ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/296997
44 | ASPIRE , Winter 2013 CBP Concrete Bridge Preservation Creative Patches Save McCullough Bridge The Conde McCullough Bridge at Coos Bay, Ore, is arguably the most exquisite showpiece in a series of historic coastal bridges along U.S. 101, the Pacifc Coast Scenic Byway. Pre- serving this signature structure of concrete arches required a variety of innovative approaches. These included a customized, self-consolidating micro-concrete; the addition of salt to the re- pair mortar; and the use of a cathodic-protection system. Designed by famed engineer Conde B. McCullough and built in 1936, the 5305-ft-long structure was the longest bridge in Oregon's highway system when constructed. It features exten- sive Art Deco ornate detailing throughout the bridge. Many sections needed to be repaired to restore the bridge. Some 20- to 30-ft-long sections received dozens of form-and-pump repairs, typically 2 to 4 in. in depth, plus numerous small hand- applied patches. These repairs required a versatile material that could adhere to the substrate, was compatible with the resistivity of the original concrete, and could be placed both in shallow and deep applications, in a confned area. salt added to Mixture Normally, it is not recommended that any salt be added to con- crete, due to the risk of propagating corrosion; however, in the case of cathodic protection, electrical resistance compatibility is important to ensure uniform current distribution to the embed- ded reinforcing steel. Electrical resistance compatibility with a concrete exposed to 80 years of marine salt was obtained by adding table salt to the repair mortar. The volume of salt added to each bag was determined by calculating the amount of chlo- rides found in the host concrete. The bridge's multilevel enclosure that housed the repair called for a lightweight, mobile concrete pump that could be used on each level and on narrow scaffold planks. Diaphragm-type, hand-operated grout pumps were placed on tight scaffold areas so the material could be pumped full depth both vertically and overhead. In these areas, complex forms were assembled to recreate the original Art Deco designs. The portable pumps necessitated the use of a self-consolidating, micro-concrete, to ensure that the top-size aggregate was small enough to be pumped while still allowing placement up to full depth without adding any pea gravel. Once the concrete cured, the repaired surface was sandblasted and a zinc coating was applied to serve as the sacrifcial com- ponent of the cathodic-protection system. This system redi- rects corrosion activity that normally would occur in the steel reinforcing bars. It requires the concrete to be uniformly and electrochemically conductive, which was enhanced by the salt in the repair mortar. A low-voltage electrical current drives the corrosion into the zinc coating rather than the reinforcing bars, protecting the bridge. It took four years to restore the south end of the bridge, and it is anticipated that the longer north end could take fve or more years to complete. Meanwhile, work continues on several oth- er Oregon bridges using this form-and-pump repair method. Through the Oregon Department of Transportations efforts to identify and prioritize needed bridge work, many historic bridges are being saved for future generations. _________ This article is an abridged version of an article that appeared in the November-December 2011 issue of Concrete Repair Bulletin and is published with permission of the International Concrete Repair Institute. For more infor - mation on the organization, visit www.icri.org. CBP CONCRETE BRIDGE PRESERVATION The Conde B. McCullough Bridge on Highway 101 at Coos Bay, Ore. Photo: International Concrete Repair Institute and BASF Corp. AspireBook_Win13.indb 44 12/28/12 11:40 AM