Concrete made with Portland cement has certain characteristics: It is relatively strong in compression but weak in tension and tends to be brittle.  

 The weakness in tension can be overcome with the use of conventional rod reinforcement and to some extent by the inclusion of a sufficient volume of certain fibres. The use of fibres also alters the behaviour of the fibre-matrix composite after it has cracked, thereby improving its toughness. 

 This excerpt from Cement & Concrete SA’s technical leaflet on fibre-reinforced concrete (FRC) aims to provide information on the properties of the more commonly available fibres and their uses to produce concrete with certain characteristics.  

 The concept of toughness 

 Toughness is defined as the area under a load-deflection (or stress-strain) curve. As can be seen from Figure 1, adding fibres to concrete greatly increases the toughness of the material. Consequently fibre-reinforced concrete can sustain load at deflections or strains much greater than these at which cracking first appears in the matrix. 

Use of fibres 

 For the effective use of fibres in hardened concrete: 

  • Fibres should be significantly stiffer than the matrix, i.e., have a higher modulus of elasticity than the matrix. 
  • The fibre content by volume must be adequate. 
  • There must be a good fibre-matrix bond. 
  • The fibre length must be sufficient. 
  • Fibres must have a high aspect ratio, i.e., they must be long relative to their diameter. 

 It should be noted that published information tends to deal with high-volume concentrations of fibre. However, for economic reasons, the current trend in practice is to minimise the fibre volume, in which case improvements in properties may be marginal. It is thus important to evaluate the published test data and manufacturer’s claims carefully. 

 It must also be noted that high-volume concentrations of certain fibres may make the plastic concrete unworkable. 

 Types of fibre 

 Glass 

Alkali-resistant glass fibre is used in the manufacture of glass-reinforced cement (GRC) products, which have a wide range of applications. 

 Claims have been made that up to 5% glass fibre by volume has been used successfully in sand-cement mortar without balling. Glass-fibre products exposed to an outdoor environment have shown a loss of strength and ductility. Because of the lack of data on long-term durability, GRC has been confined to non-structural uses where it has wide applications. GRC products are used extensively in agriculture, for architectural cladding and components, and for small containers. 

 Steel 

 Steel fibres have been used in concrete since the early 1900s. Carbon steels are most commonly used to produce fibres, but fibres made from corrosion-resistant alloys are also available. Stainless steel fibres have been used for high-temperature applications. 

 Concretes containing steel fibre have been shown to have substantially improved resistance to impact and greater ductility of failure in compression, flexure and torsion. Fatigue resistance of the concrete is reported to be increased by up to 70%. It is thought that the inclusion of steel fibre as a supplementary reinforcement in concrete could assist in the reduction of spalling due to thermal shock and thermal gradients. The lack of corrosion resistance of normal steel fibres could be a disadvantage in exposed concrete situations where spalling and surface staining are likely to occur. 

 Synthetic fibres 

 Synthetic fibres are man-made fibres resulting from research and development in the petrochemical and textile industries. There are two different physical fibre forms: Monofilament fibres and fibres produced from fibrillated tape. 

 Most synthetic fibre applications are at the 0,1% by volume level. At this level, the strength of the concrete is considered unaffected and crack control characteristics are sought. Fibre types that have been tried in cement concrete matrices include acrylic, aramid, carbon, nylon, polyester, polyethylene and polypropylene. 

  • Acrylic: Acrylic fibres have been used to replace asbestos fibre in many fibre-reinforced concrete products. They have also been added to conventional concrete at low volumes to reduce the effects of plastic-shrinkage cracking. 
  • Aramid: Aramid fibres are two and a half times as strong as glass fibres and five times as strong as steel fibres, per unit mass. Due to the relatively high cost of these fibres, aramid-fibre-reinforced concrete has been primarily used as an asbestos cement replacement in certain high-strength applications. 
  • Carbon: Carbon fibre is substantially more expensive than other fibre types. For this reason, its commercial use has been limited. It is available in a variety of forms and has a fibrillar structure similar to that of asbestos. Carbon fibre has high tensile strength and modulus of elasticity and a brittle stress-strain characteristic. Additional research is needed to determine the feasibility of carbon-fibre concrete on an economic basis. 
  • Nylon: Nylon is a generic name that identifies a family of polymers. Currently only two types of nylon fibre are marketed for concrete. It is heat-stable, hydrophilic, relatively inert and resistant to a wide variety of materials. Nylon is particularly effective in imparting impact resistance and flexural toughness, and sustaining and increasing the load-carrying capacity of concrete following first crack. 
  • Polyester: Polyester fibres are available in monofilament form and belong to the thermoplastic polyester group. They are temperature-sensitive and above normal service temperatures their properties may be altered. Polyester fibres are somewhat hydrophobic and have been used at low contents (0,1% by volume) to control plastic-shrinkage cracking in concrete. 
  • Polyethylene: Polyethylene has been produced for concrete in monofilament form with wart-like surface deformations. Polyethylene in pulp form may be an alternate to asbestos fibres. Concrete reinforced with polyethylene fibres at contents between 2% and 4% by volume exhibits a linear flexural load deflection behaviour up to first crack, followed by an apparent transfer of load to the fibres permitting an increase in load until the fibres break. 
  • Polypropylene: Polypropylene fibre is a synthetic hydrocarbon polymer, the fibre of which is made using extrusion processes by hot-drawing the material through a die. Polypropylene fibres are hydrophobic and therefore have the disadvantages of poor bond characteristics with cement matrix, a low melting point, high combustibility and a relatively low modulus of elasticity. They have been reported to reduce unrestrained plastic and drying shrinkage of concrete at fibre contents of 0,1% to 0,3% by volume. 

 Fabric and composite fibre reinforcement 

 South African manufacturers have been extremely innovative in developing versions of fibre for use with concrete. To overcome the bond and elastic modulus problem of polypropylene fibres, one development has been that of a composite of a core fibre (which can be polypropylene or a stiffer material such as acrylic, Kevlar, glass or carbon fibres) around which is spun a fluffy coating of polypropylene or cellulose. The coating can be bonded to the core at intervals to enhance the composite behaviour. 

 Natural fibres 

 Natural reinforcing materials can be obtained at a low cost and low levels of energy, using local manpower and technology. Utilisation of natural fibres as a form of concrete reinforcement is of particular interest to less developed regions, where conventional construction materials are not readily available or are too expensive. 

 Natural fibres can be either unprocessed or processed: 

  • Unprocessed natural fibres: Products made with unprocessed natural fibres such as coconut coir, sisal, sugarcane bagasse, bamboo, jute, wood and vegetable fibres have been tested in several countries. Problems have been reported with the long-term durability of some of the products. 
  • Processed natural fibres: Wood cellulose is the most frequently used natural fibre. It has relatively good mechanical properties compared with many man-made fibres such as polypropylene, polyethylene, polyester and acrylic. Results obtained from autoclaved wood-cellulose cement composites indicate that such products can be sensitive to moisture content. 

 New developments 

 A development of the last few decades has been significant research activity and increasing application of high-performance, fibre-reinforced, cement-based composites (HPFRCC). This has led to design recommendations being proposed for these materials recently in Japan. 

 Particular classes are ultra-high-performance (UHPFRC) and strain-hardening (SHCC), fibre-reinforced, cement-based composites. These composites are designed for applications varying from the requirement of high strength to that of high ductility. For instance, UHPFRC has been designed for and applied in thin bridge decks or bridge deck overlays, with compressive strengths in the range of 120MPa to 180MPa and flexural strengths in the range of 20MPa to 40MPa. 

 On the other hand, the requirement of energy dissipation in earthquake-resistant buildings has led to the use of highly ductile SHCC in coupling beams of cores of high-rise reinforced concrete buildings in Japan. Other uses of SHCC include direct exploitation of its tensile deformability in bridge deck movement joint replacement, and protection of reinforced concrete structures by its multiple, fine cracking nature, which significantly retards the ingress of moisture, gas and chlorides. An example of this application is a thin SHCC overlay of an existing dam face. 

 

Full acknowledgement and thanks go to https://cemcon-sa.org.za/ for the information in this editorial. Visit the information hub to read the full paper. 

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