The concrete with the fibre reinforcement has been widely used in construction industry recently. However, the use of fibre in cement concrete is no longer a new idea or concept. Fibre has already started to use as the concrete reinforcement since prehistoric period. In the past, horsehair was added into the mortar and straw to reinforce the mud bricks. In the early twentieth century, asbestos fibre was invent and used as the concrete reinforcement. By the 1960s, asbestos fibres were replaced with steel, glass, and synthetic fibre such as the polypropylene fibre due to the health risk consideration. Today, many researches still carry on finding out more type of fibre that is able to use as the concrete reinforcement. Research shows that the conventional civil infrastructures which made out of steel reinforced concrete can be easily deteriorated in aggressive environment which eventually leads to the failure of the structure. As to prevent the occurrence of the structural failure, constant maintenance and rehabilitation are required. Therefore, the involvement of huge amount of maintenance and rehabilitation cost is inevitable. To come out with a most economical solution, concrete specialists have invented a new method, they replaced the steel reinforcement with fibre such as polypropylene to produce a fibre reinforced concrete and it is also known as FRC. Fiber can help to increase the concrete toughness, tensile and compressive strength, provide fatigue and impact resistance as well as improve the post-cracking ductility performance. Since the elastic modulus of the fibre is relatively higher than cement concrete, the fibre is able to provide an effective reinforcement during the concrete hardened stage. The occurrence of plastic shrinkage and drying shrinkage will eventually lead to concrete cracking; therefore, using the fibre as the reinforcement can help to eliminate it. Fibres can also help to lower the permeability of concrete as well as reducing the water bleeding. However, the amount and type of fibres used may influence the level of the improvement. The study of the fiber reinforcement mechanics will be conducted during the first 5 hours after the concrete placing when the plastic shrinkage cracking usually take place.
Type of fibre used in concrete and its properties
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From the previous research, there are 5 main types of fibre that usually used as the concrete reinforcement material. They are the synthetic fibre, steel fibre, glass fibre, carbon fibre and natural fibre. Some fibre is produce by artificial materials and some is form natural materials. Different type of fibre has its own function and properties and it will vary the concrete performance level. Therefore, the selection of appropriate type of fibre has to take properties and behavior of the fibres such as the diameter, specific gravity, elastic modulus, tensile strength into consideration. It is claimed that the most common types of fibre used in united state to form the fibre reinforced concrete are steel, glass and synthetic fibre.
Synthetic fibres are produced by the artificial materials that are able to endure the alkaline environment of concrete in long term. Several types of synthetic fibre that available in the market are acrylic, aramid, carbon, nylon, polyester, polyethylene, or polypropylene. Fibre is usually added before or during the concrete mixing. The use of synthetic fibres in concrete can help to enhance the concrete structures overall durability and long-term performance. Synthetic fibres are suitable for various concrete application such as the industrial and warehouse flooring, thin-wall precast products, white-toppings and overlays, and shotcrete.
During the early age stage, the use of synthetic fibres can assist in inhibited the developing of micro shrinkage cracks and plastic settlement cracks. The fibre which dispersed and distributed equivalent throughout the concrete can help to improve the concrete properties in all direction.
Synthetic fibres have the ability to resist the shattering forces during the hardened concrete stage. Since unreinforced concrete is brittle, and its tensile strength and strain capacity is low, therefore it can be easily shatter and fail at first crack when it is subjected to compression forces. However, with the addition of the synthetic fibres in the cement concrete can help to prevent the effect of shattering forces by tightened the concrete together. Besides that, synthetic fibres can also provide abrasion and impact resistance for concrete and it minimizes the amount of plastic cracking of the concrete.
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The most commonly used of synthetic fibres in cement concrete as the concrete reinforcement are polypropylene fibres. Polypropylene fibre has various properties which make it suitable to use as the concrete reinforcement. There are two different types of polypropylene fibre which can be categorized into micro- and macro-polypropylene fibre. Micro-polypropylene fibre is usually used to control the occurring of plastic shrinkage cracking and provide abrasion and freeze-thaw resistance. However, it is weak in flexural toughness and post-crack behaviour. Previous research has proven that adding with both the micro- and macro-polypropylene fibre into the concrete mixture can overcome all the limitations and offering the best solution.
Another type of concrete reinforcement is the steel fibre. Steel fibre is manufactured from hard-drawn steel wire. It is mainly used as the reinforcement for concrete, mortar and other composite materials in many modern countries. Steel fibres are the strongest among all types of fibres. The tensile strength and elastic modulus of the steel fibre is high and it can assist in improving the concrete post-crack performance effectively.
Steel fibres can be fabricated in different shapes and sizes but the performance level will vary. Besides that, the amount of the steel fibres usage and its parameter has to be take consideration since it will differ the performance of the fibres.
The advantages of using the steel fibres as the concrete reinforcement are it can assist in strengthening the concrete structural and minimize the demand of the steel reinforcement. Steel fibres has high tensile strength and elasticity, it can also improve the concrete post-crack performance. Besides that, several studies have shown that steel fibres can help to improve ductility and energy absorption, reduce the crack widths, and provide impact & abrasion and free-thaw resistance. For certain construction projects, the conventional steel reinforcement bars are fully replace with steel fibres particularly for the industrial flooring construction and pre-casting applications.
Glass fibre, another alternative concrete reinforcement, is man-made material produced from extremely fine and high strength fibre of glass. It is widely used as a reinforcing agent for many polymer products and also as the concrete reinforcement. Those new and thinner size glass fibres have higher ductility. Once the glass fibre surface defects, the tenacity of the glass fibre will be reducing. Glass fibre can elongate more before it breaks compare to carbon fibre. With the addition of glass fibre into the cement concrete, the glass fibre reinforced concrete was formed. Glass fibre reinforced concrete is recognised as the environmentally-friendly composite material.
Glass fibre reinforced concrete offered a lot of benefits to designers and contractors. It has superior fire retardant properties. Glass fibre reinforced concrete can be form into many different products with even complex shapes. The weight of the glass fibre reinforced concrete is light; hence, the installation is easier without the required of the heavy lifting equipment and save a lot time and budget. Glass fiber-reinforced concrete (GFRC) is mainly used for production of precast architectural cladding panels, landscaping, building and site products, and many other engineering applications. It is also applicable for historical restorations and renovations. GFRC has a good resistant to corrosion and biological attack. Besides that, GFRC has low permeability and denser surface. It can also provide higher compressive and flexural strength.
Natural fibres consist of wood, sisal, coconut, bamboo, jute, akwara, and elephant grass. Natural fibre is more economical and abundant, the resources is renewable and is it non-hazardous. It will not bring harm to the human being ad environment. Furthermore, with the addition of natural fibre, the concrete is expected to become more durable due to the reduction in permeability. Natural fibre normally used to fabricate siding, backer board, roofing material, non pressure pipe. Besides that, concrete with aggressive fibre may lead to the decreasing of the concrete workability because of the increasing in the surface area. However, with too little fibre content will also reduce the slump. Lignin is normally used as concrete set retarder, however, if fibres mix up with lignin it will influence the concrete setting time adversely. Since fibres may absorb/desorbs the water from the cement matrix or atmosphere which eventually affect the setting time. Concrete mixing with aggressive fibre will not bring much effect to cement hydration. Plastic shrinkage may occur on the carbon fibre reinforced concrete due to the water evaporate faster than the water bleeding. Concrete with high fibre content can assist in impeding the dissemination of micro cracks and improve the concrete tensile strength which finally inhibits the occurrence of plastic shrinkage. In contrast, the presence of aggressive fibre in the concrete will cause a reduction in the compressive strength. Natural fibre can help to improve the concrete flexural strength and addition of fibre content does not make much difference on the concrete flexural strength. Natural fibre can also improve the concrete flexural toughness. It also provides impact resistance, reduce shrinkage and prevent cracking. After adding the natural fibre into the reinforced concrete slab, the impact resistance provided is 3-18 times higher than the plain concrete slab.
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Carbon fibre is a man-made material comprise of relatively thin fibres. The diameter of the carbon fibres is 0.005-0.010mm and it is making up with carbon atoms. The carbon atoms are bonded together in microscopic crystals that are more or less aligned parallel to the long axis of the fibre. Carbon fibre is suitable for many lightweight applications due to its lower density. Carbon fibre is also well recognised with its high tensile strength, lightweight, and low thermal expansion in many engineering applications. However, carbon is a costly technology compare to glass fibre. Carbon fibre is good in bending, but it is weak when subjected to compression forces. The way how the carbon fibre dispersed during the concrete mixing will affect the concrete performance.