Abstract
The Concrete Filled Steel Tube (CFT) structure provides the advantages of steel and concrete, including excellent strain performance, low labor cost and short term of works, but has hardly been applied to actual structures in Korea. In the CFT structure, the concrete filling cannot be visually checked, and pores may exist under the diaphragm. Accordingly, it has problems in ensuring the quality and workability. Therefore, a test was conducted with the 80MPa and 100MPa ultra-high strength concrete cast to the $400{\times}400mm$ steel tube via top casting method, and the following test results were obtained. The fluidity (690mm for 80MPa and 685mm for 100MPa) and air volume (2.1% and 2.3%, respectively) met the desired values, and the settlement was below the desired standard of 2 mm. The strength achieved at the initial aging was more than 50% for each desired strength, and was the highest in the standard water curing. As for the core strength, the strength reinforcement was low at the center of the member with a diaphragm, and the strength was higher at the lower part than at the upper part. For each desired strength, the temperature difference between the surface and center was $10^{\circ}C$ or less, and the contraction was lower at 80MPa than at 100MPa.
