GFRP (glass fibre reinforced polymer) rebar is very good at one thing steel cannot do: it does not rust. It is also light, non-conductive and non-magnetic, with high tensile strength. Its disadvantages are just as real: it is about a third as stiff as steel, it breaks without yielding, it cannot be bent or welded on site, it loses a large share of its strength at bends and at high temperatures, and it needs an engineered design rather than a bar-for-bar swap. Fiberglass rebar is a good product for the right job and a frustrating one for the wrong job.
This post goes through each advantage and disadvantage and what it means in practice. For a side-by-side table of properties, see our GFRP vs steel rebar comparison.
GFRP rebar pros and cons at a glance
| Pros | Cons |
|---|---|
| Does not corrode, even with road salt or seawater | Low stiffness: more deflection and wider cracks for the same bar |
| High tensile strength | Brittle: no yielding before rupture |
| A fraction of the weight of steel | No field bending; bends are factory-made |
| Non-conductive and non-magnetic | Bends are much weaker than straight bar |
| No coatings or cathodic protection needed | Weaker at high temperature; a fire design issue |
| Long service life in harsh exposures | Engineered design required; not in Part 9 prescriptive rules |
| Easy to cut with a saw | Not weldable; limited compression contribution in design |
| Usually higher cost per metre; longer lead times for bent shapes |
Advantages of fiberglass rebar
It does not rust. This is the reason GFRP exists. Steel in concrete corrodes once chlorides or carbonation reach it, and rust expansion cracks and spalls the concrete. GFRP takes that failure mode away, which is why Ontario's Ministry of Transportation has specified it in bridge decks since 2005 and lists it with stainless steel as premium reinforcement for exposed elements.
High tensile strength. Grade III GFRP bars have guaranteed tensile strengths well above the yield strength of Grade 400W steel. In practice this headroom is rarely used, for the reasons covered under stiffness below.
Light weight. A GFRP bar weighs roughly a quarter to a third of a steel bar of similar size. Crews carry and place more bar with less strain, and deliveries are lighter.
Electrically and magnetically neutral. GFRP does not conduct electricity or interfere with magnetic fields, which matters for MRI rooms, some electrical and utility infrastructure, and concrete near sensitive instruments.
No coatings to protect. Epoxy-coated bar has to be handled gently and patched; GFRP has no coating to damage. Coated steel needs padded slings, coated chairs and patching; GFRP needs ordinary care and good tie-downs.
Long service life where steel struggles. For a parking deck, bridge barrier or marine structure, avoiding decades of corrosion repairs can outweigh a higher bar price.
Disadvantages of fiberglass rebar
Low stiffness. Grade III GFRP has a modulus of elasticity of at least 60 GPa, against about 200 GPa for steel. Under the same stress it stretches about three times as much, so a slab or beam reinforced with the same amount of GFRP deflects more and cracks wider. Designers compensate with more bar, closer spacing or deeper sections. This is why a "15M GFRP for 15M steel" swap does not work.
No yielding. Steel stretches visibly before it fails, giving warning and letting a structure redistribute load. GFRP stays elastic until it ruptures. Design standards respond with different safety margins and by limiting stress under sustained load, because GFRP can also fail over time under high permanent stress (creep rupture).
No field bending. Every hook, corner, stirrup and dowel is formed at the factory before the resin cures. A missing or wrong bend cannot be fixed with a bender on site. That puts a premium on complete detailing and on ordering early.
Bends are weaker. The bent portion of a GFRP bar carries much less load than the straight portion. In MTO's draft GFRP material specification (OPSS.PROV 1640, November 2023 draft), the minimum bend strength for Grade III bent bars is less than half the straight-bar strength for each size. Stirrups and hooks are designed with that in mind; see our post on GFRP stirrups and dowels vs steel.
Heat and fire. The polymer resin softens at temperatures far below those that weaken steel. That limits GFRP in elements that need a fire-resistance rating unless the design addresses it, and it is one reason GFRP is more common in bridges and exposed civil work than inside buildings.
Engineered design only. GFRP bars are made to CSA S807 and designed under CSA S806 for buildings or CSA S6 for bridges. Part 9 of the Ontario Building Code writes its prescriptive reinforcement rules around steel bar, so a house or small building using GFRP needs an engineered design that the building department accepts. Our post on fiberglass rebar and CSA S807 and S806 explains which rules apply where.
Little credit in compression. Canadian FRP design rules (CSA S806 and the FRP section of CSA S6) have long ignored the compression resistance of FRP longitudinal bars, so columns and compression zones are designed as if those bars carry little or no compression. Your engineer will confirm how the current edition treats it.
Not weldable, and different to cut. GFRP is cut with a saw (a diamond or abrasive blade), not a shear or torch, and crews should control the glass dust. It cannot be welded to embedded plates or other bars.
Cost and lead time. Per metre, GFRP generally costs more than plain steel, a GFRP design may use more bar, and bent shapes have production lead times. Our guide to GFRP rebar pricing explains what drives the cost without quoting numbers that would not fit your job.
What contractors run into on site
Most of the practical issues come from treating GFRP like steel:
- Shapes missing on delivery because the bar list assumed someone would bend a few bars on site.
- Mats floating during the pour because light bar was not tied down well enough.
- Wrong accessories. Steel chairs and black tie wire defeat the point of a corrosion-free system; GFRP mats use plastic chairs and coated or nylon ties.
- Sunlight in storage. Bundles left uncovered for long periods can suffer surface damage from UV exposure.
- Size confusion. GFRP bars are identified by designated diameter and nominal area under CSA S807, and a GFRP bar labelled "10M" does not have the same area as steel 10M. Order by the area and grade on the drawings.
None of these are reasons to avoid GFRP. They are reasons to detail it completely and order it early.
So, is fiberglass rebar any good?
Worth pricing for:
- Bridge decks, barrier walls and approach slabs on salted roads.
- Parking structures, ramps and loading docks.
- Marine and waterfront work, and saltwater pools.
- Concrete near MRI rooms, transformers and other sensitive equipment.
- Elements that would be very hard or costly to repair later.
Usually not the best fit for:
- Standard house foundations and slabs built to Part 9 prescriptive rules.
- Fire-rated building elements, unless the design deals with it.
- Jobs with frequent field changes or lots of bent shapes on a tight schedule.
- Columns and other members that rely heavily on reinforcement in compression.
Residential uses such as driveways and garage floors come up often; our post on fiberglass rebar for slabs and driveways covers those separately.
Get GFRP and steel priced side by side
If your engineer is weighing GFRP against steel, a quote on both from the same drawings is the fairest test. We supply GFRP bar to your specification, and detail, cut, bend and tag steel rebar from 10M to 55M. Ask us about bent GFRP shapes and lead times for your job. Send us your drawings or see our rebar products.
Frequently asked questions
What are the disadvantages of GFRP rebar?
It is much less stiff than steel, fails without yielding, cannot be bent or welded on site, loses strength at bends and at high temperature, needs an engineered design under CSA S806 or CSA S6, and usually costs more per metre than plain steel.
Is fiberglass rebar any good?
Yes, for the jobs it suits: concrete exposed to road salt or seawater, structures near sensitive electrical or magnetic equipment, and elements that are hard to repair later. It is a poor fit where a design relies on prescriptive steel rules, lots of field changes or high fire resistance.
Is GFRP rebar stronger than steel?
Its tensile strength is higher than steel's yield strength, but it is roughly a third as stiff and does not yield. Designs are usually governed by deflection and crack width, so GFRP rarely uses its full strength.
Does fiberglass rebar meet code?
It can, under an engineered design. GFRP bars are made to CSA S807 and designed under CSA S806 for buildings or CSA S6 for bridges. The prescriptive Part 9 rules in the Ontario Building Code are written around steel bar, so a house using GFRP needs a designer and building department acceptance.
