Uses and Applications for Steel Fiber Concrete
Steel fiber concrete flooring can provide superior resistance to minimize cracks in hardened concrete, as well as maximum resistance to withstand heavy loads, either dynamic or static. If you decide to use steel fiber concrete flooring, you can select to use a ‘joint-less floor’. Joint-less floors are floors that have minimal joints, providing spaces without joints as large as 40 or 50-meter span wide.
Steel fiber dosage will vary greatly upon the project intended use, and the types of mesh being replaced. Common dosages are in the range between 20-30kg/m3 to 40-50kg/m3 for joint-less floors. Trowelling concrete will help to embed steel fibers into the concrete surface producing a better-finished product. Steel fibers will enhance the crack resistance of the concrete, and they can also be used to replace or supplement structural reinforcement. It only can be done through a structural engineer and with proper guidance.
How and When the Fiber is Added
Typically the fibers are added at the batch plant, just after all concrete aggregates are being mixed. Some people would request to have the fiber added at the job site, but then the QA/QC should have more control on how much fiber is added. The steel fiber manufacturer can provide guidance on how to mix and the amount needed to obtain the desired results. Be aware that if you add too much fiber, it might show up at the surface when finishing the concrete so be cautious about the amounts being mixed.
Costs of Steel Fiber Concrete Flooring
In general, and depending on the type of steel fiber used, it can add between $6 to $10 per cubic yard on top of the ready-mix concrete cost. This cost is based on the assumption that you are using 1.5 pounds per cubic yard of concrete.
How Workability is Affected
If you opt-in to add fiber to your concrete mix, be aware that there will be some changes in the way you manage this concrete. First of all, the slump will be affected, and it is recommended to add a superplasticizer to enhance the slump and make the concrete a little more fluid. Not all steel fiber can be used as a substitute for steel reinforcement, so make sure that your structural engineer has reviewed and analyzed the loads before proceeding.
Where to Use Steel Fiber Concrete Floors
Typical applications for steel fiber concrete flooring can be found on parking lots, playgrounds, airport runways, taxiways, maintenance hangars, access roads, and workshops. This method is also widely used for port pavements, container storage and handling areas, bulk storage warehouses, and military warehouses. Steel fiber reinforced concrete is commonly used in tunnel construction, as it provides additional flexural strength, reduces shrinkage cracking and reduces permeability.
Steel Fiber Reinforced Concrete (SFRC)
Steel fiber reinforced concrete is a composite material having fibers as the additional ingredients, dispersed uniformly at random in small percentages, i.e. between 0.3% and 2.5% by volume in plain concrete. SFRC products are manufactured by adding steel fibers to the ingredients of concrete in the mixer and by transferring the green concrete into moulds. The product is then compacted and cured by the conventional methods. Segregation or balling is one of the problems encountered during mixing and compacting SFRC. This should be avoided for uniform distribution of fibers. The energy required for mixing, conveying, placing and finishing of SFRC is slightly higher. Use of pan mixer and fiber dispenser to assist in better mixing and to reduce the formation of fiber balls is essential. Additional fines and limiting maximum size of aggregates to 20mm occasionally, cement contents of 350 kg to 550 kg per cubic meter are normally needed.Steel fibers are added to concrete to improve the structural properties, particularly tensile and flexural strength. The extent of improvement in the mechanical properties achieved with SFRC over those of plain concrete depends on several factors, such as shape, size, volume, percentage and distribution of fibers. Plain, straight and round fibers were found to develop very weak bond and hence low flexural strength. For a given shape of fibers, flexural strength of SFRC was found to increase with aspect ratio (ratio of length to equivalent diameter). Even though higher ratios of fibers gave increased flexural strength, workability of green SFRC was found to be adversely affected with increasing aspect ratios. Hence aspect ratio is generally limited to an optimum value to achieve good workability and strength. Grey suggested that aspect ratio of less than 60 are best from the point of handling and mixing of fibers, but an aspect ratio of about 100 is desirable from strength point of view. Schwarz however suggested aspect ratio between 50 and 70 is more practicable value for ready mix concrete. In most of the field applications tried out to date, the size of the fibers varies between 0.25 mm and 1.00mm in diameter and from 12mm to 60mm in length, and the fiber content ranged from 0.3 to 2.5 percent by volume. Higher contests of fiber up to 10% have also been experimented. Addition of steel fibers up to 5% by volume increased the flexural strength to about 2.5 times that of plain concrete. As explained above, mixing steel fibers considerably improves the structural properties of concrete, particularly tensile and flexural strength. Ductility and post cracking strength, resistance to fatigue, spalling and wear and tear of SFRC are higher than in the case of conventional reinforced concrete. SFRC is therefore found to be a versatile material for the manufacture of wide varieties of precast products such as manhole covers, slab elements for bridge decks, highways, runways, and tunnel linings, machine foundation blocks, door and window frames, piles, coal storage bunkers, grain storage bins, stair cases and break waters. Technology for this manufacture of SFRC light, medium and heavy duty manholes covers has been developed in India by Structural Engineering Research Centre, Chennai. Field experiments with two percent of fiber content indicated that SFRC runway slabs could be about one half the thickness of plain concrete slabs for the same wheel load coverage. Cement Research Institute of India (CRI) also demonstrated the use of SFRC in one of the jet bays at Delhi airport.
Ultra-High Performance Concrete
Advances in the science of concrete materials have led to the development of a new class of cementitious composites called ultra-high performance concrete (UHPC). The links above will direct you to pages detailing UHPC projects, bridges with UHPC components, articles that focus on UHPC research, and the main contact at Turner-Fairbank Highway Research Center (TFHRC) for UHPC.
The mechanical and durability properties of UHPC make it an ideal candidate for use in developing new solutions to pressing concerns about highway infrastructure deterioration, repair, and replacement. Since 2000, when UHPC became commercially available in the United States, a series of research projects has demonstrated the capabilities of the material. A handful of State departments of transportation have deployed UHPC components within their infrastructure, and many more are actively considering the use of UHPC.
UHPC is a cementitious composite material composed of an optimized gradation of granular constituents, a water-to-cementitious materials ratio less than 0.25, and a high percentage of discontinuous internal fiber reinforcement. The mechanical properties of UHPC include compressive strength greater than 21.7 kilopounds per square inch (ksi) (150 Megapascals (MPa)) and sustained postcracking tensile strength greater than 0.72 ksi (5 MPa). Ultra-high performance concrete has a discontinuous pore structure that reduces liquid ingress, significantly enhancing durability compared to conventional and high-performance concretes.
UHPC is being considered for use in a wide variety of highway infrastructure applications. The high compressive and tensile strengths allow for the redesign and optimization of structural elements. Concurrently, the enhanced durability properties facilitate a lengthening of design life and allow for potential use as thin overlays, claddings, or shells. In the United States, UHPC has been used in prestressed concrete girder simple-span bridges, precast concrete deck panels, and field-cast connections between prefabricated bridge components. Figure 1 shows the first UHPC highway bridge constructed in the United States.
Advantages of Fibre-Reinforced Concrete
Concrete is an integral part of any construction project, whether you are building roads, a a ground floor in your private home, or a power plant. This is because concrete is very durable, but it can be susceptible to fractures as subsoil frosts and thaws, if it shifts, or if tree roots press upward on the concrete. This can be a major problem for construction projects, as fractures lead to costly repairs and could spell disaster. The solution to this problem is the introduction of reinforced concrete.
What Does It Mean for Concrete to Be Reinforced?
Concrete is reinforced when it has suitable fibres in the mix to increase its toughness and ductility. Unlike non-reinforced concrete that is likely to break down when it fractures and cracks, reinforced fibre concrete will maintain its structural integrity, as it is held together by these fibres when a crack develops.
Suppose you are looking to embark on a construction project. In that case, you must analyse the advantages and disadvantages of using fibre-reinforced concrete, and the different types of fibres used. Trusted professionals like SB Civil Engineering would take this burden off you and ensure your project is completed hitch-free.
The advantages of fibre-reinforced concrete include the following:
Fibre-reinforced concrete has more tensile strength when compared to non-reinforced concrete.
It increases the concrete’s durability.
It reduces crack growth and increases impact strength.
Fibre-reinforced concrete improves resistance against freezing and thawing.
Reinforcing concrete with fibre increases fatigue strength.
The disadvantages of fibre-reinforced concrete include the following:
Rain might expose the fibres.
Fibres randomly orient in the concrete and could result in poor quality concrete, if not uniform.
Reinforced concrete is about 10% to 15% costlier than non-reinforced concrete.
Types of Fibre-Reinforced Concrete
There are several types of fibres used in reinforced concrete. Descriptions of the most common types follow.
Cellulose Fibres. These fibres are made with esters of cellulose or ethers obtained from plant leaves, wood, tree barks, or other plant material. The mechanical properties of these fibres can be altered by introducing lignin and hemicellulose in varying proportions. Cellulose fibres are majorly used in the textile industry as fibre reinforcement composites and chemical filters.
Natural Fibres. This fibre type is efficient and cost-effective; it is highly recommended as it is easy to source locally and readily available. Natural fibres can be obtained from a mineral source, animals, or vegetables, and processed into nonwoven fabric. The use of fibre in construction is not a new development, as horsehair and straws have been used in making plaster and bricks.
Carbon Fibres. This type of fibre is composed mainly of carbon atoms with a diameter of 5 micrometres to 10 micrometres. There are many advantages to using carbon fibres; they include, but are not limited to, the following:
It has a low thermal expansion.
It has high chemical resistance.
Carbon fibres have a high temperature tolerance.
They are stiff, low weight, and have a high tensile strength.
Polyester Fibres. Polyester fibres are the preferred choice for warehouse and other industrial floors, pavements, and precast products. Polyester macro and microfibres are mixed with concrete to ensure structural integrity, toughness, and to protect against plastic shrinkage cracks.
Glass Fibres. Glass fibre shares many mechanical features and properties with other fibres like carbon fibre and polymer fibre. This type of fibre is less brittle when used in composites but not as rigid compared to carbon fibre. As a result, glass fibres are used as reinforcing agents for many polymer products, an example of glass-reinforced plastic, also known as fibreglass.
Polypropylene Fibre. Polypropylene, or PP, is a type of fibre used in concrete because it is resistant to drying shrinkage and plastic shrinkage. This fibre helps reduce water bleeding in concrete and reduces the concrete’s permeability significantly. Polypropylene fibre is a synthetic white rugged material with good insulating properties and has high resistance against chemicals like organic solvents, acids, and alkalis.
Steel Fibre. The right amount of steel fibre in concrete can change the concrete’s physical properties qualitatively. Adding steel fibre to the mix significantly increases the concrete’s durability, resistance to cracking, resistance to bending, strength, and toughness.