Electrochemical Treatments Significantly Extend the Service Life of Reinforced Concrete Structures by Brian Pailes INTRODUCTION CHLORIDE INDUCED CORROSION Corrosion of steel reinforcement is one of the most significant resiliency challenges that concrete structures face throughout the world. Corrosion typically results from the ingress of chlorides or carbonation which breaks down the naturally passive state of steel in concrete. There are a multitude of preservation options for reinforced concrete including galvanic anodes and impressed current cathodic protection. However, electrochemical treatments can also provide significant service life extensions with minimal maintenance and upkeep, making them very attractive options for infrastructure preservation. In the preservation of reinforced concrete structures, there are two primary electrochemical treatments that are implemented, electrochemical chloride extraction and re-alkalization. Electrochemical chloride extraction (ECE) draws chlorides away from the embedded steel reinforcement and out of the concrete thereby restoring the steel’s passivity. Re-alkalization increases the alkalinity of carbonated concrete and restores the passivity of the steel reinforcement. Both these processes are achieved by temporarily installing an externally mounted anode and applying an electric field between the temporary anode and the steel reinforcement. In ECE, this electric field transports chloride ions away from the steel, and in re-alkalization, it transports an alkaline solution into the concrete. These methods are also covered in ICRI 510.1, Guide for Electrochemical Techniques to Mitigate the Corrosion of Steel for Reinforced Concrete Structures ¹, that provides a great resource regarding electrochemical preservation techniques. This article will focus on two electrochemical treatment case histories; Interstate 480 (I-480) through downtown Omaha, Nebraska in which ECE was conducted on the substructure and University Hall at the University of Chicago in which re-alkalization was implemented on the façade. 14 CONCRETE REPAIR BULLETIN MARCH/APRIL 2019 Reinforcing steel in concrete is protected from corrosion by the high alkalinity of the concrete pore solution, typically greater than a pH of 12. The high pH of the pore solution causes the formation of a passivating film on the surface of rebar, thereby preventing corrosion. Reinforced concrete structures exposed to chloride ions from seawater, de-icing salt application or chloride-containing soil can have the passivating film destroyed. Chloride ions diffuse from the concrete surface, and once their concentration at reinforcement depth reaches a threshold value, corrosion is initiated. The generally accepted chloride threshold for the initiation of corrosion at the depth of steel in reinforced concrete is between 1 to 2 lbs (0.45 to 0.9 kg) of chloride per cubic yard (0.76 cubic meter) of concrete. Concrete can also contain background chlorides, which are either admixed into fresh concrete or are naturally present in cement products or aggregates. Admixed chlorides could be added to the concrete mix through the use of chloride-containing chemical admixtures or the use of seawater instead of potable water. Admixed chlorides and chloride ions that diffuse into the concrete from the environment are referred to as “free” chlorides and are responsible for chloride-induced corrosion in reinforced concrete. Chemically bound chlorides present in aggregate are not able to initiate corrosion. CARBONATION INDUCED CORROSION Carbonation lowers concrete’s pH as carbon dioxide diffuses into moist concrete. The carbon dioxide reacts with the free lime present in the concrete pore structure, thereby reducing the OH-concentration within the concrete. If the pH of the concrete surrounding the reinforcing steel is lowered below pH 11, depassivation of the reinforcing begins and general corrosion initiates. Carbonation can cause corrosion in concrete that has not been contaminated with chlorides and can also propagate through cracks. In WWW.ICRI.ORG