The Role of Corrosion Inhibitors and Applications for Concrete Repair
The need to extend building longevity is universal, and society has created vast skylines and massive concrete structures that are seemingly impermeable and able to withstand whatever forces Mother Nature can dish out. But there’s a reality to the biggest concrete assets; concrete—even when it seems perfect—is subject to risk through the introduction of ions and contaminants that move through the micropores in the material and lead to corrosion of reinforcing steel rebar. The right strategy for extending concrete longevity must begin with preventing the degradation of the metals contained within it that add to its tensile strength. That is where corrosion inhibitors become vital to building management, and as reported by Kathy Riggs Larsen of Materials Performance, “To control and remediate rebar corrosion in new and existing concrete structures, multiple methods are available. These range from changing the concrete’s cement chemistry to using corrosion inhibiting techniques such as pozzolanic cement, surface sealers, corrosion inhibitor admixtures, cathodic protection, and surface-applied corrosion inhibitors.” But to understand how the different approaches to corrosion inhibitors work, it’s first important to understand how corrosion occurs and its real impact on the future and integrity of your concrete structures.
How Does Concrete Corrosion Occur?
Concrete corrosion refers to the degradation of reinforcing steel within a structure. As the importance of corrosion inhibition has increased, more companies have prioritized this area during the design phase of building assets. However, corrosion is inevitable. It is accelerated by harsh environments, including those with extreme temperatures or high salt levels. Furthermore, concrete corrosion is comparable to cancer, gradually spreading throughout a concrete structure and often invisible until cracks form on the surface.
It affects buildings and structures around the globe, and it usually begins with an improper pH balance on reinforcing steel rods. Now, concrete usually forms a film on the surface of steel rods that prevents corrosion, but with time, degradation of the concrete may lead to the exposure of additional materials, including water, chlorides, and other salts that result in the degradation of this passive film.
Carbonation is another catalyst of corrosion. Carbonation results when carbon dioxide dissolves within concrete pore fluids. This reaction is inevitable, created by the natural presence of carbon dioxide in the atmosphere. The reaction forms carbonic acid that then reacts with materials in the cement paste to create calcium carbonate.
While this may not necessarily seem like a major issue, it effectively lowers the total pH of the concrete. And since the passive film on the steel rebar depends on a highly alkaline environment to maintain its integrity, the rebar becomes more susceptible to the effects of moisture and ions in the environment. As a result, the rebar begins to react to other contaminants, like oxygen, salts and water, producing damaging results.
It’s at this point when degradation begins to affect the reinforcing steel. It’s the presence of oxygen and moisture that result in rusting, which is where the true cause of concrete degradation begins to grow.
Rusting steel rebar expands due to the chemical changes within it. The expansion is up to six times greater than the original volume occupied by the intact rebar. As a result, this forces an expansion within the concrete that will slowly crack and open the door to additional means of entry for water, salts and oxygen. The problem becomes self-propagating, weakening the concrete infrastructure and leading to spalling. Furthermore, this is a process that takes years to occur, and by the time it becomes evident, the problem is vast and much more severe than many realize.
What Is a Corrosion Inhibitor?
Left unchecked, corrosion of steel within concrete reinforcing structures will lead to massive costs for your company and may require complete replacement. However, the use of corrosion control, leveraging inhibitors for corrosion, early in the design and throughout the maintenance program of your building is the best way to extend the life of your concrete assets. A typical approach to repairing concrete that has cracked is to remove the damaged sections, replace the rebar and patch where needed. This helps to reduce the advancing of oxygen, water or salts into the concrete. Therefore, it is best to use a corrosion inhibitor in tandem with a waterproof seal to prevent exposure of rebar to the elements.
There are also products manufactured specifically to reduce the risk of chloride ions from coming into contact with reinforcing steel and even modifying the concrete mixture to increase its impermeability. Essentially, waterproofed concrete means little risk for entry of corrosive agents. However, highly corrosive environments, like the salt-laced winds along coastlines, require a specialized approach to preventing degradation.
This includes materials applied directly to rebar to increase the strength of the passive film on the metal, leveraging chemical agents to mix with concrete that will help to strengthen that film and even increase the electrical resistance of the metallic surface of rebar. Obviously, early protection at the time of design and construction is preferred. However, repairing damaged concrete and sealing cracks with corrosion-inhibiting materials is necessary when spalling or other signs of degradation appear.
A few examples of companies that manufacture concrete corrosion inhibitors are Cortec, Degussa, Sika, and Masterbuilders. However, there are always differences in product specifications and formulation. Thus, it’s important to have a quality engineer evaluate the unique structures and concrete requirements prior to selecting a product. This ensures that the chosen inhibitor will serve its purpose the best and avoid future repairs or failure.
Another way to approach corrosion prevention is through electrochemical means, using both anodic and cathodic materials. These electrochemical means are usually found in concrete applications that are subject to continuous exposure to different liquids, such as the holding tanks in sewage or water treatment plants.
Such applications require a means for managing the flow of electrons associated with certain ions, like those of salts, and rendering them inert. Other options for adding the electrochemical protection include an impressed current cathodic protection (ICCP) that can effectively stop spalling and prevent further damage to rebar.
Both still require a power supply, even if temporary, and cables to supply power to the anodes/cathodes. As a result, it creates an added expense and complicates the process of preventing corrosion. Of course, this is a permanent solution that requires the proper selection of both anodes and cathodes, as well as in-depth specifications for current strength, to cause ions to move away from rebar and effectively prevents future oxidation (rusting) of the steel. However, this is perhaps the costliest approach to corrosion prevention other than total replacement due to the added equipment it entails.
Thus, working with a chemical-based inhibitor for concrete that leverages an applied material or chemical additive within a concrete mix is usually preferred. For reference, here are some of the most common admixtures used that may be referenced in engineer repair specifications:
- Amine carboxylate
- Amine-ester organic emulsion
- Calcium nitrite
- Organic alkenyl dicarboxylic acid salt
There are also materials that can be applied directly to the surface of the concrete to provide corrosion resistance to rebar. Ironically, this method takes advantage of slightly porous concrete to provide a way for such materials to penetrate concrete and restore the integrity of the passive film on the surface of rebar. In a sense, these corrosion prevention materials take on a dual-purpose with waterproofing as well.
What to Consider When Choosing a Concrete Inhibitor or Sealer
Waterproofing membranes are perhaps the overlooked side of concrete repair and damage prevention strategies. While focusing on corrosion inhibitors is great, the waterproofing membrane that’s applied to the surface of the concrete is the first line of defense against the penetration of corrosion-inducing materials. Furthermore, the waterproof seal on the surface of concrete actually serves a purpose in inhibitors for concrete too. As explained by Adhesives & Sealants Industry (ASI), “Concrete sealers come with a variety of different features. The most basic function is to protect against the ingress of moisture, carbonation, and other potentially damaging elements. Others add stain resistance or corrosion inhibition.
Inhibiting corrosion is especially needed in applications where there is exposure to sea air with elevated humidity and chloride levels or in northern climates where there is a need for deicers.
As cities, counties, and states that experience harsh winters are switching to liquid deicing brines instead of granular ones, more chlorides are entering concrete surfaces at a faster rate than ever. This trend is accelerating damage due to freeze-thaw cycles and the corrosion of reinforcing steel, leading to the heightened need for sealers with corrosion inhibitors.”
To ensure your team chooses the right sealant or other admixture for preventing corrosion, it’s important to take a few considerations into account. These include:
- Liquid vs. powdered corrosion inhibitors. Depending on the scale and specifications of the repair, a liquid or powdered corrosion inhibitor may be appropriate. Of course, both materials are usually mixed with concrete or grouting materials to fill cracks and damage. Regardless, the key is to ensure your team can store and transport the materials properly based on their liquid or powdered state.
- Substrate integrity and extensivity of water penetration. Another critical consideration is the extent of damage to the substrate. If excessive water penetration has occurred, a corrosion inhibitor may need to be sprayed directly onto the replaced rebar prior and allowed to set prior to completing the repair.
- Setting time and weather conditions necessary for application. Like all types of concrete repairs, the setting time and weather conditions will affect the efficacy of corrosion inhibitors, so planning is critical for ensuring the products work as expected.
- Effective against carbonation and chlorides. Given that carbonation and chloride are two of the primary causes of corrosion, it’s important to choose your inhibitors for corrosion based on the conditions that the concrete is subjected to. This may include corrosion inhibitors against both or one type of corrosion agent.
- Ease of application and reapplication expectations. It’s always important to think about the future repairs and whether corrosion inhibitors will require reapplication. This is especially true when looking at corrosion inhibitors applied directly to the exposed surface of concrete, taking all above considerations into account along the way.
- Other specifications of the repair as determined by the engineer. Even if everything looks great, the most important consideration when selecting an inhibitor is how well it aligns with engineer specifications for the repair. Fortunately, the engineer will usually specify the specific chemical and physical components of each inhibitor to eliminate all uncertainty.
Advantages of Corrosion Inhibitors in Concrete Repair
The advantages of using corrosion inhibitors for concrete repair are very easy to understand and include:
- Less spalling and fewer repairs. As rebar remains strong, the concrete is less likely to become subject to damage, resulting in lower total maintenance costs, provided the proper waterproofing barrier and surface corrosion inhibitors for concrete are used. This effectively prevents the further degradation of steel and concrete structures too.
- Ability to target multiple causes of degradation. While some buildings may be subject to more of a specific type of corrosion-causing material, there are some that will always be present, such as carbonation. As a result, a core advantage of using an inhibitor is targeting multiple causes of degradation and corrosion in the same repair. Furthermore, the corrosion inhibitor admixtures can also offer increased adhesion between substrates to increase the impermeability of the concrete and prevent future corrosion.
Extend the Longevity of Your Concrete Assets by Partnering With an Expert in Concrete Repair and Maintenance
While seemingly the strongest building material, concrete is actually a protective covering for the real strength of a building—its rebar reinforcements. Without that reinforcement, concrete will inevitably fail and even worse, the rebar itself can cause untold damage to your concrete structures if left unchecked. As a result, it’s important to extend the longevity of your building assets by choosing a top-quality partner for handling concrete repairs, corrosion prevention programs, and all other maintenance needs.
Contact HSG Inc., to learn more about the role of corrosion inhibitors in maintaining your buildings.