Preservation Briefs: 15 Preservation Of Historic Concrete: Problems And General Approaches
PRESERVATION BRIEFS: 15
PRESERVATION OF HISTORIC CONCRETE: PROBLEMS AND GENERAL
The link immediately below connects to the latest version of National Park Service Preservation Brief 15:
William B. Coney, AIA
This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:
Superintendent of Documents
P.O. Box 371954
Pittsburgh, PA 15250-7954
Available ONLY in packaged set
Briefs 15-23 - $5.00
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or FAX 202/512-2250 to verify price and availability.
THE SECRETARY OF THE INTERIOR'S "STANDARDS FOR REHABILITATION"
REQUIRE THAT DETERIORATED ARCHITECTURAL FEATURES SHALL BE REPAIRED
RATHER THAN REPLACED. WHEN THE SEVERITY OF DETERIORATION REQUIRES
REMOVAL OF HISTORIC MATERIAL, ITS REPLACEMENT SHOULD MATCH THE
MATERIAL BEING REPLACED IN COMPOSITION, DESIGN, COLOR, TEXTURE, AND
OTHER VISUAL QUALITIES.
"Concrete" is a name applied to any of a number of compositions
consisting of sand, gravel, crushed stone, or other coarse
material, bound together with various kinds of cementitious
materials, such as lime or cements. When water is added, the mix
undergoes a chemical reaction and hardens. An extraordinarily
versatile building material, concrete is used for the utilitarian,
the ornamental, and the monumental. While early proponents of
modern concrete considered it to be permanent, it is, like all
materials, subject to deterioration. This Brief surveys the
principal problems posed by concrete deterioration, their likely
causes, and approaches to their remedies. In almost every
instance, remedial work should only be undertaken by qualified
professionals. Faulty concrete repair can worsen structural
problems and lead to further damage or safety hazards. Concrete
repairs are not the province of do-it-yourselfers. Consequently,
the corrective measures discussed here are included for general
information purposes only; they do not provide "how to" advice.
The Romans found that the mixture of lime putty with pozzolana, a
fine volcanic ash, would harden under water. The result was
possibly the first hydraulic cement. It became a major feature of
Roman building practice, and was used in many buildings and
engineering projects such as bridges and aqueducts. Concrete
technology was kept alive during the Middle Ages in Spain and
Africa, with the Spanish introducing a form of concrete to the New
World in the first decades of the 16th century. It was used by
both the Spanish and English in coastal areas stretching from
Florida to South Carolina. Called "tapia," or "tabby," the
substance was a creamy white, monolithic masonry material composed
of lime, sand, and an aggregate of shells, gravel, or stone mixed
with water. This mass of material was placed between wooden forms,
tamped, and allowed to dry, the building arising in layers, about
one foot at a time.
Despite its early use, concrete was slow in achieving widespread
acceptance as a building material in the United States. In 1853,
the second edition of Orson S. Fowler's, A Home for All, publicized
the advantages of "gravel wall" construction to a wide audience,
and poured gravel wall buildings appeared across the United States.
Seguin, Texas, 35 miles east of San Antonio, came to be called "The
Mother of Concrete Cities" for some 90 concrete buildings made from
local "lime water" and gravel. Impressed by the economic
advantages of poured gravel wall or "lime-grout" construction, the
Quartermaster General's Office of the War Department embarked on a
campaign to improve the quality of building for frontier military
posts. As a result, lime-grout structures were built at several
western posts, such as the buildings that were constructed with 12-
or 18-inch thick walls at Fort Laramie, Wyoming between 1872 and
1885. By the 1880s sufficient experience had been gained with
unreinforced concrete to permit construction of much larger
buildings. The Ponce de Leon Hotel in St. Augustine, Florida, is
a notable example from this period.
Reinforced concrete in the United States dates from 1860, when S.T.
Fowler obtained a patent for a reinforced concrete wall. In the
early 1870s William E. Ward built his own house in Port Chester,
New York, using concrete reinforced with iron rods for all
structural elements. Despite these developments, such construction
remained a novelty until after 1880, when innovations introduced by
Ernest L. Ransome made reinforced concrete more practicable. The
invention of the horizontal rotary kiln allowed production of a
cheaper, more uniform and reliable cement, and led to the greatly
increased acceptance of concrete after 1900.
During the early 20th century Ransome in Beverly, Massachusetts,
Albert Kahn in Detroit, and Richard E. Schmidt in Chicago promoted
concrete for utilitarian buildings with their "factory style,"
featuring an exposed concrete skeleton filled with expanses of
glass. Thomas Edison's cast-in-place reinforced concrete homes in
Union Township, New Jersey, proclaimed a similarly functional
emphasis in residential construction. From the 1920s onward,
concrete began to be used with spectacular design results: in James
J. Earley and Louis Bourgeois' exuberant, graceful Baha'i Temple in
Wilmette, Illinois; and in Frank Lloyd Wright's masterpiece
"Fallingwater" near Mill Run, Pennsylvania. Eero Saarinen's
soaring Terminal Building at Dulles International Airport outside
Washington, D.C. exemplifies the masterful use of concrete achieved
in the Modern era.
***CAUSES OF CONCRETE DETERIORATION***
Deterioration in concrete can be caused by environmental factors,
inferior materials, poor workmanship, inherent structural design
defects, and inadequate maintenance.
Environmental factors are a principal source of concrete
deterioration. Concrete absorbs moisture readily, and this is
particularly troublesome in regions of recurrent freeze-thaw
cycles. Freezing water produces expansive pressure in the cement
paste or in nondurable aggregates. Carbon dioxide, another
atmospheric component, can cause the concrete to deteriorate by
reacting with the cement paste at the surface.
MATERIALS AND WORKMANSHIP:
Materials and workmanship in the construction of early concrete
buildings are potential sources of problems. For example,
aggregates used in early concrete, such as cinders from burned coal
and certain crushed brick, absorb water and produce a weak and
porous concrete. Alkali-aggregate reactions within the concrete
can result in cracking and white surface staining. Aggregates were
not always properly graded by size to ensure an even distribution
of elements from small to large. The use of aggregates with
similarly sized particles normally produced a poorly consolidated
and ,therefore, weaker concrete.
Early builders sometimes inadvertently compromised concrete by
using seawater or beach sand in the mix or by using calcium
chloride or a similar salt as an additive to make the concrete more
"fireproof." A common practice, until recently, was to add salt to
strengthen concrete or to lower the freezing point during
cold-weather construction. These practices cause problems over the
In addition, early concrete was not vibrated when poured into forms
as it is today. More often it was tamped or rodded to consolidate
it, and on floor slabs it was often rolled with increasingly
heavier rollers filled with water. These practices tended to leave
voids (areas of no concrete) at congested areas, such as at
reinforcing bars at column heads and other critical structural
locations. Areas of connecting voids seen when concrete forms are
removed are known as "honeycombs" and can reduce the protective
cover over the reinforcing bars.
Other problems caused by poor workmanship are not unknown today.
If the first layer of concrete is allowed to harden before the next
one is poured next to or on top of it, joints can form at the
interface of the layers. In some cases, these "cold joints"
visibly detract from the architecture, but are otherwise harmless.
In other cases, "cold joints" can permit water to infiltrate, and
subsequent freeze-thaw action can cause the joints to move. Dirt
packed in the joints allows weeds to grow, further opening paths
for water to enter. Inadequate curing can also lead to problems.
If moisture leaves newly-poured concrete too rapidly because of low
humidity, excessive exposure to sun or wind, or use of too porous
a substrate, the concrete will develop shrinkage cracks and will
not reach its full potential strength.
STRUCTURAL DESIGN DEFECTS:
Structural and design defects in historic concrete structures can
be an important cause of deterioration. For example, the amount of
protective concrete cover around reinforcing bars was often
insufficient. Another design problem in early concrete buildings
is related to the absence of standards for expansion-contraction
joints to prevent stresses caused by thermal movements, which may
result in cracking.
Improper maintenance of historic buildings can cause long-term
deterioration of concrete. Water is a principal source of damage
to historic concrete (as to almost every other material) and
prolonged exposure to it can cause serious problems. Unrepaired
roof and plumbing leaks, leaks through exterior cladding, and
unchecked absorption of water from damp earth are potential sources
of building problems. Deferred repair of cracks allowing water
penetration and freeze-thaw attacks can even cause a structure to
collapse. In some cases the application of waterproof surface
coatings can aggravate moisture-related problems by trapping water
vapor within the underlying material.
***MAJOR SIGNS OF CONCRETE DETERIORATION***
Cracking occurs over time in virtually all concrete. Cracks vary
in depth, width, direction, pattern, location, and cause. Cracks
can be either active or dormant (inactive). Active cracks widen,
deepen, or migrate through the concrete. Dormant cracks remain
unchanged. Some dormant cracks, such as those caused by shrinkage
during the curing process, pose no danger, but if left unrepaired,
they can provide convenient channels for moisture penetration,
which normally causes further damage.
Structural cracks can result from temporary or continued overloads,
uneven foundation settling, or original design inadequacies.
Structural cracks are active if the overload is continued or if
settlement is ongoing; they are dormant if the temporary overloads
have been removed, or if differential settlement has stabilized.
Thermally-induced cracks result from stresses produced by
temperature changes. They frequently occur at the ends or corners
of older concrete structures built without expansion joints capable
of relieving such stresses. Random surface cracks (also called
"map" cracks due to their resemblance to the lines on a road map)
that deepen over time and exude a white gel that hardens on the
surface are caused by an adverse reaction between the alkalies in
a cement and some aggregates.
Since superficial repairs that do not eliminate underlying causes
will only tend to aggravate problems, professional consultation is
recommended in almost every instance where noticeable cracking
Spalling is the loss of surface material in patches of varying
size. It occurs when reinforcing bars corrode, thus creating high
stresses within the concrete. As a result, chunks of concrete pop
off from the surface. Similar damage can occur when water absorbed
by porous aggregates freezes. Vapor-proof paints or sealants,
which trap moisture beneath the surface of the impermeable barrier,
also can cause spalling. Spalling may also result from the
improper consolidation of concrete during construction. In this
case, water-rich cement paste rises to the surface (a condition
known as laitance). The surface weakness encourages scaling, which
is spalling in thin layers.
Deflection is the bending or sagging of concrete beams, columns,
joists, or slabs, and can seriously affect both the strength and
structural soundness of concrete. It can be produced by
overloading, by corrosion, by inadequate construction techniques
(use of low strength concrete or undersized reinforcing bars, for
example), or by concrete creep (long-term shrinkage). Corrosion
may cause deflection by weakening and ultimately destroying the
bond between the rebar and the concrete, and finally by destroying
the reinforcing bars themselves. Deflection of this type is
preceded by significant cracking at the bottom of the beams or at
column supports. Deflection in a structure without widespread
cracking, spalling, or corrosion is frequently due to concrete
Stains can be produced by alkali-aggregate reaction, which forms a
white gel exuding through cracks and hardening as a white stain on
the surface. Efflorescence is a white, powdery stain produced by
the leaching of lime from Portland cement, or by the preWorld War
II practice of adding lime to whiten the concrete. Discoloration
can also result from metals inserted into the concrete, or from
corrosion products dripping onto the surface.
Erosion is the weathering of the concrete surface by wind, rain,
snow, and salt air or spray. Erosion can also be caused by the
mechanical action of water channeled over concrete, by the lack of
drip grooves in beltcourses and sills, and by inadequate drainage.
Corrosion, the rusting of reinforcing bars in concrete, can be a
most serious problem. Normally, embedded reinforcing bars are
protected against corrosion by being buried within the mass of the
concrete and by the high alkalinity of the concrete itself. This
protection, however, can be destroyed in two ways. First, by
carbonation, which occurs when carbon dioxide in the air reacts
chemically with cement paste at the surface and reduces the
alkalinity of the concrete. Second, chloride ions from salts
combine with moisture to produce an electrolyte that effectively
corrodes the reinforcing bars. Chlorides may come from seawater
additives in the original mix, or from prolonged contact with salt
spray or de-icing salts. Regardless of the cause, corrosion of
reinforcing bars produces rust, which occupies significantly more
space than the original metal, and causes expansive forces within
the concrete. Cracking and spalling are frequent results. In
addition, the load carrying capacity of the structure can be
diminished by the loss of concrete, by the loss of bond between
reinforcing bars and concrete, and by the decrease in thickness of
the reinforcing bars themselves. Rust stains on the surface of the
concrete are an indication that internal corrosion is taking place.
***PLANNING FOR CONCRETE PRESERVATION***
Whatever the causes of deterioration, careful analysis,
supplemented by testing, is vital to the success of any historic
concrete repair project. Undertaken by experienced engineers or
architects, the basic steps in a program of testing and analysis
are document review, field survey, testing, and analysis.
While plans and specifications for older concrete buildings are
rarely extant, they can be an invaluable aid, and every attempt
should be made to find them. They may provide information on the
intended composition of the concrete mix, or on the type and
location of reinforcing bars. Old photographs, records of previous
repairs, documents for buildings of the same basic construction or
age, and news reports may also document original construction or
changes over time.
A thorough visual examination can assist in locating and recording
the type, extent, and severity of stress, deterioration, and
Two types of testing, on-site and laboratory, can supplement the
field condition survey as necessary. On-site, non-destructive
testing may include use of a calibrated metal detector or sonic
tests to locate the position, depth, and direction of reinforcing
bars. Voids can frequently be detected by "sounding" with a metal
hammer. Chains about 30 inches long attached to a 2-foot-long
crossbar, dragged over the slabs while listening for hollow
reverberations, can locate areas of slabs that have delaminated.
In order to find areas of walls that allow moisture to penetrate to
the building interior, areas may be tested from the outside by
spraying water at the walls and then inspecting the interior for
water. If leaks are not readily apparent, sophisticated equipment
is available to measure the water permeability of concrete walls.
If more detailed examinations are required, nondestructive
instruments are available that can assist in determining the
presence of voids or internal cracks, the location and size of
rebars, and the strength of the concrete. Laboratory testing can
be invaluable in determining the composition and characteristics of
historic concrete and in formulating a compatible design mix for
repair materials. These tests, however, are expensive. A
well-equipped concrete laboratory can analyze concrete samples for
strength, alkalinity, carbonation, porosity, alkali-aggregate
reaction, presence of chlorides, and past composition.
Analysis is probably the most important step in the process of
evaluation. As survey and test results are revised in conjunction
with available documentation, the analysis should focus on
determining the nature and causes of the concrete problems, on
assessing both the short-term and long-term effects of the
deterioration, and on formulating proper remedial measures.
Repairs should be undertaken only after the planning measures
outlined above have been followed. Repair of historic concrete may
consist of either patching the historic material or filling in with
new material worked to match the historic material. If replacement
is necessary, duplication of historic materials and detailing
should be as exact as possible to assure a repair that is
functionally and aesthetically acceptable. The correction and
elimination of concrete problems can be difficult, time-consuming,
and costly. Yet the temptation to resort to temporary solutions
should be avoided, since their failure can expose a building to
further and more serious deterioration, and in some cases can mask
underlying structural problems that could lead to serious safety
Principal concrete repair treatments are discussed below. While
they are presented separately here, in practice, preservation
projects typically incorporate multiple treatments.
REPAIR OF CRACKING:
Hairline, nonstructural cracks that show no sign of worsening
normally need not be repaired. Cracks larger than hairline cracks,
but less than approximately one-sixteenth of an inch, can be
repaired with a mix of cement and water. If the crack is wider
than one-sixteenth of an inch, fine sand should be added to the mix
to allow for greater compactibility, and to reduce shrinkage during
drying. Field trials will determine whether the crack should be
routed (widened and deepened) minimally before patching to allow
sufficient penetration of the patching material. To ensure a
long-term repair, the patching materials should be carefully
selected to be compatible with the existing concrete as well as
with subsequent surface treatments such as paint or stucco.
When it is desirable to re-establish the structural integrity of a
concrete structure involving dormant cracks, epoxy injection repair
should be considered. An epoxy injection repair is made by sealing
the crack on both sides of a wall, or a structural member, with an
epoxy mortar, leaving small holes, or "ports" to receive the epoxy
resin. After the surface mortar has hardened, epoxy is pumped into
the ports. Once the epoxy in the crack has hardened, the surface
mortar can be ground off, but the repair may be visually
noticeable. (It is possible to inject epoxy without leaving
noticeable patches but the procedure is much more complex.)
Other cracks are active, changing their width and length. Active
structural cracks will move as loads are added or removed. Thermal
cracks will move as temperatures fluctuate. Thus,
expansion-contraction joints may have to be introduced before
repair is undertaken. Active cracks should be filled with sealants
that will adhere to the sides of the cracks and will compress or
expand during crack movement. The design, detailing, and execution
of sealant-filled cracks require considerable attention or else
they will detract from the appearance of the historic building.
Random (map) cracks throughout a structure are difficult to
correct, and may be unrepairable. Repair, if undertaken, requires
removing the cracked concrete. A compatible concrete patch to
replace the removed concrete is then installed. For some buildings
without significant historic finishes, an effective and economical
repair material is probably a sprayed concrete coating, troweled or
brushed smooth. Because the original concrete will ultimately
contaminate new concrete, buildings with map cracks will present
continuing maintenance problems.
REPAIR OF SPALLING:
Repair of spalling entails removing the loose, deteriorated
concrete and installing a compatible patch that dovetails into the
existing sound concrete. In order to prevent future crack
development after the spall has been patched and to ensure that the
patch matches the historic concrete, great attention must be paid
to the treatment of rebars, the preparation of the existing
concrete substrate, the selection of compatible patch material, the
development of good contact between patch and substrate, and the
curing of the patch.
Once the deteriorated concrete in a spalled area has been removed,
rust on the exposed rebars must be removed by wire brush or
sandblasting. An epoxy coating applied immediately over the
cleaned rebars will diminish the possibility of further corrosion.
As a general rule, if the rebars are so corroded that a structural
engineer determines they should be replaced, new supplemental
reinforcing bars will normally be required, assuming that the rebar
is important to the strength of the concrete. If not, it is
possible to cut away the rebar.
Proper preparation of the substrate will ensure a good bond between
the patch and the existing concrete. If a large, clean break or
other smooth surface is to be patched, the contact area should be
roughened with a hammer and chisel. In all cases, the substrate
should be kept moist with wet rags, sponges, or running water for
at least an hour before placement of the patch. Bonding between
the patch and substrate can be encouraged by scrubbing the
substrate with cement paste, or by applying a liquid bonding agent
to the surface of the substrate. Admixtures such as epoxy resins,
latexes, and acrylics in the patch may also be used to increase
bonding, but this may cause problems with color matching if the
surfaces are to be left unpainted.
Compatible matching of patch material to the existing concrete is
critical for both appearance and durability. In general, repair
material should match the composition of the original material (as
revealed by laboratory analysis) as closely as possible so that the
properties of the two materials, such as coefficient of thermal
expansion and strength, are compatible. Matching the color and
texture of the existing concrete requires special care. Several
test batches of patching material should be mixed by adding
carefully selected mineral pigments that vary slightly in color.
After the samples have cured, they can be compared to the historic
concrete and the closest match selected.
Contact between the patch and the existing concrete can be enhanced
through the use of anchors, preferably stainless-steel hooked pins,
placed in holes drilled into the structure and secured in place
with epoxy. Good compaction of the patch material will encourage
the contact. Compaction is difficult when the patch is "laid-up"
with a trowel without the use of forms; however, by building up
thin layers of concrete, each layer can be worked with a trowel to
achieve compaction. Board forms will be necessary for large
patches. In cases where the existing concrete has a significant
finish, care must be taken to pin the form to the existing concrete
without marring the surface. The patch in the form can be
consolidated by rodding or vibration.
Because formed concrete surfaces normally develop a sheen that does
not match the surface texture of most historic concrete, the forms
must be removed before the patch has fully set. The surface of the
patch must then be finished to match the historic concrete. A
brush or wet sponge is particularly useful in achieving matching
textures. It may be difficult to match historic concrete surfaces
that were textured, as a result of exposed aggregate for example,
but it is important that these visual qualities be matched. Once
the forms are removed, holes from the bolts must also be patched
and finished to match adjacent surfaces.
Regardless of size, a patch containing cement binder (especially
Portland cement) will tend to shrink during drying. Adequate
curing of the patch may be achieved by keeping it wet for several
days with damp burlap bags. It should be noted that although
greater amounts of sand will reduce overall shrinkage, patches with
a high sand content normally will not bond well to the substrate.
REPAIR OF DEFLECTION:
Deflection can indicate significant structural problems and often
requires the strengthening or replacement of structural members.
Because deflection can lead to structural failure and serious
safety hazards, its repair should be left to engineering
REPAIR OF EROSION:
Repair of eroded concrete will normally require replacing lost
surface material with a compatible patching material (as outlined
above) and then applying an appropriate finish to match the
historic appearance. The elimination of water coursing over
concrete surfaces should be accomplished to prevent further
erosion. If necessary, drip grooves at the underside of
overhanging edges of sills, beltcourses, cornices, and projecting
slabs should be installed.
Many early concrete buildings in the United States are threatened
by deterioration. Effective protection and maintenance are the
keys to the durability of concrete. Even when historic concrete
structures are deteriorated, however, many can be saved through
preservation projects involving sensitive repair, or replacement of
deteriorated concrete with carefully selected matching material.
Successful restoration of many historic concrete structures in
America demonstrates that techniques and materials now available
can extend the life of such structures for an indefinite period,
thus preserving significant cultural resources.
END OF SECTION