Precasting of standardized sections is bringing the economic and aesthetic advantages of the arch to short span highway structures in the United States. With a proprietary system developed in Switzerland in the 1960s, site work and casting of the bridge parts proceed simultaneously. Final erection of all precast arch segments and headwalls typically takes one day or less, while overall construction time is about a month. Computer programs reduce to a matter of hours the needed engineering work on hydraulic and geotechnical problems of the individual site.
HOW THE BRIDGE WORKS
Standardized precast concrete arch segments 6 feet wide and 10 inches thick are erected side by side as required to provide the desired roadway width. The air-entrained concrete is specified at 4200 psi and Grade 60 reinforcement, either bars or welded wire fabric, is used. Arch segments are set on prepared footings and earth fill is placed over them--at least 14 inches deep, but as much as 15 to 20 feet in some cases. Standard roadway paving is placed on top of the fill. The fill works in two ways: to help distribute and resist loads, and to protect both roadway and arch from the temperature extremes and weathering common to typical bridge installations.