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New types of structural columns are being developed for new construction. They are made of concrete-encased fiber reinforced polymer (FRP) tubes. The concrete-filled FRP tubes are cast in place. The tube acts as formwork, protective jacket, confinement, and shear and flexural reinforcement. It can also be used to complement or replace conventional steel reinforcement of the column. This paper presents the results of experimental and analytical studies of the performance of concrete columns confined with carbon and glass FRP composite tubes. Concrete-filled FRP tubes are instrumented and tested under uniaxial compressive load. Test variables include type of fiber, thickness of tube, and concrete compressive strength. Results show that external confinement of concrete by FRP tubes can significantly enhance the strength, ductility, and energy absorption capacity of concrete. Equations to predict the compressive strength and failure strain, as well as the entire stress-strain curve of concrete-filled FRP tubes were developed. A comparison between the experimental results and those of analytical results indicate that the proposed model provides satisfactory predictions of ultimate compressive strength, failure strain, and stress-strain response. The study shows that the available models generally overestimate the strength of concrete confined by FRP tubes, resulting in unsafe design.
Saafi et al. (1999) studied this question.