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chemical analyses to determine chloride content, sulfate surface treatments using very fine particulates, such
content, and alkali levels of the concrete; identification as dolomitic limestone powder, can sometimes clean
of deleterious aggregates; and determination of depth effectively. However, micro abrasive cleaning may alter
of carbonation. Compressive strength studies can the texture and surface reflectivity of concrete. Some
be conducted to evaluate the strength of the existing concrete can be damaged even by fine particulates
concrete and provide information for repair work. The applied at very low pressures.
laboratory studies provide a general identification of
Chemical surface treatments can clean effectively
the original concrete's components and aggregates,
but may also alter the appearance of the concrete by
and evidence of damage due to various mechanisms
bleaching the concrete, removing the paste, etching
including cyclic freezing and thawing, alkali-aggregate
the aggregate, or otherwise altering the surface.
reactivity, or sulfate attack. Information gathered
Detergent cleaners or mild, diluted acid cleaners may
through laboratory studies can also be used to help
be appropriate for removal of staining or severe soiling.
develop a mix design for the repair concrete.
Cleaning products that contain strong acids such as
hydrochloric (muriatic) or hydrofluoric acid, which will
Cleaning
damage concrete and are harmful to persons, animals,
site features, and the environment, should not be used.
As with other historic structures, concrete structures are
cleaned for several reasons: to improve the appearance For any cleaning process, trial samples should be
of the concrete, as a cyclical maintenance measure, or performed prior to full-scale implementation. The
in preparation for repairs. Consideration should first be intent of the cleaning program should not be to return
given to whether the historic concrete structure needs to the structure to a like new appearance. Concrete can
be cleaned at all. If cleaning is required, then the gentlest age gracefully, and as long as soiling is not severe or
system that will be effective should be selected. deleterious, many structures can still be appreciated
without extensive cleaning.
Three primary methods are used for cleaning concrete:
water methods, abrasive surface treatments, and Methods of Maintenance and Repair
chemical surface treatments. Low-pressure water (less
than 200 psi) or steam cleaning can effectively remove
The maintenance of historic concrete often is thought of
surface soiling from sound concrete; however, care is
in terms of appropriate cleaning to remove unattractive
required on fragile or deteriorated surfaces. In addition,
dirt or soiling materials. However, the implementation
water and steam methods are typically not effective in
of an overall maintenance plan for a historic structure is
removing staining or severe soiling. Power washing
the most effective way to help protect historic concrete.
with high-pressure water is sometimes used to clean or
For examples, the lack of maintenance to roofs and
remove coatings from sound, high-strength concrete, but
drainage systems can promote water related damage
high-pressure water washing is generally damaging to
to adjacent concrete features. The repeated use of
and not appropriate for concrete on historic structures.
deicing salts in winter climates can pit the surface of old
concrete and also may promote decay in embedded steel
When used with proper controls and at very low
reinforcements. Inadequate protection of concrete walls
pressures (typically 35 to 75 psi), microabrasive
adjacent to driveways and parking areas can result in the
need for repair work later on.
The maintenance of historic concrete involves the regular
inspection of concrete to establish baseline conditions
and identify needed repairs. Inspection tasks involve
monitoring protection systems, including sealant joints,
expansion joints, and protective coatings; reviewing
existing conditions for development of distress such as
cracking and delaminations; documenting conditions
observed; and developing and implementing a cyclical
repair program.
Sealants are an important part of maintenance of historic
concrete structures. Elastomeric sealants, which have
replaced traditional oil-resin based caulks for many
applications, are used to seal cracks and joints to keep
Figure 16. Impact echo testing is performed on a concrete structural
slab to help determine depth of deterioration. In this method, a short out moisture and reduce air infiltration. Sealants are
pulse of energy is introduced into the structure and a transducer commonly used at windows and door perimeters,
mounted on the impacted surface of the structure receives the at interfaces between concrete and other materials,
reflected input waves or echoes. These waves are analyzed to help and at attachments to or through walls or roofs, such
identify flaws and deterioration within the concrete. as with lamps, signs, or exterior plumbing fixtures.

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