FAQ - Fiber reinforced concrete

All you need to know about Steel Fiber Reinforced Concrete

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Yes, but expect a 0.4” to 1.2” slump loss through the hose depending on the steel fiber dose rate, ambient temperatures and hose length. A midrange water reducing agent (MrWr) is commonly used to enhance workability and ease of flow through pump lines. High-range water reducers (HrWr) may be required in some cases. Typically, a 4” diameter hose is required.

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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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Fibers can only protrude from forms where there is a joint. They can not protrude in the middle of a form. This can be minimized if the joints are caulked before concrete placement. However, it is not always possible to calk every joint. The number of protruding fibers is a function of the precision of the joints and the fiber dosage.

Wider joints will catch more fibers than tighter joints. After the formwork is removed, the fibers can be quickly knocked down with a hand sanding block or a small angle grinder.

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For cast insitu, internal vibration is the most used option to consolidate the concrete. Form vibration is generally used in the precast industry.

When steel fiber concrete is cast into form work a small amount of vibration of the forms helps keep the fibers from touching the forms and thereby from being visible when the forms are removed. For example, during casting of steel fiber reinforced precast structures, the forms are vibrated to consolidate the concrete. This action results in an almost fiber free surface of the structures. So allowing a short period of form vibration in the all cast-in-place structures, in addition to internal vibration where possible, will provide the best finished surface.

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No failures of the plastic liner due to fiber punctures have ever been identified. the abrasion from sharp aggregates during placement of the concrete poses just as big of a threat to the liner as do the steel fibers. After placement the fibers tend to move around and re-orient themselves during vibration which relieves any pressure of an individual fiber on the liner created during placement. Many projects using SFRC are constructed with cast-in-place and sprayed shotcrete directly in contact with waterproof membranes.

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Typical steel fiber reinforced concrete contains less than 0.5% vol. Steel fibers and hardly more than 0.75% vol. Those fibers are discontinuous and not connected to each other. Tests only show a slight decrease in electrical resistivity due to the addition of steel fibers. However, the resistance to current flow is still substantial. Effects from moisture content and aggregate composition are much more dominant than the addition of steel fibers.

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Not more than 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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