Page 257 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION CONTROL OF BRIDGES 235

2. Zinc ribbons embedded in grooves cut into the concrete.

Both systems performed well for 14 years prior to removal because of failure of the
asphalt overlay and the necessity of widening the structure. Although the sacrificial
anode CP systems perform well, the majority of the CP systems on bridge decks are
impressed-current systems.
Because of the relatively high resistivity of atmospherically exposed concrete sub-
structures, most anodes utilize impressed current to achieve the necessary driving
voltages to supply the current required for corrosion control. However, an exception
to this is the use of sacrificial zinc anodes for CP of coastal bridges in Florida, which
have a relatively low concrete resistance. However, studies continue to examine the
use of sacrificial anodes because of the benefit of its low maintenance compared to
impressed-current CP systems. Two of these studies are the following:

1. Hydrogen-gel anode system.
2. Thermal-sprayed alloy anode system

The zinc–hydrogen anode system uses 10–20 mm thick zinc sheet anodes attached
to the concrete with ionically conductive hydrogel adhesive. Field trials have shown
that this system is capable of supplying sufficient current for effective corrosion con-
trol. The thermal-sprayed alloy anode system utilizes a metallization (flame or arc
spraying) process to form a metallized coating on the concrete surface. The two most
promising anode materials were Al–Zn–In alloy and zinc (16).
The cost of CP systems varies depending on the type of system used. In this
regard, Virginia DOT has published a report entitled, “Evaluation of Anodes for Gal-
vanic Cathodic Prevention of Steel Corrosion in the Prestressed Concrete Piles in
Marine Environments in Virginia” (30). The data in this report and the data in the
literature published by Virmani (2) suggest that the sprayed Al–Zn–In alloy or the
2
zinc-hydrogel systems with a life of 10–20 years cost $108–129/m .
4.9.1.9.3 Cathodic Protection for Prestressed Concrete Bridge Members
The primary concern for CP of prestressed concrete members is the possible
hydrogen-induced cracking failure (also known as hydrogen embrittlement) of the
tendons at operating loads. Hydrogen production at the steel surface is because of
CP at potentials more negative than −0.9 V saturated calomel electrode. Because
of this reason, CP for prestressed concrete has focused on the use of the sacrificial
anode system and constant current or constant voltage rectifier impressed current
systems. Another concern is the application on bridge members that have an uneven
electrical resistivity across the concrete surface. This leads to uneven distribution of
the CP current and the possibility of overprotection in the low-resistivity regions. It
is generally agreed that CP of prestressed concrete members can be accomplished
safely and reliably if proper care is taken to maintain minimum CP requirements and
to prevent overprotection.