Concrete Tip of the Day #8: Fibre Reinforced Concrete – What You Need to Know

Blog Title: Understanding Fibre Reinforced Concrete – Don’t Swap Without a Plan

Fibre reinforced concrete (FRC) has become increasingly common on construction sites, with a wide range of options now available—from cellulose and polypropylene to aramid and steel fibres. While they might all seem similar at a glance, don’t be fooled: not all fibres behave the same way in concrete.

Why Use Fibres at All?

Concrete on its own is brittle and weak in tension. Reinforcement—whether it’s steel mesh or discrete fibres—is used to limit crack widths and provide ductility. But the real trick is in understanding when the reinforcement kicks in. It only becomes active after cracking starts. That’s why the stiffness of the fibre, known as Young’s Modulus, matters so much—it determines how well it limits the width of those cracks.

Here’s a quick look at typical Young’s Modulus values for different fibre types:

In short, polypropylene fibres are about 3% as stiff as steel—meaning they allow much wider cracks under the same load. So swapping steel with synthetic fibres (or even switching between synthetics) without engineering approval can spell trouble.

The Golden Rule

🛑 Never change from one type of fibre to another—or from mesh to fibre—without the approval of the designer or supplier. Even at the same dosage rate, results can differ significantly depending on fibre material, shape, and bond characteristics.

Fibre Types in Practice

1. Micro-synthetic (Polypropylene) Fibres

• Control plastic shrinkage cracking

• Dosage: ~0.9 kg/m³

• Reduce bleed water but may decrease slump

• Incorrect jointing = wide uncontrolled cracks

2. Cellulose Fibres

• Also target plastic shrinkage

• Dosage: Up to 1.8 kg/m³

• Enhance freeze-thaw durability

• No joint spacing benefits—must cut joints properly

3. Macro-synthetic Fibres (Aramid, Nylon)

• Used as an alternative to mesh or steel

• Non-corrosive but prone to wider cracks due to creep

• Design methods still developing—handle with care

4. Steel Fibres

• High stiffness, excellent post-crack control

• Not interchangeable across brands or types

• May show on the surface

• Prefer pump-grade mixes and vibrating screeds for best finish

  
  

Best Practices

✅ Use the correct mix for fibre type—don’t just toss fibres into a standard mix✅ Control slump—especially with steel fibre mixes✅ Cut joints early and accurately to manage shrinkage✅ Ask for documentation from the fibre supplier (Producer Statement, design details)✅ Avoid site modifications to fibre dosage or type without technical advice

Final Thought

Fibres are powerful tools in the hands of knowledgeable professionals—but risky shortcuts or substitutions can lead to cracking, durability loss, and liability headaches. When it comes to fibre reinforced concrete, stick to the plan, follow the spec, and don’t guess.

✅ Stay tuned for the next episode of Concrete Tip of the Day!