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Avoiding Corrosion Damage in Reinforced Concrete
Corrosion-inhibiting admixtured protect steel reinforcing. By James M. Gaidis and Arnie M. Rosenberg November 2001 Concrete International, The Magazine of ACI, vol 23, No. 11. Concrete structures such as bridges and buildings normally contain steel reinforcement. The main benefit is an economical increase of safe load-carrying capacity and ductility for the combination. Thermal coefficients of expansion for steel and concrete are well matched. In addition, we have known for almost a hundred years that the alkaline nature of the surrounding concrete assures protection of the steel from corrosion.1 Generally, steel is chemically passive in concrete because of concrete's alkalinity, but it isn't immune to corrosion. That is, the rate of corrosion can be so low that normally we can neglect it for all practical purposes. But if conditions change, the corrosion rate could increase to problematic proportions. The corrosion process requires water, oxygen from air, and a catalyst such as salt.2,3 Steel won't corrode in dry reinforced concrete, even if salt is present, nor will it in wet concrete, even in the presence of chloride ions, which are in salt solutions, if oxygen is essentially unavailable, for instance, at great depths under the ocean surface. In the U.S., the major impetus for large-scale use of corrosion inhibitors in concrete arose from premature failures of reinforced concrete bridges on the interstate highway system. Road crews salt these bridges in winter to melt snow and/or prevent icing.4 The problem was severe, and continues to be a major concern.5 Studies into this began in the 1960s and led to the commercialization of a calcium nitrite admixture in 1980 by W.R. Grace, and of other admixtures based on organic inhibitors, such as those from Axim Concrete Technologies, Cortec Corp., BASF Admixtures, and Sika Corp. Use of corrosion inhibitors in concrete structures is quite common, based on the successful performance of these products. Corrosion Problems Damage caused by corrosion of reinforcement in concrete results from oxidation due an electrochemical process. In this process, water is the solvent medium, which allows transportation of ions. Oxygen is the reactant, which takes electrons from iron atoms, causing iron to go into solution. Through a very similar process, flashlight batteries produce current by the corrosion of zinc anodes in an aqueous medium containing manganese dioxide. The occasional burst battery is suggestive of the damage that can occur in reinforced concrete after corrosion has occurred. In reinforced concrete, a catalyst, like chloride ion from salt that has diffused into concrete, is the critical component in the corrosion process. This accelerates the deterioration and keeps it going. Mechanistic details are difficult to pin down, but the chloride ion is able to penetrate and weaken the thin film of iron oxides coating the reinforcing steel. This covering is normally passive in concrete. Chloride ions are also uniquely able to react with ferrous ions (iron in solution) after they are liberated from the rebar, allowing them to travel some distance before precipitating as iron hydroxide. When the ferrous ions are finally deposited, they oxidize further by reaction with oxygen and become very insoluble hydrated ferric oxides, with a volume much larger than that of the original iron in the rebar. The chloride ions are then released to return to the rebar and continue the corrosion cycle.
Stresses built up by the volume increase, as predicted by Fig. 1, lead to disruption of the concrete surface in the form of delaminations, or highly visible staining from cracks that will form. Inspectors tend to notice such effects well before the whole structure suffers a reduction in strength. Still, the appearance or use of the structure may be greatly impaired, requiring repairs well before expected (Fig. 3).  Fig. 3: Corrosion staining in concrete Categorizing damaged structures Concrete structures undergoing chloride ion-induced damage fall into two main categories:
Since structures exposed to deicing salts usually have the advantage of colder temperatures for at least part of the year, compared to marine structures, corrosion rates may be lower. There is also a greater likelihood of more complete drying of the structure in the summer, which could reduce corrosion rates by diminishing the water needed to transport ions. There is a disadvantage for these structures serving in cold weather environments, however, over and above the chloride from deicing salt. Damage due to corrosion may be abetted by freezing water trapped in tiny cracks, which are initiated by summertime corrosion and kept open by expanded corrosion products. Then, during the following summer, the cracks would permit deeper access to water and oxygen, and would fill more easily with corrosion debris. Reducing corrosion damage Some ways of preventing or reducing corrosion damage in concrete are:
A consortium, consisting of Master Builders, Grace Construction Products, the Silica Fume Association, and the University of Toronto, was established under ACI's Strategic Development Council to fund the development of an initial service-life prediction and life-cycle cost model. After further development, the consortium intends to turn over this model, Life-365, to ACI Committee 365, Service Life Prediction, for review and possible adoption. Life-365 takes into account variables such as level of corrosion inhibitors and other treatments such as sealants; membranes; and rebar made from black steel, stainless steel, or with epoxy-coating. Within the model, menus permit data entry for concrete properties, inhibitor level, structural dimensions (thickness and cover), and geographical location (to incorporate the temperature variable). The model computes the chloride level expected at the rebar based on Fick's Law 6 for the exposure defined. It also compares it to the protection level of the inhibitor. When the chloride level reaches the appropriate threshold level, corrosion begins and eventually the concrete will require repair.7 The initial cost and repair costs may be estimated and compared for several approaches (addition rate of inhibitors, concrete permeability, etc.) over the lifetime of the structure (as defined by the user). Corrosion of steel in concrete is an electrochemical phenomenon that can be controlled. One of the best ways to control it is use of a corrosion inhibitor. Inhibitors have proven themselves beneficial in concrete for over 20 years.8 Our understanding of corrosion and inhibitor action has reached the stage where the Life-365 model now computes service life based on variables relating to the structure, the concrete, the steel, the inhibitor, and the geographical location of a structure to be built, Protection can be specified to prevent corrosion or minimize its effect for the lifetime of a structure. The maximum benefit from an inhibitor will be obtained by allowing the manufacturer to examine its final application and to make recommendations based on the model and related experience. References
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Axim Concrete Technologies | Euclid Chemical Company | Grace Construction Products
BASF Admixtures | Sika Corporation
Arnie Rosenberg, Executive Director
Email: info@corrosioninhibitors.org
Phone: 301-340-7368