Page 253 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION CONTROL OF BRIDGES 231

3. Altering the concrete environment to make it less corrosive. One way would be
to extract the chlorides from concrete such as electrochemical chloride removal.

Each state department of transport (DOT) has its own specifications and criteria
for rehabilitation of deteriorated concrete bridge components. The following example
illustrates the decision process for rehabilitation or deck replacement.

• If spalling of concrete is observed, the surface is checked for delamination by
chain drag, and core samples are taken to determine the chloride concentration.
3
• If the chloride concentration is greater than 1.8 kg/m , the coverage is removed
3
to depths where the concentration of chloride is less than 1.2 kg/m .
• If less than 75 mm is removed to reach an acceptable chloride level, the removed
concrete is replaced by an overlay.
• If more than 75 mm is removed, the entire deck must be replaced.
• CP is applied only in the case of partial disbondment of the concrete and when
there is no extensive spalling.

4.9.1.8 Surface Barriers The application of an overlay of low-slump concrete,
latex-modified concrete (LMC), high-density concrete, polymer concrete, or bitumi-
nous concrete with membrane on the existing concrete provides a barrier that impedes
continued intrusion of chloride anions, moisture, and oxygen that are necessary for
continued corrosion. These barrier systems may be used only after decontamination
because corrosive agents get trapped in the concrete, leading to loss in its capacity to
function properly.
Traditionally, more than 90% of the rehabilitation jobs used low water-to-cement
ratio concrete or LMC overlay as the preferred method. The estimated lifetime of the
rehabilitation methods is about 15 years (16).
Many studies have reported performance and cost data for different overlay and
patching systems (25–28). Table 4.17 gives the cost and life expectancy for overlay
and patching options for concrete bridges (27).

4.9.1.9 Cathodic Protection This is a corrosion control method involving imposi-
tion of an external voltage on the steel surface such that the steel becomes cathodic
and favoring cathodic reaction and decreasing the anodic reaction, that is, metal loss
and hence mitigating corrosion. In other words, CP transfers the oxidation (anodic)
reaction involving metal loss and hence corrosion of rebar, over to the anode of CP
system. Thus, the selection of proper anode is critical.
The primary strength of CP is that it can mitigate corrosion after it has been ini-
tiated. Although CP is often placed on pipelines, underground storage tanks (USTs),
and other structures during construction, it is generally installed on bridge members
only after corrosion has initiated and some amount of deterioration has occurred. The
primary reason for not installing a CP system on bridge components during construc-
tion is that corrosion often does not initiate for 10–20 years following construction.
Therefore, the CP system maintenance and a large portion of CP system design life