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Corrosion is a huge problem for engineers who frequently use metal products in their structures, because it can be a safety hazard. According to engineering regulations, the lifetime durability and design of a structure is around 50 years. This lifetime can be extended by regular maintenance, or the structure should be demolished and rebuilt when the 50 years have passed.

The recent collapse of a bridge in Italy demonstrated the severe impact of corrosion on structures – and the results of a lack of maintenance. The Morandi bridge in Genoa is the fourth bridge that has collapsed over the last few years.

MCI inhibitors extend bridge lifespanCompleted in 1967, engineers have been raising concerns about the bridge for a number of years, specifically with regards to its unusual concrete-encased common steel cables. In fact, the designer of the Genoa bridge warned four decades ago that it would require constant maintenance to remove rust, given the effects of corrosion from sea air and pollution on the concrete. In addition, the problem of fatigue corrosion of metal elements, which is particularly insidious in steels of high mechanical strength such as strands, is still not well understood.

“As this reinforced and pre-stressed concrete bridge has been there for more than 35 years, it is very possible that corrosion of tendons or reinforcement is a contributory factor to the collapse,” Ivana Liposcak, Cortec’s MCI technical support manager for Europe says.

“At the time of planning, many concepts about the sustainability of the bridge were not known and considered, such as the long-term behavior of viaducts subjected to heavy traffic and situated in aggressive environments. Due to the vibration in concrete, microcracks occur, through which the moisture leads to oxidation and corrosion of steel and in that way, the structure gradually loses its capacity due to corrosion.”

Liposcak says that by using MCI technology in severely corrosive environments, structures will have a stronger resistance to corrosion and therefore longer durability. Increased durability means fewer repairs, enhanced structural integrity and a longer service life, all leading to greater sustainability.

She adds that crumbling infrastructure is a worldwide problem. In the spring of 2000, for example, a concrete and steel walkway snapped in half in North Carolina. After an inspection, it was determined that all 11 steel cables that were holding the bridge together were corroded, and the bridge buckled under the weight. The corrosion was caused by too much calcium chloride, an inorganic salt compound that’s highly corrosive to steel, mixed into the grout that cemented the bridge’s steel cables in place. Nearly 50 lawsuits were filed against the speedway and the construction company, with settlements of millions of Dollars being granted. A report from the American Road & Transportation Builders Association states that more than 15% of that country’s bridges are “structurally deficient”.

Similarly, in 2007, a Concorde overpass collapsed in Canada. The commission of enquiry found one of the major causes was improper rebar support for the design, which caused a “plane of weakness” where cracks eventually occurred.

The difficulty is that concrete, or rather the steel used to reinforce it, can fail in a number of ways. Salt, ice and the pounding of weather can cause fractures in the concrete’s surface. Once the water reaches the steel reinforcing or tendons, it corrodes them. This enlarges the cracks, which can cause the concrete to fall apart. “Other factors also compound the deterioration of bridges, such as a constant traffic. This is a problem for bridges designed 50 years ago, when traffic was lower, cars were smaller and lorries much lighter,” Liposcak says.

Harsh weather conditions, such as heat, cold, floods and high winds also contribute to this corrosion of bridges. This is why regular inspections and maintenance are essential, she adds.

“During last two decades there have been huge advances in technology to extend the lifespan of structures and avoid possible tragedies happening. One of the most efficient uses of migrating corrosion inhibitors (MCIs) is when they are applied directly during the construction phase as well as a part of the maintenance repair system in existing structures. For pre-stressed structures for bridges, MCIs are recommended for the protection of pre-stressed cables before grouting.”

MCI inhibitors are made from a renewable raw material, enabling users to earn certain LEED credits. These products form a protective layer on the surface of embedded steel reinforcement.

MCIs are based on amine technology. They are classified as mixed inhibitors, meaning they affect both anodic and cathodic portions of a corrosion cell. MCI can be applied in many forms, including as a concrete admixture or a topical treatment. It moves as a liquid through the concrete matrix via capillary action and migrates in a vapour phase throughout the concrete pore structure. When MCI comes in contact with embedded metals, it has an ionic attraction to it and forms a protective molecular layer. This film prevents corrosive elements from further reacting with the reinforcement and reduces existing corrosion rates, greatly extending concrete service life.

Liposcak says there are many projects around the word currently using migrating inhibitor technology. For example, erecting the new Frederikssund bridge in Denmark. The aim is to replace the old bridge, built in 1935, by providing an alternative to the only currently active bridge over the Roskilde Fjord. The project includes design and construction of an 8-km-long dual-carriageway highway bridge over the fjord.

The Maslenica Bridge in Croatia is also using MCIs. One of the largest bridges of its type, with an arc of 200 meters in diameter, the bridge has seen its reinforcing steel starting to corrode, causing concrete spalling, as a result of changing temperatures, constantly fluctuating humidity, and strong wind containing salt from seawater.

To restore the bridge and prevent future corrosion from happening, all spalling concrete was water-blasted off, along with dirt and corrosion on the rebar. CorrVerter MCI Rust Primer was brushed on exposed rebar to passivate the metal from further corrosion, and MCI-2020 was applied to the entire concrete structure using an airless sprayer to prevent any potential corrosion that was not apparent.

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