An expansion of Sea-Tac Airport’s parking garage makes it the largest single parking structure in the country, with 12,700 parking stalls.

The $59.5 million Sea-Tac Parking Terminal expansion project added 3,000 parking stalls to the airport garage’s existing 9,700 stalls over a period of three years. That feat required placing about 780,000 cubic yards of concrete to create what is reportedly the largest single parking structure in the United States.

The project included a three-level vertical expansion of the portion of the existing garage that includes helixes 7 and 8. This part of the garage only had five levels while the rest of the structure had eight. The expansion brought the entire structure to the same height.

After the vertical expansion was finished, a new adjoining parking structure standing eight stories was built. The foundation for that structure used spread footings, running footings and a retaining wall to create space for the new slab-on-grade. Decks on the new structure used post-tensioned construction with low water-to-cement ratio, air-entrained 5,000 psi concrete.

One of the challenges of the project was the mixture and availability of concrete. The high air content, use of state sand, and mixed fractured rock created a mixture that was hard to control with respect to air content and slump. It also was tough to pump and hard to finish. Between 500 and 800 yards of concrete were poured on an average day, and at times pumped over 700 feet through slick line.

A unique aspect of the project was the placement of perimeter columns. To limit the potential for cracking at the base of the perimeter columns due to the stressing of the overhead post-tensioned decks, over 280 column pourstrips were used.

Column pourstrips required a multi-step column placement process. The columns were first formed full height, level-to-level. Then, sand was poured into the formworks to a depth of 2 feet. Concrete was then poured into the column forms. After the concrete set, the formworks were removed and the sand blown from the base of the columns with compressed air — resulting in a 2-foot “voids” at the bases of the columns, with just the rebar remaining in place.

The voids were left until the placement/post-tension stressing of the overhead decks was complete, allowing columns to freely adjust to minor movements associated with post-tensioned concrete construction.

Once the overhead decks were completed, formworks were placed around the voids and concrete was poured back in. About 2 inches of the 2-foot voids required pressure-grouting due to space constraints caused by the rebar.