Constitutive concrete modelling and time-dependent seismic fragility for aging RC elements under coupled non-uniform corrosion and freeze-thaw cycles

Offshore RC structures located in cold marine regions are prone to deterioration effects caused by chloride-induced corrosion and freeze-thaw cycles. This study firstly investigates the mechanical behavior and deterioration mechanisms of marine concrete specimens through rapid salted freeze-thaw cycling tests. Six groups of unconfined and confined elements are considered. The main test parameters include the confinement condition, reinforcement ratio, stirrup ratio, and number of salted freeze-thaw cycles. The effects of these parameters on the surface damage modes, damage evolution, mass loss, and freeze-thaw resistance of specimens, are examined in detail. Uniaxial compressive tests are also performed to obtain the stress-strain curves of specimens. Stress-strain models are proposed for both unconfined and confined concrete specimens under coupled chloride-induced corrosion and freeze-thaw cycles, and their effectiveness is validated against test results. Subsequently, deterioration models are developed for aging RC structures under coupled non-uniform chloride-induced corrosion and freeze-thaw cycles. The developed stress-strain and deterioration models for aging RC members are then employed for time-dependent seismic fragility assessment for the piers of a coastal highway bridge in order to evaluate the influence in realistic engineering structures. The results indicate that: (i) the mass loss ratios in confined concrete specimens are generally smaller than that in their unconfined counterparts; (ii) the unconfined concrete specimens exhibited brittle splitting failure while, in addition to the enhanced strength, relatively ductile failure modes occurred in the confined cases, with the ultimate behavior characterized by buckling or rupture of stirrups; (iii) time-dependent seismic fragility of aging RC piers exhibited a nonlinear increasing trend over the service life and severity of damage states; and (iv) neglecting the influence of freeze-thaw damage can lead to notable underestimation of time-dependent seismic fragility of aging RC piers.