Spiral Post-Tensioning System for Concrete Members

Four thick-walled 1008 mm (39.7 in.) long cylindrical specimens with an inside diameter of 304.8 mm (12 in.), a wall thickness of 76.2 mm (3 in.), and a monolithically cast 250 mm (9.8 in.) long flange at either end were fabricated. For three of the specimens, a spiral post-tensioning system consisting of four clockwise and four counterclockwise outside wrapping strands was used. The fourth specimen had eight post-tensioned straight strands. Specimens were fabricated using high-performance, self-consolidating concrete and 12.5 mm (0.5 in.) post-tensioning tendons. Each specimen was supported at the ends and point loaded at the midspan. Spiral specimens were post-tensioned to 25, 50, and 75% of strand capacity, whereas the specimen with straight strands was post-tensioned to 50% strand capacity. The spiral post-tensioning system increased ultimate capacity by 45% when compared with the specimen having longitudinally aligned strands.