Coupling effect of freeze-thaw cycles and multiaxial compression on concrete-filled steel tube (CFST): Insights from simplified meso-scale axisymmetric finite element model

With increasing applications in cold regions, concrete-filled steel tube (CFST) has become one of the favorite structural types due to the prompting load-carrying capacity and construction efficiency, where the core concrete is generally under multiaxial stress states. However, the damage mechanism for the coupling effect of freeze--thaw (FT) cycles and multiaxial stress states on the core concrete has not been yet distinctly clarified. This may be attributed to the difficulties in explaining the macro-scale (>10 mm) structural phenomenon (i.e., strength reduction) through efficiently modeling the corresponding micro-scale (10-3-1 mm) damage process considering the spatiotemporal coupling effect. To overcome this limitation, a simplified meso-scale axisymmetric finite element (FE) model has been firstly developed through the rational exploitation of pore expansion-shrink cycles under external multiaxial loads and proved capable of efficiently simulating the concrete damage under the coupling effect. The FT damage evaluations of concrete under various types of multiaxial active compressions have been comprehensively analyzed. The direction of the FT damage distribution proved being consistent with that of external load, and the stress states combining high axial compression with low radial compression show an adverse effect on the FT resistance. On this basis, a multiaxial compression-dependent FT damage model has been proposed and verified against experimental results. Thereafter, the FT resistance of CFST under external load and its corresponding confinement effect have been studied. The results show that, similar to plain concrete, the FT resistance of CFST will firstly increase and then decrease with the increasing axial compression. Mean-while, the promoting FT resistance mechanism of CFST has been clarified, which originates from two stages as the FT damage mitigation during FT cycles and the damage effect reduction after FT cycles. This analytical method for the coupling effect is also beneficial for other types of confined concretes.