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.
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 (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.
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, 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.
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.
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 underway.
