Combined degradation mechanisms of reinforced concrete structures from the perspective of steel rebar corrosion: Current findings, needs, and innovative mechanistic pathways and assessment methodologies

This review critically examines the corrosion durability of reinforced concrete (RC) structures under combined degradation mechanisms (CDM), including chloride, carbonation, sulfate, freeze–thaw, and alkali–silica reaction attack. While single degradation mechanisms (SDM) have long underpinned durability design and service-life modeling, field RC structures rarely experience them in isolation. Instead, simultaneous, sequential, chemical–physical interactions govern corrosion initiation and propagation beyond SDM-based assumptions. Synthesizing experimental and field studies, this review demonstrates that CDM fundamentally alter corrosion-resistant properties ( e.g., E corr , i corr , R p ) and corrosion-controlling service-life parameters ( e.g., t i , C crit , CCT, CPR ). Depending on degradation pairing and exposure sequence (e.g., chloride→carbonation or carbonation→chloride), CDM exhibit distinct mechanistic pathways or corrosion signatures, leading to damage deceleration (e.g., via concrete pore refinement, binding reactions, rebar-passivity refinement) or acceleration (e.g., via pH reduction, bound chloride release, microcracking, moisture redistribution). Consequently, corrosion evolution under CDM is nonlinear and exposure-history–sensitive. The framework shows that tᵢ and CPR are particularly sensitive to CDM, while C crit and CCT are no longer fixed material constants under multi-mechanism damage, challenging conventional SDM-based service-life models. Despite growing recognition of CDM effects, a critical methodological gap remains. Existing corrosion test standards remain largely SDM-oriented and fail to reproduce realistic CDM exposures, limiting cross-study comparability and potentially misrepresenting long-term field performance, particularly climate-variable environments. Overall, this review synthesizes current CDM studies, define mechanistic pathways at rebar level, re-evaluates corrosion parameters used in service-life analysis, and proposes reproducible methodologies to support experimentation, durability modeling, and design of resilient, low-carbon RC infrastructure.