No, fibre-reinforced concrete doesn't replace rebar in general. Micro fibres (the "fibre mesh" added at the plant) only help with early plastic shrinkage cracking and don't replace any steel. Macro synthetic and steel fibres can stand in for light crack-control steel, such as wire mesh, in some slabs on grade, but only when the slab is designed for them. They don't replace the structural rebar in footings, walls, beams, columns or suspended slabs.
Part of the confusion is the word "fibre". It gets used for three different things on Ontario job sites, and one of them, fiberglass rebar, really is a bar replacement. Here's how to tell them apart and when each one does the job.
Three different things called "fibre"
| Product | What it is | What it replaces |
|---|---|---|
| Micro synthetic fibres ("fibre mesh") | Fine polypropylene fibres, roughly 12 to 19 mm long, mixed into the concrete | Nothing. Reduces plastic shrinkage cracking while the concrete is still setting |
| Macro synthetic or steel fibres | Larger, stiffer fibres, typically 38 to 50 mm long for macro synthetics, dosed to a design | Can replace light crack-control reinforcement (wire mesh, light bar) in designed slabs on grade |
| GFRP rebar (fiberglass rebar) | Solid bars of glass fibre in resin, made to CSA S807 | Steel rebar, one bar for another, in a design made for GFRP |
Lengths for synthetic fibres are typical ranges from industry technical guidance; check the product data for the fibre actually specified.
So "fiberglass instead of rebar" and "fibre concrete instead of rebar" are two separate questions. The first is about a different bar material. The second is about whether you need bars at all.
What micro fibres do (and don't do)
Micro synthetic fibres are the most common fibre in residential concrete. They are added at the batch plant and help resist the fine plastic shrinkage cracks that form in the first hours, while the surface is drying faster than the concrete below.
What they don't do is hold cracks closed once the concrete has hardened. Drying shrinkage, temperature changes and loads can open cracks later, and micro fibres at normal dosages offer essentially no resistance to that. That's the job of reinforcement, whether bars, mesh or properly designed structural fibres.
Micro fibres and rebar aren't competitors. Plenty of reinforced slabs, driveways and garage floors use both: micro fibres in the mix to help with early surface cracking, and rebar or mesh on chairs to hold later cracks tight and carry load where the design needs it. Adding fibres to a slab detailed with rebar is usually fine; removing the rebar because fibres were added is not.
If your concrete ticket says "fibre", ask which kind and at what dosage. A small dose of micro fibres is not a substitute for the rebar or mesh on your drawings.
Macro synthetic and steel fibres
Macro synthetic fibres and steel fibres are a different class. They are larger and stiffer, they're dosed at much higher rates, and they're distributed through the full depth of the slab. After the concrete cracks, they bridge the crack and carry some tension across it, which is what lets them take over the role of light crack-control steel.
Where they're used well:
- Slabs on grade such as warehouse floors, shop floors, garage slabs and some driveways, designed for fibre.
- Places where placing steel is awkward, such as thin slabs where a mat is hard to keep at the right height.
- Shotcrete and some precast work, where fibres are a standard part of the mix design.
How fibre concrete is specified matters. Performance is normally set by testing, such as the ASTM C1609 flexural performance test, and fibre concrete is classified under ASTM C1116. The designer converts the reinforcement the slab needs into a fibre type and dosage, usually with the fibre manufacturer's data. "Add fibres" on its own isn't a specification.
Where fibres can't stand in for rebar
Even macro and steel fibres have clear limits. On houses and small buildings in Ontario, keep rebar wherever:
- Part 9 prescribes bars. Flat ICF walls, reinforced block foundation walls and cold room slabs all have bar requirements in the 2024 Ontario Building Code, and the ICF and cold room rules call specifically for steel bar to CSA G30.18. A fibre mix doesn't satisfy them. See foundation wall rebar in Ontario.
- The element is structural. Footings designed with rebar, grade beams, columns, beams, lintels and suspended slabs carry load through their bars. Replacing that steel with fibres would be a redesign, not a substitution.
- There are dowels, ties or edge bars. Dowels into a foundation wall, bars across a construction joint and continuous bars in a thickened edge are discrete bars with a specific job. A fibre slab usually still has them.
- The drawings say rebar. Any change from rebar to fibres needs the designer's or engineer's approval, and on permitted work the building department may want to see it.
For an ordinary floor on ground, Section 9.16 of the OBC doesn't require reinforcement at all, which is why fibres often show up there. Our post on slab-on-grade rebar requirements explains when a slab moves into design.
Fiberglass rebar: a bar, not a mix
GFRP rebar is the real "fiberglass instead of rebar" option. It's a solid, deformed or sand-coated bar made of glass fibres in a resin, and it's placed, tied and lapped like steel bar. It doesn't rust, which is why it's used in parking structures, bridge decks, marine work and other places where chlorides attack steel.
It isn't a drop-in swap, though:
- It's much less stiff than steel, so designs are usually governed by deflection and crack width, and often need more or larger bars.
- It can't be bent on site. Bends are made at the factory, so every L-bar, hook and stirrup has to be ordered in shape.
- It follows its own standards. Bars are made to CSA S807, and buildings are designed with CSA S806. See does fiberglass rebar meet code and GFRP vs steel rebar.
Practical trade-offs on a slab
When fibres are a permitted option, the trade-offs are mostly on site:
- No mat to place or keep at height. Fibres are already in the concrete, so there's nothing to walk into the subgrade during the pour. This is the strongest argument for them.
- Finishing. Some fibres can show at the surface, and the finisher may need to adjust methods.
- Pumping and mix. High fibre dosages change how the concrete flows; the ready-mix supplier should be part of the decision.
- Cost. Fibres move cost from steel and labour into the concrete. Whether that works out depends on the slab, the dosage and the crew. Compare it against a rebar or mesh option using the same slab plan; our wire mesh vs rebar cost comparison shows what to include.
Rebar and GFRP for your job
We supply and fabricate steel rebar in 10M to 55M, including dowels, edge bars, stirrups and custom bent shapes, and we supply GFRP rebar. If your designer has kept rebar in the slab, footings or walls, or is pricing a GFRP option, send the drawings and we'll prepare the bar list and quote.
Send your drawings for a quote, or see our rebar products, including GFRP.
Frequently asked questions
Does fibre in concrete replace rebar?
Only in limited cases and only when designed for it. Macro synthetic or steel fibres can replace light crack-control steel such as wire mesh in some slabs on grade. They don't replace structural rebar in footings, walls, beams, columns or suspended slabs, and micro fibres don't replace any reinforcement.
Does fibre concrete need rebar?
It depends on what the element does. A fibre slab on grade designed for crack control may have no bars in the field of the slab, but dowels, edge bars and any structural parts still need the rebar shown on the drawings.
Is fiberglass rebar the same as fibre concrete?
No. Fiberglass (GFRP) rebar is a solid bar made of glass fibres in resin that replaces steel bar one for one in a design. Fibre-reinforced concrete has short loose fibres mixed into the concrete. They solve different problems and follow different standards.
Can I use fiberglass instead of rebar?
GFRP bars can replace steel rebar in an engineered design made for them, typically where corrosion is the concern. They are not a simple swap on a drawing detailed for steel, and Part 9 prescriptive rules that call for rebar assume steel bar to CSA G30.18.
