Extending the service life of concrete structures is one of the major concerns of the concrete industry. Many studies have been performed in the field of predicting, preventing and reducing the corrosion of steel reinforcement induced by the attack of de-icing salts. According to design manuals (CEB, 1992) and literature (Cusson, Qian, & Chagnon, 2008), the most important factors regarding the protection of steel reinforcement in concrete are the quality of the concrete used and the thickness of concrete cover over the reinforcement.
Therefore, the most efficient solution to prevent corrosion in concrete involves using high quality concrete that has been properly cured after placement. This will ensure that the concrete possesses a dense and disconnected pore structure, with limited cracking potential and a high resistance to chloride penetration. Once concrete has cracked or aggressive agents have been able to pass through it, the use of corrosion inhibitors can act as a secondary protective barrier to help prevent corrosion.
The main goal of all corrosion inhibition solutions are to either prevent corrosion by blocking and/or delaying the initiation of corrosion (preventive applications), or to slow down corrosion once it has been initiated by reducing the corrosion rate (curative applications) (Bertolini, Elsener, Pedeferri, & Polder, 2004). Various types of inhibition products are available on the market, but not all of them are efficient in practice. According to several papers (Hansson, Mammoliti, & Hope, 1998), (Rouleau, 2013), (Cusson et al., 2008), the use of calcium nitrite based inhibitors added to fresh concrete is considered one of the most efficient solutions for corrosion prevention when it is used properly in conjunction with a high quality concrete mix.
Calcium nitrite enhances the protective layer around steel rebar (Rouleau, 2013), and slows the dissolution of iron (anodic action). However, the inhibition efficiency of calcium nitrite is predicated on dosage (Monticelli, Frignani, & Trabanelli, 2000), which must be high enough with respect to the amount of chlorides expected to be present in the concrete (Ann, Jung, Kim, Kim, & Moon, 2006).
In order to prevent corrosion initiation, several works report a threshold effect and recommend a concentration ratio [Nitrite]/[Chloride] varying from 0.5 to 1 (Bertolini et al., 2004). On the other hand, calcium nitrite is also known to act as a moderate set-accelerator (Hansson et al., 1998), which can reduce the long-term durability of the concrete if too large an amount is used (Jolin, Beaupré, Pigeon, & Lamontagne, 1997), (Dufour, Reny, Lacroix, & Morin, 2009).
Therefore, recommendations from the manufacturer in conjunction with predictions of the amount of chloride exposure during the service life of the structure should be taken into account when choosing the initial dosage of calcium nitrite. Undesirable interactions with other admixtures should also be considered during the development of a concrete mix design containing calcium nitrite.
Although the use of corrosion inhibition systems can help delay the onset of corrosion, once it begins and/or once significant cracks appear in the cover concrete it has been found that corrosion is very difficult to stop (Cusson et al., 2008).
For all of these reasons and as mentioned in the introduction of this paper, the use of a high quality concrete properly placed at sufficient cover thickness is always recommended to enhance the service-life of reinforced concrete structures.
– Nicolas Ginouse, Ph.D, Eng., June 2015