Polypropylene Fibers According to major supplies, “Use of synthetic fibers for reinforcing concrete is continuously, increasing. The increase has been considerable since 1980, but slowed somewhat in 1990, a year of substantial construction cutbacks. Apparently the construction community believes there are advantages in the use of synthetic fibers in concrete.” (Schupack) Synthetic fibers are used to improve crack control in concrete. Some reports say that synthetic fiber reinforced concrete (SFRC) would replace welded wire fabric in many slab-on-grade applications. But in reality if the welded wire fabric is placed properly it controls crack width better than the synthetic fiber reinforced concrete. In a case study of the use of synthetic fibers in reinforced concrete, the following conclusions were obtained. No matter what concrete placing job is being done, there is no substitute for good concrete practices.
The use of a low fiber volume mix will help control plastic shrinkage cracks and bleeding, but not give good cracking control once the crack forms. The synthetic fibers running through a crack have a poor bond providing no shear friction. Impact and toughness tests on synthetic fiber reinforced concrete imply less edge spalling will occur. The American Concrete Institute (ACI) conducted two studies on polypropylene fiber reinforced concrete, one dealing with plastic shrinkage cracking and the other on permeability characteristics. Plastic shrinkage cracking occurs when the surface water on the concrete evaporates faster than the bleed water reaches the surface of the concrete. It was determined by the plastic shrinking cracking study that polypropylene fibers helped reduce the total plastic shrinkage crack area on test panels.
Also determined is that the screeding rate affects the total crack area in polypropylene reinforced concrete, while finishing operations showed no significant effects. This study also suggests the use of longer fibers (about 0.75in.) will produce less crack area. “Permeability plays an important role in long-term durability of concrete materials. Permeability of concrete generally refers to the rate at which particular aggressive substances (water, sulfates, chloride ions, etc.) can flow through the concrete.” (Soroushian) As discussed in the plastic shrinkage study that polypropylene fibers reduce cracking. Less cracking in the concrete surface that surface would be less permeable. In the permeability study, they concentrated on the effects of chloride and the permeability of the concrete.
The results of this study concluded polypropylene fibers had little effects on chloride permeability of concrete. The polypropylene fibers only help reduce plastic shrinkage cracks. In residential construction, polypropylene fibers have been in use since the mid-to-late 1970s. Polypropylene fibers are used for their high tensile strength and low cost. These fibers fit into two categories, for early plastic shrinkage and for improved long-term crack control. One of the first residential applications of polypropylene fibers was in western Pennsylvania, in March of 1979.
Fibers were used in all the flat work of the residence. Polypropylene fibers have gained acceptance by residential home builders due to its flexibility and its ability to conform to forms. Home builders say polypropylene fibers help them sell more concrete by reducing the potential risks of cracking. The use of polypropylene does not compensate for the lack of good design, water/cement ratios, temperature and wind conditions. One important thing to remember is nothing replaces good concrete practices. Bibliography Schupack, Morris, and William R. Stanley. “Seven Case Studies of Synthetic Fiber Reinforced Slabs.” Concrete International Feb. 1992:50-56.
Soroushian, Parviz., Faiz Mirza, and Abdulraman Alhozaimy. “Permeability Characteristics of Polypropylene Fiber Reinforced Concrete.” ACI Materials Journal 92(1995):291-295. Mirza, Faiz., Parviz Soroushian, and Abdulraman Alhozaimy. “Plastic Shrinkage Cracking of Polypropylene Fiber Reinforced Concrete.” ACI Materials Journal 92(1995):553-560. Biddle, Daniel T. “Fiber Reinforcement in Residential Concrete.” Concrete International Feb.