Page 155 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CABLE AND SUSPENSION BRIDGES 133

give service lives of 75–100 years. In designing long-life bridge structures, it should
be noted that changing load and capacity requirements may make the structure
functionally obsolete before it becomes structurally deficient. Hence, emphasis
should be placed on forecasting traffic loads and patterns and on designs to handle
changes in traffic volume.
Prestressed concrete bridges are low in number (18%) in comparison to reinforced
concrete bridges (40%), and the total economic impact is not as large as reinforced
concrete bridges. However, on an individual basis, failure of prestressed concrete
components may have a significant impact on the structural integrity of a bridge.
Prestressed concrete bridges rely on the tensile strength of tendons to sustain the
load, and the loss of even a few tendons may lead to catastrophic failure of a bridge
component.
There were 107,700 prestressed concrete bridges in the United States of America
in 1960. In 1992, the UK Ministry of Transportation imposed a ban on the com-
missioning of grouted, bonded posttensioned bridges. This ban is the result of the
collapse of two foot bridges in 1960. The failure of posttensioned Melle Bridge across
the Scheldt River in Belgium was reported in 1956. This failure was traced to the
corrosion of posttensioned strands.
Reliable nondestructive methods for providing assurance to the owners that the
built structures have met construction specifications are not available. The main con-
cern is whether the ducts in the posttensioned bridge members have been completely
filled with the grouts and whether there is uniform coverage over the prestressing
steel.

3.6 STEEL BRIDGES

Atmospheric corrosion of exposed steel is common. Painting is a universal solution
to corrosion because of exposure to environmental conditions. Paints can deteriorate
following moisture uptake, ultraviolet exposure, wear or mechanical damage, and
exposure to chemicals. For instance, the performance of the same coatings will vary
significantly depending on exposure to industrial, urban, rural, or marine environ-
ments. Once a coating is compromised, corrosion can initiate, and often, is acceler-
ated under a deteriorated coating more than in the absence of coating. Thus selection
of proper coating for the right application is important for a long service life. Proper
and timely maintenance of the structure can extend the overall life of the coating
significantly.

3.7 CABLE AND SUSPENSION BRIDGES

These bridges comprise a small percentage of the total nation’s bridges. There are
nearly 150 cable bridges, which range between 100 and 130 years in age. Construc-
tion and corrosion control of these bridges were reviewed (10). Corrosion problems
are highly dependent on specific structural configuration, design, making general