Study on Long-Term Behavior and Ultimate Strength of CFST Columns

Long-term tests (864 days) were conducted on three types of columns, including plain concrete (PC) columns, reinforced concrete (RC) columns and concrete-filled steel tube (CFST) columns. The shrinkage and creep performance and the ultimate strength of the columns were tested and analyzed. The results show that the shrinkage strain of RC columns is about 87.6% of that of PC columns, the shrinkage strain of the CFST columns is about 34.8% of that of PC columns, and that the effect of expansion agent on shrinkage strain is about 2% for the PC columns. It was also found that the smaller the loading age, the greater the creep coefficient. Reinforcement can significantly reduce the creep deformation of PC columns. The creep coefficient of steel pipe surface is larger than that of inner concrete for CFST columns, leading to a longer life due to the creep of inner concrete to achieve stability. The loading age and the reinforcement have great impact on extreme compressive strain and ultimate strength. The hoop effect of RC columns is obvious. Loading rate influences the stress redistribution of CFST columns. The stress redistribution effect is relatively small for columns with loading course. However, the effect on the ultimate strength is very small for columns with or without loading courses. The difference of two is only 2.4%.