Precast Inc. - May/June 2008 - (Page 8) T E C H N I C A L LY S P E A K I N G Introduction to FiberReinforced Concrete By NPCA Staff he more we progress in our knowledge of concrete technology, the more we return to what we already know. So is the case of fiber reinforcement in precast concrete products. The advantages of using fibers to alter the brittle behavior of concrete products have been known since 1400 B.C. The basic principles of fiber reinforcement are still relevant today. Today’s technology and recent research demonstrates that fibers made of a variety of materials, including polypropylene, glass, steel and other products, can dramatically improve the crack resistance, impact resistance, impermeability, abrasion resistance and durability of precast products. However, due to their random orientation and discontinuous nature, their structural contribution cannot be assessed by the same methodology as continuous reinforcement. T How fibers improve concrete performance Concrete is an inherently durable material that has exceptional compressive strength. However, its tensile capacity is only about 10 percent of its compressive capacity. Consequently, concrete has a tendency to crack due to tensile stresses induced during the curing process as well as during external loading. Fibers provide three-dimensional temperature and shrinkage reinforcement and reduce this tendency, enabling the concrete to further display its versatility as a preferred material for manufactured building products. Fiber-reinforced concrete converts the sudden brittle failure of concrete to a more gradual and ductile process through the redistribution of stresses in the event of cracking. This gradual ductile failure is essential in concrete design, since failure without any warning could cause devastating results. In technical terms, fibers reduce the characteristic non-isotropic behavior of concrete. The ability to redistribute thermal stresses and consequently reduced plastic shrinkage cracking is one of the major advantages of fiberreinforced concrete. As illustrated in Figure 1, fibers can greatly reduce both the size and initiation of cracking of precast products, which has a direct effect on the ultimate durability of a product. As a rule, the higher the volume of fibers, up to an optimum level, the more improved the performance of a product. Fiber use can also improve the impermeability of precast products, as shown in Figure 2, due to a prolonged hydration period. Placement practices In North America, the use of fibers for precast concrete applications must be in compliance with ASTM C 1116, “Standard Specification for Fiber Reinforced Concrete and Shotcrete.” Mix designs and production procedures must be carefully adjusted before placing into full production. Follow the manufacturer’s recommendations regarding material handling and safety procedures. The performance of fibers in plastic- state concrete is a function of the specific placement conditions, the fiber shape and aspect ratio. In most applications, fibers specifically manufactured for precast concrete do not require exceptionally special or unique placement practices. In some instances (such as with collated fibrillated polypropylene fibers), the use of fibers has been reported to facilitate placement practices (pumping and early form removal) due to the greater uniformity and reduced segregation of fiber mixes. Typical dosages of synthetic fibers in precast concrete range from 1 to 8 pounds per cubic yard. Steel fiber dosages range from 20 to 160 pounds per cubic yard. Quantities greater than this usually do not result in further improved properties. To ease handling and improve aesthetics, the use of formmounted vibrators is encouraged to produce surfaces that are free of excess fibers. When higher steel fiber concentrations are specified, an adjustment of the fiber-mix proportion may be needed to accommodate the higher surface area of the fibers with additional cement paste. Steel fibers usually require additional energy in the mixing and placement process. They also have a tendency to protrude at sharp corners of formwork. This condition can be alleviated by chamfering or rounding these sharp corners. In some cases, a reduced batch size or an increased mixing time (or both) may be needed to ensure uniform dispersion of the steel fibers. 8 MAY/JUNE 2008 | PRECAST INC.
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