Technical data.
What an Endurobar bar must meet, how it is made and traced, and how to handle and design with it — all referenced to IS 18256, IS 18255 and IRC 137.
Sizes & Guaranteed Strength
Nine standard sizes, G4 to G32
Each size has a nominal area and a minimum guaranteed ultimate tensile force set by IS 18256. Strength is guaranteed per size, so designers work from kilonewtons, not a single headline MPa.
| Size | Ø mm | Nominal area mm² | Measured area mm² | Min. tensile force kN | Bent bar* kN | Min. bend radius mm |
|---|---|---|---|---|---|---|
| G4 | 4 | 12.6 | 13 – 15 (18) | 11 | 6 | 12 |
| G6 | 6 | 28.3 | 29 – 34 (40) | 23 | 14 | 18 |
| G8 | 8 | 50.3 | 51 – 60 (70) | 40 | 24 | 24 |
| G10 | 10 | 78.5 | 79 – 94 | 60 | 35 | 30 |
| G12 | 12 | 113.1 | 115 – 136 | 80 | 50 | 36 |
| G16 | 16 | 201.1 | 207 – 241 | 130 | 80 | 48 |
| G20 | 20 | 314.2 | 316 – 377 | 190 | 120 | 60 |
| G25 | 25 | 490.9 | 493 – 589 | 270 | 160 | 100 |
| G32 | 32 | 804.2 | 806 – 965 | 420 | — | — |
*Minimum guaranteed force in the straight portion of a bent bar. Bracketed areas apply to sand-coated G4–G8. Bend radius is 3× diameter up to G20 and 4× above. Source: IS 18256:2023, Table 1.
What Every Bar Must Meet
Material & performance limits
Glass fibre
Unidirectional E-CR (chemical-resistant) or R (high-strength) glass rovings, sized to bond with the resin.
Resin
Vinylester or epoxy. The base polymer must contain no polyester at all; added styrene stays under 10% of the resin by mass.
Fillers & additives
Only commercial-grade inorganic fillers and process additives, capped at 15% of the resin by mass in total.
Surface enhancement
Sand-coated, braided, spiral-wound, indented or milled surfaces give the mechanical grip that transfers load to concrete.
Behaviour Under Load
Strong, linear — and different from steel
GFRP climbs in a straight line to its tensile limit and then fails. Steel yields and stretches first. Designing with GFRP means respecting that difference.
Illustrative curves for a typical bar — not a guarantee. Guaranteed strength is per size and lot (see table above).
Design Honestly
What to design around
- Lower stiffness
Modulus is about a quarter of steel's, so deflection and crack width often govern the design.
- No yield point
GFRP is linear-elastic to failure — it does not yield or give plastic warning, so designs use conservative limits.
- Brittle failure
It ruptures at its tensile limit instead of stretching, which is why strength is guaranteed per size and lot.
- Not weldable or field-bendable
Shapes are formed at the factory; there is no welding and no on-site bending.
- Lower heat endurance
It tolerates elevated temperature less well than steel, so fire design relies on adequate concrete cover.
- Not one-for-one with steel
Design by GFRP rules (ACI 440 / IRC 137) — bar size and spacing come from the loads, not from a steel schedule.
Made & Traced
From glass fibre to a lot-tagged bar
Bars are made by pultrusion — a continuous process in which nothing is changed mid-lot.
Glass rovings
Continuous fibre is drawn from creels and aligned straight, because fibre straightness drives strength.
Resin impregnation
Rovings pass through a resin bath so every fibre is fully wetted.
Forming die
A calibrated die squeezes out excess resin and sets the bar's diameter.
Heat cure
An oven cures the resin to the required degree of cure and glass transition temperature.
Surface & pull
The bar is pulled continuously while its surface enhancement is applied.
Cut, coil & tag
Bars are cut or coiled, then tagged by lot for traceability.
Manufacturer's Test Certificate
Every consignment carries a certificate, traceable to its production lot, listing materials, tests and results against IS 18256.
Marked along the bar
Brand, size designation and guaranteed tensile force are marked on the bar at regular intervals — at least every two metres.
Tagged bundles & coils
Each bundle or coil carries a tag with brand, size, guaranteed force, lot number and month and year of manufacture.
Lot-wise testing
Samples are drawn per production lot at set tonnage frequencies and any failing lot is rejected as a whole.
Requalification
Product qualification is repeated on any change of materials or process, and at least every three years for all sizes.
On-Site Handling
Eight rules for the site crew
Drawn from IRC 137-2022, clause 6 — the rules that keep GFRP performing as designed.
No bending on site
Bars are factory-bent only. Bending, straightening or re-shaping on site is not permitted.
Cut with care
Field cutting is limited to what ACI 440.5 allows. Thermal cutting and shear cutting are not permitted.
Tie with plastic
Use self-locking plastic straps or plastic-coated tie wire. No other tying method.
GFRP chairs only
Metal bar supports and chairs are prohibited; use GFRP chairs.
No electrical earthing
Electrical grounding of GFRP bars is prohibited.
Shade from sun & heat
Store off the ground and cover with opaque sheeting in direct sun or above 45 °C ambient.
No diameter swaps
Never substitute a different diameter from the one specified on the drawing.
Wear gloves
Loose glass fibres can irritate bare skin, so handle with gloves.
Highway & Road Projects
Where IRC 137 permits GFRP
IRC 137-2022 limits GFRP in road projects to these concrete components, using vinylester-resin bars with E-CR or R glass. The MoRTH value-engineering circular (30 Aug 2022) separately encourages GFRP for crash barriers, drains and minor culverts.
- Approach slabs
- Bridge decks & deck overlays
- Foot-over-bridge walkways & slab culverts
- Bridge-cum-bandhara deck slabs & barriers
- Concrete roads — JPCP, CRCP & short-panel (cast-in-situ and precast)
- Retaining walls
- Noise barriers
- Box culverts
- Crash barriers & bridge parapets
- Pedestrian parapets & railings
- Bulkheads & bulkhead copings
- MSE wall panels & copings
- Drainage structures
Supply
How bars are supplied
Store bundles separated, off the ground and out of direct sunlight to avoid abrasion and UV damage.
Beyond The Obvious
More places it works
Standards & References
What we build and test to
- IS 18256 : 2023
Solid round GFRP bars for concrete reinforcement — specification
- IS 18255 : 2023
FRP bars for concrete reinforcement — methods of test
- IRC : 137-2022
Guidelines on use of FRP bars in road projects (Part 1: GFRP)
- MoRTH circular, 30 Aug 2022
Value-engineering practices for National Highway projects — GFRP for crash barriers, drains and minor culverts
- ACI 440.1R-15 · 440.5-18
Design and construction guides for FRP-reinforced concrete
- ASTM D7957 · D7205 · D7617 · D7913 · D7705
Bar specification, tensile, shear, bond and alkali-resistance test methods
Technical FAQs
Questions from spec engineers
Yes. Bond and stress transfer differ between the two, so the designer accounts for each, and each bar type gets its own chairs.
Yes in the sense that it ruptures without yielding. That is why strength is guaranteed per size and lot, and why designs use conservative limits.
The bar is a cured composite, not a yielding metal. Bends are formed in the factory before the resin fully cures, and a bent bar keeps at least 60% of the straight bar's guaranteed force.
GFRP has good fatigue and cyclic behaviour, but it does not yield, so seismic detailing needs a design report from your structural engineer.
Standard structural concrete mixes work; the mix is chosen for the exposure and strength the project needs.
IRC 137-2022 lists approach slabs, bridge decks, concrete pavements, retaining walls, culverts, crash barriers, parapets, MSE panels and drainage structures.
Its corrosion resistance suits piles in aggressive ground, subject to a design by your structural engineer.
Per IS 18255. Because GFRP slips in ordinary grips, test specimens are anchored with couplers acting as grippers on both ends.