These fibres enhance the tensile properties of concrete, helping it withstand various stresses during service. The type and amount of fibres used depend on the specific application, as FRC is designed for multiple purposes. In some cases, composite fibres are used to target different performance outcomes of the concrete.

Key Benefits: 

• Crack Control – Helps limit the width of cracks during plastic shrinkage and drying phases.
• Increased Flexural Strength – Enhances post-crack load-bearing capacity of the concrete elements. 
• Reduced Early-Age Deformation – Minimises shrinkage and settlement issues. 
• Improved Impact Resistance – Provides toughness to withstand impact and blast forces. 
• Can be designed for Fire Resistance – Polypropylene fibers can be used to reduce explosive spalling in high strength concrete.
• Secondary Reinforcement – Can be used in place of traditional reinforcing in some applications. 

Types of Fibres in Concrete 

Fibre materials vary widely, but they are generally categorised into three main types: 

• Steel Fibres – High strength and stiffness, commonly used for structural applications. 

• Macro-Synthetic Fibres – Moderate strength, used for secondary reinforcement and crack control. 

• Micro-Synthetic Fibres – Primarily used to reduce plastic shrinkage cracking. 

Choosing the Right Fibre Type 

Different fibre types offer specific reinforcing properties. In some cases, a hybrid fibre blend is used to optimise performance across multiple service conditions. 

Fibre Geometry and Workability 

The shape and size of fibres significantly impact both fresh and hardened concrete performance. Key considerations include: 

• Aspect Ratio – Defined as fibre length divided by diameter, this affects how well fibres disperse in concrete. 

• Profile – Fibres may be straight, twisted, fibrillated, or ribbed, influencing bond strength and anchorage. 

• Packing and Dispersion – Fibres can be loose, bundled, or collated to optimise mixing and placement. 

While all fibres impact concrete workability, adjustments in mix design are undertaken to compensate for impact on workability. Typical rates of application can be up to 30kg/m3 for steel fibers and 4kg/m3 for synthetic fibers but will vary accordingly to specific project requirements.

Structural Performance 

Modern steel fibres offer improved anchorage, tensile strength, and ductility, allowing for enhanced structural performance. At moderate-high dosages (e.g., 25-30 kg per cubic metre), fibre reinforced concrete can achieve strain-hardening characteristics similar to conventional reinforced concrete. 

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