September 19, 2019
by Julie Holmquist, Andrea Moore, and Casey Heurung
Marine environments are known for posing an elevated threat of corrosion to metal. Engineers take special precautions to address this problem when designing reinforced concrete structures in these regions. Bio-based migrating corrosion inhibitors can be an important design factor when building a sustainable structure in a corrosive marine environment.
The first phase of the Gulf State Park master plan and enhancement project in Baldwin County, Alabama, provides an example of how bio-based migrating corrosion inhibitors are able to mitigate corrosion with an eye to sustainability and resiliency. The project was recognized by Partners for Environmental Progress (PEP) with a 2019 Environmental Stewardship Award, in part for the use of a bio-based corrosion inhibiting admixture in the buildings of the Lodge at Gulf State Park. The project is pending the Leadership in Energy and Environmental Design (LEED) Gold certification. In addition to being in line with the project’s goals for sustainability and resiliency, the admixture had the benefit of providing significant cost savings that helped keep the project within budget. The Lodge serves as a model for other project owners building with similar goals in corrosive environments.
Corrosion problems for reinforced concrete
Marine environments, especially those found in subtropical climates like the Alabama Gulf Coast, have a very well-known problem with corrosion. The corrosive combination of high temperatures, humidity, and chloride-ridden sea spray can take a toll on reinforced concrete structures faster than building owners would like.
Andrew Marlin, PE, senior principal at MBA Engineers and the primary structural engineer working on the Lodge at Gulf State Park, explained, “Concrete is going to have cracks, which allow in moisture, and moisture on the Gulf Coast has a high salt content. When the moisture gets on the reinforcing and post-tensioning, corrosion can happen very quickly. When it starts corroding, it spalls the concrete, and then you have got a huge mess.”
The Lodge was one of Marlin’s first projects located directly on the coast. However, from working with those who are regularly involved in coastal repair projects, he already knew millions of dollars go into coastal buildings dealing with corrosion issues, particularly on exposed areas like condominium and hotel balconies. His awareness of the significant corrosion problems on the previous Gulf State Park structure built in the 1960s factored into the search for enhancing durability of reinforced concrete in the new lodge. The original hotel was in serious disrepair, including corrosion damage, by the time that Hurricane Ivan helped finalize its destruction in 2004. Since the structure was so deteriorated, it had become a high priority for the publicly funded project to be durable and long lasting.
Available methods for corrosion protection
There are a variety of ways and different schools of thought on how best to combat corrosion in reinforced concrete. Some methods, such as creating a thicker concrete cover or increasing concrete mix quality, can be effective but prohibitively expensive or impractical. Epoxy-coated rebar is a common strategy for corrosion protection, but adds greater expense that may not fit within the project budget. As Marlin noted, epoxy coating can also chip off this type of rebar when it is being moved around and stored, opening up an eventual corrosion site on the rebar. Cathodic protection is also used to prevent corrosion by either electrically charging a rebar grid from a power source or attaching sacrificial anodes at regular spacing in a rebar grid prior to pouring the concrete. Finally, corrosion-inhibiting admixtures can be easily utilized to prevent corrosion on rebar by being dosed into a concrete batch at a ready-mix facility or onsite.
There are two main types of corrosion-inhibiting admixtures. Calcium nitrite (CNI) is one of the earlier ones introduced in the 1970s. This was found to increase the chloride threshold of reinforced concrete, competing with chloride ions to create a passivating layer on the rebar and thus inhibiting corrosion. A major disadvantage is admixture dosage increases significantly based on expected chloride loading, and it is generally 10 to 30 L (2 to 6 gal) per cubic yard. In addition to the large amount of material used, CNI can also accelerate set time, especially with higher doses (read the Improving Durability of Infrastructure with Migratory Corrosion Inhibitors [MCI] handbook by Boris Miksic).
Migrating corrosion inhibitors came on to the scene in the 1980s in the form of amine alcohols with vapor pressure allowing them to travel through concrete pores and form a protective layer at the level of the rebar. The technology reached an even higher level in the 1990s with the development of second-generation amine carboxylate migrating corrosion inhibitors showing a stronger adsorption to the metal surface. These inhibitors are said to offer ‘mixed’ protection by inhibiting both the anodic and cathodic corrosion reactions. Some of these admixtures offer the additional benefit of being partially derived from renewable materials. Several are also certified to meet the American National Standards Institute/National Sanitation Foundation (ANSI/ NSF), Drinking Water System Components – Health Effects, for use in potable water structures. Dosage rate is generally fixed at 0.6 L/m³ (1 pt/yd³). Rather than accelerating concrete set time, migrating corrosion-inhibitor admixtures tend to have a set-retarding quality (unless normal set is desired).
Corrosion inhibitor admixture testing
Concrete chemistry is complex, requiring the engineer or contractor to check compatibility whenever an admixture is included in the mix design. ASTM C1582, Standard Specification for Admixtures to Inhibit Chloride-Induced Corrosion of Reinforcing Steel in Concrete, is used to test-screen corrosion-inhibiting admixtures. One portion of the test evaluates how the admixture will affect the physical properties of the concrete while another section evaluates the corrosion-inhibiting performance of the admixture. Bio-based migrating corrosion inhibitor A, which was ultimately used in the Lodge, passed both test phases, as detailed below.
During ASTM G180, Standard Test Method for Corrosion Inhibiting Admixtures for Steel in Concrete by Polarization Resistance in Cementitious Slurries, testing (one of two optional methods for the corrosion inhibiting portion of ASTM C1582), four corrosion cells treated with bio-based migrating corrosion inhibitor A were compared to 13 control cells that had no inhibitor. On average, samples treated with the bio-based corrosion inhibitor were able to increase the corrosion resistance by a factor of 10, thus meeting ASTM C1582/G180 requirements that the inhibited samples show a corrosion resistance eight times the value of the control specimens in testing (Figure 1) (see “Re: Evaluation of Corrosion Inhibiting Admixture According to ASTM G180, TCG Project 17057” by Neal S. Burke). In the physical properties portion of the ASTM C1582 test, concrete samples treated with the bio-based corrosion inhibitor showed good results, indicating the admixture did not negatively affect physical properties of the concrete mix (consult “Admixture to Inhibit Chloride-Induced Corrosion of Reinforcing Steel in Concrete (ASTM C1582) Concrete Properties Testing Final Report” by Glenn Schaefer). They did tend to show a delay in set time, which can offer a benefit to placement in warmer climates.
Gulf State Park lodge project
The Lodge at Gulf State Park is an example of how a structural engineering firm worked to ensure a quality design with corrosion precautions that would fit into the project’s overall goals and budget.
“This project, more than any other project I have ever worked on, was very concerned about resiliency,” Marlin said. He added the previous hotel (built in 1974) had many corrosion problems and was irreversibly damaged during Hurricane Ivan. The new structure required a longer service-life design that would demonstrate wise stewardship of public funds. Due to the harsh coastal location, many resiliency and sustainability issues were considered, including flooding, storms, strong winds, and of course, corrosion.
Reinforced concrete was the main construction material chosen for the hotel complex. Hence, early discussions of corrosion control focused on the use of epoxy-coated rebar to discourage corrosion, despite the significant cost expected. However, as the project progressed, it was evident this option was not going to fit with the budget, no matter how hard the structural engineering firm tried. Marlin said a new opportunity opened up when the project’s LEED consultant connected him with a local supplier of bio-based migrating corrosion-inhibitor admixtures, who did cost analysis and life-cycle analyses of the different options.
Life-cycle analyses included using service-life prediction modeling. This modeling analyzes a variety of factors such as climate, splash zone, concrete-mix design, concrete coverage, etc. to predict the length of project service life before repairs will be needed. For uncoated rebar, the expected service life before repairs was 11 years, according to the analysis. Epoxy-coated rebar boosted the service-life estimate to 25 years. Data used to represent the effect of the bio-based migrating corrosion-inhibitor admixture brought the service life up to a projected 40 years before repairs.
The admixture cost analysis also promised to bring the project within budget, so Marlin investigated deeper, following up with other engineers and consultants who had used the admixture, before finally presenting the option to the construction manager and ownership team. Ultimately, the admixture was adopted, saving an estimated $250,000 to $300,000 and representing less than half a percent of the project’s total multimillion-dollar construction cost (special thanks to Andrew Marlin (senior principal, MBA Engineers) for project background).
In addition to cost savings and corrosion-inhibitor benefits, the bio-based admixture had several other features aligning with the project’s goal to be LEED Gold certified. Two factors in particular helped the project earn points toward the LEED certification:
Another positive ‘eco-friendly’ aspect of the admixture included its certification to meet ANSI/NSF. This certification involves testing to make sure the admixture will not leach harmful contaminants into the water. Although the standard is intended for guidance on materials specifically used in drinking water structures, by extension, it suggests an overall friendlier profile for humans and the environment.
In recognition of the project’s sustainable and environmental focuses, including and also going beyond the corrosion protection aspect, Gulf State Park and the admixture supplier received the Community Partner Award from PEP, a local coalition of businesses and educational leaders seeking to implement environmental best practices.
The Lodge at Gulf State Park is a model for countless other coastal reinforced structures for which builders are looking to achieve sustainability and durability. The same bio-based admixture that was used in the new lodge and surrounding structures is a viable strategy for minimizing corrosion on other reinforced concrete structures in similar environments, with the purpose of extending service life in severe conditions. It has, in fact, recently been used in the coastal port renovations and seawater aquariums in the Gulf Coast area. The multitude of new projects will show how the bio-based admixture stands the test of time and also reduces long-term repair costs for owners.
Julie Holmquist is a content writer at Cortec Corporation where she researches and writes about corrosion-inhibiting technology for concrete, electronics, metalworking, oil and gas, and other industries. She can be reached via e-mail at firstname.lastname@example.org.
Andrea Moore is chief operating officer and co-owner of M2 Solutions, Inc. in Mobile, Alabama, where she assists owners, engineers, and contractors with service-life solutions for infrastructure through technologically advanced products. She is a graduate of Vanderbilt University’s School of Engineering, and the Technology, Operations & Business Education (T.O.B.E.) program at the University of Maryland’s Robert H. Smith School of Business with more than a decade of experience in industrial projects and service life solutions. She can be reached via e-mail at email@example.com.
Casey Heurung is a technical service engineer at Cortec Corporation. He has four-and-a-half years of experience in the corrosion industry and holds a B.Sc. degree in chemistry. He performs routine testing, offers technical recommendations, and develops products for the company. He can be reached at firstname.lastname@example.org.
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