Fiber Reinforced Concrete is a composite concrete material consisting of mixtures of cement, mortar, and discrete, discontinuous, uniformly dispersed suitable fibrous material that increases its structural strength and integrity. Fibers are commonly used in concrete to control and minimize cracks on a structure as a result of drying shrinkage, or plastic shrinkage. They can also effectively reduce the permeability of concrete and therefore reduces water bleeding in concrete.
Historical Perspective of Fiber Applications
Fibers have been used in ancient times as reinforcement materials, the use of fibers as reinforcement is not a new concept. Historically, straws and horsehair were used in mudbricks and mortar in masonry works. In the 1900s, asbestos became popular fiber material used in concrete as reinforcement. In the 1950s, when the health risks of asbestos were first discovered, and the need to find a replacement for the substance in concrete and other building materials, the concept of concrete composite materials became a popular topic of interest of which fiber-reinforced concrete was one of the topics of interest. By the fall of the 1960s, glass, steel, and synthetic materials such as polypropylene fibers were used in concrete. To date research into finding new fiber-reinforced concretes is ongoing.
Effect of Fibers in Concrete
The quantity of fibers added to a concrete mix is usually expressed in the percentage of the total volume of the composite material (concrete and fibers), usually known as “volume fraction” (Vf). The volume fraction Vf ranges from 0.1 – 3%. The aspect ratio (L/D) of fiber reinforced concrete is calculated by dividing fiber length (L) by its diameter (d). Non-circular cross-section fibers use an equivalent diameter for calculating the aspect ratio. When the modulus of elasticity of the fiber is higher than the concrete or mortar binder, this helps in bearing the load by increasing the tensile strength of the composite material. Likewise, increasing the aspect ratio of the fiber usually separates the flexural strength and toughness of the concrete. Therefore longer length results in a better matrix inside the concrete and a finer diameter increases the count of fibers as well. In ensuring that each fiber strand is effective, it is highly recommended to use fibers longer than the maximum aggregate size. Normal concrete aggregates are 20mm equivalent in diameter which forms about 35-45% of concrete, therefore fibers longer than 20mm will be more effective in the composite material. However, fibers that are too long and are not properly treated and processed tend to ball in the mix and create workability problems during preparation.
Fibers are usually added to concrete for durability and better performance of the composite material.
Benefits of Fiber Reinforced Concrete
- Fiber-reinforced concrete limits the crack growth improves the impact strength of concrete and leads to a greater strain capacity of the composite material
- For large industrial projects, macro-synthetic fibers can be used to improve concrete’s durability. Long and thick fibers made from synthetic materials may serve as a replacement for steel bars or fabric reinforcement.
- Fiber reinforcement in concrete improves the concrete’s freeze-thaw resistance and helps keep the concrete durable and attractive for long periods.
- Fibers are usually used in the concrete mix to improve mix cohesion and pumpability over long distances
- Fibers help in increasing the resistance of concrete to plastic shrinkage during curing
- The use of fibers in concrete composite material minimizes steel reinforcement requirements
- Fibers in concrete control and minimizes crack widths, which improves the durability of the concrete.
- Application of fibers in concrete mix reduces segregation and bleed-water
- The toughness of fiber-reinforced concrete is about 10 to 40 times compared of that of plain concrete
- Fibers in concrete composite tend to increase the fatigue strength of the concrete.
- The sheer capacity of reinforced concrete beams can be improved by using fibers
- Fibers reinforced concrete is useful where high tensile strength and reduced cracking are desirable or when conventional steel reinforcement cannot be achieved.
Different Types of Fiber Reinforced Concrete
The following are the different types of fibers commonly used in the construction industry for fiber-reinforced concrete.
- Carbon Fiber Reinforced Concrete
- Asbestos Fiber Reinforced Concrete
- Steel Fiber Reinforced Concrete
- Polypropylene Fiber Reinforced Concrete
- GFRC Glass Fiber Reinforced Concrete
- Macro-synthetic Fiber Reinforced Concrete
- Micro-synthetic Fiber Reinforced Concrete
- Organic Fiber Reinforced Concrete
Carbon Fiber Reinforced Concrete
Carbon fibers are fibers made up of mostly carbon atoms, about 5–10 micrometers in diameter. This type of fiber has high tensile strength, high stiffness, high chemical resistance, high-temperature tolerance, low thermal expansion, and low weight. Carbon fibers are commonly combined with other materials to form a composite. When treated with a plastic resin and properly baked, it forms a carbon-fiber-reinforced polymer commonly known as carbon fiber with a very high strength-to-weight ratio and is highly rigid but somewhat brittle. Carbon fibers are also made up of other materials, such as graphite, which forms reinforced carbon composites, with a very high tolerance to heat.
Asbestos Fiber Reinforced Concrete
One of the naturally available and inexpensive minerals for fiber is asbestos, which has been successfully used in combination with Portland cement paste that forms a widely used product known as asbestos cement. Asbestos fibers are usually thermal-mechanical and chemical resistant making them perfectly suitable for tiles, corrugated roofing elements, and sheet product pipes.
Steel Fiber Reinforced Concrete
Steel fiber is simply a metal reinforcement used in concrete to improve its properties. A certain quantity of steel fiber in concrete can result in qualitative changes in the concrete’s physical properties which can greatly increase resistance to bending, cracking, fatigue, impact, tenacity, and durability. To enhance strength, toughness, improve long-term behavior, and stress resistance, SFRC can be used in structures such as housing, flooring, precast, heavy-duty pavement, bridges, mining, and tunneling.
Polypropylene Fiber Reinforced Concrete
Polypropylene fiber reinforced concrete is a synthetic fiber, transformed from propylene, and used in a variety of applications in concrete, popularly known as polypropene or PP. These fibers are mostly used in concrete to prevent cracks due to plastic shrinkage and drying shrinkage. Polypropylene fiber is in the polyolefins group and is non-polar and partially crystalline. The properties are similar to that of polyethylene, but it is more heat resistant and harder. It is a white rugged and tough material highly resistant to chemicals. Polypropylene is produced from propylene gas by a catalyst such as titanium chloride. It also reduces the permeability of concrete and thus minimizing the bleeding effect of water.
GFRC Glass Fiber Reinforced Concrete
Glass fiber reinforced concrete is a composite concrete material consisting of several extremely fine glass fibers. The mechanical properties of glass fiber are comparable to other fibers such as carbon and polymer fibers. Although glass fiber is not as rigid as carbon fiber, it is quite cheaper and less brittle compared to carbon when used in concrete. Glass fibers are mostly used as a reinforcing element for many polymer products; to create a relatively lightweight and very strong fiber-reinforced polymer (FRP) composite material known as glass-reinforced plastic (GRP), also popularly called “fiberglass”. This material is denser, contains little or no air or gas, and is a poorer thermal insulator when compared to glass wool.
Macro-synthetic Fiber Reinforced Concrete
Macro synthetic fibers are usually employed in structural concrete as a replacement for nominal steel bar or fabric reinforcement; they are not meant to replace structural steel. Macro synthetic fibers are produced from a blend of polymers and were initially developed to provide a replacement for steel fibers in some applications. Originally, macro synthetic fibers were identified as a potential replacement for steel fibers in sprayed concrete, but progressive research and development have shown that they had a role to play in the design and construction of ground-supported slabs and a lot of other applications. They are usually the perfect option for providing nominal reinforcement in highly aggressive environments, such as coastal and marine structures, due to their resistance to staining and spalling as a result of corrosion of steel. They have been used in tram and light railway developments because they are poor conductors.
When subjected to intense fire, usually macro fibers will tend to melt as the temperature increases within a temperature range of 150 to 200 degrees C. They will often lose their mechanical properties under fire and will no longer provide structural capacity.
Micro-synthetic Fiber Reinforced Concrete
Micro-synthetic fibers in concrete provide adequate resistance to the formation of plastic shrinkage cracks against welded wire reinforcement, they have limitations in providing resistance to crack width openings as a result of structural load, drying shrinkage, or other forms of stress. Micro-synthetic fibers enhance the toughness of the concrete without losing flexibility, therefore they provide better strength and the perfect finished concrete surface.
Natural Fiber Reinforced Concrete
The natural fibers are obtained directly from animals, plants, or mineral resources and converted into nonwoven fabrics such as paper or, into woven cloth or felt. Natural fiber can also be referred to as a collection of different organic cells in which the diameter is negligible when compared to the entire length. Using natural fibers in making concrete is highly recommended due to their availability locally and are plentiful. Natural fibers are perfectly suitable for reinforced concrete and are richly available in most developing countries.
Application of Fiber Reinforced Concrete
Fiber Reinforced concrete is ideally suited for concrete applications that require adequate protection from plastic and drying shrinkage, increased service life, reduced construction costs, and improved durability. It can be applied to a variety of projects and construction works such as follows:
- Agricultural: Farm and animal storage structures, paving, walls, silos, etc
- Structural Reinforcement: Minimum shear reinforcement in components such as elevated slabs, beams, and walls.
- Commercial: Exterior and interior floors, polished concrete, car parking areas, and roadways.
- Elevated Decks: Industrial and commercial composite steel deck construction and elevated formwork at sports centers, airports, commercial buildings, shopping centers, etc.
- Highways, Roadways & Bridges: Curb and drainage work, white-toppings, barrier rails, conventional concrete paving, pervious concrete, sound attenuation barriers, etc.
- Mining & Tunneling: In precast segments and shotcrete, which may include slope stabilization, tunnel lining, sewer work, shafts, etc.
- Ports & Airports: They can be applied in runways, aprons, seawalls, taxiways, dock areas, packing, and loading ramps.
- Precast Concrete & Products: Architectural panels, walls, bank vaults, fencing, septic tanks, tilt-up construction, burial vaults, sculptures, and grease trap structures.
- Residential: Driveways, pool construction with shotcrete, basements, foundations, drainage, colored concrete, sidewalks, etc.
- Waterways: Dam structures, channel linings, lock structures, storm-water structures, ditches, etc.
Warehouse & public buildings: Light- to heavy-duty loaded floors, roadways, and parking stores.