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Preservation of Historic Concrete:
Problems and General Approaches
William B. Coney, AIA
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.
Historical Overview
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 (see fig. 1). 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 (see fig. 2). Impressed by the economic advantages
of poured gravel wall or "limegrout" 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 (see fig. 3).
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 (see fig. 4).
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 (see cover); and in Frank Lloyd Wright's masterpiece
"Fallingwater" near Mill Run, Pennsylvania (see fig. 5). 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.
Types of Concrete
Unreinforced concrete is
a composite material containing aggregates (sand, gravel,
crushed shell, or rock) held together by a cement combined
with water to form a paste, and gets its name from the fact
that it does not have any iron or steel reinforcing bars. It
was the earliest form of concrete. The ingredients become a
plastic mass that hardens as the concrete hydrates, or
"cures." Unreinforced concrete, however, is relatively weak,
and since the turn of the century has largely been replaced
by reinforced concrete. Reinforced concrete is concrete
strengthened by the inclusion of metal bars which increase
the tensile strength of concrete. Both unreinforced and
reinforced concrete can be either cast in place or
precast.
Cast-in-place concrete is
poured onsite into a previously erected formwork that is
removed after the concrete has set. Precast concrete is
molded offsite into building components. More recent
developments in concrete technology include post-tensioned
concrete and prestressed concrete, which feature greater
strength and reduced cracking in reinforced structural
elements.
Causes of Concrete
Deterioration
Deterioration in concrete
can be caused by environmental factors, inferior materials,
poor workmanship, inherent structural design defects, and
inadequate maintenance (see figs. 6, 7, and 8).
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
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 long term.
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
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 alkalis 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
occurs.
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 creep.
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 pre-World
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 deicing 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.
Document Review. 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.
Field Survey. A thorough
visual examination can assist in locating and recording the
type, extent, and severity of stress, deterioration, and
damage.
Testing. Two types of
testing, onsite and laboratory, can supplement the field
condition survey as necessary. Onsite, nondestructive testing
may include use of a calibrated metal detector or sonic tests
to locate the position, depth, and
direction of reinforcing bars (see fig. 9). 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 (see fig. 10). 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 compostion.
Analysis. 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.
Concrete Repair
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 (see
fig. 11). 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 hazards (see fig. 12).
Principal concrete repair
treatments are discussed below. While they are presented
separately here, in practice, preservation projects typically
incorporate multiple treatments (see figs. 13a-I).
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
reestablish 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 professionals.
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.
Summary
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 (see figs. 14a-c). or replacement of deteriorated
concrete with carefully selected matching material (see figs.
15a-c). 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.
Bibliography
Concrete Repair and
Restoration. ACI Compilation No. 5. Detroit: American
Concrete Institute, 1980. Reprint of Concrete International:
Design & Construction. Vol. 2, No. 9 (September
1980).
Condit, Carl W. American
Building: Materials and Techniques from the First Colonial
Settlements to the Present. Chicago: University of Chicago
Press, 1968.
Hime, W.G.
"Multitechnique Approach Solves Construction Materials
Failure Problems." American Chemical Society. Vol. 46, No. 14
(1974).
Huxtable, Ada Louise.
"Concrete Technology in U.S.A." Progressive Architecture.
(October 1960).
Onderdonk, Francis S. The
Ferro-Concrete Style. New York: Architectural Book Publishing
Company, Inc., 1928.
Perenchio, W.F., and
Marusin, S.L. "ShortTerm Chloride Penetration into Relatively
Impermeable Concretes." Concrete International. Vol. 5, No. 4
(April 1983), pp. 34-41.
Pfiefer, D.W., Perenchio,
W.F., and Marusin, S.L. "Research on Sealers, Coatings and
Specialty Concretes for Barrier Films and Layers on Concrete
Structures." Proceedings of the RILEM Seminar on the
Durability of Concrete Structures Under Normal Outdoor
Exposure, Hanover, Federal Republic of Germany, March 2629,
1984.
Pfiefer, D.W. "Steel
Corrosion Damage on Vertical Concrete, Parts I & II."
Concrete Construction. (February 1981).
Prudon, Theodore.
"Confronting Concrete Realities." Progressive Architecture.
(November 1981), pp. 131137.
Ropke, John C. Concrete
Problems, Causes & Cures. New York: McGrawHill,
1982.
Sabnis, Gajanan, ed.
Rehabilitation, Renovation and Preservation of Concrete and
Masonry Structures. ACISP85. Detroit: American Concrete
Institute, 1985.
This Preservation Brief
was prepared under contract with the National Park Service by
William B. Coney, Senior Architect for Wiss, Janney, Elstner
Associates, Inc. in Northbrook, Illinois. The author would
like to thank others who aided in the research and writing of
the Brief: William F. Perenchio, Thomas L. Rewerts, Rexford
L. Selbe, John Fraczek, and Bruce S. Kaskell, all of Wiss,
Janney, Elstner Associates, Inc. Architects Gordon D. Orr,
Jr., and Robert A. Bell provided information on the
restoration of Milton House and Unity Temple, respectively.
Barbara M. Posadas, Department of History, Northern Illinois
University, lent her considerable editorial skill to the
entire Brief. Tony C. Liu, James R. Clifton, and Michael J.
Paul of the American Concrete Institute Committee 364,
reviewed and commented on the manuscript, along with Lee H.
Nelson, H. Ward Jandl, Kay D. Weeks, Sharon C. Park, and
Michael J. Auer of the National Park Service. Washington,
D.C. September, 1987
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