Confinement model for concrete columns internally reinforced with glass fiber reinforced polymer spirals

This research investigates confinement of concrete using glass fiber reinforced polymer (GFRP) spirals. Concrete prisms 10 in. in diameter and 30 in. high were internally reinforced with GFRP spirals. Using different configurations of GFRP spirals, 21 prisms were built; in addition, three prisms were built without any reinforcement. The different series of specimens with GFRP spirals were created by varying the bar diameter and pitch. The bar sizes used for spirals were #3, #4 and #5. The pitch used for #3 spirals was 1.5 in., 2 in. and 3 in. The pitch used for #4 spirals was 1.5 in. and 2 in. The pitch used for #5 spirals was 1.5 in., 2 in. and 2.5 in. Wooden dowels were used to hold the spirals at the required pitch. Compression tests were conducted for each specimen and results were obtained in the form of axial load, axial stress, axial strain and hoop strain. A concrete confinement model was obtained which describes the increase in both compressive strength and axial strain of concrete confined internally with GFRP spirals. The confinement model was verified with tests conducted on four concrete columns reinforced with GFRP spirals and GFRP longitudinal bars and similar specimens from the literature. The four columns were 8 in. in diameter and 30 in. high reinforced with #3 GFRP spirals at a pitch of 1.5 in. and had either four or six #5 longitudinal GFRP bars. The agreement between the model and the columns was satisfactory for both confined concrete strength and ultimate axial compressive strain.