One of the largest general-aviation airports in the Midwest, Spirit of St. Louis Airport accommodates 1,000 takeoffs and landings daily. The airport opened in 1964 to relieve air traffic at Lambert Airport, the major airport in the St. Louis area. Over the years, the heavy plane traffic caused severe deterioration of the asphalt. In some sections, exposure to jet fuel turned the asphalt to mush. A devastating flood in 1993 that submerged the ramp under 9 feet of water aggravated the damage, making the apron unusable.

Last February, however, the Spirit of St Louis apron reopened to air traffic - after being restored in just three months by the use of a concrete overlay. What makes this project noteworthy is the versatility of the overlay in accommodating aircraft of significantly different weights. It also marks the first use of ultra-thin whitetopping at a U.S. general-aviation airport. Whitetopping of airport runways and aprons has been performed in the past, but only at thicknesses of 5 inches or greater. Ultra-thin overlays are generally less than 4 inches thick.

A design feature of the ultra-thin overlay was the 4-foot, 2-inch joint spacing. The smaller slabs minimized random cracking in the overlay and will be easier to remove and replace should cracking occur. The mix design called for 3 pounds per cubic yard of polypropylene fibers. The small slab-section size and use of polypropylene fibers yielded a durable surface.