Bond behavior of steel-fiber-reinforced self-stressing and self-compacting concrete-filled steel tube columns for a period of 2.5 years

The aim of this study is to explore the influence of concrete age on the bond behavior of steel-fiber-reinforced self-stressing and self-compacting concrete-filled steel tube (FSSCFST) columns. The push-out tests of 36 FSSCFST columns are conducted at ages corresponding to 28 d or 2.5 y. All specimens with an age of 2.5 y are tested under four cycles of repeated loading. The variables considered in the test are as follows: (a) concrete age (28 d and 2.5 y); (b) thickness of the steel tube (2.5 mm, 3.5 mm, and 4.25 mm); (c) steel-fiber volume percentage (0% and 1.2%). The experimental results indicate that an increase in the concrete age increases the bond strength and corresponding slip (Su) of FSSCFST columns. The addition of steel fibers improves the bond behavior at an early age and it generally loses its effect with increases in the concrete age. With increases in the loading cycle, the bond strength in the same direction exhibits a decreasing trend. However, steel fibers delay the degradation in bond strength.