to stop cracks from propagating by holding the concrete together so cracks cannot spread wider or grow longer. However, since polypropylene fibers are distributed throughout the concrete, they are effective close to where cracks start at the aggregate-paste interface.