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concrete during its placement in forms, or in molds in the case
of precasting. This problem is especially prevalent in highly
ornamental units. Early twentieth century concrete was often
tamped or rodded into place, similar to techniques used in
forming cast stone. Poorly consolidated concrete often contains
voids (lfbugholes" or "honeycombs"), which can reduce the
protective concrete cover over the embedded reinforcing
bars, entrap water, and, if sufficiently large and strategically
numerous, reduce localized concrete strength. Vibration
technology has improved over time and flowability agents are
also used today to address this problem.

A common type of deterioration observed in concrete is the
effect of weathering from exposure to wind, rain, snow, and
salt water or spray. Weathering appears as erosion of the
cement paste, a condition more prevalent in northern regions
where precipitation can be highly acidic. This results in the
exposure of the aggregate particles on the exposed concrete
surface. Variations may occur in the aggregate exposure due
to differential erosion or dissolution of exposed cement paste.
Erosion can also be caused by the mechanical action of water
channeled over concrete, such as by the lack of drip grooves in
belt courses and sills, and by inadequate drainage. In addition,
high-pressure water when used for cleaning can also erode the
concrete surface.

In concrete structures built prior to the First World War,
concrete was often placed into forms in relatively short
vertical lifts due to limitations in lifting and pouring
techniques available at the time. Joints between different
concrete placements (often termed cold joints or lift lines) may
sometimes be considered an important part of the character of
a concrete element (Fig. 13). However, wide joints may permit
water to infiltrate the concrete, resulting in more rapid paste
erosion or freeze-thaw deterioration of adjacent concrete in
cold climates.
Figure 12. The concrete lighthouse at the Kilauea Point In the early twentieth century, concrete was sometimes placed
Light Station, Kilauea, Kauai, Hawaii, was constructed in several layers parallel to the exterior surface. A base concrete
circa 1913. The concrete, which was a good quality, high was first created with form work and then a more cement rich
strength mix for its day, is in good condition after almost mortar layer was applied to the exposed vertical face of the
one hundred years in service. Deterioration in the form of
spalling related to corrosion of embedded reinforcing steel
has occurred primarily in areas of higher ornamentation
such as projecting bands and brackets (see close-up photo).

the concrete tends to be weak and porous
because these aggregates absorb water. Some of
these aggregates can be extremely susceptible
to deterioration when exposed to moisture
and cyclic freezing and thawing. Concrete
was sometimes compromised by inclusion of
seawater or beach sand that was not thoroughly
washed with fresh water, a condition more
common with coastal fortifications built prior to
1900. The sodium chloride present in seawater
and beach sand accelerates the rate of corrosion
of the reinforced concrete.
Figure 13. Fort Casey on Admiralty Head, Fort Casey, Washington, was
Another problem encountered with historic constructed in 1898. The lift lines from placement of concrete are clearly
concrete is related to poor consolidation of the visible on the exterior walls and characterize the finished appearance.
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