Fiber reinforced concrete

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The concept of using fibers in concrete as reinforcement is not new. For the last three decades numerous studies were performed on Fiber Reinforced Concrete (FRC).

Historical perspective

Fibers have been used as reinforcement since the Middle Ages. Horsehair was used in mortars and straw was used in mud bricks as reinforcement. In the beginning of 1900 asbestos fibers were being used. In the 1950’s the concept of composite materials came into being and fiber reinforced concrete was one of the topics of interest. There was also a need for the search of materials to replace asbestos used in concrete and other building materials. By the 1960’s steel, glass and synthetic fibers such as polypropylene fibers were used in concrete. Currently further study is being performed on different aspects of fiber reinforced concrete.

Effect of fibers in concrete

Fibers are usually used in concrete to control plastic shrinkage cracking and drying shrinkage cracking. They also lower the permeability of concrete and thus reduce bleeding of water. Some types of fibers produce greater impact, abrasion and shatter resistance in concrete. Generally fibers do not increase the flexural strength of concrete, so it can not replace moment resisting or structural steel reinforcement. Some fibers reduce the strength of concrete.

Types of fibers

• Steel fibers
• Glass fibers - Glass Fiber Reinforced Concrete (GFRC)
• Synthetic fibers
• Natural fibers

Some developments in fiber reinforced concrete

Newly developed FRC named Engineered Cementitious Composite (ECC) is 500 times more resistant to cracking and 40 percent lighter than usual concrete. ECC can sustain strain-hardening up to several percent strain, resulting in a material ductility of at least two orders of magnitude higher in comparison to normal concrete or standard fiber reinforced concrete. ECC also has unique cracking behavior. When loaded to beyond the elastic range, ECC maintains crack width to below 100 µm, even when deformed to several percent tensile strains. Recent studies performed on high-performance fiber-reinforced concrete in a bridge deck found that adding fibers provides residual strength and controls cracking. There were fewer and narrower cracks in the FRC even though the FRC had more shrinkage than the control. The residual strength is directly proportional to the fiber content. A new kind of natural fiber reinforced concrete (NFRC) made of cellulose fibers processed from genetically modified slash pine trees are giving good results. The cellulose fibers are longer and greater in diameter than other timber sources. Some studies were performed on using waste carpet fibers in concrete in an environment friendly approach to recycle carpet waste. A carpet typically consists of two layers of backing (usually fabrics from polypropylene tape yarns), joined by CaCO3 filled styrene-butadiene latex rubber (SBR), and face fibers (majority being nylon 6 and nylon 66 textured yarns). Such nylon and polypropylene fibers can be used for concrete reinforcement. Studies have shown that FRC containing carpet waste show adequate structural qualities to make it a feasible choice of recyling and thus reducing need for landfilling.