Chloride-induced corrosion in steel-reinforced concrete under wet-dry conditions: Corrosion mechanism and critical chloride content

Chloride-induced corrosion of steel is a major durability issue for reinforced concrete (RC) structures. The critical chloride content ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>C</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">crit</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> ), a key parameter in engineering, is generally studied under simplified exposure conditions. However, data on how wet-dry cycles – common in practice – affect <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>C</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">crit</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> and the corrosion mechanism are lacking. Thus, in this study, RC specimens were subjected to permanent immersion, daily wet-dry cycles (6/18 hours), and weekly cycles (2/5 days). Mass transport was studied by chloride profiling and moisture sorption tests, corrosion initiation was determined electrochemically, and corrosion products were identified by Raman spectroscopy. Cyclic exposure accelerated moisture and chloride ingress relative to immersion, reducing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>C</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">crit</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> testing from 2–5 years to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo>∼</mml:mo> </mml:math> 1 year. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>C</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">crit</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> scattered widely in all exposure regimes (0.5 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo>…</mml:mo> </mml:math> 3.5% chloride by cement weight), and statistics showed at most weak inter-regime differences in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>C</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">crit</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> . Post-autopsy Raman consistently identified chloride green rust near anodic zones and Fe(III) oxyhydroxides further away. Combined with thermodynamic modelling, these observations are consistent with established chloride-assisted pathways that can sustain localised corrosion under the exposure regimes tested. Finally, implications for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>C</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">crit</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> laboratory testing are discussed.