A building is an investment in the future. For concrete parking garages, many considerations owners make during the design process can affect the life of the structure. Throughout design and after, keep the following in mind for a parking garage that stands the test of time:
- Select a qualified design team and contractor with expertise and experience in the chosen construction type.
- Select a designer that focuses on thoroughness and attention to detail—details matter when it comes to design and construction. A thorough and complete set of construction documents yields tighter bid results and reduces the chance of surprises (change orders) during construction. Also, a carefully detailed structure that addresses concrete shrinkage, thermal movement, restraining effects, deflection, steel corrosion and other serviceability concerns will go a long way toward increasing the life expectancy of the structure. Engage the design team throughout construction and incorporate the provisions of Chapter 17 of the International Building Code (Structural Tests and Special Inspections) to evaluate the contractor’s work for compliance with the design intent.
- In a precast concrete garage, there are frequent joints and exposed steel connections. Make sure they are properly detailed and constructed to reduce opportunities for deterioration from movement or exposure to de-icing chemicals and other harsh elements. At a minimum, all exposed steel connections and embed plates should be hot-dipped galvanized.
- For cast-in-place, post-tensioned concrete structures, make sure encapsulated tendons are used and the concrete mix design is proportioned for durability, with admixtures that resist freeze-thaw action and reduce permeability. Various combinations of low water-cement ratio (0.40 or less), air entrainment (5 to 7%), calcium nitrite corrosion inhibitor (3 gal/cy), penetrating silane sealer (40% solids or more) and micro silica fume (50 lb/cy) are commonly used to address these issues. If using micro silica fume and the parking garage will be exposed to de-icing chemicals or other harsh elements within its first year of service, consider applying a penetrating silane sealer to the concrete surface to protect the concrete until the micro silica fume has an opportunity to fully activate. This usually takes approximately one year.
- Increasing the protective concrete cover over the embedded reinforcing steel is another easy way of achieving improved durability. A thickness of 2 inches is recommended for surfaces exposed to de-icing chemicals or other harsh elements. “Black” mild steel reinforcement with increased cover is preferred over epoxy coated reinforcement with less cover.
- Don't allow electrical conduit to be embedded in the cast-in-place concrete portions of a structure. Many a parking garage has been severely damaged from embedded electrical conduit that has corroded and expanded, causing significant delamination and spalling.
- The minimum floor slope for positive drainage is 1.5% or approximately 3/16 inch per foot. Ideally, 2% or 1/4 inch per foot is recommended to effectively increase the life expectancy of a parking structure by reducing the potential for contaminated water leaching into the concrete and causing corrosion of mild reinforcing steel, prestressed strands or post-tensioned tendons.
- The value of regular maintenance and cleaning of the parking structure cannot be overemphasized. Removing harmful elements brought into the garage before they have an opportunity to do harm, touching up galvanized coating on exposed steel connections and repairing failed joint seals and sealants go a long way toward extending the life of the structure.
- It is best practice to design a parking garage for durability without the need for a surface-applied traffic coating. Traffic coatings are an excellent option for extending the life of an existing garage that lacks the above best practices for durable design. The traffic coating can also be used as a strategy to reduce energy, depending on the goals and maintenance of the facility. For example, black traffic coating on the upper decks of a parking garage can assist in heat retention in the winter to help snow and ice melt in mild winter weather events, reducing the need for plow trucks, deicing-chemicals, etc. On the other hand, white traffic coatings reflect the sun’s energy, rather than absorb it, and may be more suitable in areas of intense heat. When selecting traffic coatings for the upper decks, make sure to select ones that won’t degrade when exposed to ultraviolet light. Aliphatic epoxies or urethanes are an excellent choice for this condition.
- For more information regarding parking garage durability, refer to the recommendations of the American Concrete Institute in their ACI 362 publication “Guide for the Design of Durable Parking Structures,” the Post-Tensioning Institute in their publication “Design Fundamentals of Post-Tensioned Concrete Floors” and the Precast/Prestressed Concrete Institute in their publications “Precast Prestressed Concrete Parking Structures: Recommended Practice for Design and Construction” and “Maintenance Manual for Precast Parking Structures.”
I welcome feedback and the opportunity to discuss this topic further should you have questions about how to extend the life of your parking structure.
All the best,
Steve Osborn, PE, SE, FSMPS, CPSM
President, CE Solutions