Materials Performance

NOV 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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46 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11 CHEMICAL TREATMENT T The Severn Bridge in the United King- dom is a 988-m span suspension bridge built in 1966. An interior in- spection of the main suspension ca- bles in 2006-2007 revealed corrosion and reduced strength. Acoustic mon- itoring, dry air injection, and vapor corrosion inhibitors were installed. A subsequent inspection showed that these protective measures had stabi- lized the condition of the cables. The Severn Bridge (Figure 1) is a 988-m span suspension bridge located in the Unit ed Kingdom that carries th e M48 motorway across the River Severn estuary between Bristol and South Wales. It was built in 1966 and features many innova- tions such as inclin ed han gers and a pioneering use of streamlined box girder d e c k c o n str u c t i o n . In 2 0 0 6 , th e U. K . Highways Agency commissioned a program of intr usive examinations of th e main suspension cables. These investigations provided data used for strength assessment of the cables. This article describes the use of a degradation model to predict the performance of the cables, and demon- strates the effectiveness of a dry air injec- tion system combined with vapor corrosion inhibitors in stabilizing the condition of the main suspension cables. The first intrusive inspection of the main cables, carried out in 2006-2007, found the cables were corroded and had reduced structural strength. Following this investigation and assessment, an acoustic monitoring system and dry air injection system were installed on both main sus- pension cables to control the deterioration. The purpose of the dry air injection sys- tem was to reduce the relative humidity (RH) within the cable to <40%, an accepted percentage to prevent corrosion of metals. To enhance the dry air injection system, a corrosion inhibitor was included. T h e d r y a i r i n j e c t i o n sy st e m w a s installed during 2008 and the drying phase had been completed by September 2009, as demonstrated by the achievement of a RH of 40%. A s re c o m m e n d e d by th e Na t i o n a l Cooperative Highway Research Program (NCHRP), 1 further intrusive inspection and a corresponding load assessment were undertaken in 2010, the results of which have b e en pre v iou sly publi sh ed . 2 Th e assessment showed that the strength of the cables was no worse than during the origi- nal assessment in 2006. In addition to an acoustic monitoring system, sensors were installed to monitor vehicle weights and th e air c ondition within the suspension cables. Using this data, Mott MacDonald developed an over- all monitoring system for cables based on t h e p r i n c i p l e s c o n t a i n e d i n t h e U. K . Highways Agency's Standard BD79/13. 3 The data from the monitoring system were used to supplement results of an assessment of Corrosion Control of the Severn Bridge Main Suspension Cables Paul lambert, Mott MacDonald, Altrincham, United Kingdom Jeffrey fisher, Mott MacDonald, Croydon, United Kingdom

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