REPORT
protection against corrosion. Cathodic protection, chloride
extraction, and corrosion-inhibitor additives in repair materials
can also be useful to prevent or delay future corrosion.
4.3.3 Water penetration—Water may penetrate into
concrete by hydrostatic pressure, moisture vapor pressure,
capillary action, wind-driven rain, or any combination of
these. Cracks, concrete density, porous concrete, lack of
entrained air, structural defects, or improperly designed or
functioning joints all contribute to the movement of moisture.
Water penetration into concrete contributes to corrosion of
reinforcement, freezing-and-thawing damage, leakage into
the interior of the structure or occupied levels beneath decks,
and possible structural damage. A properly designed protection
system should address any or all of these issues.
4.3.4 Carbonation—Carbonation is the reduction of the
protective alkalinity of concrete, caused by the absorption of
carbon dioxide and moisture. In normal concrete, the reinforcing
steel is protected by the naturally high alkalinity of the
concrete around the reinforcement, usually a pH above 12. A
passivating oxide layer is formed around the reinforcing
steel that acts as a protective coating. The oxide coating
helps prevent the reinforcing steel from corroding as long as
the high alkalinity is maintained. When carbonation occurs
the alkalinity falls and, once it goes below a pH of 10, the
embedded reinforcing steel is subject to corrosion. Because
carbonation occurs from the face of the concrete inwards,
any bars close to the exterior surface are subject to the effects
of carbonation and are not protected against corrosion.
Barrier coatings may provide protection against future
carbonation where concrete coverage is insufficient. Many
of these barrier coatings are relatively new and do not have
any field performance data. Any barrier coating should be
carefully evaluated before use. The owner should be
informed of these limitations and written consent should be
obtained before use. Other systems available to reestablish
the protection of the reinforcing steel in carbonated concrete
include the use of a cathodic protection system or the realkalization
of concrete.
4.3.5 Anodic ring (halo effect)—On many concrete repair
projects, situations arise where existing reinforcement
extends from the parent concrete into a repair mortar or new
concrete. Quite frequently, failures occur due to accelerated
corrosion of the reinforcement in the parent concrete, just
beyond the edge of the repair. It is common to see delamination
of concrete around the perimeter of new repair patches in
spite of the fact that good-quality materials, workmanship,
and methods were used.
This is commonly referred to as an anodic ring or halo
effect. It occurs because the same bar extends into two
distinctly different environments, setting up conditions that
could result in an increase of the differences in electrical
potential at the bond line between the new and the parent
concrete. Corrosion occurs at the anode, usually in the parent
concrete, as electrons are attracted to the cathodic portion of
the reinforcement in the uncontaminated repair material. The
build-up of rust results in spalling of concrete due to the large
internal forces developed at the surface of the reinforcement.
The presence of chlorides accelerates this process.
This is a difficult problem to solve. Sealers or barrier
coatings help, to some extent, to slow down the process but
do not stop it. Barrier coatings on the reinforcing steel, such
as epoxies, latex slurries, or zinc-rich coatings, can inhibit
the corrosion activity; however, there are field-application
problems that can significantly reduce their effectiveness.
Cathodic protection and chloride extraction are alternatives
that should be considered where economically feasible.
Recently, several highway departments have used galvanic
anodes to reduce corrosion in and around patches and in
concrete overlays (Section 4.7.4).
4.3.6 Cracks—All concrete repair programs and protection
systems should address the proper treatment or repair of
cracks. Intrusion of water into cracks can result in corrosion
and freezing-and-thawing problems in cold climates.
Only after determining the reason for the occurrence of a
crack can a proper repair technique be developed. Structural
cracks requiring repair should be able to reestablish load
transfer across the crack, usually coupled with epoxy
injection to ensure a positive repair.
Active cracks, especially those due to thermal changes on
exterior exposures, should be repaired to allow for future
movements. Techniques involving caulking, chemical
grouts, elastomeric coatings, and high elongation epoxies
have proven to be useful in addressing moving cracks.
The repair of active cracks on exterior exposures can be
difficult. Most of the materials used for crack repair are
temperature-sensitive and cannot be installed much below
4 °C (40 °F). It is most common to repair cracks at temperatures
above the manufacturer’s recommended minimum. Although
this facilitates the installation of the repair material, active
cracks due to temperature variations tend to close in warm
weather. Cracks that open in the winter are closed in the
summer. Both the contractor and the engineer should be
aware of this before commencing with the repair process. If
feasible, an inspection of the structure should be conducted
in cold weather to document the location of the cracks
requiring repair (ACI 224.1R). It is also desirable to conduct
repairs when the crack is near its maximum width, because
most flexible materials used in repair of active cracks
perform better in compression than in tension (Emmons
1994; ACI 504R).
4.3.7 Chloride/chemical attack—Penetration of chemical
or salt solutions through concrete contribute to the corrosion
of the embedded steel. In addition, chemical attack,
including acids, alkalis, and sulfates, may have a detrimental
effect on the concrete itself. Barrier protection systems are
commonly used to minimize the intrusion of chemicals into
concrete. This is thoroughly discussed in ACI 515.1R, and it
provides a summary of the effects of a variety of materials on
concrete. Refer also to Portland Cement Association’s
Publication IS001 (1997).
4.3.8 Surface erosion—Erosion of concrete at the surface
is a major concern on dams, spillways, and other waterfront
structures, as well as on bridge decks, ramps, parking decks,
industrial floors, and other traffic-bearing structures. Usually
to a lesser extent, it can also be a concern on buildings
exposed to acid rain and severe weather conditions. Concrete