Why Florida Oceanfront Condominium Concrete Repairs Fail And Three Things You Can Do To Stop It

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Ever hear a Florida oceanfront condominium owner say: "We just spent a fortune to repair our building and now we have more concrete spalling. We can't afford to keep repairing this way. Why don't these repairs last? Isn't there a way to repair the building once and for all?" The frustration of condominium owners on the coast over this issue is well documented.

Many condominiums are coming into what has been dubbed "second generation repairs". The fact is, some of these repairs, are third or fourth generation repairs. Yet, the concrete repair materials available today are better than ever and the International Concrete Repair Institute has outlined sound repair methods. Contractors wanting to do a good job, with good repair materials, are not succeeding to even their own expectations. Why? Because the chemical differences between the old concrete and the new concrete is affecting the durability of the repair.

Corrosion is accelerated by differences within the steel’s environment. If I take a steel reinforcing bar and throw it out on the beach, it will corrode very slowly and uniformly. That’s ...
... called “macro” corrosion. The environment is uniform so corrosion is very gradual. If, on the other hand, I take a steel bar and place one end in a corrosive environment and the other end in new, uncontaminated concrete, the bar will rapidly corrode. That’s called “micro” corrosion. Micro corrosion is concentrated and rapid. It is concentrated between two different materials, like new concrete and old concrete.

In your Florida oceanfront condominium, the old concrete, exposed to air and moisture, gradually changes chemically by the natural aging process of carbonation. Carbon dioxide, plentiful in the atmosphere, reacts with hydroxides in concrete to form calcium carbonates and water. This reaction causes the concrete alkalinity to drop from its normal 12.6 - 13.2 to as low as about 9. The reduction of Ph below 11 impairs the protective passivating layer on the steel, resulting in the steels increased propensity to corrode in the presence of chloride ions. The resulting rust occupies a greater volume than the original steel, causing pressure on the surrounding concrete, causing the concrete to crack.

Second, concrete structures on the coast are exposed to the salt laden air which covers balconies and walkways with chloride ions that seep through the concrete pore structure and initiate micro corrosion cells of highly charged ions on the reinforcing steel. The chloride ions do two destructive things. First, they cause an electric polarity difference in reinforcing steel and second, with moisture, they make the concrete electrolytic.

This makes a significant difference between the chemistry of the new concrete patch material adjacent to the old concrete, with the reinforcing steel acting as a conductor between them. The new concrete has a high PH and no chloride ions. The old concrete has a reduced PH and negatively charged chloride ions throughout its pore structure. If you define an alkaline battery, "Energizer" for example, (the one that just keeps on going) it is a highly alkaline material connected to a lower alkaline material by a conductor. The "Energizer" battery is a more efficient battery than the one created by the patch, but the principal and the effect are the same; the creation of an electric charge that accelerates corrosion. This phenomenon has been called the "Incipient Battery Effect" so called as an unintended result that, if not countered, will accelerate corrosion in the reinforcing steel adjacent to the repair.

Lastly, corrosion cells in structures are commonly overlooked and not addressed in the repair. Corrosion cells occur between dissimilar metals in the concrete, such as aluminum rails in concrete and the aluminum threshold on the concrete under the sliding glass door. In this situation one metal acts as a sacrificial anode to the other and oxidizes. These corrosion cells can be powerful and destructive.

So, what is the solution for your Florida oceanfront condominium? Understanding why the problem exists helps explain what can be done.

First, the differences between the old concrete and the new concrete have to be mitigated. Chemical corrosion inhibitors have been used successfully to reduce corrosion in areas surrounding repairs. The application needs encompass all of the old concrete, which means stripping the deck coatings off the entire balcony or walkway for application of the inhibitors over the entire surface. Spot treatment or localized treatment with corrosion inhibitors is ineffective.

Secondly, keep moisture out of the concrete. Apply crack bridging waterproof urethane coatings to the top surface to keep water from penetrating into the concrete. Keep the stucco on the edge and bottom of the slab painted with an exterior grade acrylic pant to reduce absorption form moisture in the air and protect the concrete from carbonation.

Third, isolate dissimilar metals. Aluminum thresholds can be isolated from the concrete via plastic or rubber shims or sealants. Aluminum handrails should be removed from concrete and surface mounted on rubber or neoprene gaskets or other inert materials.

Robert A. Fitzpatrick, P.E. is a professional engineer registered in the State of Florida with 30 years of engineering and construction experience. He is past President of the Florida Chapter of the International Concrete Repair Institute and the Society of American Military Engineers. You can learn more about United Engineering’s services, qualifications and project history at http://UnitedEngineeringConsultants.com..