Cathodic Prevention-Protection of Steel in Cracked Concrete

ABSTRACT

Reinforcement steel in concrete corrodes severely in marine service when chloride ion concentration exceeds a critical threshold level (CT). Cathodic prevention (CPrev) has been proposed as a supplemental corrosion management approach to extend service life of structures under this severe environment. This work seeks to determine the performance of CPrev and cathodic protection (CP) systems applied to cracked concrete in a simulated marine environment. Experiments use reinforced concrete blocks with controlled-width cracks placed along the length of a central reinforcing steel bar, with initial cyclic exposure to a 5% NaCl solution. Crack widths ranging from 0.01 in to 0.04 in (0.25 mm to 1 mm) and polarization levels ranging from -330 mV to -540 mV (SCE) were evaluated for cathodic prevention over a 3.5 year exposure period. Specimens without any protection served as open circuit potential (OC) controls. Time to corrosion initiation increased substantially as polarization reached -540 mV (SCE), with 4 instances of no activation at that potential after >1300 days of continuing exposure, including even one instance with the widest crack (0.04 in). Interpretation of the results with an empirical equation fit procedure tentatively projected extended cathodic prevention in the presence of cracks up to 0.04 in at a polarization potential somewhat beyond (e.g., ~ -0.56 V (SCE)) the most negative values examined here. Long term verification of that projected limit is needed.

INTRODUCTION

The corrosion of steel in concrete, especially in marine environment, can have an adverse effect on the service life of a structure. The large inventory of reinforced steel bridges on salt water of some major transportation agencies is subject to corrosion of the embedded steel due mostly to chloride induced corrosion. It is common today to design bridges to achieve a minimum 75- year service life and the main design approach to that end has been to improve quality of concrete to make it less permeable, and to increase the clear concrete cover.¹ Both steps greatly extend the time to initiation of corrosion ², but the added cover places construction constraints and some high performance concretes may be more susceptible to cracking requiring special handling with associated risk and cost. These cracks in concrete structures can cause major localized durability problems because the transport of chloride ions to the reinforcement is greatly enhanced. Chloride concentrations of about 2.4 kg/m³ were found for example at the steel/concrete interface in cracked concrete locations in some bridges build with low permeability concrete, compared to about 0.24 kg/m³ or less at the steel/concrete interface in sound concrete locations.³ Thus early corrosion in high performance concretes is likely to develop mainly at crack locations. For this reason, performance in cracked concrete is becoming a dominant concern in modern design.^{4, 5}