Adding fibres to concrete is straightforward once you know the sequence: mix to a normal consistency first, add fibres gradually, then extend the mixing time. Get the order or timing wrong and fibres clump together ("ball up") instead of dispersing evenly, which weakens the reinforcement exactly where you need it. This guide covers the correct process step by step.
Why Does the Order Matter?
Fibres, whether polypropylene micro-fibres, steel fibres, or structural macro-synthetic fibres, need to be introduced into a mix that already has enough water and workability to accept them. Adding fibres to dry aggregate and cement before water goes in, or dumping the full dose into a running mixer all at once, are the two most common causes of clumping.
Step-by-Step: Adding Fibres to Concrete
1. Confirm the Fibre Type and Dosage First
Check the manufacturer's recommended dosage rate for your specific fibre type and application. Dosage is specified in kg per m³, and it varies significantly between micro-synthetic, structural macro-synthetic, and steel fibres, so this is not a step to estimate by eye.
2. Mix to a Normal Workable Consistency First
Get the concrete mixed to a normal, workable slump before introducing any fibres. This gives the fibres something to disperse through, rather than fighting a dry, stiff mix.
3. Add Fibres Gradually
Sprinkle the fibres into the running mixer steadily over 1-2 minutes rather than adding the full dose in one go. This single change is the biggest factor in avoiding fibre balling, since a sudden dump of fibres tends to tangle before the mixer can separate them.
4. Extend the Mixing Time
Keep mixing for an extra 3-5 minutes after the last fibres go in, longer than you would for a standard unreinforced batch. This gives the fibres time to fully disperse through the mix rather than settling in clumps.
5. Check for Balling Before You Place
Inspect the discharge for visible fibre clumps before placing. If balling has occurred, correct the process on the next batch (slower addition, more mixing time) rather than placing a clumped mix, which will not reinforce evenly.
The More Reliable Alternative
Adding fibres correctly on-site is achievable but leaves room for error, particularly under time pressure on a live pour. Specifying fibre-reinforced concrete when ordering means the dose is batched and mixed correctly before the truck arrives, removing the mixing risk entirely. Our concrete admixtures guide covers how fibre reinforcement fits alongside other admixture types.
Frequently Asked Questions
Why do fibres clump together in concrete?
Fibre balling almost always comes down to three causes: adding the full dose too quickly, mixing a batch that is too dry when the fibres go in, or not mixing long enough afterward to disperse them. Adding fibres gradually to an already-workable mix, then extending mixing time, solves most balling problems.
Can I add fibres to ready-mixed concrete after it arrives?
It is possible on-site with a mixer truck's drum, adding fibres gradually while the drum continues rotating and extending mixing time before discharge. It is far more reliable to specify fibre reinforcement when ordering, so the correct dosage is batched in and mixed properly before the truck leaves the plant.
Do fibres replace the need for steel mesh?
In many applications, yes. Fibre-reinforced concrete is commonly used to replace mesh in ground-supported slabs. The fibre dosage and type must be engineered for the specific application, and a structural engineer should confirm equivalence with the mesh specification it is replacing. See our full guide to concrete reinforcement types for the detail.
Does Procon 24/7 supply fibre-reinforced concrete ready-mixed?
Yes. Our fibre-reinforced concrete service doses steel or polypropylene fibres on-site, so they are distributed evenly through every cubic metre before it reaches your pour, without the mixing risk of adding fibres on-site yourself.
