By: Jimmy Monahan
As Building Envelope Consultants, we thoroughly inspect all exposed reinforced concrete structures that are a part of a building. Evaluating the condition of exposed concrete balconies is a major component of our FISP investigations and building envelope surveys. Balconies provide outdoor living space to building occupants and are much sought after amenities in the NYC real estate market. While balconies can provide many benefits, they can lead to long-term repair issues if not properly maintained.
During our inspections, we often observe cracks and spalls in the concrete perimeters of the balconies. These cracks and spalls are often due to an inappropriate amount of concrete cover over the balcony reinforcement. “Concrete cover, in reinforced concrete, is the least distance between the surface of embedded reinforcement and the outer surface of the concrete (ACI 130).” Concrete cover protects the reinforcement from corrosion, insulates the steel from extreme heat such as fire, and it ensures the reinforcement can be actively engaged without slipping when loaded.
Concrete cover is crucial in order to protect the reinforcing steel from corrosion caused by environmental effects. When the steel reinforcement is not properly placed or protected from the environment, it will begin to corrode due to oxidation. Oxidation of the reinforcing steel often occurs from moisture infiltration and/or carbon dioxide intrusion through the concrete. When steel reinforcement corrodes, it will rust and expand to a greater volume than the steel itself. This action results in the cracking, spalling and delamination of the concrete covering the steel. The more concrete cover provided, the longer it will take moisture infiltration or carbon dioxide intrusion to occur.
Although concrete cover protects reinforcement from environmental factors, it also protects the structural reinforcement during a fire. Concrete can undergo extreme surface temperatures, exceeding 1500° F. Reinforcing steel, however, will lose 50% of its steel strength when subjected to temperatures exceeding 1200° F. Concrete cover is essential for keeping reinforcing steel cooler than outside elements, and the thicker the concrete cover, the greater the fire rating that’s achieved.
A fundamental importance of reinforced concrete design is the bond strength between the concrete and the reinforcing steel. It’s crucial that tensile stresses can be transferred from concrete to its reinforcing steel, otherwise failures can occur. The concrete cover over the reinforcing steel is essential for ensuring the bond strength between concrete and steel. Increasing concrete cover also improves the bond strength and slip resistance between the materials.
It’s important that Building Envelope Consultants identify signs of insufficient concrete cover and design our repairs with the appropriate concrete cover in mind. If insufficient cover is left in place, failures will continue to occur, costing an owner more time and money and leading to more frustration.