Page 152 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

130 CORROSION CAUSES

to ∼8. The steel corrodes around pH 11.0. Carbonation rates follow parabolic
kinetics:
d = At 0.5
where d is carbonation depth, t is time, and A is constant.
Steel bridges corrode on exposure of steel to atmospheric conditions. The extent
of corrosion is greatly enhanced because of marine (salt spray) exposure and other
corrosive industrial environments. The only corrosion preventive method is to provide
a barrier coating such as paint.
There is considerable need for additional studies on innovative construction mate-
rials such as corrosion-resistant alloy/clad rebars (both metallic and nonmetallic) and
more durable concretes with inherent corrosion-resistant properties. Further research
and development in rehabilitation technologies that can mitigate corrosion with min-
imal maintenance requirements such as sacrificial cathodic protection (CP) systems
is desirable.
According to the National Bridge Inventory Database, there are nearly 600,000
bridges in the United States of America of which nearly 300,000 were built between
1950 and 1995. The materials of construction for these bridges are concrete, steel,
timber, masonry, timber/steel/concrete combinations, and aluminum. Reinforced
concrete and steel bridges are in the majority and built since 1950, which can
deteriorate because of corrosion. According to a report by the American Society
of Civil Engineers, the condition of the bridge structures was rated as “poor”
and was found to be the largest contributor to the US infrastructure cost of
corrosion.
Because of the specific concrete property of weak tensile strength as compared to
its compressive strength, steel reinforcing is placed in the tension regions in concrete
members, such as decks and pilings. The two primary forms of steel reinforcing in
concrete bridges are “conventional” reinforcing bar (rebar) and prestressed tendons.
The difference between conventional reinforcement and prestressed tendon reinforce-
ment is that the prestressed tendons are loaded in tension (prestressed) either prior to
placing the concrete (pretensioned) or after placing and curing of the concrete (post-
tensioned). It is useful to note that prestressed tendon generally has a higher tensile
strength than conventional rebar steel.
The majority of concrete deterioration leading to reduced service life and/or
replacement is associated with conventional reinforced steel bridge structures. This
is simply because the large majority of the reinforced concrete bridges and the
longer in-service times experienced by these bridges. Although conventional rebar
and prestressed tendon bridge structures have specific design, construction, and
corrosion-related concerns and consequences, the basic corrosion mechanism is
similar, and many control methods are applicable to both.

3.5.3 Conventional Reinforced Concrete
Reinforced concrete bridges suffer from corrosion of the reinforcement and,
consequently, concrete degradation because of the high tensile forces exerted by