Experimental study on seismic performance of hybrid steel-polypropylene fiber-reinforced recycled aggregate concrete-filled circular steel tube columns

In this paper, 1 recycled aggregate concrete-filled circular steel tube column (RACFST) and 11 hybrid steel -polypropylene fiber-reinforced recycled aggregate concrete-filled circular steel tube (HFRACFST) columns were designed given the variables of steel fiber (SF) volume content (Vsf = 0.6 %,1.2 %,1.8 %), polypropylene fiber (PPF) content fractions (Vpf = 0.05 %,0.10 %,0.15 %), concrete strengths (C40, C50, C60), thickness of steel tube (t = 2.70 mm, 3.65 mm, 4.50 mm), axial compression ratios (n = 0.25, 0.35, 0.45). Then the influence of variables on hysteretic curves, ductility, energy dissipation and stiffness degradation were studied experimen-tally using the pseudo-static tests. Subsequently, the restoring force models of HFRACFST were proposed based on the experimental results. The results indicated that: (1) The incorporation of HF (hybrid steel-polypropylene fiber) could enhance the hysteretic behaviour of HFRACFST columns. An increase in PPF content had little effect on the lateral bearing capacity and energy dissipation capability. Nevertheless, it could improve the ductility to some extent. As the SF content increased, the lateral bearing capacity, energy dissipation capability and ductility of the specimens increased. (2) An increase in concrete strength resulted in the increase of the bearing capacity and energy dissipation capability, but led to inferior ultimate deformation and ductility. (3) The comprehensive hysteretic behaviour was improved significantly as the thickness of steel tube increased, especially in the energy dissipation capability. (4) The specimens with higher axial compression ratio experienced inferior ultimate deformation and ductility. (5) The ductility indexes of all specimens were higher than 3.0. The yielding elastic and ultimate elastoplastic drift ratios were in the ranges 0.012-0.013 and 0.038-0.054, respectively. The hys-teretic performance of HFRACFST columns could satisfy the requirement of seismic design. It is feasible to apply the HFRACFST columns to vertical load-bearing structures in the seismic zones.