Precast Pretensioned Concrete for Parking Garages

Precast pretensioned concrete is concrete cast in a factory around high-strength steel strands that are stretched before the concrete is placed and released after it cures, putting the member into compression.

In precast pretensioning the strands are stretched first, in a casting bed, and the concrete is poured around them. Once it cures, the strands are released and their pull squeezes the concrete, leaving the member precompressed with a slight upward camber, made in a plant and trucked in, versus post-tensioning done in place. Illustrative.

The finished pieces, typically double tees for the floors plus beams, columns, spandrels and walls, are trucked to the site and erected by crane. Precast is one of the most common systems for U.S. parking structures because it is fast to build, produces long clear spans and gives consistent quality. It is the main alternative to cast-in-place post-tensioned concrete.

Advantages

Plant casting gives controlled curing and quality.
Fast erection, since pieces are made while the foundations are built.
Long, column-free spans that suit efficient parking layouts. See clear-span construction.
Less on-site formwork and labor.

What to watch

Joints and connections between pieces, which must be sealed and maintained to keep water and salt out. See chloride-ion contamination.
Camber, the built-in upward bow of prestressed members, which affects drainage slopes. See deflection.
Crane access and delivery routes on tight urban sites.
Seismic connection design in high-seismic regions.

The Parkonomics view

Precast builds a garage quickly; its joints decide how long the garage stays dry. A funded sealant and joint maintenance plan belongs in the same budget as the structure.