Approaching concrete longevity in corrosive water environments

by brittney_cutler | October 19, 2021 9:24 am

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by Julie Holmquist

In 1971, Israel began using its first Olympic-sized swimming pool; the Wingate pool played a significant role in Israeli athletics as swimmers spent countless hours splashing through its water for daily practice. Athletes came from around the world to participate in international competitions held there.

In 2013, a new Olympic pool was built to take its place as the primary venue for competitions, while the first pool continued to be used for practice. Half a century of exposure to a naturally corrosive environment took its toll on the concrete elements, causing leaks and concrete delamination requiring repeated repairs. Not until 2016, when the pool was diagnosed with active rebar corrosion in its walls, was anything more than local repairs considered.

In hindsight, one wonders if proactive corrosion mitigation could have led to fewer repairs in the first place. Whatever the case, the story underscores the importance of understanding heightened corrosion risk factors for certain concrete water handling structures—from swimming pools to seawalls to desalination plants—and not overlooking longevity in the face of a short-term fix.

Many possible design strategies exist to maximize the service life of a concrete structure, delay corrosion, and minimize repair time. While these should be fairly considered, one of the most practical and economical technologies for construction specifiers, engineers, and building owners to be aware of is migrating corrosion inhibitors (MCIs) as this technology can be particularly helpful when it comes to corrosive water handling environments.

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Corrosive concrete water handling environments

Pools are a highly corrosive environment requiring special corrosion mitigation considerations for a few reasons; one is convection in the indoor pool environment that can deposit wet films with dissolved salts on surfaces in the facility. Another is hypochlorous acid (HOCl)—a strong oxidizer that forms in pools as a byproduct of adding chlorine gas to the water for disinfection. Combined with high humidity, these elements create an extremely harsh environment and accelerate corrosion. Maintenance corridors and cellars around and under pools are as harsh as the facilities above the water, facilitating a corrosive environment on both sides of the concrete.

Seawalls are also exposed to extreme conditions. Built along shorelines to resist erosion, seawalls are in direct contact with saltwater, which, when absorbed, can be a harsh electrolyte to accelerate the corrosion of embedded steel reinforcement. A humid environment with high temperatures will cause the corrosion rate to accelerate even more. The seawall owner can expect repairs at an early age, unless special precautions are taken.

Even more extreme are conditions encountered by concrete elements in a desalination plant. Hadar Halperin, a construction consultant familiar with the heightened corrosion risks of desalination and other harsh environments in the Middle East, explained that the salinity level of the Mediterranean Sea near Israel is around 3.8 percent. Designers of two large desalination plants in Israel had to compensate for this high salinity level when constructing jack-pipes to feed seawater into the plant and discharge brine back to the sea.

During the desalination process, the seawater doubles in salinity to around seven percent in the resulting brine wastewater, Halperin explained. Those dealing with desalination water in the Red Sea region have to battle even higher baseline salinity of about 4.1 percent. Reinforced concrete used in brine water holding tanks or disposal channels at desalination plants therefore likely face double the threat of corrosion experienced by a seawall.

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Impact of local climates and chlorides on concrete

Service life prediction modeling software[4] can offer a snapshot of how various climates and chloride levels can affect concrete structures differently. Inputting the same hypothetical concrete mix into the program yields a variety of different results based on expected risk factors and surface chloride concentration for each location. For example, in one modeling session, a parking garage in either Minneapolis or San Diego was expected to reach its maximum surface chloride concentration in 13.3 years at either location. However, corrosion initiation was predicted after 10.8 years in Minneapolis (in the middle of the continent) and more than two years earlier in San Diego, an overall warmer climate near the coast.

Modeling for marine spray zones showed even more extremes. In these locations, the software assumes surfaces will reach maximum surface chloride content (one percent) sooner than in other environments. For a hypothetical marine spray zone structure in Boston, Massachusetts, corrosion initiation was predicted in 8.1 years, while heading south to the warmer subtropical climate of West Palm Beach, Florida, brought a prediction of corrosion initiation in only 5.5 years (see Figure 1).

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Strategies to counter corrosion

There are many approaches to countering corrosion and extending the service life of concrete structures in corrosive water environments. These include the concrete mix design, the type of reinforcement used, corrosion inhibitors, and the thickness of the concrete cover.

According to Penn State College of Engineering, extremely low water-cement ratios result in dense concrete, making it more difficult for chlorides to penetrate.⁷ However, this type of concrete enhances thermal cracking which lessens the actual concrete cover. Penn State also notes denser concrete can be more difficult to work with, and requires water-reducing admixtures. Adding more pozzolans such as silica fume and fly ash is another way to reduce permeability. Epoxy coated rebar offers protection from water and oxygen by means of a physical barrier but can experience accelerated corrosion where coating imperfections, such as cracks and holes, are present.

Corrosion inhibiting admixtures, often calcium-nitrite (CNI) based, are another option. CNI reacts with free iron ions to form a protective oxide layer on the rebar and raise the chloride threshold; but they must be dosed based on expected chloride loading, requiring increasingly higher dosage rates that can be ineffective if predictions are wrong. Theoretically, a thicker concrete cover will also delay time to corrosion, but additional concrete can be prohibitively expensive and does not solve the problem of corrosion once cracks allow an access point to the rebar embedded within the concrete.

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MCI technology

MCIs are an especially practical and cost-effective method of extending concrete service life. They are typically ‘mixed’ inhibitors that form a protective molecular layer at the anode and cathode of a corrosion cell. The latest generation of MCIs comprise amine carboxylate chemistry. Several MCI treatments are certified to meet NSF/ANSI Standard 61, Drinking Water System Components—Health Effects for use in potable water structures[7].

Unlike CNI admixtures, which must be dosed based on expected chloride threshold, MCI admixtures are dosed at a much lower rate independent of expected chloride content[8] (0.6 L per m³ [1 pt per yd³] versus 10 or more L/m³ [several gallons per yd³]). MCI admixtures do not compromise concrete physical properties such as compressive or flexural strength and freeze-thaw durability[9].

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Rather than causing workability challenges by accelerating set time, as CNI has been known to do, set time with MCIs remain normal and can even be delayed, allowing more flexibility for workers. Further, there is no problem with adding MCIs to the concrete mix at either the batching plant or onsite.

While incorporating MCIs from the start as an admixture is ideal, MCIs can be applied topically to existing concrete surfaces. As a liquid surface treatment, MCIs penetrate the concrete pores through a combination of capillary action and vapor diffusion to reach the surface of the embedded rebar. Testing has indicated surface-applied MCIs can penetrate sound concrete up to approximately 76 mm (3 in.) deep.

There are significant potential economic and service life benefits of MCIs. Amine carboxylate based MCIs have been seen to double or triple time to corrosion and reduce corrosion rates by five to 15 times once started. In using software prediction modeling as mentioned above[11], MCIs were able to significantly increase the service life prediction of a hypothetical Minneapolis parking garage from 16.8 to 51.4 years. For the structure in West Palm Beach, Florida, the service life prediction jumped from 11.5 to 40.5 years (see Figure 1). This is typically at a fraction of the total project cost. For example, use of MCIs in the foundation of the Princess Tower residential skyscraper in the Marina district of Dubai, UAE, added 0.07 percent to the total cost while more than doubling the service life prediction from 48 to 103 years. When used in the Lodge at Gulf State Park, MCIs exceeded the service life prediction of epoxy-coated rebar and offered an estimated direct savings of $250,000 to $300,000.

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MCI applied at desalination plants

The Sorek Desalination Plant near Tel Aviv, Israel, was an exciting development of its time as one of the world’s largest saltwater reverse osmosis (SWRO) desalination plants. It was built from 2010 to 2013 with a capacity to process 624,000 m³ (164,843,361 gal) of water per day and provide enough fresh water for 1.5 million people, equivalent to 20 percent of Israel’s municipal water demand[13].

The extreme saltwater conditions were a notable threat to the durability of the plant’s most vulnerable concrete elements: jack-pipe segments (carrying seawater and brine to and from the plant), pretreatment bins, and brine water reservoirs used post-treatment. For extra protection, an MCI admixture was combined with a waterproofing admixture for corrosion inhibition and water resistance in the jack-pipes and filtration bins.

MCI was also used in the brine water reservoirs. When some of the desalinated water reservoirs and columns ended up having a lower concrete cover than desired, a topical MCI liquid was applied to the surface to help compensate for the deficiency. Eight years later, the project was still considered a durability success, so much so that the same MCI and waterproofing admixtures were specified in some of the concrete elements cast for Sorek II, an extension of the first plant currently[14] underway.

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Use of MCIs in seawalls

A recently built seawall in Longboat Key, Florida, also had to face the severe challenge of direct contact with seawater in a warm subtropical climate where corrosion quickly accelerates. Modeling software predictions for a standard seawall concrete mix forecasted a service life of only 15.2 years before the first repair would be needed. Adding an MCI admixture to the design tripled the projected service life to 46.9 years before repair.

However, the owner had an even stronger vision in mind to construct the wall with a 100-year service life (time before first repair) so it would last for future generations. The first mix design was replaced with a concrete mix used by the Florida Department of Transportation (FDOT), which brought the service life prediction beyond the required 100 years. The addition of MCI extended the prediction even farther to more than 150 years.

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One port wall in Tel Aviv, Israel, is a perfect example of what can happen to normal seawalls with constant exposure to seawater and marine conditions. The wall has already reached the end of its service life and is set for repair later in 2021. It is showing signs of deterioration with an exposed network of rusted reinforcement in some places clearly revealing an underlying problem. Due to the corrosiveness of the environment, a corrosion consultant has been involved in the project, and both a topical MCI and an MCI admixture have been specified for use in the 150-m² (1615-sf) repair. MCI chemistry will be a critical factor in delaying and reducing corrosion, so the next repair does not have to happen so soon.

“The considerations in projects like these usually focus on longevity with minimal maintenance, since shutting down for repairs is extremely difficult,” said Halperin, whose firm was called on to supply MCI technology for the damaged wall.

 Addressing root causes of swimming pool damage with MCI

The potential benefits of MCI chemistry are now being sought after in the case of the Wingate pool deterioration although previous repairs may have overlooked long-term corrosion mitigation. The consultant involved in the 2016-2020 repair took a different tack by incorporating MCI technology into the repair mortar to enhance its durability. Further, MCI was inserted in tablet form inside the walls of the pool and beyond the rebar depth to leave an ongoing source of MCI diffusing throughout the concrete pores and protecting adjacent reinforcement from future corrosion. By going beyond the surface issues of temporary patching, this repair is much more forward-looking with the intent to slow corrosion and delay time to the next repair. As a tribute to the practicality of this approach, the project was completed during 2019-2020 without having to close the pool.

Conclusion

MCI technology is an important option for concrete structural designers to consider. MCI has shown itself to be a viable tool to reduce corrosion and extend service life. Its benefits in terms of practicality, cost-effectiveness, ease of use, and service life potential make it worthy of further investigation by any engineer, contractor, builder, or construction professional seeking time-tested, innovative ways to enhance concrete longevity in corrosive water handling environments.

Julie Holmquist has been a content writer at Cortec Corporation for more than five years. She specializes in writing about corrosion-inhibiting technology for concrete, electronics, manufacturing, oil and gas, and many other industries. Her articles have been published in dozens of industry magazines. She can be reached via email
at jholmquist@cortecvci.com.

Source URL: https://www.constructionspecifier.com/approaching-concrete-longevity-in-corrosive-water-environments/

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