Page 246 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

224 CORROSION CONTROL AND PREVENTION

conventional reinforced concrete decks or substructures. Thus, corrosion control
practices for conventional reinforced concrete are applicable to components of many
other bridge structures.
Corrosion protection can be incorporated into new bridge structures by proper
design and construction practices, including the use of high-performance concrete
such as silica fume additions, low-slump concrete, and an increase in concrete cover
thickness. Each of these attempts to impede migration of chloride and oxygen or other
corrosive agents through the concrete to the steel rebar surface. However, eventually,
the corrosive agents penetrate through the concrete cover and cracks, necessitating
other corrosion control processes. A widely used method of corrosion prevention is
the use of coated carbon steel rebar and to some degree, corrosion-resistant alloy/clad
rebars. The typical organic rebar coating is fusion-bonded epoxy, while the metallic
rebar coating is galvanizing (very limited use in bridge structures). Rebar cladding
with a corrosion-resistant alloy such as stainless steel is relatively new. Solid rebars
made out of stainless steel alloys have been used to a limited extent. In addition,
nonmetallic composite materials have been used. Another corrosion control practice
involves the addition of corrosion-inhibiting admixture to the concrete.

4.9.1.1 Epoxy-Coated Rebars A technical note prepared by the FHWA provides
a discussion of the use of epoxy-coated rebar in bridge decks. Epoxy coatings (also
referred to as powders or fusion-bonded coatings) are 100% solid, dry powders. The
dry epoxy powders are electrostatically sprayed over clean, preheated rebar to pro-
vide a tough, impermeable coating. The coatings achieve the toughness and adhesion
to the metal sample because of a reaction caused by heat. As epoxy powders are ther-
mosetting materials, they are not affected by temperature. The epoxy coating provides
a physical barrier between aggressive chloride ions and the steel bar.
For some time, bridge deck deterioration because of corrosion of reinforcing bars
has been the main problem. Prior to 1970, Portland cement itself was considered to
give protection from corrosion. Later, it was found that deicing salts caused corrosion
of reinforcing steel bars. Many thousands of bridge decks containing corroded black
reinforcing steel showed signs of spalling in 7–10 years after construction. Corrosion
of the substructure was also observed because of the leakage of deicing salt solution.
Epoxy coated rebar was introduced in the 1970s as a means of extending the life of
reinforced concrete bridge components by minimizing concrete deterioration caused
by corrosion of reinforcing steel. The epoxy coatings prevent the moisture and chlo-
rides from reaching the surface of the reinforcing steel and reacting with the steel.
Since the 1970s, the highway industry has widely used epoxy coatings. Just as any
protective system, the coatings will degrade over time, leading to corrosion of rebar.
In the case of many substructures exposed to a severe corrosive marine environ-
ment epoxy-coated rebars did not perform as well as in bridge deck applications.
This was encountered in bridges located in the Florida Keys. Significant corrosion
was observed in substructure members of the bridges in 6–9 years. These members
are subjected to salt spray in the splash zone which is highly corrosive. The amount
and the degree of corrosion observed in the Florida Keys bridges raised questions
concerning the durability of epoxy-coated rebar. The performance of epoxy coated