Carbon, steel or glass fibre reinforcement: the comparison

Continuous glass fibres give GFRP a higher tensile strength than steel (often 2× the yield strength of ordinary steel). However, its elastic modulus is 3 to 5 times lower (~40–60 GPa versus 200 GPa). As a result, GFRP deforms much more under the same stress. In practice, deflection and crack width govern design, not ultimate strength. Behaviour is linear up to brittle failure (no plastic plateau). Elements are therefore often designed so that concrete crushing controls failure, with increased minimum reinforcement ratios (approximately 1.6× those for steel according to ACI 440).

Carbon remains clearly superior in strength and especially stiffness. Steel retains the advantage of ductility (warning before failure).

GFRP weighs approximately a quarter as much as steel (density ~1,900–2,100 kg/m³). An equivalent diameter is therefore 4 times lighter. This reduces transport costs and energy, makes manual handling easier (less crane use and less crew fatigue) and lightens the structure. Carbon is even lighter (~1,800 kg/m³). The weight advantage is decisive on hard-to-access sites, long spans or renovation projects where every kilogram counts.

Neither GFRP nor carbon undergoes electrochemical corrosion → concrete cover can be reduced compared with steel (often 15–25 mm for GFRP according to guidance and practice, versus 30–50 mm for steel in exposed conditions). However, codes (ACI 440.11, etc.) still require minimum cover for bond, fire (resin degrades at a lower temperature than steel) and sometimes transverse thermal expansion. In practice, the main gains are reduced element thickness and concrete quantities. Textile carbon reinforcement (such as CARBOrefit®) allows the thinnest cover (10–15 mm).

Values vary considerably with LCA boundaries (A1–A3 cradle-to-gate, grid electricity, fibre content, etc.). Steel is typically around 1,500–2,000 kg CO₂eq/t (or less for recycled electric arc furnace steel, EAF). GFRP often falls in the 600–1,200 kg CO₂eq/t range depending on the source; some LCAs show a comparable or slightly higher impact per kg, but a substantially lower total impact because much less mass is required and corrosion maintenance is avoided. Carbon has a high production impact per kilogram (fibres carbonised at high temperature), but the very small volume used in textile solutions makes it competitive, hence the table’s ~450 kg (an optimistic value requiring context). Over the full life cycle (avoided maintenance), both non-metallic solutions often have a clear advantage.

• Initial material cost: GFRP is often 1.5–3× more expensive per kilogram than steel, but because approximately 4× less mass is used, the installed material cost can be similar or lower. Carbon is significantly more expensive per kilogram.
• Installation: savings on transport, handling and sometimes labour (lighter bars, sometimes supplied in coils for small diameters).
• Life cycle: GFRP and carbon have a major advantage in aggressive environments (de-icing salts, marine exposure, wastewater) because corrosion and repairs are avoided. Over 30–50 years, total cost can be 30–50% lower than steel according to life-cycle cost studies.
• Limitations: no on-site bending (factory bends only), sometimes longer lead times and specific design requirements.

• Steel: the default solution for most structures in mildly aggressive environments. Better ductility, easy on-site bending, mature codes, the lowest initial cost and well-understood behaviour. Choose it when corrosion is not the dominant risk and ductility is important (seismic zones, redistribution).
• Carbon fibre: when maximum performance (strength + stiffness) is needed with minimum thickness and weight. Ideal for thin strengthening layers (carbon textile + fine concrete such as CARBOrefit®), long spans, lightweight structures, highly aggressive environments, or where the overall carbon footprint and long-term durability are priorities. The higher cost is justified by gains in thickness, weight and durability.
• Glass fibre (GFRP): an excellent cost/performance compromise for structures exposed to corrosion (bridges, car parks, marine structures, wastewater treatment plants, road infrastructure). Low weight and immunity to corrosion are its strengths. Avoid it or assess it carefully where stiffness is critical (long spans without adequate deflection control), in seismic zones requiring ductility, or under severe fire exposure without adequate protection. Dedicated codes exist (ACI 440.11, etc.).

Practical recommendation: for a given project, start with the dominant risk (corrosion? weight? thickness? ductility? total cost?) and validate the values against supplier datasheets and the applicable code. All three solutions have useful roles; textile carbon reinforcement pushes the limits of thickness and performance, GFRP makes corrosion-free reinforcement more affordable, and steel remains the robust, ductile standard.