The Blue Ridge Parkway (Watauga County Bridge 359) over Blackberry Road east of Blowing Rock, built ca. 1939. Marks of the timber formwork are still visible at the reinforced concrete slab of the bridge (source: NCDOT bridge inspection files). Slab bridges are monolithic, flat concrete beams (slabs) with twisted or roughened reinforcing steel rods concentrated in the lower portion and at either end of the slab, where tensile forces and sheer are the greatest. The amount of steel and depth of the slab are based on its length and live-load capacity.
Slab spans are rarely more than 35 feet long. For lengthier spans, other bridge types offer a more economical use of material.
The earliest identified reinforced concrete slab highway bridges in the inventory are a pair of similar bridges carrying Wentworth Road over the North and South Prongs of Terry’s Creek in Reidsville (Rockingham County Bridges 238 and 239). The multiple-span bridges were built in 1910 by the county and widened to one side in 1922 by the State Highway Department.
The design is unusual; the reinforced slab is combined with short arches of less than 15 feet. This combination reflects early thinking about reinforced concrete, when builders did not yet have full understanding of the capabilities of the material and a greater variety of reinforcing patterns and designs were attempted.
A section of reinforced concrete slab with reinforcing steel shown as black circles (source: FHWA, Bridge Inspector's Reference Manual, 2012). In late 1919, the State Bridge Department adopted a standard slab design for spans in the range of 13-to 24-feet in length. Among early prototypical examples are the bridge carrying Old U.S. 21 over Carson Branch near Glade Valley (Alleghany County Bridge 212), and a pair of bridges carrying Old Liberty Road over branches of Bush Creek near Randleman (Randolph County Bridges 435 and 436).
Other early 1920s examples are scattered across the state. The Historic Bridge Inventory includes 115 pre-1961 reinforced concrete slab bridges, but this is but a small slice of the statewide number since it includes only those, with the exception of one Blue Ridge Parkway bridge, over 20 feet in length