Steel-Fibre-Reinforced Concrete: Post-Cracking Strength and Behaviour

The steel fibres used in concrete are short steel elements obtained by various production methods. Fibres produced from cold-drawn wire are one such type. The fibres are added to the mix; their distribution and orientation are affected by the mix, the mould geometry and the placing conditions. The length, diameter, aspect ratio, tensile strength and end geometry of the fibre are checked against the technical specifications of the selected product.

The aspect ratio is the ratio of the fibre length to its diameter. The geometry of the fibre, its end shape, tensile strength, bond with the concrete and distribution together affect performance. Hooked ends can improve anchorage; the appropriate fibre is not selected on the basis of aspect ratio or end shape alone.

The main contribution of steel fibre is post-cracking load-bearing capacity and toughness. Adding fibres does not by itself raise the compressive strength class of the concrete. Compressive strength is determined by testing and depends on the mix and production conditions; the post-cracking performance of fibre-reinforced concrete is also assessed separately.

The contribution of the fibres becomes evident particularly after a crack has formed: by bridging the crack, they limit its opening and help to maintain load transfer. The level of this contribution depends on the fibre type, dosage and orientation and on the properties of the concrete.

Transport and installation damage can be reduced. With appropriate design and production, fibres can help to reduce the risk of edge breakage and chipping. This contribution does not replace correct lifting and installation practice.

Crack opening can be limited. Crack control can help to reduce the ingress of water and harmful substances into the concrete. Watertightness and service life are assessed together with the cross-section, joints, environmental effects and installation conditions.

Early-age cracking is assessed separately. In the control of plastic shrinkage, the fibre type, size and dosage are important. Fibres used for this purpose are considered together with appropriate curing and evaporation control; the same effect is not to be expected from every steel fibre.

The production process can be adapted. In some applications verified by design and testing, fibres can be used in place of part or all of the conventional reinforcement. In some products, on the other hand, they are considered together with reinforcement. Reducing or eliminating the reinforcement cage is not decided on the basis of fibre addition alone.

Fibre dosage is stated in kg/m³; the appropriate quantity is determined according to the cross-section of the product, its manufacturability and the targeted post-cracking performance. When the fibre content changes, workability and distribution are also assessed.

To assess the post-cracking flexural performance of fibre-reinforced concrete, the notched beam test under TS EN 14651 can be used. The properties of the fibres, the standards to which the product is subject and the acceptance tests of the final element are considered together; a single material test does not on its own demonstrate the suitability of the product.

• Precast concrete and reinforced concrete pipes: Particularly in microtunnelling (jacking) pipes.
• Inspection manhole and property connection chamber units: In base units, shaft units and cones.
• Box culvert and U-channel units: In applications whose suitability has been verified by section design and performance testing.
• Industrial concrete floors: Warehouse and facility floors where joint spacing can be increased.
• Other applications: Airport aprons, port areas, tunnel shotcrete, dam and canal linings.

For buried infrastructure products, in addition to fill and traffic loads, the stresses during installation, ground settlement and compaction of the fill are also assessed.

In microtunnelling jacking pipes, alignment deviations and the high forces transferred to the end faces can cause local stresses. Fibres can contribute to crack control in these zones. The permissible jacking force, the end face and joint design and the installation tolerances are determined separately.

None of this replaces the standard. TS 821 EN 1916 remains in force for concrete pipes and TS EN 1917 for inspection manhole units; steel fibre is assessed within these standards as part of the design.

With appropriate design and production, steel fibres can improve post-cracking behaviour and load-bearing performance. Appropriate fibre selection, section design and performance checks are considered together.

The fibre type and dosage suitable for your product are determined according to the cross-section and loading conditions; you can obtain support from our technical team on this matter.