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to cement ratio. Some admixtures, including polymer modifiers,
may change the appearance of the concrete mix. Design of the
concrete patching material should address characteristics required
for durability, workability, strength gain, compressive strength,
and other performance attributes. During installation of the
repair, skilled workmanship is required to ensure proper mixing
procedures, placement, consolidation, and curing.

Matching and Repair Techniques for Historic Concrete

Repair measures should be selected that retain as much of the
• • I '{\ I ., •
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• , -II t, . \ ',~,' j , ~ ;.\ '\ -.: original material as possible, while providing for removal of an
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. "
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I·, ... ~ t' I'!, ' ;. ' "w --. I '.' adequate amount of deteriorated concrete to provide a sound
. I, .',: "i '" 1 ~ ) .' substrate for a durable repair. The installed repair must visually
'. j I I - • , , match the existing concrete as closely as possible and should be
b .• I similar in other aspects such as compressive strength, permeability,
and other characteristics important in the mix design of the
concrete (Fig. 18).
Understanding the original construction techniques often provides
opportunities in the design of repairs. For example, joints between
the new and old concrete can be hidden in changes in surface
profile and cold joints. The required patching mix for the concrete
to be used in the repair will likely need to be specially designed
to replicate the appearance of the adjacent historic concrete. A
high level of craftsmanship is required for finishing of historic
concrete, in particular to create the sometimes inconsistent finish
and variation in the original concrete in contrast to the more even
appearance required for most non-historic repairs.

To match the various characteristics of the original concrete, trial
mixes should be developed. These mixes need to take into account
the types and colors of aggregates and paste present in the original
concrete. Different mixes may be needed because of variations
in the appearance and composition of the historic concrete. The
trials should utilize different forming and finishing techniques
to achieve the best possible match to the original concrete. Initial
trials should first take place on site but off the structure. The mix
designs providing the best match are then installed as trial repairs
on the structure, and assessed after they have cured.

Achieving compatibility between repair work and original
concrete may be difficult, especially given the variability often
present in historic concrete materials and finishes. Formed rather
than trowel-applied patch repairs are recommended for durability,
Figure 18. (a) Exposed aggregate precast concrete is as forming permits better ranges of mix ingredients (such as coarse
sounded with a hammer to detect areas of deterioration. aggregates) and improved consolidation as compared to trowel-
Corrosion of the exposed reinforcing steel bar has led to applied repairs. Parge coatings usually are not recommended
spalling of the adjacent concrete. (b) Samples of aggregate as they do not provide as durable repair as formed concrete.
considered for use in repair concrete are compared to the However, in some cases parge coatings may be appropriate to
original concrete materials in terms of size, color, texture, match an original parged surface treatment Proper placement
and reflectance. (c) Various sample panels are made using and finishing of the repair are important to obtain a match with
the selected concrete repair mix design for comparison to
the original concrete. To minimize problems associated with rapid
the original concrete on the building, and the mix design is
adjusted based on review of the samples. (d) After removal curing of concrete, such as surface cracking, it is important to use
of the spall, the concrete surface is prepared for installation proper curing methods and to allow for sufficient time.
of a formed patch. (e) Prior to placement of the concrete,
a retarding agent is brush-applied to the inside face of the Hairline cracks that show no sign of increasing in size may often
formwork to slow curing at the surface. After the concrete be left unrepaired. The width of the crack and the amount of
is partially cured, the forms are removed and the surface movement usually limits the selection of crack repair techniques
of the concrete is rubbed to remove some of the paste and that are available. Although it is difficult to determine whether
expose the aggregate to match the original concrete. cracks are moving or non-moving, and therefore most cracks

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