e-mosty June 2017: Osman Gazi B. US Suspension. Hålogaland B. e-mosty June 2017: Suspension Bridges | Page 51

Figure 2: Helical strand cable, Eastbound Bridge (left); PPWS cable, Westbound Bridge (right) Following internal cable inspections and a thorough evaluation of options, the Maryland Transportation Authority adopted a proactive approach to the preservation of the main cables, by deciding to implement a main cable dehumidification strategy. Although cable dehumidification had been implemented in Asia and Europe, this would become the first application of main cable dehumidification in the United States and North America. 2. MAIN CABLE DEHUMIDIFICATION The Maryland Transportation Authority engaged AECOM for the design and development of the contract documents for the main cable dehumidification, based on our international cable dehumidification expertise. AECOM had previously performed the cable investigations, design, and supervision for the installation of main cable dehumidification systems on three signature bridges in the United Kingdom including Forth Road, M48 Severn and Humber (Figure 3). All three bridges had parallel galvanized wire cables and a conventional corrosion protection system of red lead paste, galvanized wrapping wire and paint. The extent of corrosion discovered was greater than originally anticipated, and it became evident that intervention was required. It was determined that the exposure of the main cables in high levels of humidity and corrosive contaminants such as chlorides, carbon dioxide and noxious gases was resulting in a continuous oxidization process of the main cables, and the traditional protection systems adopted during their construction were not as effective as originally tho ught. The main cable dehumidification system on Forth Road Bridge was installed and fully operational in 2009, while the systems on M48 Severn and Humber Bridges became fully operational in 2009 and 2010 respectively (Cocksedge & Bulmer, 2009; Cocksedge et al, 2011; Christodoulou et al. 2011). The premise of cable dehumidification is to reduce the relative humidity (RH) inside the cables below 60% where corrosion becomes thermodynamically negligible (Figure 4). The relationship between corrosion rate of ferrous steel and humidity was investigated by Vernon in the 1930s (Vernon, 1935). AECOM performed the internal cable inspections and strength evaluation of the main cables using the NCHRP 534 Guidelines (TRB 2004) for all three of the bridges between 2007 and 2010. 2/2017