Introduction to FRC
Fiber reinforced concrete is a new class of strong, tough and highly durable material. Fiber in the cement based matrix acts as a crack preventer which resists the growth of flow in the matrix and preventing these from widening under load in to cracks which cause failure. Randomly oriented, discrete fibers in concrete, mortar and cement paste have capability to enhance many engineering properties. It strengthens properties such as fracture toughness, flexural strength and resistance to fatigue, impact, thermal shock. Fiber reinforcement is very useful if used to manipulate the properties of materials in required direction to its optimum use in construction and re-fabrication of structures.
The concept is not new. Fibers have been used as reinforcement since ancient times. Horsehairs in mud bricks, asbestos Fibers in concrete and by the time in the 1950s the concept of Fiber reinforced concrete was one of the topics of interest. When there was a need to find a replacement for the asbestos used in concrete and other building materials as the risk to health occurred. By the 1960s, steel, glass (GFRC), and synthetic Fibers such as polypropylene Fibers were being used in concrete, and research still continues.
Fiber reinforced concrete (FRC) is concrete containing fibers which increases its structural strength and bonding. It contains short discrete Fibers that are uniformly distributed and randomly oriented. Fibers include steel Fibers, glass Fibers, synthetic Fibers and natural Fibers. Within these different Fibers that character of Fiber reinforced concrete changes with varying concretes, Fiber materials, geometries, distribution, orientation and densities.
For a strong construction the reinforcement shall have the following properties:
• High strength
• High tensile strain
• Good bond to the concrete
• Thermal compatibility
• Durability in the concrete environment
Effect of Fibers in concrete
Fibers are usually used in concrete to control cracks due to shrinkage. Also to reduce 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 and thus cannot replace structural steel reinforcement. Indeed, some Fibers actually reduce the strength of concrete. A number of Fibers added to a concrete mix is expressed as a percentage of the total volume of the composite (concrete and Fibers), termed volume fraction (Vf).
Vf typically ranges from 0.1 to 3%. The aspect ratio (l/d) is a ratio of Fiber length (l) and its diameter (d). Fibers with a non-circular cross section use an equivalent diameter for the calculation of aspect ratio. The increase in the aspect ratio of the Fiber usually segments the flexural strength and toughness of the matrix. However, Fibers which are too long tend to “ball” in the mix and create workability problems.
Research indicates that using Fibers in concrete has limited effect on the impact resistance of the materials. This research is very important because people used to think that ductility increases when concrete is reinforced with Fibers. The results also indicate that the using micro Fibers, concrete offers better impact resistance compared with the longer Fibers.
Benefits of using Fibers
Polypropylene and Nylon fibers can:
• Improve mix cohesion, improving pumpability over long distances
• Improve freeze-thaw resistance
• Improve resistance to explosive spalling in case of a severe fire
• Improve impact resistance
• Increase resistance to plastic shrinkage during curing
Steel fibers can:
• Improve structural strength
• Reduce steel reinforcement requirements
• Improve ductility
• Reduce crack widths
• Improve impact & abrasion resistance
• Improve freeze-thaw resistance
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