Steel fibers can be categorized into five major categories, according to their method of production:
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Group I: cold-drawn wire
The most used group in various applications as it is the best type in performing with concrete.
Group II: cut sheet
Used in a considerable types of applications.
Group III: melt extracted
Not used in much applications
Group IV: shaved cold drawn wire
Used in a considerable types of applications.
Group V: milled from blocks
Not used in much applications.
This paper is focused on Group I as it’s the most common type for reinforced concrete use. Steel fiber could come in different shapes and sizes as follows:
-Shape: straight, hooked, undulated, crimped, twisted, coned …
-Length: typically, from 30 mm to 60 mm
-Diameter: typically, from 0.4 mm to a maximum of 1.3 mm
The performance of steel fiber reinforced concrete is affected by the shape and the length of steel fibers. Longer fibers and smaller diameters would have better performance as they have more anchorage length. The ratio of fiber length to diameter could provide ideal approximate calculation of fiber performance. Anchorage type change could change the shape of the load deflection curve of the steel fiber concrete. The product standards for steel fiber makes it easy to find a summary of fiber properties quickly as well as performance from the mandatory CE-label. For additional information.
Form Factors: The higher the aspect ratio between length/diameter the better the performance of the mix. However, this could cause balling which would limit the ability of mixing higher dosage
As a result, a relationship between high l/d rations and the great quantity of single fibers is derived. Hence more attention is required to mix design. 20 kg/m³ or 15kg/m³ of high- perform fibers could easily do the work of 40 Kg/m3 of easy-mix fibers (low l/d). Systems have been developed to avoid balling and to insure optimal distribution which can secure mixing 100 kg/m3 of high-performing fibers perfectly with concrete.
(A) Loose Steel Fiber
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With low aspect ratio loose steel fiber should not face the problems of workability and balling.
Example of Loose steel fiber:
The easy-mix type offers a length of 50 mm at a theoretical l/d of 45. Since its low aspect ratio, its performance is relatively low and its workability is relatively high.
Using blower blast equipment to add fibers to concrete: For high aspect ratios like, extra ways should be used so that can be effortlessly and efficiently added to the concrete like blower blast. Blowing loose fibers with aspect ratios more than 70 usually tends to cause issues like balling. Blowing is different from dosing as dosing requires precise weight of fibers.
(B) Glued Steel Fiber
Glued fiber technology is developed to prevent balling due to additional loose fiber of high aspect ratio. Once the mixing begins, the glued steel fiber starts slowly to separate to insure a homogeneous mix.
Example of glued steel fiber bundles
It is possible to add glued steel fiber bundles from the bag directly to the central mixer or mixing truck. They can also be added indirectly through a conveyor belt. Automatic dosing is likewise reachable. There are two methods to categorize fibers according to their modulus of elasticity or their origin. In the view of modulus of elasticity, fibers can be classified into two basic categories, namely, those having a higher elastic modulus than concrete mix (called hard intrusion) and those with lower elastic modulus than the concrete mix (called soft intrusion). Steel, carbon and glass have higher elastic modulus than cement mortar matrix, and polypropylene and vegetable fibers are classified as the low elastic modulus fibers. High elastic modulus fibers simultaneously can improve both flexural and impact resistance; whereas, low elastic modulus fibers can improve the impact resistance of concrete but do not contribute much to its flexural strength. According to the origin of fibers, they are classified in three categories of metallic fibers (such as steel, carbon steel, and stainless steel), mineral fibers (such as asbestos and glass fibers), and organic fibers. Organic fibers can be further divided into natural and man-made fibers. Natural fibers can be classified into vegetable origin or sisal (such as wood fibers and leaf fibers), and animal origin (such as hair fibers and silk). Man-made fibers can also be divided into two groups as natural polymer (such as cellulose and protein fibers), and synthetic fibers (such as nylon and polypropylene).
The Benefits of Steel Fiber
For the same mixing and construction, there is no need to add equipment or increase the construction cost of metal fibers for concrete due to the low steel fibre cost.
Under the same strength requirement, metal fibers for concrete can replace steel mesh and reduce the pouring thickness of concrete by 15-25%.
Improve the construction conditions, mixing vehicles and construction personnel can reach the site directly, and shorten the construction period by 30%. Steel fiber reinforced concrete has high tensile strength, which can prolong the distance between contraction joints and facilitate continuous and rapid pouring of concrete.
Convenient construction, saving labor, greatly shortening the construction period and saving the total steel fibre cost.
The Working Principle of Steel Fiber
The working principle of steel fibers for concrete reinforcement involves reinforcing the concrete matrix in three dimensions using steel fibers. These metal fibers, with their specific tensile strength, aspect ratio (length/diameter), and anchorage, play a crucial role in enhancing concrete performance.
By being distributed throughout the concrete, steel fibers effectively restrain the formation of micro-cracks and help redistribute the accumulated stress resulting from applied loads and shrinkage. When cracks do occur, the steel fibers intercept them promptly and inhibit their growth, significantly reducing the likelihood of further crack propagation. This proactive approach ensures that the concrete remains strong and durable, minimizing the risk of structural failure.
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