Construction Fiber for Concrete

Construction fiber is mixed into concrete to control cracking. Pick the type by job.

Updated Oct 28, 2026. Edited by: Ilyass Ben Hakim.

Where to buy

Home centers sell small bags of micro fiber for DIY mixes. For truck loads, the ready mix plant can add fiber. Masonry suppliers stock macro and steel fibers.

Professional insight

Concrete contractors pick fiber by the crack they want to stop. Shrinkage cracks and structural cracks need different fibers.

Micro fibers control plastic shrinkage, not crack widening.
Macro fibers give residual strength after cracking.
Fiber never replaces engineered rebar unless the design says so.
Dose by pounds per cubic yard, per the bag.

Check the label

Fiber material: polypropylene, nylon, glass or steel.
Micro or macro type and fiber length.
Compliance with ASTM C1116, and A820 for steel.
Dose rate in lbs per cubic yard or kg per cubic metre.
Bag size matched to the volume you will mix.
Degradable bag type, if it goes straight in the drum.
Safety data sheet available from the maker.

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Types

Concrete fiber types
Type	Typical length	Main job
Micro synthetic	Under 1 inch	Plastic shrinkage cracks
Macro synthetic	1.5 to 2 inches	Post crack strength in slabs
Steel fiber	1 to 2.5 inches	Industrial floors and shotcrete
Glass fiber	Short strands	Thin precast and GFRC

How to use it

1Check the dose for your slab or mix design.
2Add fiber with the aggregate or into the running drum.
3Mix for the time the maker states.
4Look for clumps before you pour.
5Finish as normal; a few surface fibers are expected.

Common questions

Can fiber replace wire mesh?: Macro fiber can in some slabs, if the design allows.
Will fibers show on the surface?: Some may. Proper finishing and timing keep most of them below.
Can I add fiber to bagged concrete?: Yes. Use a micro fiber sized for small batches.