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Steel fibres

Steel fibres mixed into the concrete can provide an alternative to the provision of conventional steel bars or welded fabric in some applications. The concept has been in existence for many years (the first patent was applied for in 1874) and it has been used in a limited range of applications: among the first major uses was the patching of bomb craters in runways during World War II. However, it was during the 1970s that commercial use of this material began to gather momentum, particularly in Europe, Japan and the USA.

Today, industrial floors and pavements are major applications for steel-fibre-reinforced concrete. In the United Kingdom, several million m² of steel-fibre-reinforced slabs have been installed over the past ten years, both for ground-supported and pile-supported floors. Other major applications for fibre-reinforced concrete include external paved areas, sprayed concrete, composite slabs on steel decking and precast elements.

Fibres are often used to replace the nominal conventional steel fabric in ground bearing slabs. Steel fibres are increasingly being used in suspended ground floor slabs on piles to replace much, and in many cases all, of the reinforcement. Savings in the cost of supplying and fixing the conventional welded fabric reinforcement that is replaced can offset the extra cost of adding fibres to the concrete. There may also be health and safety benefits resulting from the reduced handling of reinforcement. In addition, problems caused by misplacement of conventional steel in the depth of the slab are avoided.

BS EN 14889-1, Fibres for concrete, Steel fibres - Definitions, specifications and conformity, classifies steel fibres into five Groups, according to the method of manufacture, as follows:

Group I  Cold-drawn wire
Group II  Cut sheet
Group III  Melt extracted
Group IV  Shaved cold drawn wire
Group V  Milled from blocks

Further methods of characterisation along with typical properties include:

Cross-section: .............................. Round, flat, crescent, etc
Deformations: ............................... Straight, wavy, end hook, etc.
Length:..........................................19-60 mm
Aspect Ratio (length/diameter):....... 30-100
Tensile strength............................. 345-1700 N/mm²
Young´s modulus........................... 205 kN/mm²

Some of the physical characteristics directly affect key aspects of performance while others are less important.

The properties of the fibres are covered by BS EN 14889, Fibres for concrete, Part 1, Steel fibres – Definition, specifications and conformity. One requirement is for the supplier to declare the unit volume of fibres in kg/m³ to achieve a residual flexural strength of 1.5 MPa at 0.47mm central deflection and/or a residual flexural strength of 1MPa at 3.0mm central deflection when tested in accordance with the proposed standard beam test.

Further information on the use of steel fibres may be found in Concrete Society Technical Report 63, Guidance for the design of steel-fibre-reinforced concrete.

Acknowledgement: The Concrete Society

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TR63 Guidance for the design of steel-fibre-reinforced concrete