Fiber reinforced polymers in seismic upgrading of existing reinforced concrete structures

Externally-bonded Fiber Reinforced Polymers (FRPs) when used as jackets in reinforced concrete (r.c.) members are effective in increasing shear, lap-splice strength and deformation capacity. For this reason they are currently used extensively as a fast remedy in earthquake-damaged structures, or to strengthen under-designed structures in areas of high seismicity. FRP jackets impart little or no stiffness to the encased element, whereas experimental evidence indicates that the demand on bar anchorages increases after jacketing of the adjacent plastic hinges near supports. Under excessive rotation demand the jackets, being susceptible to stress concentrations, are at risk of rupture due to buckling of primary reinforcement, when the embedded stirrups are very sparse (substandard detailing). These performance issues are explored in the paper through a combined evaluation of published experimental evidence and simple mechanistic constructs that highlight the mechanical contribution of the jacket to the various failure modes of an r.c. element. Dependable deformation capacity at yield and ultimate and the various strength components are examined through collective evaluation of available tests and design lower bound expressions are derived. Criteria that should be considered as part of the upgrading strategy when FRP jacketing is used so as to control the deformation demand of the structure are also discussed.