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concrete. In cases where the color of a substrate needs to Impressed-current cathodic protection is the most
be changed, such as to modify the appearance of existing effective means of mitigating steel corrosion and has
repairs, an alternative to pigmented film-forming coatings is been used in practical structural applications since the
the use of pigmented stains. 1970s. However, impressed-current cathodic protection
systems are typically the most costly to install and
Many proprietary clear, penetrating sealers are currently
require substantial ongoing monitoring, adjustment,
available to protect concrete substrates. These products
and maintenance to ensure a proper voltage output
render fine cracks and pores within the concrete
hydrophobic; however, they do not bridge or fill cracks. (protection current) over time. Sacrificial anode cathodic
protection dates back to the 1800s, when the hulls of
Clear sealers may change the appearance of the concrete in
that treated areas become more visible after rain in contrast ships were protected using this technology. Today
to the more absorptive areas of original concrete. Once many industries utilize the concept of sacrificial anode
applied, penetrating sealers cannot be effectively removed cathodic protection for the protection of steel exposed
and are therefore considered irreversible. They should to corrosive environments. It is less costly than an
not be used on historic concrete without thorough prior impressed-current system, but is somewhat less effective
consideration. However, clear penetrating sealers provide and requires reapplication of the anode when it becomes
an important means of protection for historic concrete that depleted.
is not of good quality and can help to avoid more extensive
future repairs or replacement. Thus they are sometimes Re-alkalization. Another technique currently available
appropriate for use on historic concrete. Once applied, these to protect concrete is realkalization, which is a process
sealers will require periodic re-application. to restore the alkalinity of carbonated concrete. The
treatment involves soaking the concrete with an alkaline
Waterproofing membranes are systems used to protect
solution, in some cases forcing it into the concrete to
concrete surfaces such as roofs, terraces, plazas, or balconies,
the level of the reiniorcing steel by passage of direct
as well as surfaces below grade. Systems range from coal
tar pitch membranes used on older buildings, to asphalt or current. These actions increase the alkalinity of the
concrete around the reiniorcement, thus restoring the
urethane-based systems. On historic buildings, membrane
systems are typically used only on surfaces that were protective alkaline environment for the reiniorcement.
originally protected by a similar system and surfaces that are Like impressed-current cathodic protection methods, it
not visible from grade. Waterproofing membranes may be is costly. Other corrosion methods are also available but
covered by roofing, paving, or other architectural finishes. have a somewhat shorter history of use.

Laboratory and field testing is recommended prior to Careful evaluation of existing conditions, the causes and
application of a protection system or treatment on any nature of distress, and environmental factors is essential
concrete structure; testing is even more critical for historic before a protection method is selected and implemented.
structures because many such treatments are not reversible. Not every protection system will be effective on each
As with other repairs, trial samples are important to structure. In addition, the level of intrusion caused by
evaluate the effectiveness of the treatment and to determine
the protection system must be carefully evaluated before
whether it will harm the concrete or affect its appearance.
it is used on a historic concrete structure.
Cathodic Protection. Corrosion is an electrochemical
process in which electrons flow between cathodic (positively Summary
charged) and anodic (negatively charged) areas on a
metal surface; corrosion occurs at the anodes. Cathodic In the United States, concrete has been a popular
protection is a technique used to control the corrosion of construction material since the late nineteenth century
metal by making the whole metal surface the cathode of and recently has gained greater recognition as a historic
an electrochemical cell. This technique is used to protect material. Preservation of historic concrete requires a
metal structures from corrosion and is also sometimes
thorough understanding of the causes and types of
used to protect steel reinforcement embedded in concrete.
deterioration, as well as of repair and replacement
For reiniorced concrete, cathodic protection is typically
materials and methods. It is important that adequate
accomplished by connecting an auxiliary anode to the
time is allotted during the planning phase of a project
reiniorcing so that the entire reiniorcing bar becomes a
to provide for trial repairs and mock-ups in order
cathode. In sacrificial anode (passive) systems, current flows
to evaluate the effectiveness and aesthetics of the
naturally by galvanic action between the less noble anode
(such as zinc) and the cathode. In impressed-current (active) repairs. Careful design is essential and, as with other
systems, current is impressed between an inert anode preservation efforts, the skill of those performing
(such as titanium) and the cathode. Cathodic protection is the work is critical to the success of the repairs. The
intended to reduce the rate of corrosion of embedded steel successful repair of many historic concrete structures
in concrete, which in turn reduces overall deterioration. in recent years demonstrates that the techniques and
Protecting embedded steel from corrosion helps to prevent materials now available can extend the life of such
concrete cracking and spalling. structures and help ensure their preservation.
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