Concrete is generally thought to be a resilient material. Yet, those responsible for maintaining today’s concrete structures are finding their buildings and bridges may not last as long as they’d like, partly due to the destructive effects of corrosion.
Rest assured, corrosion issues can be successfully addressed. Long-term durability in modern concrete structures is frequently impacted by corrosion of the embedded steel used to reinforce and limit cracking. Steel, when exposed to oxygen, has a thin protective layer on the surface that helps to prevent the steel from corroding. Concrete can help maintain this protective layer by maintaining a high pH level. If this pH drops, the protective layer can diminish and allow the steel corrosion process to begin. As steel corrodes, it begins to expand and is capable of expanding up to six to 10 times in volume. Long before reaching this level of corrosion, the steel will cause the concrete covering the steel to crack and potentially spall.
One of the more common causes of corrosion, and eventual spalling of concrete, comes from the use of de-icing salts and a chemical process known as carbonation. De-icing salts generally consist of chlorides, electrolytes that react with other components in the concrete to form an acid, which can cause areas of severe corrosion. Carbonation of concrete develops when carbon dioxide in the atmosphere reacts with components in cement—the bonding agent of concrete— to form a molecule with a lower pH. This reaction affects the concrete from the concrete surface to the interior, or through cracks in the concrete cover. It may take years for these chemical reactions to reach the steel, and several more years to see the effects of the corrosion. However, if not treated early, the reaction may cause severe and dangerous conditions.
Terracon recently helped a client address deterioration of concrete used in a 60-year-old residence hall at a university. It was suspected that salts containing chlorides were used within the concrete mixture to help accelerate the concrete curing time when it was placed during the winter months. A combination of chlorides, moisture, and carbonation eventually caused the steel reinforcement to corrode the concrete on the underside of the facility’s roof decks.
Our team provided the owner with a survey and testing of the concrete to map the distress and determine the cause of the deterioration. Based on our findings, we developed repair options and an engineer’s estimate for completing the repairs. Due to the age of the building, the budget, and the long-term plan for the building, only a portion of the decks was selected for repair. Areas selected for repairs utilized sacrificial anodes, tabs of zinc connected to the steel meant to corrode before the steel corrodes, allowing for extended service life of the steel. Observations were performed to check the connection between the sacrificial anodes and the steel, the cleaning and coating of the steel reinforcement, and the installation of the mortar to confirm the repair mortar was well bonded to the substrate.
The repairs performed are expected to provide an additional 10 years of service life to allow the structure to be used while the building is slowly decommissioned.
Jeff Poe, Jr., P.E., MSCE, RRO is an engineer in the Charlotte, N.C., office. He has six years experience in building enclosure design and testing, structural material diagnostics and repairs, and contract administration.