Frequently asked questions

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Steel fiber reinforced concrete is an alternative to traditional reinforced concrete for certain application areas. Steel fibers are a discontinous, 3-dimensionally orientated, isotropic reinforcement, once they are mixed into the concrete. Steel fibers bridge the crack at very small crack openings, transfer stresses and develop post crack strength in the concrete.

A variety of fiber types (material, shape, size...) are available, their effect in concrete varies. Therefore steel fiber reinforced concrete shall never be simplified as a “concrete with steel fibers”. Steel fibers may be divided into five groups:

  • Type I - cold-drawn wire
  • Type II - cut sheet
  • Type III - melt-extracted
  • Type IV - shaved cold drawn wire
  • Type V - milled from blocks

The vast majority belongs to group I. the common and most performing anchorage type is the “hooked end”. For the same type of fiber, length/diameter and tensile strength have the biggest influence on fiber performance. The higher the l/d ratio, the better the performance of the steel fiber reinforced concrete.

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For indoor applications such as tunnels and warehouses, no. For outdoor applications such as pavements some minor rusting may occur. Experience in highways and industrial pavements indicate that while individual fibers corrode at the surface, staining of the concrete surface does not occur. Overall aesthetics and serviceability are maintained even with the presence of individual fiber corrosion. Indoor Applications-Surface fibers in typical indoor tunnels or manufacturing floor applications remain bright and shiny under normal environmental conditions.

Outdoor Applications Without cracks-experience has shown that concrete specified with a 28-day compressive strength over 3000 psi, mixed with standard water/cement ratios, and installed with methods that provide good compaction, limit the corrosion of fibers to the surface skin of the concrete. When surface fibers corrode, there is no propagation of the corrosion more than 0.008” beneath the surface. Since the fibers are short, discontinuous, and rarely touch each other, there is no continuous path for stray or induced currents between different areas of the concrete. Outdoor Applications With cracks-laboratory and field-testing of cracked SFRC in environments containing chlorides has indicated that the cracks in concrete can lead to corrosion of the fibers passing across the crack. However, small cracks (crack widths < 0.008”) do not allow corrosion of steel fibers passing across the crack. If the cracks wider than 0.008” and are limited in depth, the consequences of this localized corrosion are not structurally significant.

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The post crack strength of steel fiber concrete is a material property which is commonly used to differentiate fiber performance. This will typically be determined with a bending test and is often referred to as the residual flexural strength (see below). For the same concrete composition, steel fiber performance is a function of fiber length, diameter, aspect ratio, anchorage and tensile strength. A fiber dosage alone has no performance related value at all. AStM international has two flexural testing procedures for fiber reinforced concrete. The two testing procedures are AStM c1399 Standard test Method for obtaining Average residual-Strength of Fiber-reinforced concrete and AStM c1609 Standard test Method for Flexural Performance of Fiber-reinforced concrete. Bekaert propose the use of AStM c1609. AStM c1399 can lead to inflated post crack flexural strengths due to favorable fiber orientation and the use of a steel plate to control the first crack’s energy release.

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The residual flexural strength is equal to the post crack flexural strength of steel fiber concrete corresponding to a certain deflection in a beam bending test. It is a value from testing which has been introduced for the design of steel fiber concrete.

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Steel fibers are not a replacement of synthetic micro fibers and vice versa. Both fiber types provide very different properties to concrete so that the applications fields do not overlap. Rather than a substitute, both fiber types may be used complementary. While steel fibers offer post crack strength and thus act as reinforcement, synthetic micro fibers reduce cracking due to plastic shrinkage and improve the fire resistance of concrete. They do not provide any reinforcing effect.

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Mechanically anchored steel fibers have been proven as reinforcement, even for structural application. Steel fibers are made from a material with well known engineering properties; e modulus, Poisson’s ratio, tensile strength and creep. The e-modulus of steel is greater than that of concrete. Thus, the steel fibers pick up the stresses quickly and affect the cracking process immediately. The long term load carrying capacity of the steel fiber reinforced concrete is significant. Steel fibers have a material specification of AStM A820. Macro synthetic fibers come in a large variety and have very different material properties. Macro synthetic fibers do not have a material specification in AStM. All macro synthetic fibers do have an e-modulus lower then that of concrete and relatively low tensile strengths. Thus, macro synthetic fibers need a certain crack width to occur prior to being able to engage in the concrete and then only moderate post crack strength values can be achieved. Macro synthetic fibers are also subject to creep which makes the long term loading capacity of the fiber lower or non existent. The rate of creep can be increased with increased ambient temperatures.

There are at least four factors to review when considering reinforcement; Modulus of elasticity, Poisson’s ratio, tensile strength and creep.

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Steel fiber concrete compresses the construction schedule, allows for alternative construction methods or design solutions and increases durability. When a project is delivered quicker with fewer efforts and labor, the higher costs of the steel fibers are overcompensated by the savings.

In certain applications the volume weight of steelfibers is lower as the rebar for a similar reinforcing effect. For those applications the cost/reinforcement is lower for steelfibers.

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