The need for more parking spaces results in many decisions a building owner must face. Should you construct a parking garage or a surface parking lot? What project delivery method should you use? Which team of consultants and builders should you select? And when building a parking garage—what type of construction makes the most sense?
The answer to this last question depends a lot on the owner’s goals and priorities. Today, the two most common construction types for standalone parking garages in the Midwest are:
1) cast-in-place, post-tensioned concrete, or
2) precast, prestressed concrete.
Both methods provide different advantages.
1. Cast-in-place, post-tensioned – These garages are constructed using ready-mix concrete poured into removable forms on-site. High-strength tendons (typically sheathed with lithium-based grease and unbonded) in the concrete are anchored at the outer edges of the concrete and tensioned after the concrete has gained sufficient strength.
Durability and potential for lower long-term costs
This method of casting the garage on the site creates many benefits that relate to durability. With fewer joints and mechanical connections, and built-in protection systems such as epoxy-coated reinforcing steel and concrete admixtures (like air entraining agents, water reducers, corrosion inhibitors, silica fume, etc.), this type of garage traditionally requires fewer repairs than a precast garage. This results in lower maintenance costs over the life of the structure. The monolithic construction inherent in post-tensioned garages helps reduce building movement from both gravity and lateral loads. The more movement a structure experiences, the more likely cracking and connection distress will occur.
Comfort and design
Cast-in-place, post-tensioned garages mean fewer joints—a major source of maintenance in concrete structures. This provides a smoother ride and less noise when drivers are traveling through the garage. In addition, with a post-tensioned garage, beams can be spaced further apart, giving a more open and safe feel by increasing light distribution and depth of views into the garage without increasing floor-to-floor heights. Often fewer fixtures are needed to illuminate the space because of how the structural beam location improves light distribution. Cast-in-place garages also offer greater design flexibility and customization.
2. Precast, prestressed – Panels, ledger beams, columns and double-tee members are fabricated off-site and assembled on-site. High-strength tendons (typically unsheathed and bonded) in the concrete are anchored at the outer edges of the concrete forms and pre-tensioned before the concrete is placed, then released after the concrete has gained sufficient strength.
Timing, speed and concrete quality
The primary components of the garage are fabricated off-site and assembled on-site, so the garage can be field-assembled quickly and in nearly any kind of weather. Depending on fabrication time, this can potentially allow an owner to gain occupancy faster. Because the components are produced in a controlled environment, concrete quality is more easily achieved.
Potential for lower initial costs
The initial construction cost is sometimes lower with precast construction because of reduced labor and erection costs and shorter construction duration. Design costs are usually lower with precast construction because the structural design is usually provided by the fabricator, consistent with design criteria established by the design team. Long-term costs however, are usually higher due to more frequent joints and mechanical connections that require regular maintenance and repairs over the life of the structure.
I welcome feedback and the opportunity to discuss this topic further should you have additional thoughts or questions about precast versus post-tensioned parking garages and how to extend their useful life economically.
In my next blog post, I’ll address considerations for improved durability in your parking garage, regardless of construction type.
Until next time,
Steve Osborn, PE, SE, FSMPS, CPSM
President, CE Solutions