CORROSION-INHIBITING ADMIXTURES (CIAs)
Corrosion of reinforcing steel in concrete has always been a problem. The related cost for corrosion damage of infrastructure alone exceeds billions of dollars each year.
When reinforcing steel corrodes, one of the primary substances produced is ferric oxide, more commonly known as rust. Rust occupies several times the initial volume of the steel. This increase in volume creates stresses within the concrete surrounding the corroding reinforcing steel. These stresses eventually cause spalling of the protective concrete cover.
While unsightly, this also may create the possibility of catastrophic structural failures. As the corrosion process accelerates, the cross-section of good reinforcing steel decreases. This increases stresses in the steel that can exceed safe limits. Therefore, several methods have been developed to reduce the corrosion rate of reinforcing steel and extend the service life of the structure. These include the use of corrosion-inhibiting admixtures (CIAs).
Corrosion-inhibiting admixtures are used in concrete that is exposed to external chloride sources. External chloride sources include deicing salts (commonly used to melt ice in winter) and marine environments where concrete is either submerged in or located close to seawater. Other applications include structures near chloride contaminated soils or concrete containing admixed chlorides, such as calcium chloride.
How they work
Corrosion-inhibiting admixtures work by elevating the chloride threshold. The chloride threshold is defined as the concentration of chloride ions necessary at the reinforcing steel surface to initiate active corrosion. This is usually done by reducing the anodic reaction or the availability of ferrous and ferric ions to react with chloride. Typically in the high pH environment of concrete, a passive (protective) layer (ferrous hydroxide) is formed and remains fairly stable, protecting the reinforcing steel from active corrosion. In the presence of chloride ions or carbonation, this passive layer breaks down and corrosion of the reinforcing steel is accelerated.
Materials used to make corrosion-inhibiting admixtures include calcium nitrite, sodium nitrite, dimethyl ethanolamine, amines, phosphates and ester amines. Of these, calcium nitrite is the most widely used and is classified as an active, anodic corrosion inhibitor. Calcium nitrite reacts with the iron ions, increasing the stability of the passive layer. In essence, the iron is attracted to the nitrite more than the chloride. Since the efficiency ratio (the effective ratio of the nitrite to the chloride) of the nitrite is approximately equal, the concentration of the chloride ion must exceed that of the nitrite ion in order to break down the passive layer and begin actively corroding.
Other corrosion-inhibiting admixtures have mechanisms that reduce the rate at which chloride gets into the concrete (chloride ingress). These are sometimes referred to as passive systems. Some of these are considered to be chloride screening admixtures in that they have a hydrophobic (repels water) component, which lines the concrete’s pore structure and reduces moisture ingress. Chloride ions are transported by moisture in the pore structure of concrete. By reducing the moisture ingress, they also reduce the chloride ingress. While chloride screening admixtures are not technically corrosion inhibitors, they are effective at increasing the time-to-corrosion initiation, which ultimately increases the service life of a structure.
Finally, some corrosion-inhibitors are cathodic-based. They interfere with the reduction of oxygen, which is necessary for the corrosion process. These types of admixtures are not commonly used. However, cathodic protection systems (designed for use after concrete has hardened or been exposed to chloride) are more commonly used to extract chlorides or reduce the rate of corrosion for existing structures.
Effects on concrete
Corrosion inhibiting admixtures, such as calcium nitrite, can increase the chloride threshold by five times. Calcium nitrite is also considered an accelerator. This may be of concern in warmer climates or with increased dosages.
Nitrite is water-soluble and therefore present in the pore solution, increasing its conductivity. As a result, concrete containing calcium nitrite will have increased Rapid Chloride Permeability (RCP) values Calcium nitrite has very little effect on air-entrainment.
Some organic corrosion inhibitors or pore lining CIAs will require much greater dosages of air-entraining and water-reducing admixtures. They may also slightly reduce compressive strength results.
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