Early-age Creep Of Axial Compression Concrete Filled Steel Tubular Members

Though Concrete-Filled Steel Tubular (CFST) structures have achieved massive application and great development in China, many problems about long-term behavior are still unclear so far, such as creep effects of the CFST. In actual engineering, since steel tubes play roles as both components to bear loads and formwork of core concrete, once core concrete reached its strength, it will work together with steel tube, i.e. loading age of the concrete core is earlier than that of other concrete structures. Early-age creep has noteworthy influence towards CFST’s stress-redistribution and deformation so that it should be emphasized. Therefore, this paper did a research about property of CFST’s early-age creep.The main contents in this paper are as follows:1) Experiments on Early-age creep of axial compression CFST members are carried out. The CFST specimens are loaded at ages of one-day, three-day, seven-day and twenty-eight-day separately, to acquire the strain change caused by creep of concrete core, and strength and elasticity modulus of concrete core at corresponding age are tested. Furthermore the specimens are loaded continuously including the unloading stage to obtain concrete’s load-and-unload curve. Result shows that, under constant loading condition, CFST’s creep decreases with the increase of loading ages and the speed of creep’s increase is faster at the early stage of loading. With the increase of ages, concrete strength and elasticity modulus show a development tendency which is fast increasing at beginning stage followed by a gentle and stable ending. After unloading, concrete shows a recovery of deformation on the whole, and the rate of recovery becomes slow with the increase of loading ages.2) Applying cluster model for early-age concrete creep, coefficients in hydration dynamic model, and elastic, viscoelastic and viscoplastic coefficients in Nishihara Model are regressed according to creep loading-unloading curve. Creep ratio curve of early-age concrete core is obtained.3) Based on mechanical relationship between the steel tube and concrete core in axial compressed CFST members, early-age creep model for axial compression CFST members is proposed. By introducing the creep ratio curve of early-age concrete core, early-age creep of axial compression CFST members are predicted, and results show that the predicted creep values agree well with the experimental data. 4) Results obtained by the proposed model are compared with creep values of axial compression CFST members obtained by using ACI209R Model, Model BP2, Model B3, GL2000Model, CEB-FIP90model, as well as CEB-FIP2010model, together with experimental values. The results showed that several common-used creep models, such as Model B3, ACI209R Model, CEB-FIP90Model, and GL2000Model are not suitable for early-age concrete creep analysis.