Influence of chloride cations on pore solution chloride and critical chloride threshold of carbon steel rebar

ABSTRACT

As part of ongoing research to validate the efficacy of a potentiodynamic polarization technique of determining the critical chloride threshold (C_CRIT) value of steel reinforcing bars exposed to chloride-contaminated concrete, the present work reports C_CRIT values of carbon steel exposed to NaCl, CaCl₂ and MgCl₂. The increasing use of locally available multi-chloride anti-icing agents in Ontario during the winter season, which are brine solution with ~21% chloride as NaCl, CaCl₂ and MgCl₂, resulted in a need to understand the impact of these salts on the threshold values. The potentiodynamic polarization method allowed the influence of these salts on C_CRIT values of carbon steel rebar to be determined in a significantly shorter period than existing standard methods.

Cement pastes with varying admixed chloride as NaCl, CaCl₂ and MgCl₂ were cast with 0.5 w/cm ratio and their pore solution content was expressed after 28 days curing for cation and anion analyses. The resulting pore solution composition revealed increasing Ca, K and Na cations and increasing chloride and sulphate anions when the cement pastes were admixed with chloride as NaCl. However, sulphates decreased in the pore solution when the pastes were prepared with CaCl₂ and MgCl₂. The consequence of the latter findings was that the threshold values by mass of pore solution of the carbon steel rebar exposed to these chloride compounds were significantly lower than bars exposed to pore solution with NaCl. Nevertheless, the conversion of these values to percentage by mass of cementitious materials gave surprisingly similar values for all three chlorides

KEYWORDS:

• Carbon steel rebar
• concrete pore solution
• critical chloride threshold
• potentiodynamic polarization
• pitting potential

Acknowledgments

The authors appreciate the funding and support provided for this project by the Ministry of Transportation of Ontario (MTO) and steel donations from Max Aicher North America.

Disclosure of potential conflicts of interest

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

I.G. Ogunsanya

ACI member Ibrahim G Ogunsanya, PhD, P.Eng, is a licensed professional engineer in Ontario affiliated with the Department of Mechanical and Mechatronics Engineering at the University of Waterloo.

L. Kristufek

L. Kristufek received her BSc and MASc from theUniversity of Waterloo and was affiliated with the Department of Mechanical and Mechatronics Engineering at the time this work was carried out.

P. Le Meur

P. Le Meur was an exchange student from Polytech Nantes in France at the time this research was conducted in the Department of Mechanical and Mechatronics Engineering, University of Waterloo.

C.M. Hansson

Carolyn M. Hansson, FACI, is Professor of materials engineering in the Mechanical and Mechatronics Engineering Department at the University of Waterloo. She is a member of ACI Committees 222, Corrosion of Metals in Concrete, and 365, Service Life Prediction. She received her PhD and is alicensed professional engineer.