Fiber reinforced concrete (FRC) is an advanced form of the reinforced concrete cast by mixtures of cement, mortar, or concrete and discontinuous, discrete, uniformly distressed suitable fibers. Many researchers prove that the addition of small, closely spaced, and uniformly dispersed fibers to concrete plays the role of cracker arrester and substantially enhance its static and dynamic properties. Here we discuss briefly all the types, uses, properties, microstructure, and advantages of FRC.
Effect of fibers in concrete
Fiber-reinforced concrete is used to overcome the difficulty of plain cement concrete which gives very low tensile strength, low ductility strength, and a little strength to cracking. Also in plain cement concrete is a chance of brittle fracture because of the propagation of micro-cracks present in the concrete that makes poor tensile strength.
By using conventional in first steel bars and by applying restraint techniques engineers and scientists want to improve the tensile property of concrete. Both of the above meters increase compressive strength to concrete members but do not increase the inherent tensile strength of concrete in their own way.
Followings are the main problems in plain concrete and similar type of brittle materials: there is a possibility of the existence of structural cracks (micro-cracks) even before loading and causes of volume change due to drying shrinkage or other reasons.
These microcracks propagate and open up by the application of external loading. This micro-crack propagation is a danger for inelastic deformation in concrete.
Types of Fiber used in fiber reinforced concrete
Fiber is circular or flat in shape and has a certain property.
The generally used fibers in fiber reinforced concrete are:
- Steel fibers,
- Polypropylene fiber,
- Nylons fiber,
- Asbestos Fibers,
- Coir fibers,
- Glass fibers, and
- Carbon Fiber.
Steel Fiber for FRC concrete
The most commonly used fiber is steel fiber in round shape. The diameter of the fibre is in the range of 0.25 to 0.75 mm. Sometimes fiber gets rushed due to the presence of moisture and loses some of its strengths but this is possible only on the surface.
Some examples of the use of steel fiber reinforced concrete are overlays of road payments, and bridge decks and Airfield payments, where they improve the flexural, impact, and fatigue property of concrete.
Steel fibers are also used for customer shells and plates.
Among the several types of steel fibers recently develop steel fibers is “Dramix glued steel fiber” as shown in fig. In this fiber, the fiber structure is in the bunch so the detachment and dispersion are in a regulated manner avoiding ballooning of fibers.
Duramax glued steel fiber can we use to produce high strength concrete up to M60 grade. This is used for tunnel lining that can protect the fire hazard in the tunnel.
Polypropylene and Nylon Fiber used in fiber reinforced concrete
Polypropylene and nylon fibers are found to be suitable to increase the impact strength but it has a low modulus of elasticity so it is not good for flexural strength.
Asbestos cement is one of the products of Portland cement and asbestos mixed and its tensile strength is in the range of 560 to 980 N/mm2 (81221 Psi to 142137 Psi). Asbestos cement has higher flexural strength so it is the most successful material.
Organic Fibre used in Fiber Reinforced concrete
Sometimes organic fibers like coir, jute, cane splits are also used for unimportant fiber concrete. Organic fibers or natural fibres (see spelling differences) are fibers that are produced by plants, animals, and geological processes. This type of fiber can be used as a component of composite materials in less important fibre concrete, where the orientation of fibers impacts the properties. Organic fibers can also be oriented into sheets to make paper or felt.
Glass fibers used in fiber reinforced concrete
Glass fiber is one of the modern methods of making glass fiber reinforced concrete (GFRC Concrete). It has a very high tensile strength in the range of 1020 to 4080 N/mm2. There is composite material trade name “CEM-FIL” developed as alkali-resistant glass fiber because glass fibre (GRFC) in consumption with cement is affected by the alkaline condition of cement. It is a durable material as compared to conventional E-glass fiber.
Carbon fiber develops high modulus of elasticity and flexural strength frame it is in composition with cement as reinforced material. The tensile strength of coronavirus are ranges of 2110 to 2815 N/mm 2.
Nowadays it is using structures like cladding, panels, and shells.
Factors affecting properties of fiber reinforced concrete
The main properties of fibre reinforced concrete depend upon the transfer of stress between the cement matrix and the fibers because it is a composite material of cement matrix and fiber-reinforced both distributed in a randomly or orderly manner. Its properties also depend on the compaction technique of concrete, size, and shape of the aggregate, amount of fibers, type of fibers, orientation, and distribution of the fibers.
Relative fiber matrix stiffness
Researchers show that the modulus of elasticity of the cement matrix should be lower than that of fibers for effective stress transfer. Steel, glass, carbon is the high modulus fibers impart strength in stiffness to the composite.
The bond between the cement matrix and the fibers should be sufficient for high tensile strength of composite and also is effective for stress transfer.
Volume of fibers
The strength and toughness of the fiber and cement composite are depended upon the volume of the fibers used, the relation is generally linear, means a form of fibers increases the strength and toughness of the composite also increases. The drawbacks of the large value of fiber cause segregation of concrete and mortar.
The aspect ratio of the fiber
The aspect ratio (ratio of its length to its diameter) (l/d) of the fibre is one of the important properties. Its value is in the range of 30 to 150. The properties and behavior of the fiber composite are also dependent upon the aspect ratio. According to research the relation between aspect ratio and ultimate strength of the composite is linear up to an aspect ratio of 75. But more than 75 of the aspect ratio relative strength and toughness is reduced. As shown in the table below.
Table: the relation between aspect ratio and strength and toughness.
|Types of Concrete
|Plain cement concrete
Orientation of fibers
The orientation of fibers is random, it is totally difference from conventional reinforcement where bars are oriented in the direction desired.
Note: the alignment of fibers parallel to the applied load offered more tensile strength and toughness as compared to randomly distributed perpendicular fibers.
Workability and compaction of concrete
The steel fiber decreases the workability of concrete, it is also difficult to compact the concrete when the fiber is introduced. The Non-Uniform distribution of fiber is also a major reason for poor workability. So the water-cement ratio can be increased by adding suitable water-reducing admixtures.
Size of coarse aggregate
The minimum size of course aggregate should be listening to 10 mm.
Mixing of fibre reinforced concrete
The mixing of fibre reinforced concrete should be done in such a way that there should be prevention from segregation, balling of fibers, and the difficulty of mixing the materials uniformly.
The mixing of steel fibre in content more than two aspect ratio more than 100 is difficult to mix.
Following are the typical proportion used for mixing fiber reinforced concrete:
Cement content : 325 to 550 kg/m3
W/C : 0.4 to 0.6
Percentage of Saint to total aggregate: 50 to 100 %
Maximum aggregate size : 10 mm
Air content : 6 to 9 percent
Fibre content : 0.5 to 2.5 percent by volume of the mix
: Steel- 1% 78 kg/m3
: Glass- 1% 25 kg/m3
: Nylon- 1 percent 11kg/m3
Precaution: The fiber should be added before the addition of water because that makes uniform dispersion of fibres throughout the mix.
Application of fiber reinforced concrete
Fiber-reinforced concrete increases the static and dynamic tensile strength, energy-absorbing characteristics, and better fatigue strength so, it is now a day using overlays of the airfield, road pavement, refractory linings, etc.
The isotropic properties of concrete are provided by uniform dispersion of fibers compared to conventional reinforced concrete so, show fiber reinforced concrete is also nowadays using on the fabrication of precast members like pipes, boats, beams, staircase steps, wall panels, roof panels, sanitary manholes covers, etc.
The trade name of fibre reinforced concrete in the United States is “Wirand concrete”. Another use of this type of concrete is for the production of prefabricated formwork molds of “U” shape for casting lintels and small beams.
Glass Fiber Reinforced Cement (GFRC)
Hybrid concrete is using in in many parts of building construction nowadays.
Alkali resistance glass fiber is developed by UK building research establishment and Peking Tom class UK.
Cement or cement sand mortar is mixed with 4 to 4.5 % by volume of glass fiberglass reinforced cement. Glass fibers are too many applications as building components. eg. below.
Uses of glass fiber reinforced concrete(GFRC):
- The cladding of building;
- Permanent and temporary formwork;
- Pressure pipe manufacturing;
- Fabrication of doors and doors frame;
- Decorative grills,
- Sun breakers,
- Bus shelters, and
- For fabrication of park benches.
Current development in fibre reinforced concrete (FRC)
The New technology developed in FRC are:
- High fiber volume micro-fiber systems.
- Slurry Infiltrated fiber concrete (SIFCON).
- Compact Reinforced composites
A brief discussion about these are given below:
High fibre volume micro fiber systems
The physical properties of this microfibers are:
- Size about 3mm long and
- Cross-section area 5 to 25 micron,
- Specific surface area 200 cm2/gram.
The conventional method of mixing it is not used in proportioning of microfiber cement because of the balling of fiber, improver dispersion with less workability.
The mixing technique used is Omni mixer with the use of admixtures such as carboxyl methylcellulose, silica fume, and ground granulated blast furnace slag.
For Supreme Performance high doses of superplasticizers, low sand cement ratio, standard sand particles of size less than 1 mm mixing with a long time is obtained.
Due to its high toughness and much impact strength, it is used in thin precast products like roofing sheets, cladding panels, etc. It is also so popular in use for repair and rehabilitation works.
Use of plastic fiber to improve the fire 🔥resistance of high strength concrete
Recently plastic fibers like polypropylene fibers are incorporated in the high-performance concrete mix to take care of the brittle behavior and to improve the fire resistance property of high strength concrete.
Concrete made with a very low w/c ratio (w/c ratio of 0.30 or less) is inferior with respect to fire resistance to normal concrete with a w/c ratio of 0.5 or more.
When w/c ratio is 0.5 or more the microstructure of such concrete is likely to be more porous on account of the existence of large capillary cavities formed by the excess water not used in the hydration process.
In the case of high strength concrete with very low w/c ratio the microstructures is virtually dense and there are no capillary cavities.
When such high strength concrete is subjected to fire, the water vapor will exert pressure and make the cover concrete to spall off exposing reinforcement directly to the fire effect.
With the incorporation of plastic fibers, the fibers melt at high-temperature and create voids in the surface portion of concrete which will absorb water vapor pressure to reduce the spalling of cover concrete and thereby protecting the steel reinforcement from direct fire effect.
In fact, the melting of plastic fibers makes the high strength concrete a porous material like ordinary concrete whose good fire-resistant property is unquestionable.
Slurry infiltrated fiber concrete (SIFCON)
Slurry infiltrated fibre concrete was invented by Lakard in 1979. In this method microfiber in concrete is maintained about 20% by volume with a method of preparing steel fiber bed and cement slurry is infiltrated. This process can increase in properties of concrete such as load-carrying capacity and toughness.
The high volume of fibre high compressive strength properties can be also achieved.
Nowadays, blast-resistant structures and burglar-proof safe vaults in banks, residential buildings SIFCON performance is better.
Compact reinforced composites (CRC)
The following materials compositions are used in compact reinforced composites:
- Dense cement matrix,
- 20 to 30 % silica fume by weight of cement,
- 10 to 20% by volume of conventional reinforcement,
- 5 to 10% of fine fibers of 6 mm long and 0.15 mm in diameter.
CRC is an extremely strong material that has flexural strength up to 260 MPa and compressive strength off to 200 MPa.
Compact reinforced composite is a versatile material that can be molded and fabricated at the site, and it is almost as strong as structural steel.
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