Page 5 - doi_preservationbrief15

While reinforcing steel has played a pivotal role in Lack of proper maintenance of building elements
expanding the applications of concrete in twentieth such as roofs and drainage systems can contribute to
century architecture, corrosion of this steel has also water-related deterioration of the adjacent concrete,
caused deterioration in many historic structures. particularly when concrete is saturated with water
Reinforcing steel embedded in the concrete is normally and then exposed to freezing temperatures. As water
surrounded by a passivating within the concrete freezes, it
oxide layer that, when present, expands and exerts forces on
protects the steel from corrosion the adjacent concrete. Repeated
and aids in bonding the freezing and thawing can result
in the concrete cracking and
steel and concrete. When the
concrete's normal alkaline delaminating. Such damage
environment (above a pH appears as surface degradation,
of 10) is compromised and including severe scaling and
micro-cracking that extends
the steel is exposed to water,
into the concrete. The condition
water vapor, or high relative
is most often observed near
humidity, corrosion of the
the surface of the concrete but
steel reinforcing takes place. A
can also eventually occur deep
reduction in alkalinity results
within the concrete. This type
from carbonation, a process that
occurs when the carbon dioxide Figure 8. The Chess Pavilion in Chicago'S Lincoln Park of deterioration is usually most
in the atmosphere reacts with was designed by architect Morris Webster and constructed severe at joints, architectural
in 1956. The pavilion is a distinctive landscape feature, details, and other areas with
calcium hydroxide and moisture
with its reinforced concrete cantilevered slab that provides more surface exposure to
in the concrete. Carbonation
cover for chess players. weather. In the second half of
starts at the concrete's exposed
the twentieth century, concrete
surface but may extend to the
has utilized entrained air (the
reinforcing steel over time.
incorporation of microscopic
When carbonation reaches
air bubbles) to provide
the metal reinforcement, the
enhanced protection against
concrete no longer protects the
damage due to cyclic freezing
steel from corrosion.
of saturated concrete.
Corrosion of embedded The use of certain aggregates
reinforcing steel may be can also result in deterioration
initiated and accelerated if
of the concrete. Alkali-
calcium chloride was added to
aggregate reactions-in some
the concrete as a set accelerator
cases alkali-silica reaction
during original construction
(ASR)-occur when alkalis
to promote more rapid curing.
Figure 9. The Ira C. Keller Fountain in Portland, Oregon, normally present in cement
It may also take place if the was designed by Lawrence Halprin and constructed in react with certain aggregates,
concrete is later exposed to 1969. The fountain is constructed primarily of concrete leading to the development of
deicing salts, as may occur pillars with formboard textures and surrounding elements, an expansive crystalline gel.
during the winter in northern patterned with geometric lines, which facilitate the path When this gel is exposed to
climates. Seawater or other of water. Photo: Anita Washko, Wiss, Janney, Elstner moisture, it expands and causes
marine environments can Associates, Inc.
cracking of the aggregate and
also provide large amounts concrete matrix. Deleterious
of chloride, either from
aggregates are typically found only in certain areas of
inadequately washed original aggregate or from
the country and can be detected through analysis by an
exposure of the concrete to seawater.
experienced petrographer. Low-alkali cements as well
as fly ash are used today in new construction to prevent
Corrosion-related damage to reinforced concrete is
such reactions where this problem may occur.
the result of rust, a product of the corrosion process of
steel, which expands and thus requires more space in
the concrete than the steel did at the time of installation. Problems Specifically Encountered with
This change in volume of the steel results in expansive Historic Concrete
forces, which cause cracking and spalling of the
adjacent concrete (Fig. 12). Other signs of corrosion of Materials and workmanship used in the construction
embedded steel include delamination of the concrete of historic concrete structures, particularly those built
(planar separations parallel to the surface) and rust before the First World War, sometimes present potential
staining (often a precursor to spalling) on the concrete sources of problems. For example, where the aggregate
near the steel. consisted of cinder from burned coal or crushed brick,
5