Specifying corrosion inhibiting admixtures for concrete structures

by arslan_ahmed | October 3, 2023 12:39 pm

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By Julie Holmquist, Ash Hasania, and Lisa Marston

In construction, service life demands are increasingly rising. It is becoming more common to receive design requests for bridges, high-rises, and other concrete structures with service lives of 100 or 150 years.

How is an engineer to keep up with these demands while staying within a limited budget? If funding is plentiful, what additional steps can be taken to maximize reinforced concrete durability? Corrosion inhibiting admixtures can be an important piece to this puzzle, but their selection should be made while carefully understanding the characteristics of each type. This article discusses some key considerations engineers should make first when choosing whether or not to use a corrosion inhibiting admixture and subsequently choosing which admixture to specify.

Why is concrete corrosion a concern?

Corrosion protection has a huge impact on the service life of reinforced concrete and should be considered for any concrete structure meant to last more than 10 years. Concrete creates a naturally protective environment for reinforcing metal because of its high pH. This alkaline environment initially inhibits corrosion of embedded rebar. However, as time goes on, the concrete reacts with atmospheric CO2 and ultimately replaces calcium hydroxide (Ca[OH]₂) with calcium carbonate (CaCO₃), which lowers the pH of the concrete so the rebar is no longer protected. This process is called carbonation and typically occurs at a rate of about 1 mm (0.039 in.) per year.¹ It therefore may not reach the depth of the rebar for several decades. However, some environments, such as industrial parks, may have higher CO2 levels that accelerate carbonation and shorten time to corrosion.

While carbonation can play a role in concrete corrosion, chlorides are often the foremost concern because they can incite corrosion even before carbonation sets in. Chlorides from sea spray, deicing salts, or saline groundwater leach in through concrete pores and combine with moisture to create an electrolyte that speeds up rebar corrosion rates. When concrete begins to crack due to physical damage or freeze-thaw cycles, matters get worse because the chlorides have a new path that makes it even easier for them to reach the surface of the steel.

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Once corrosion initiates, it starts a chemical reaction in which the iron in the steel reinforcement oxidizes. The resulting iron oxides, or rust, represent metal loss on the rebar. They also have a higher volume than the original reinforcing steel. This “swelling” of corrosion products puts pressure on the overlying concrete, leading to cracking and spalling, which exposes the reinforcement to more corrosives. The vicious cycle continues deteriorating the structure until replacement or proper repairs take place.

Aside from the safety concerns of an unsound structure, the economic and environmental costs are chief concerns because of the high investment in energy-intensive raw materials needed to build a new structure. Putting preventative measures in place to delay the time to corrosion and reduce the rate of corrosion that has already started is therefore one of the key strategies an engineer can take in designing a structure to last longer.

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Where is corrosion a concern?

Since different environments are more corrosive than others, an engineer should take a structure’s location and intended purpose into account when specifying materials for the project. Regions that use a high volume of deicing salts in the winter are especially notorious for corrosion problems on reinforced concrete parking ramps. Bridges and seawalls in seaside environments are also under greater attack—sometimes from humid, salt-laden air, and other times by direct immersion in saltwater. Elevated temperatures also play a role, often accelerating corrosion in tropical or subtropical climates. In places such as the Middle East, high mineral sabkha soil and/or high-water tables (possibly with saltwater) add another dimension of corrosiveness. Functional factors can also play into the equation, as when concrete elements are built to handle seawater or brine at desalination plants, creating an obvious risk for an early corrosion attack.

Why use corrosion inhibiting admixtures?

There are many strategies to mitigate corrosion. These include using a lower water to cement ratioto decrease permeability, increasing concrete cover, using galvanized or epoxy-coated rebar or an alternative type of reinforcement, adding cathodic protection, and others. Many of these approaches have their tradeoffs—most often economically. For example, using epoxy-coated rebar, swapping black steel rebars with stainless steel, installing sacrificial anodes, or connecting to a power source for impressed current cathodic protection (ICCP) can add significant cost to the project price tag.

Corrosion inhibiting admixtures are typicallya more cost-effective approach for tight budgets. Similar to other admixtures, they are liquid or powder additive formulations that are mixed into the concrete batch at the ready-mix plant or onsite. When properly mixed, they are dispersed throughout the concrete batch, allowing them to either delay the onset of corrosion, slow down active corrosion rates, or both. Admixtures often constitute a very small percentage of the total building cost while significantly adding to service life predictions.

Economic issues aside, investing extra money in “more durable” materials is not failsafe: cracks can still occur in a thick concrete cover, leading to chloride ingress, and epoxy-coated rebar can corrode where the coating has chipped off. On the other hand, if funds are not lacking and maximum durability is required, corrosion inhibitors can add another level of protection when used in conjunction with one of these other corrosion inhibiting strategies.

What can qualify as a corrosion inhibiting admixture?

When it comes to industry standards, engineers often look to guidelines from the American Concrete Institute (ACI) for direction. While these guidelines do not give the final answer, they do compile information and opinions from experts in their fields. The “ACI PRC-222-19: Guide to Protection of Metals in Concrete Against Corrosion”² outlines a variety of industry-accepted best practices for inhibiting corrosion. The “ACI 212.3R-16: Report on Chemical Admixtures for Concrete” focuses on concrete admixtures in general, with an entire chapter dedicated to admixtures that inhibit corrosion.³ These two guidelines define corrosion inhibiting admixtures as those that delay time to corrosion onset and/or reduce the corrosion rate. They refer to ASTM C1582, Standard Specification for Admixtures to Inhibit Chloride-Induced Corrosion of Reinforcing Steel in Concrete,⁴ which requires a corrosion inhibitor to reduce the rate and amount of corrosion, compared to a control. The ACI standards also imply that simply reducing chloride intrusion is not enough for a product to qualify as a corrosion inhibiting admixture.

The reference to ASTM C1582 brings up a good point; it is the most widely accepted standard for qualifying a corrosion inhibiting admixture. This benchmark requires that a corrosion inhibiting admixture pass either ASTM G109, a multi-year test, or ASTM G180, a three-day test, in order to be considered a chloride corrosion inhibiting admixture. While some experts feel there is room for improvement on how well these tests simulate actual corrosion and protective mechanisms in concrete, it is an important baseline to start the screening process until more relevant standards can be designed and implemented. The types of admixtures currently recognized as commercially available corrosion inhibitors in ACI 212.3R-16 are amine carboxylates, calcium nitrite (Ca[NO₂]₂), and amine esters.⁵ Their elaborate definitions in the ACI document can be summarized as follows:

• Amine carboxylates: Mixed inhibitors that adsorb onto metal and create a hydrophobic protective layer that protects against anodic and cathodic corrosion reactions.⁶
• Calcium nitrite: An inhibitor that raises the chloride threshold by reacting with the anode of the corrosion cell.⁷
• Amine esters: Pore-blocking elements that form
  a protective film on metal surfaces.⁸

What are the key differentiators and factors among admixtures?

Some characteristics to look at include biobased content, dosage rate, impact on set time and air content, acceptability for potable water certification, performance after cracking, and the ability to inhibit corrosion caused by both carbonation and chlorides. ACI 212 covers many of these factors and is a good reference to consult for additional information after reviewing the basics covered below.

Calcium nitrite

Calcium nitrites (CNI) are perhaps the oldest corrosion inhibiting admixtures in existence. These raise the chloride threshold and therefore must be dosed according to expected chloride loading. The higher the expected chloride content, the higher the dose.⁹ This chloride threshold mechanism also means calcium nitrite is suitable for chloride-induced corrosion, but not for carbonation-induced corrosion. Calcium nitrite is a set accelerator, which poses challenges for high temperature pours. It does not have biobased content or NSF/ANSI 61 certification.

Amine carboxylates

A key advantage of amine carboxylates is that they are dosed independently of chloride concentration—they have one of the lowest doses at 0.6 to 1 L/m³ (1 to 1.5 pt/yd³). Another important feature is amine carboxylates do not accelerate set time, fostering good workability especially when pouring concrete in the middle of a heatwave. They often contain a percentage of renewable content. To date, the only corrosion inhibiting admixture on the market that is a United States Department of Agriculture (USDA) Certified Biobased Product comprises amine carboxylate chemistry.¹⁰ The use of biobased content can help projects earn credit toward Leadership in Energy and Environmental Design (LEED) certification, so this aspect is especially attractive to those considering sustainable construction.

Amine carboxylate technology has also shown good protection in cracked beam testing,¹¹ an important characteristic for ongoing protection beneath cracked or damaged concrete. Several amine carboxylate admixtures are certified to meet ANSI/NSF 61 for use in drinking water system components, which is a relevant feature for those designing concrete water tanks. There is also good evidence that amine carboxylate admixtures will inhibit carbonation-induced corrosion as well as chloride-induced corrosion.¹²

Amine esters

Amine esters are dosed at 5 L/m³ (1 gal/yd³)¹³ and do not significantly affect set time.¹⁴ Amine esters are likely to block the ingress of contaminants to slow the corrosion process. However, since pore-blocking is the primary mechanism of amine esters, it is difficult to evaluate their corrosion inhibiting efficiency per standard tests.

Estimating service life

Once the engineer has identified which admixture characteristics are preferred for the project, computer modeling can be used to estimate and compare projected service lives for different combinations of protective materials and concrete mix types. Several service life prediction models exist, including STADIUM,¹⁵ WJE CASLE,¹⁶ and Life-365¹⁷; the latter is open-access and therefore more commonly used. These models take into account many different factors, such as environmental conditions (e.g. marine, high temperature), structural component (e.g. beam, column, slab, etc.), mix type, corrosion inhibitors, and the use of other corrosion-reducing features (e.g. epoxy-coated rebar or galvanized rebar). Based on these and many other parameters, the service life prediction software estimates both the time to corrosion initiation and propagation. Together, these estimates give the expected years of service life.

Some corrosion inhibitors are already entered as values into the models. In the case of Life-365, calcium nitrite and an amine ester corrosion inhibiting admixture are included in the drop-down list. Values for other corrosion inhibitors must be entered manually. In the case of amine carboxylates, the chloride corrosion threshold limit and the propagation period can be customized in the model based on the results of ASTM G109.

Dr. Mohamad Nagi, PE, Ph.D., of the American University in Dubai (AUD), took some of these findings and concluded the chloride corrosion threshold of one amine carboxylate corrosion inhibiting admixture was 0.18 percent by the weight of concrete.¹⁸ This became the manufacturer’s recommended input for amine carboxylate admixtures in Life-365 modeling. Since ASTM G109 testing has shown a corrosion rate reduction of five times for this specific amine carboxylate formula,¹⁹ the manufacturer also recommends multiplying the corrosion propagation period by a factor of five to calculate time to the first repair.²⁰ Other variations on the same chemistry may have slightly different inputs based on test results.

Service life predictions can be very insightful when compared side by side, showing the potential time gained from different types of admixtures and protective mechanisms. For example, one hotel project situated on the Gulf Coast had an estimated service life of 11 years when using uncoated rebar, 25 years with epoxy-coated rebar, and 40 years with a biobased amine carboxylate. The project ended up choosing a biobased amine carboxylate, saving six figures by eliminating the cost of epoxy-coated rebar in favor of the admixture.²¹

Making use of admixture selection tools

Service life demands continue to increase as customers realize the importance of building lasting structures for economic, safety, and environmental reasons. While there are multiple approaches to boost service life, one of the most economical methods continues to be corrosion inhibiting admixtures. Ultimately, evaluating project priorities, understanding admixture characteristics with the help of industry guidelines, and comparing service life prediction results are excellent tools to help engineers specify the right corrosion inhibiting admixture for the job.

Notes

1 Read the article by Kayla Hanson, PE, “Understanding Carbonation.” Precast Inc. Magazine, July-August (Carmel, IN: National Precast Concrete Association, 2015).

2 Refer to the guide, “ACI PRC-222-19: Guide to Protection of Metals in Concrete Against Corrosion” (Farmington Hills, MI: American Concrete Institute, 2019).

3 Refer to the guide, “ACI 212.3R-16: Report on Chemical Admixtures for Concrete,” (Farmington Hills, MI: American Concrete Institute, March 2016): page 35-41.

4 See the standard, “ASTM C1582/C1582M-11(2017)e1 Standard Specification for Admixtures to Inhibit Chloride-Induced Corrosion of Reinforcing Steel in Concrete.” (West Conshohocken, PA: ASTM International, July 28, 2017).

5 Refer to ACI 212.3R-16, page 36.

6 Refer to ACI 212.3R-16, page 36.

7 Refer to ACI 212.3R-16, page 37.

8 Refer to ACI 212.3R-16, page 36-37.

9 See Table 13.3 on ACI 212.3R-16, page 39.

10 MCI®-2005; learn more and browse the product catalog at www.biopreferred.gov/BioPreferred/.

11 Consult the American Engineering Testing, Inc. “Report of Concrete Corrosion Inhibitor Testing,” AET Report 05-01171 for Cortec Corporation (St. Paul, MN: August 13, 2003).

12 Refer to the document by Xu Yongmo, She Hailong, and Boris A. Miksic. “Comparison of Inhibitors MCI and NaNo2 in Carbonation Induced Corrosion.” Materials Performance (Houston, TX: NACE International, January 2004), page 42-46.

13 Refer to ACI 212.3R-16, page 38.

14 Refer to ACI 212.3R-16, page 40.

15 Find STADIUM prediction modeling software at simcotechnologies.com/what-we-do/stadium-technology-portfolio/[4].

16 Find CASLE prediction modeling software at www.wje.com/expertise/services/detail/corrosion-assessment-and-service-life-evaluation-casle[5].

17 Find LIFE-365 prediction modeling software at www.life-365.org/[6].

18 See the research by Nagi, Mohamad, PE, Ph.D. “Report on the corrosion threshold of MCI-2005 in normal concrete reinforced with black steel based on ASTM G109 results.” (American University in Dubai, December 17, 2012).

19 Consult the American Engineering Testing, Inc. “Report of Corrosion Inhibitor Testing,” AET Report 05-00777 for Cortec Corporation (St. Paul, MN: August 8, 2011).

20 For instructions on MCI inputs for LIFE-365, see www.cortecmci.com/wp-content/uploads/2017/09/LIFE-365-Inputs-for-MCI2c-5.27.15.pdf[7]

21 Read the article by Julie Holmquist, Andrea Moore, and Casey Heurung, “Bio-based Corrosion Inhibitors: Building for resiliency in marine environments[8],” The Construction Specifier (September 2019), page 18-24.

Authors

  Julie Holmquist has been writing content for Cortec Corporation since November 2015. She enjoys learning about the science behind corrosion prevention and the many industries affected. She can be reached via email at jholmquist@cortecvci.com.

Ash Hasania is the Migrating Corrosion Inhibitors (MCI) technical sales and market manager for Cortec Corporation. He is a civil, water, and environmental engineer who has been focusing on the use of MCI for concrete since 2013. He can be reached via email at ahasania@cortecvci.com.

Lisa Marston is a regional technical service engineer at Cortec Corporation, specializing in MCI for concrete. Marston holds a BSE in chemical engineering. She can be reached at Lmarston@cortecvci.com.

Source URL: https://www.constructionspecifier.com/concrete-admixtures/

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