Review of 60 studies: design codes give conservative development lengths for GFRP bars in concrete

Szabolcs Szinvai, Tamás Kovács and Jiao-Long Zhang published a systematic review of the bond behavior of glass fiber-reinforced polymer (GFRP) bars in concrete in Discover Applied Sciences in 2026 (doi.org/10.1007/s42452-026-09867-5) springer.comdoi.orgspringer.comOpen source ↗. The authors compared the provisions of major international design guidelines. They searched the Web of Science database and selected 60 relevant publications for detailed assessment.

Why GFRP bars need separate detailing

GFRP bars are increasingly used as internal concrete reinforcement because of their corrosion resistance and non-magnetic properties. Compared with steel bars, they have lower stiffness, brittle behavior, diverse surface treatment options and different bond mechanisms. According to the authors, these differences complicate the design of anchorages, lap splices and detailing in general.

The review covers bond mechanisms and failure modes, bond stress–slip models, and the effects of surface treatment, concrete properties and confinement. It also covers development and lap-splice length, bent-bar detailing and long-term bond performance.

What the review found in the guidelines

Surface properties are among the most influential parameters governing bond strength. Their effects are not adequately represented by the limited surface coefficients used in current design provisions, the authors report springer.comdoi.orgspringer.comOpen source ↗.

As a result, existing development-length equations are generally conservative, particularly for modern GFRP products and well-confined reinforcement. Unconservative predictions may still occur under certain conditions.

Most guidelines rely on empirical average bond-stress concepts and do not explicitly account for non-uniform bond-stress distributions. The authors name insufficient knowledge about the reduction of bond strength under environmental exposure, sustained loading and long-term loading as a reason for the excessive conservatism.

The paper lists research priorities: surface characterization, confinement, non-uniform bond models, environmental exposure, sustained loading and bent-bar detailing springer.comdoi.orgspringer.comOpen source ↗.