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Planning for Concrete Preservation
The significance of a historic concrete building or
structure-including whether it is important for its
architectural or engineering design, for its materials
and construction techniques, or both-guides decision
making about repair and, if needed, replacement
methods. Determining the causes of deterioration is also
central to the development of a conservation and repair
plan. With historic concrete buildings, one of the more
difficult challenges is allowing for sufficient time during
the planning phase to analyze the concrete, develop
mixes, and provide time for adequate aging of mock-ups
for matching to the original concrete.
An understanding of the original construction
techniques (cement characteristics, mix design, original
intent of assembly, type of placement, precast versus
cast in place, etc.) and previous repair work performed
on the concrete is important in determining causes
of existing deterioration and the susceptibility of the
structure to potential other types of deterioration.
For example, concrete placed in short lifts (individual
concrete placements) or constructed in precast segments
will have numerous joints that can provide entry points
for water infiltration. Inappropriate prior repairs, such
as installation of patches using an incompatible material,
can affect the future performance of the concrete. Such
prior repairs may require corrective work.
As with other preservation projects, three primary
approaches are usually considered for historic concrete
structures: maintenance, repair, or replacement.
Maintenance and repair best achieve the preservation
goal of minimal intervention and the greatest retention
of existing historic fabric. However, where elements of
the building are severely deteriorated or where inherent
problems with the material lead to ongoing failures,
replacement may be necessary.
During planning, information is gathered through
research, visual survey, inspection openings, and
laboratory studies. The material should then be
reviewed by professionals experienced in concrete
deterioration to help evaluate the nature and causes of
the concrete problems, to assess both the short-term and
long-term effects of the deterioration, and to formulate
proper repair approaches.
Condition Assessment
A condition assessment of a concrete building or
structure should begin with a review of all available
documents related to original construction and prior
repairs. While plans and specifications for older
concrete buildings are not always available, they can
be an invaluable resource and every attempt should be
made to find them. They may provide information on
the composition of the concrete mix or on the type and
location of reinforcing bars. If available, documents
related to past repairs should also be reviewed to
8
Figure 15. Evidence of
moisture movement through
concrete is apparent
in the form of mineral
deposits on the concrete
surface. Cyclic freezing
and thawing of entrapped
moisture, and corrosion of
embedded reinforcement,
have also contributed to
deterioration of the concrete
column on this fence at
Crocker Field in Fitchburg,
Massachusetts, designed by
the Olmsted Brothers.
understand how the repairs were made and to help
evaluate their anticipated performance and service life.
Archival photographs can also provide a valuable source
of information about original construction.
A visual condition survey will help identify and
evaluate the extent, types, and patterns of distress
and deterioration. The American Concrete Institute
offers several useful guides on how to perform a visual
condition survey of concrete. Generally, the condition
assessment begins with an overall visual survey,
followed by a close-up investigation of representative
areas to obtain more detailed information about modes
of deterioration.
A number of nondestructive testing methods can be
used in the field to evaluate concealed conditions. Basic
techniques include sounding with a hand-held hammer
(or for horizontal surfaces, a chain) to help identify areas
of delamination. More sophisticated techniques include
impact-echo testing (Fig. 16), ground penetrating radar,
pulse velocity, and other methods that characterize
concrete thickness and locate voids or delaminations.
Magnetic detection instruments are used to locate
embedded reinforcing steel and can be calibrated to
identify the size and depth of reinforcement. Corrosion
measurements can be taken using copper-copper
sulfate half-cell tests or linear polarization techniques to
determine the probability or rate of active corrosion of
the reinforcing steel.
To further evaluate the condition of the concrete,
samples may be removed for laboratory study to
determine material components and composition,
and causes of deterioration. Samples need to be
representative of existing conditions but should be taken
from unobtrusive locations. Laboratory studies of the
concrete may include petrographic evaluation following
ASTM C856, Practice for Petrographic Examination
of Hardened Concrete. Petrographic examination,
consisting of microscopical studies performed by a
geologist specializing in the evaluation of construction
materials, is performed to determine air content, water-
cement ratio, cement content, and general aggregate
characteristics. Laboratory studies can also include