Glass fiber-reinforced polymer (GFRP) reinforcing bars do not yield. This is the fundamental fact that separates GFRP-reinforced concrete design from conventional steel-reinforced concrete design. A steel rebar loaded in tension reaches its yield point, elongates plastically, and provides the ductile warning that structural codes rely upon. A GFRP bar, by contrast, is linear-elastic to failure. It stretches in proportion to load, and when the load reaches the ultimate tensile strength — typically 600 MPa to 900 MPa for a high-quality vinyl ester GFRP bar — it ruptures suddenly, with no yield plateau. ACI 440.1R, the guide for the design and construction of structural concrete reinforced with FRP bars, builds its entire design philosophy around managing this behavioral difference.
Flexural Design: Over-Reinforced Sections and Compression-Controlled Failure
In steel-reinforced concrete, the desired flexural failure mode is tension-controlled: the steel yields before the concrete crushes, providing visible cracking and deflection as warning. In GFRP-reinforced concrete, the desired failure mode is compression-controlled: the concrete crushes before the GFRP ruptures. This is achieved by designing the section to be over-reinforced — providing more GFRP reinforcement than the balanced reinforcement ratio, so that the concrete reaches its ultimate compressive strain of 0.003 while the GFRP strain remains well below its rupture strain. This gives a more gradual failure with concrete spalling, rather than a sudden, brittle tensile rupture of the bars.
The balanced reinforcement ratio for GFRP is substantially lower than for steel because GFRP has a much lower modulus (approximately 40 GPa to 60 GPa, compared to 200 GPa for steel). For a typical 30 MPa concrete and GFRP with a guaranteed tensile strength of 700 MPa and modulus of 50 GPa, the balanced ratio is approximately 0.0035 — far below the typical 0.015 to 0.025 used for steel. Achieving an over-reinforced section with GFRP requires careful calculation, but is generally feasible for most beam and slab geometries.
Shear Design: The Crack Width Constraint
GFRP bars do not yield, but GFRP-reinforced concrete does crack. The crack widths tend to be wider than in steel-reinforced concrete because the lower modulus of GFRP means the bars stretch more under service loads. Wider cracks reduce the aggregate interlock contribution to shear resistance, which is why ACI 440.1R reduces the concrete shear strength Vc for GFRP-reinforced members compared to steel-reinforced members. The reduction factor is a function of the axial stiffness of the GFRP reinforcement; the lower the stiffness, the greater the reduction. For typical GFRP bars, Vc is typically 30% to 50% lower than the steel-reinforced value for the same section dimensions.
This means that GFRP-reinforced beams generally require more shear reinforcement (GFRP stirrups) or closer stirrup spacing than an equivalent steel-reinforced beam. The stirrups themselves are GFRP, with a bend radius that must be large enough to avoid stress concentration at the bend — a minimum bend diameter of 6 times the bar diameter is commonly specified.
Environmental Reduction Factors
GFRP bars are chemically resistant, but the alkaline environment of concrete (pH > 12.5) is aggressive to glass fiber. All GFRP bars for concrete reinforcement use a vinyl ester resin matrix specifically formulated for alkali resistance, and the design tensile strength is reduced by an environmental reduction factor CE per ACI 440.1R. For GFRP bars in concrete exposed to earth and weather, the typical CE is 0.7 to 0.8, applied to the guaranteed ultimate tensile strength. This accounts for long-term strength degradation in the alkaline pore solution. Testing per ASTM D7705 (alkaline resistance) and ACI 440.3R (test methods) provides the data for manufacturer-specific CE values.
For detailed product specifications of GFRP rebar, see our GFRP Rebar page.