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base concrete. The higher cement content in the facing
concrete provided a more water-resistant outer layer
and finished surface. The application of a cement-rich
top layer, referred to in some early concrete publications
as "waterproofing," was also used on top surfaces of
concrete walls, or as the top layer in sidewalks. With this
type of concrete construction, deterioration can occur
over time as a result of debonding between layers, and
can proceed very rapidly once the protective cement-rich
layer begins to break down.

It is common for historic concrete to have a highly
variable appearance, including color and finish texture.
Different levels of aggregate exposure due to paste
erosion are often found in exposed aggregate concrete.
This variability in the appearance of historic concrete
increases the level of difficulty in assessing and repairing
weathered concrete.

Signs of Distress and Deterioration
Characteristic signs of failure in concrete include
cracking, spalling, staining, and deflection. Cracking
occurs in most concrete but will vary in depth, width,
direction, pattern, and location, and can be either active
or dormant (inactive). Active cracks can widen, deepen,
or migrate through the concrete, while dormant cracks
remain relatively unchanged in size. Some dormant
cracks, such as those caused by early age shrinkage of
the concrete during curing, are not a structural concern
but when left unrepaired, can provide convenient
channels for moisture penetration and subsequent
Figures 14. Layers of architectural concrete that have debonded
damage. Random surface cracks, also called map cracks (spaUed) from the surface were removed from a historic water tank
due to their resemblance to lines on a map, are usually during the investigation performed to assess existing conditions.
related to early-age shrinkage but may also indicate Photos: Anita Washko, Wiss, Janney, Elstner Associates, Inc.
other types of deterioration such as alkali-silica reaction.
Structural cracks can be caused by temporary or
phenomena when water-rich cement paste (laitance)
continued overloads, uneven foundation settling, seismic
rises to the surface. The resulting weak material is
forces, or original design inadequacies. Structural cracks
vulnerable to spalling of thin layers, or scaling. In some
are active if excessive loads are applied to a structure, if
cases, spalling of the concrete can diminish the load-
the overload is continuing, or if settlement is ongoing.
carrying capacity of the structure.
These cracks are dormant if the temporary overloads
have been removed or if differential settlement has
Deflection is the bending or sagging of structural beams,
stabilized. Thermally-induced cracks result from
joists, or slabs, and can be an indication of deficiencies in
stresses produced by the expansion and contraction
the strength and structural soundness of concrete. This
of the concrete during temperature changes. These
condition can be produced by overloading, corrosion
cracks frequently occur at the ends or re-entrant corners
of embedded reinforcing, or inadequate design or
of older concrete structures that were built without
expansion joints to relieve such stress. construction, such as use of low-strength concrete or
undersized reinforcing bars.
Spalling (the loss of surface material) is often associated
with freezing and thawing as well as cracking and Staining of the concrete surface can be related to soiling
delamination of the concrete cover over embedded from atmospheric pollutants or other contaminants,
reinforcing steel. Spalling occurs when reinforcing dirt accumulation, and the presence of organic growth.
bars corrode and the corrosion by-products expand, However, stains can also indicate more serious
creating high stresses on the adjacent concrete, which underlying problems, such as corrosion of embedded
cracks and is displaced. Spalling can also occur when reinforcing steel, improper previous surface treatments,
water absorbed by the concrete freezes and thaws (Fig. alkali-aggregate reaction, or efflorescence, the deposition
14). In addition, surface spalling or scaling may result
of soluble salts on the surface of the concrete as a result
from the improper finishing, forming, or other surface
of water migration (Fig. 15).
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