Fiber reinforced polymer (FRP) composites in retrofitting of concrete structures: Polyurethane systems versus epoxy systems

Fiber reinforced polymer (FRP) composites are now common structural materials for both new construction and repair/rehabilitation of existing structures. Since the 1980s researchers have developed a significant body of knowledge on externally-bonded composites for infrastructure repair; however, with emphasis on the use of epoxy systems (matrix and adhesives). Externally-bonded FRP composites with polyurethane matrices and adhesives have recently been investigated due to advantages in constructability and mechanical properties. However, little research is available on bond of polyurethane composites to concrete infrastructure, and direct comparisons between performance of epoxy and polyurethane systems. This paper presents several small-scale experiments to characterize the mechanical properties of the bond to concrete of polyurethane FRP composites alongside with epoxy composites. The tests include 3-point bending tests of concrete beams reinforced with the composite materials, lap shear tests, and coupon tensile tests. Strain data collected from the lap shear experiments were used to develop bond-slip relationships of the composite materials that were then implemented in a finite element model and compared with the experimental flexural results. While polyurethane matrices and adhesives are typically characterized by lower shear and normal strengths, results demonstrate the flexibility of the polyurethane matrix proved advantageous in spreading the bond stresses over a larger area compared with epoxy composites. Therefore polyurethane-reinforced concrete beam stiffness and strength properties are comparable with the epoxy counterparts.