Fiber reinforced concrete
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Fibre reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibres that are uniformly distributed and randomly oriented. Fibres include steel fibres, glass fibres, synthetic fibres and natural fibres. Within these different fibres that character of fibre reinforced concrete changes with varying concretes, fibre materials, geometries, distribution, orientation and densities.
Contents
Historical perspective
The concept of using fibres as reinforcement is not new. Fibres have been used as reinforcement since ancient times. Historically, horsehair was used in mortar and straw in mud bricks. In the early 1900s, asbestos fibres were used in concrete, and in the 1950s the concept of composite materials came into being and fibre reinforced concrete was one of the topics of interest. There was a need to find a replacement for the asbestos used in concrete and other building materials once the health risks associated with the substance were discovered. By the 1960s, steel, glass (GFRC), and synthetic fibres such as polypropylene fibres were used in concrete, and research into new fibre reinforced concretes continues today.
Effect of fibres in concrete
Fibers are usually used in concrete to control cracking due to both plastic shrinkage and drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of water. Some types of fibres produce greater impact, abrasion and shatter resistance in concrete. Generally fibres do not increase the flexural strength of concrete, and so cannot replace moment resisting or structural steel reinforcement. Indeed, some fibres actually reduce the strength of concrete. The amount of fibres added to a concrete mix is expressed as a percentage of the total volume of the composite (concrete and fibres), termed volume fraction (Vf). Vf typically ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividing fibre length (l) by its diameter (d). Fibres with a non-circular cross section use an equivalent diameter for the calculation of aspect ratio. If the modulus of elasticity of the fibre is higher than the matrix (concrete or mortar binder), they help to carry the load by increasing the tensile strength of the material. Increase in the aspect ratio of the fibre usually segments the flexural strength and toughness of the matrix. However, fibres which are too long tend to "ball" in the mix and create workability problems.
Some recent research[where?] indicated that using fibres in concrete has limited effect on the impact resistance of the materials[1 & 2]. This finding is very important since traditionally, people think that ductility increases when concrete is reinforced with fibres. The results also indicated out that the use of micro fibres offers better impact resistance compared with the longer fibres.[1]
The High Speed 1 tunnel linings incorporated concrete containing 1 kg/m³ of polypropylene fibres, of diameter 18 & 32 μm, giving the benefits noted below.[1]
Benefits
Polypropylene fibres 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 fibres can:
- Improve structural strength
- Reduce steel reinforcement requirements
- Improve ductility
- Reduce crack widths
- Improve impact & abrasion resistance
- Improve freeze-thaw resistance
Blends of both steel and polymeric fibres are often used in construction projects in order to combine the benefits of both products; structural improvements provided by steel fibres and the resistance to explosive spalling and plastic shrinkage improvements provided by polymeric fibres.
In certain specific circumstances, steel fibre can entirely replace traditional steel reinforcement bar in reinforced concrete. This is most common in industrial flooring but also in some other precasting applications. Typically, these are corroborated with laboratory testing to confirm performance requirements are met. Care should be taken to ensure that local design code requirements are also met which may impose minimum quantities of steel reinforcement within the concrete. There are increasing numbers of tunnelling projects using precast lining segments reinforced only with steel fibres.
Useful standards:
- EN 14889-1:2006 Fibres for Concrete. Steel Fibres. Definitions, specifications & conformity
- EN 14889-2:2006 Fibres for Concrete. Polymer Fibres. Definitions, specifications & conformity
- EN 14845-1:2007 Test methods for fibres in concrete
- ASTM A820-06 Standard Specfication for fibres in Fibre Reinforced Concrete
- ASTM C1018-07 Standard test methods for flexural toughness & first crack strength
Some developments in fibre reinforced concrete
The newly developed FRC named Engineered Cementitious Composite (ECC) is 500 times more resistant to cracking and 40 percent lighter than traditional concrete.[citation needed] ECC can sustain strain-hardening up to several percent strain, resulting in a material ductility of at least two orders of magnitude higher when compared to normal concrete or standard fibre reinforced concrete. ECC also has unique cracking behaviour. 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 a high-performance fibre-reinforced concrete in a bridge deck found that adding fibres provided residual strength and controlled cracking. There were fewer and narrower cracks in the FRC even though the FRC had more shrinkage than the control. Residual strength is directly proportional to the fiber content.
A new kind of natural fibre reinforced concrete (NFRC) made of cellulose fibres processed from genetically modified slash pine trees is giving good results[citation needed]. The cellulose fibres are longer and greater in diameter than other timber sources. Some studies were performed using waste carpet fibers in concrete as an environmentally friendly use of recycled carpet waste. A carpet typically consists of two layers of backing (usually fabric from polypropylene tape yarns), joined by CaCO3 filled styrene-butadiene latex rubber (SBR), and face fibres (majority being nylon 6 and nylon 66 textured yarns). Such nylon and polypropylene fibres can be used for concrete reinforcement.
For statical calculations there is a new modelling in the book: B.Wietek, Stahlfaserbeton, edited by Vieweg + Teubner, 2008, ISBN 978-3-8348-0592-8.
See also
References
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