Page 14

2017SummerShotcreteEMag

Aiming for Extended Service Life with Migrating Corrosion Inhibitors By Ray Schallom III, Jessi Meyer, and Julie Holmquist Corrosion of embedded steel reinforcement is a threat to all types of reinforced concrete, leading to cracking, spalling, and a shortened service life of the structure. While the pH of new concrete initially protects embedded steel reinforcement by creating a natural passive oxide layer, it inevitably gives way to the effects of carbonation, chloride exposure, and acid rain or other industrial pollutants. The corrosion process initiates, followed by deterioration that challenges the durability and safety of structures. A variety of technologies have been developed to counteract this process, ranging from corrosion-inhibiting admixtures to cathodic protection (CP). Calcium nitrite (CNI), an older admixture technology, is effective against chloride-induced corrosion, but has disadvantages in that its dosage rate is variable depending on expected exposure to chlorides (the more chloride exposure, the higher the dosage rate). As dosage rates increase, so do CNI’s adverse effects on the physical properties of concrete, such as dramatically increased shrinkage and accelerated setting time. CP technologies require reinforcement to be continuous and involve installation of many embedded galvanic anodes, or application of an impressed current, which must be continuously monitored. MIGRATING CORROSION INHIBITOR TECHNOLOGY Migrating corrosion inhibitors, which are based on organic amine alcohol and amine carboxylate technology, offer many advantages for extending the service life of concrete structures. They have a set dosage rate, which is five to 30 times lower than that of CNI, and generally have no effect on the physical properties of concrete (strength, shrinkage, and freezing and thawing). Some versions can delay setting time, whereas others are normal set. Migrating corrosion inhibitors are available in both liquid and powder formulations for ease of application, and are also available as topical treatments for existing structures. When added to the concrete mixture, they are dispersed through the mixing process but also have the ability to migrate through concrete pores by capillary action and vapor diffusion. They travel randomly from areas of high concentration to areas of low concentration, according to Fick’s Second Law, until they come into contact with embedded reinforcement. Migrating corrosion inhibitors are considered mixed corrosion inhibitors, meaning they provide protection to both anodic and cathodic areas. They are attracted to embedded metals and adsorb onto them. Figures 1 and 2 demonstrate the chemistry of amine alcohols and amine carboxylates on the metal surface. This chemistry allows migrating inhibitors to form a tenacious bond that protects the metal from interacting with corrosive elements at both the anode and cathode of a corrosion cell. This protective layer also keeps the pH at the reinforcing bar surface at an optimal level for inhibiting corrosion and reduces existing corrosion rates.1 ADVANTAGES OF MIGRATING CORROSION INHIBITORS Migrating corrosion inhibitor technology has many advantages in terms of effectiveness and user friendliness. As an admixture, migrating corrosion inhibitors can be readily added to shotcrete or other concrete mixtures at a relatively small dosage rate compared to other admixtures (1 to 1.5 pints/yd3 0.6 to 1 L/m3 for liquids, 1 lb/yd3 0.6 kg/m3 for powders). They do not negatively affect concrete properties, and they meet ASTM C1582/C1582M2 requirements such as flexural strength, compressive strength, setting time, and freezing-and-thawing durability.3 Migrating corrosion inhibitors are nonhazardous and environmentally friendly and include bio-based options made from by-products of corn. The latest generation amine-carboxylate-based migrating Fig. 1: Illustration of amine alcohol inhibitor, where R1, 2, 3 is H or alkyl group OH tail end group Fig. 2: Illustration of amine carboxylate inhibitor, where R1, 2, 3 is H or alkyl group OH tail end group 12 Shotcrete | Summer 2017 www.shotcrete.org


2017SummerShotcreteEMag
To see the actual publication please follow the link above