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should be assumed to be moving, it is possible to repair
non-moving cracks by installation of a cementitious
repair mortar matching the adjacent concrete. It is
generally desirable not to widen cracks prior to the mortar
application. Repair mortar containing sand in the mix may
be used for wider cracks; unsanded repair mortar may be
used for narrower cracks.

When it is desirable to re-establish the structural integrity
of a concrete structure involving dormant cracks, epoxy
injection repair has proven to be an effective procedure.
Such a repair is made by first sealing the crack on both sides
of a wall or structural member with epoxy, polyester, wax,
tape, or cement slurry, and then injecting epoxy through
small holes or ports drilled in the concrete. Once the epoxy
in the crack has hardened, the surface sealing material
may be removed; however, this type of repair is usually
quite apparent. Although it may be possible to inject epoxy
without leaving noticeable residue, this process is difficult
and, in general, the use of epoxy repairs in visible areas of
concrete on historic structures is not recommended.

Active structural cracks (which move as loads are
added or removed) and thermal cracks (which move as
temperatures fluctuate) must be repaired in a manner that
will accommodate the anticipated movement. In some more
extreme cases, expansion joints may have to be introduced
before crack repairs are undertaken. Active cracks may
be filled with sealants that will adhere to the sides of
the cracks and will compress or expand during crack
Figure 19. A high-speed grinder ia used to widen a crack in
movement. The design, detailing, and execution of sealant
preparation for installation of a sealant. This process is called
repairs require considerable attention, or they will detract "routing. " After the crack is prepared, the sealant is installed to
from the appearance of the historic building. The routing prevent moisture infiltration through the crack. Although sealant
and cleaning of a crack, and installation of an elastomeric repairs can provide a durable, watertight repair for moving cracks,
sealant to prevent water penetration, is used to address they tend to be very visible.
cracks where movement is anticipated. However, unless
located in a concealed area of the concrete, this technique due to original construction techniques, architectural
is often not acceptable for historic structures because the design, or differential exposure to weather. Trial repairs
repair will be visually intrusive (Fig. 19). Other approaches, and mock-ups are used to evaluate the proposed
such as installation of a cementitious crack repair, may need
replacement concrete work and to refine construction
to be considered even though this type of repair may be less
techniques (Fig 20).
effective or have a shorter service life than a sealant repair.
Protection Systems
Replacement
Coatings and Penetrating Sealers. Protection systems
If specific components of historic concrete structures are
such as a penetrating sealers or film forming coating
beyond repair, replacement components can be cast to
are often used with non-historic structures to protect
match historic ones. Replacement of original concrete
the concrete and increase the length of the service life
should be carefully considered and viewed as a method of
of concrete repairs. However, film-forming coatings
last resort. In some cases, such as for repeated ornamental
are often inappropriate for use on a historic structure,
units, it may be more cost-effective to fabricate precast
unless the structure was coated historically. Film-
concrete units to replace missing elements. The forms
forming coatings will often change the color and
created for precast or cast-in-place units can then be used
appearance of a surface, and higher build coatings can
again during future repair projects.
also mask architectural finishes and ornamental details.
Careful mix formulation, placement, and finishing are For example, the application of a coating on concrete
required to ensure that replacement concrete units will having a formboard finish may hide the wood texture
match the historic concrete. There is often a tendency to of the surface. Pigmented film-forming coatings are
make replacement concrete more consistent in appearance also typically not appropriate for use over exposed
than the original concrete. The consistency can be in aggregate concrete, where the uncoated exposed surface
stark contrast with the variability of the original concrete contributes significantly to the historic character of
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