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The Use of Substitute Materials
on Historic Building Exteriors
Sharon C. Park, AIA
The Secretary of the Interior's Standards for Rehabilitation
require that "deteriorated architectural features be repaired
rather than replaced, wherever possible. In the event that
replacement is necessary, the new material should match the
material being replaced in composition, design, color,
texture, and other visual properties." Substitute materials
should be used only on a limited basis and only when they
will match the appearance and general properties of the
historic material and will not damage the historic resource.
Introduction
When deteriorated, damaged, or lost features of a historic
building need repair or replacement, it is almost always best
to use historic materials. In limited circumstances
substitute materials that imitate historic materials may be
used if the appearance and properties of the historic
materials can be matched closely and no damage to the
remaining historic fabric will result.
Great care must be taken if substitute materials are used on
the exteriors of historic buildings. Ultraviolet light,
moisture penetration behind joints, and stresses caused by
changing temperatures can greatly impair the performance of
substitute materials over time. Only after consideration of
all options, in consultation with qualified professionals,
experienced fabricators and contractors, and development of
carefully written specifications should this work be
undertaken.
The practice of using substitute materials in architecture is
not new, yet it continues to pose practical problems and to
raise philosophical questions. On the practical level the
inappropriate choice or improper installation of substitute
materials can cause a radical change in a building's
appearance and can cause extensive physical damage over time.
On the more philosophical level, the wholesale use of
substitute materials can raise questions concerning the
integrity of historic buildings largely comprised of new
materials. In both cases the integrity of the historic
resource can be destroyed.
Some preservationists advocate that substitute materials
should be avoided in all but the most limited cases. The fact
is, however, that substitute materials are being used more
frequently than ever in preservation projects, and in many
cases with positive results. They can be cost-effective, can
permit the accurate visual duplication of historic materials,
and last a reasonable time. Growing evidence indicates that
with proper planning, careful specifications and supervision,
substitute materials can be used successfully in the process
of restoring the visual appearance of historic resources.
This Brief provides general guidance on the use of substitute
materials on the exteriors of historic buildings. While
substitute materials are frequently used on interiors, these
applications are not subject to weathering and moisture
penetration, and will not be discussed in this Brief. Given
the general nature of this publication, specifications for
substitute materials are not provided. The guidance provided
should not be used in place of consultations with qualified
professionals. This Brief includes a discussion of when to
use substitute materials, cautions regarding their expected
performance, and descriptions of several substitute
materials, their advantages and disadvantages. This review of
materials is by no means comprehensive, and attitudes and
findings will change as technology develops.
Historical Use
of Substitute Materials
The tradition of using cheaper and more common materials in
imitation of more expensive and less available materials is a
long one. George Washington, for example, used wood painted
with sand-impregnated paint at Mount Vernon to imitate cut
ashlar stone. This technique along with scoring stucco into
block patterns was fairly common in colonial America to
imitate stone (see illus. 1, 2).
Molded or cast masonry substitutes, such as dry-tamp cast
stone and poured concrete, became popular in place of
quarried stone during the 19th century. These masonry units
were fabricated locally, avoiding expensive quarrying and
shipping costs, and were versatile in representing either
ornately carved blocks, plain wall stones or rough cut
textured surfaces. The end result depended on the type of
patterned or textured mold used and was particularly popular
in conjunction with mail order houses (see illus. 3). Later,
panels of cementitious permastone or formstone and less
expensive asphalt and sheet metal panels were used to imitate
brick or stone.
Metal (cast, stamped, or brake-formed) was used for
storefronts, canopies, railings, and other features, such as
galvanized metal cornices substituting for wood or stone,
stamped metal panels for Spanish clay roofing tiles, and
cast-iron column capitals and even entire building fronts in
imitation of building stone (see illus. no. 4).
TERRA-- cotta, a molded fired clay product, was itself a
substitute material and was very popular in the late 19th and
early 20th centuries. It simulated the appearance of
intricately carved stonework, which was expensive and
time-consuming to produce. Terra cotta could be glazed to
imitate a variety of natural stones, from brownstones to
limestones, or could be colored for a polychrome effect.
Nineteenth century technology made a variety of materials
readily available that not only were able to imitate more
expensive materials but were also cheaper to fabricate and
easier to use. Throughout the century, imitative materials
continued to evolve. For example, ornamental window hoods
were originally made of wood or carved stone. In an effort to
find a cheaper substitute for carved stone and to speed
fabrication time, cast stone, an early form of concrete, or
cast-iron hoods often replaced stone. Toward the end of the
century, even less expensive sheet metal hoods, imitating
stone, also came into widespread use. All of these materials,
stone, cast stone, cast iron, and various pressed metals were
in production at the same time and were selected on the basis
on the basis of the availability of materials and local
craftsmanship, as well as durability and cost (see illus. 5).
The criteria for selection today are not much different.
Many of the materials used historically to imitate other
materials are still available. These are often referred to as
the traditional materials: wood, cast stone, concrete, terra
cotta and cast metals. In the last few decades, however, and
partly as a result of the historic preservation movement, new
families of synthetic materials, such as fiberglass, acrylic
polymers, and epoxy resins, have been developed and are being
used as substitute materials in construction. In some
respects these newer products (often referred to as high tech
materials) show great promise; in others, they are less
satisfactory, since they are often difficult to integrate
physically with the porous historic materials and may be too
new to have established solid performance records.
When to
Consider Using Substitute Materials in Preservation Projects
Because the overzealous use of substitute materials can
greatly impair the historic character of a historic
structure, all preservation options should be explored
thoroughly before substitute materials are used. It is
important to remember that the purpose of repairing damaged
features and of replacing lost and irreparably damaged ones
is both to match visually what was there and to cause no
further deterioration. For these reasons it is not
appropriate to cover up historic materials with synthetic
materials that will alter the appearance, proportions and
details of a historic building and that will conceal future
deterioration (see illus. 6).
Some materials have been used successfully for the repair of
damaged features such as epoxies for wood infilling,
cementitious patching for sandstone repairs, or plastic stone
for masonry repairs. Repairs are preferable to replacement
whether or not the repairs are in kind or with a synthetic
substitute material (see illus. 7).
In general, four circumstances warrant the consideration of
substitute materials: 1) the unavailability of historic
materials; 2) the unavailability of skilled craftsmen; 3)
inherent flaws in the original materials; and 4)
code-required changes (which in many cases can be extremely
destructive of historic resources).
Cost may or may not be a determining factor in considering
the use of substitute materials. Depending on the area of the
country, the amount of material needed, and the projected
life of less durable substitute materials, it may be cheaper
in the long run to use the original material, even though it
may be harder to find. Due to many early failures of
substitute materials, some preservationist are looking abroad
to find materials (especially stone) that match the historic
materials in an effort to restore historic buildings
accurately and to avoid many of the uncertainties that come
with the use of substitute materials.
1. The unavailability of the historic material.
The most common reason for considering substitute materials
is the difficulty in finding a good match for the historic
material (particularly a problem for masonry materials where
the color and texture are derived from the material itself).
This may be due to the actual unavailability of the material
or to protracted delivery dates. For example, the local
quarry that supplied the sandstone for a building may no
longer be in operation. All efforts should be made to locate
another quarry that could supply a satisfactory match (see
illus. 8). If this approach fails, substitute materials such
as dry-tamp cast stone or textured precast concrete may be a
suitable substitute if care is taken to ensure that the
detail, color and texture of the original stone are matched.
In some cases, it may be possible to use a sand-impregnated
paint on wood as a replacement section,
achieved using readily available traditional materials,
conventional tools and work skills. (see illus. 9). Simple
solutions should not be overlooked.
2. The unavailability of historic craft techniques and lack
of skilled artisans. These two reasons complicate any
preservation or rehabilitation project. This is particularly
true for intricate ornamental work, such as carved wood,
carved stone, wrought iron, cast iron, or molded terra cotta.
However, a number of stone and wood cutters now employ
sophisticated carving machines, some even computerized. It is
also possible to cast substitute replacement pieces using
aluminum, cast stone, fiberglass, polymer concretes, glass
fiber reinforced concretes and terra cotta. Mold making and
casting takes skill and craftsmen who can undertake this work
are available. (see illus. 10, 11). Efforts should always be
made, prior to replacement, to seek out artisans who might be
able to repair ornamental elements and thereby save the
historic features in place.
3. Poor original building materials.
Some historic building materials were of inherently poor
quality or their modern counterparts are inferior. In
addition, some materials were naturally incompatible with
other materials on the building, causing staining or galvanic
corrosion. Examples of poor quality materials were the very
soft sandstones which eroded quickly. An example of poor
quality modern replacement material is the tin coated steel
roofing which is much less durable than the historic tin or
terne iron which is no longer available. In some cases, more
durable natural stones or precast concrete might be available
as substitutes for the soft stones and modern terne-coated
stainless steel or lead-coated copper might produce a more
durable yet visually compatible replacement roofing (see
illus. 12).
4. Code-related changes.
Sometimes referred to as life and safety codes, building
codes often require changes to historic buildings. Many
cities in earthquake zones, for example, have laws requiring
that overhanging masonry parapets and cornices, or
freestanding urns or finials be securely re-anchored to new
structural frames or be removed completely. In some cases, it
may be acceptable to replace these heavy historic elements
with light replicas (see illus. 13). In other cases, the
extent of historic fabric removed may be so great as to
diminish the integrity of the resource. This could affect the
significance of the structure and jeopardize National
Register status. In addition, removal of repairable historic
materials could result in loss of Federal tax credits for
rehabilitation. Department of the Interior regulations make
clear that the Secretary of the Interior's Standards for
Rehabilitation take precedence over other regulations and
codes in determining whether a project is consistent with the
historic character of the building undergoing rehabilitation.
Two secondary reasons for considering the use of substitute
materials are their lighter weight and for some materials, a
reduced need of maintenance. These reasons can become
important if there is a need to keep dead loads to a minimum
or if the feature being replaced is relatively inaccessible
for routine maintenance.
Cautions and Concerns
In dealing with exterior features and materials, it must be
remembered that moisture penetration, ultraviolet
degradation, and differing thermal expansion and contraction
rates of dissimilar materials make any repair or replacement
problematic. To ensure that a repair or replacement will
perform well over time, it is critical to understand fully
the properties of both the original and the substitute
materials, to install replacement materials correctly, to
assess their impact on adjacent historic materials, and to
have reasonable expectations of future performance.
Many high tech materials are too new to have been tested
thoroughly. The differences in vapor permeability between
some synthetic materials and the historic materials have in
some cases caused unexpected further deterioration. It is
therefore difficult to recommend substitute materials if the
historic materials are still available. As previously
mentioned, consideration should always be given first to
using traditional materials and methods of repair or
replacement before accepting unproven techniques, materials
or applications.
Substitute materials must meet three basic criteria before
being considered: they must be compatible with the historic
materials in appearance; their physical properties must be
similar to those of the historic materials, or be installed
in a manner that tolerates differences; and they must meet
certain basic performance expectations over an extended
period of time.
Matching the Appearance of the Historic Materials
In order to provide an appearance that is compatible with the
historic material, the new material should match the details
and craftsmanship of the original as well as the color,
surface texture, surface reflectivity and finish of the
original material (see illus. 14). The closer an element is
to the viewer, the more closely the material and
craftsmanship must match the original.
Matching the color and surface texture of the historic
material with a substitute material is normally difficult. To
enhance the chances of a good match, it is advisable to clean
a portion of the building where new materials are to be used.
If pigments are to be added to the substitute material, a
specialist should determine the formulation of the mix, the
natural aggregates and the types of pigments to be used. As
all exposed material is subject to ultraviolet degradation,
if possible, samples of the new materials made during the
early planning phases should be tested or allowed to weather
over several seasons to test for color stability.
Fabricators should supply a sufficient number of samples to
permit onsite comparison of color, texture, detailing, and
other critical qualities (see illus. 15, 16). In situations
where there are subtle variations in color and texture within
the original materials, the substitute materials should be
similarly varied so that they are not conspicuous by their
uniformity.
Substitute materials, notably the masonry ones, may be more
water-absorbent than the historic material. If this is
visually distracting, it may be appropriate to apply a
protective vapor-permeable coating on the substitute
material. However, these clear coatings tend to alter the
reflectivity of the material, must be reapplied periodically,
and may trap salts and moisture, which can in turn produce
spalling. For these reasons, they are not recommended for use
on historic materials.
Matching the Physical Properties
While substitute materials can closely match the appearance
of historic ones, their physical properties may differ
greatly. The chemical composition of the material (i.e.,
presence of acids, alkalines, salts, or metals) should be
evaluated to ensure that the replacement materials will be
compatible with the historic resource. Special care must
therefore be taken to integrate and to anchor the new
materials properly (see illus. 17). The thermal expansion and
contraction coefficients of each adjacent material must be
within tolerable limits. The function of joints must be
understood and detailed either to eliminate moisture
penetration or to allow vapor permeability. Materials that
will cause galvanic corrosion or other chemical reactions
must be isolated from one another.
To ensure proper attachment, surface preparation is critical.
Deteriorated underlying material must be cleaned out.
Noncorrosive anchoring devices or fasteners that are designed
to carry the new material and to withstand wind, snow and
other destructive elements should be used (see illus. 18).
Properly chosen fasteners allow attached materials to expand
and contract at their own rates. Caulking, flexible sealants
or expansion joints between the historic material and the
substitute material can absorb slight differences of
movement. Since physical failures often result from poor
anchorage or improper installation techniques, a structural
engineer should be a member of any team undertaking major
repairs.
Some of the new high tech materials such as epoxies and
polymers are much stronger than historic materials and
generally impermeable to moisture. These differences can
cause serious problems unless the new materials are modified
to match the expansion and contraction properties of adjacent
historic materials more closely, or unless the new materials
are isolated from the historic ones altogether. When stronger
or vapor impermeable new materials are used alongside
historic ones, stresses from trapped moisture or differing
expansion and contraction rates generally hasten
deterioration of the weaker historic material. For this
reason, a conservative approach to repair or replacement is
recommended, one that uses more pliant materials rather than
high-strength ones (see illus. 19). Since it is almost
impossible for substitute materials to match the properties
of historic materials perfectly, the new system incorporating
new and historic materials should be designed so that if
material failures occur, they occur within the new material
rather than the historic material.
Performance Expectations
While a substitute material may appear to be acceptable at
the time of installation, both its appearance and its
performance may deteriorate rapidly. Some materials are so
new that industry standards are not available, thus making it
difficult to specify quality control in fabrication, or to
predict maintenance requirements and long term performance.
Where possible, projects involving substitute materials in
similar circumstances should be examined. Material
specifications outlining stability of color and texture;
compressive or tensile strengths if appropriate; the
acceptable range of thermal coefficients, and the durability
of coatings and finishes should be included in the contract
documents. Without these written documents, the owner may be
left with little recourse if failure occurs (see illus. 20,
21).
The tight controls necessary to ensure long-term performance
extend beyond having written performance standards and
selecting materials that have a successful track record. It
is important to select qualified fabricators and installers
who know what they are doing and who can follow up if repairs
are necessary. Installers and contractors unfamiliar with
specific substitute materials and how they function in your
local environmental conditions should be avoided.
The surfaces of substitute materials may need special care
once installed. For example, chemical residues or mold
release agents should be removed completely prior to
installation, since they attract pollutants and cause the
replacement materials to appear dirtier than the adjacent
historic materials. Furthermore, substitute materials may
require more frequent cleaning, special cleaning products and
protection from impact by hanging window-cleaning
scaffolding. Finally, it is critical that the substitute
materials be identified as part of the historical record of
the building so that proper care and maintenance of all the
building materials continue to ensure the life of the
historic resource.
Choosing an
Appropriate Substitute Material
Once all reasonable options for repair or replacement in kind
have been exhausted, the choice among a wide variety of
substitute materials currently on the market must be made
(see illus. 22). The charts at the end of this Brief describe
a number of such materials, many of them in the family of
modified concretes which are gaining greater use. The charts
do not include wood, stamped metal, mineral fiber cement
shingles and some other traditional imitative materials,
since their properties and performance are better known. Nor
do the charts include vinyls or molded urethanes which are
sometimes used as cosmetic claddings or as substitutes for
wooden millwork. Because millwork is still readily available,
it should be replaced in kind.
The charts describe the properties and uses of several
materials finding greater use in historic preservation
projects, and outline advantages and disadvantages of each.
It should not be read as an endorsement of any of these
materials, but serves as a reminder that numerous materials
must be studied carefully before selecting the appropriate
treatment. Included are three predominantly masonry materials
(cast stone, precast concrete, and glass fiber reinforced
concrete); two predominantly resinous materials (epoxy and
glass fiber reinforced polymers also known as fiberglass),
and cast aluminum which has been used as a substitute for
various metals and woods.
Summary
Substitute materials--those products used to imitate historic
materials--should be used only after all other options for
repair and replacement in kind have been ruled out. Because
there are so many unknowns regarding the longterm performance
of substitute materials, their use should not be considered
without a thorough investigation into the proposed materials,
the fabricator, the installer, the availability of
specifications, and the use of that material in a similar
situation in a similar environment.
Substitute materials are normally used when the historic
materials or craftsmanship are no longer available, if the
original materials are of a poor quality or are causing
damage to adjacent materials, or if there are specific code
requirements that preclude the use of historic materials. Use
of these materials should be limited, since replacement of
historic materials on a large scale may jeopardize the
integrity of a historic resource. Every means of repairing
deteriorating historic materials or replacing them with
identical materials should be examined before turning to
substitute materials.
The importance of matching the appearance and physical
properties of historic materials and, thus, of finding a
successful longterm solution cannot be overstated. The
successful solutions illustrated in this Brief were from
historic preservation
projects involving professional teams of architects,
engineers, fabricators, and other specialists. Cost was not
necessarily a factor, and all agreed that whenever possible,
the historic materials should be used. When substitute
materials were selected, the solutions were often expensive
and were reached only after careful consideration of all
options, and with the assistance of expert professionals.
Pros and
Cons of Various Substiture Materials
Cast Aluminum
Material: Cast aluminum is a molten aluminum alloy cast in
permanent (metal) molds or onetime sand molds which must be
adjusted for shrinkage during the curing process. Color is
from paint applied to primed aluminum or from a factory
finished coating. Small sections can be bolted together to
achieve intricate or sculptural details. Unit castings are
also available for items such as column plinth blocks.
Application: Cast aluminum can be a substitute for cast iron
or other decorative elements. This would include grillwork,
roof crestings, cornices, ornamental spandrels, storefront
elements, columns, capitals, and column bases and plinth
blocks. If not self-supporting, elements are generally
screwed or bolted to a structural frame. As a result of
galvanic corrosion problems with dissimilar metals, joint
details are very important.
Advantages:
-
light weight (1/2 of castiron)
-
corrosion-resistant, noncombustible
-
intricate castings possible
-
easily assembled, good delivery time
-
can be prepared for a variety of colors
-
long life, durable, less brittle than cast iron
Disadvantages:
-
lower structural strength than castiron
-
difficult to prevent galvanic corrosion with other metals
-
greater expansion and contraction than castiron; requires
-
gaskets or caulked joints
-
difficult to keep paint on aluminum
Checklist:
-
Can existing be repaired or replaced inkind?
-
How is cast aluminum to be with other metals attached?
-
Have full-size details been developed for each piece to be
cast?
-
How are expansion joints detailed?
-
Will there be a galvanic corrosion problem?
-
Have factory finishes been protected during installation?
-
Are fabricators/installers experienced?
Cast Stone (dry tamped):
Material: Cast stone is an almost-dry cement, lime and
aggregate mixture which is dry-tamped into a mold to produce
a dense stone-like unit. Confusion arises in the building
industry as many refer to high quality precast concrete as
cast stone. In fact, while it is a form of precast concrete,
the drytamp fabrication method produces an outer surface
resembling a stone surface. The inner core can be either
drytamped or poured full of concrete. Reinforcing bars and
anchorage devices can be installed during fabrication.
Application: Cast stone is often the most visually similar
material as a replacement for unveined deteriorated stone,
such as brownstone or sandstone, or terra cotta in imitation
of stone. It is used both for surface wall stones and for
ornamental features such as window and door surrounds,
voussoirs, brackets and hoods. Rubberlike molds can be taken
of good stones on site or made up at the factory from shop
drawings.
Advantages:
-
replicates stone texture with good molds (which can come
from extant stone) and fabrication
-
expansion/contraction similar to stone
-
minimal shrinkage of material
-
anchors and reinforcing bars can be built in
-
material is firerated
-
range of color available
-
vapor permeable
Disadvantages:
-
heavy units may require additional anchorage
-
color can fade in sunlight
-
may be more absorbent than natural stone
-
replacement stones are obvious if too few models and molds
are made
Checklist:
-
Are the original or similar materials available?
-
How are units to be installed and anchored?
-
Have performance standards been developed to ensure color
stability?
-
Have large samples been delivered to site for color,
finish and absorption testing?
-
Has mortar been matched to adjacent historic mortar to
achieve a good color/tooling match?
-
Are fabricators/installers experienced?
Glass Fiber Reinforced Concretes (GFRC)
Material: Glass fiber reinforced concretes are lightweight
concrete compounds modified with additives and reinforced
with glass fibers. They are generally fabricated as thin
shelled panels and applied to a separate structural frame or
anchorage system. The GFRC is most commonly sprayed into
forms although it can be poured. The glass must be alkaline
resistant to avoid deteriorating effects caused by the cement
mix. The color is derived from the natural aggregates and if
necessary a small percentage of added pigments.
Application: Glass fiber reinforced concretes are used in
place of features originally made of stone, terra cotta,
metal or wood, such as cornices, projecting window and door
trims, brackets, finials, or wall murals. As a molded product
it can be produced in long sections of repetitive designs or
as sculptural elements. Because of its low shrinkage, it can
be produced from molds taken directly from the building. It
is installed with a separate noncorrosive anchorage system.
As a predominantly cementitious material, it is vapor
permeable.
Advantages:
-
lightweight, easily installed
-
good molding ability, crisp detail possible
-
weather resistant
-
can be left uncoated or else painted
-
little shrinkage during fabrication
-
molds made directly from historic features
-
cements generally breathable
-
material is firerated
Disadvantages:
-
non-loadbearing use only
-
generally requires separate anchorage system
-
large panels must be reinforced
-
color additives may fade with sunlight
-
joints must be properly detailed
-
may have different absorption rate than adjacent historic
material
Checklist:
-
Are the original materials and craftsmanship still
available?
-
Have samples been inspected on the site to ensure
detail/texture match?
-
Has anchorage system been properly designed?
-
Have performance standards been developed?
-
Are fabricators/installers experienced?
Precast Concrete
Material: Precast concrete is a wet mix of cement and
aggregate poured into molds to create masonry units. Molds
can be made from existing good surfaces on the building.
Color is generally integral to the mix as a natural
coloration of the sand or aggregate, or as a small percentage
of pigment. To avoid unsightly air bubbles that result from
the natural curing process, great care must be taken in the
initial and longterm vibration of the mix. Because of its
weight it is generally used to reproduce individual units of
masonry and not thin shell panels.
Application: Precast concrete is generally used in place of
masonry materials such as stone or terra cotta. It is used
both for flat wall surfaces and for textured or ornamental
elements. This includes wall stones, window and door
surrounds, stair treads, paving pieces, parapets, urns,
balusters and other decorative elements. It differs from cast
stone in that the surface is more dependent on the textured
mold than the hand tamping method of fabrication.
Advantages:
-
easily fabricated, takes shape well
-
rubber molds can be made from building stones
-
minimal shrinkage of material
-
can be load bearing or anchorage can be cast in
-
expansion/contraction similar to stone
-
material is firerated
-
range of color and aggregate available
-
vapor permeable
Disadvantages:
-
may be more moisture absorbent than stone although
coatings may be applied
-
color fades in sunlight
-
small air bubbles may disfigure units
-
replacement stones are conspicuous if too few models and
molds are made
Checklist:
-
Is the historic material still available?
-
What are the structural/anchorage requirements?
-
Have samples been matched for color/texture/absorption?
Have shop drawings been made for each shape?
-
Are there performance standards?
-
Has mortar been matched to adjacent historic mortar to
achieve good color/tooling match?
-
Are fabricators/installers experienced?
Fiber Reinforced Polymers (FRP, Fiberglass)
Material: Fiberglass is the most well known of the FRP
products generally produced as a thin rigid laminate shell
formed by pouring a polyester or epoxy resin gelcoat into a
mold. When tack-free, layers of chopped glass or glass fabric
are added along with additional resins. Reinforcing rods and
struts can be added if necessary; the gel coat can be
pigmented or painted.
Application: Fiberglass, a non load-bearing material attached
to a separate structural frame, is frequently used as a
replacement where a lightweight element is needed or an
inaccessible location makes frequent maintenance of historic
materials difficult. Its good molding ability and versatility
to represent stone, wood, metal and terra cotta make it an
alternative to ornate or carved building elements such as
column capitals, bases, spandrel panels, beltcourses,
balustrades, window hoods or parapets. Its ability to
reproduce bright colors is a great advantage.
Advantages:
-
lightweight, long spans available with a separate
structural frame
-
high ratio of strength to weight
-
good molding ability
-
integral color with exposed high quality pigmented
gel-coat or takes paint well
-
easily installed, can be cut, patched, sanded
-
non-corrosive, rot-resistant
Disadvantages:
-
requires separate anchorage system
-
combustible (fire retardants can be added); fragile to
impact.
-
high coefficient of expansion and contraction requires
frequently placed expansion joints
-
ultraviolet sensitive unless surface is coated or pigments
are in gelcoat
-
vapor impermeability may require ventilation detail
Checklist:
-
Can original materials be saved/used?
-
Have expansion joints been designed to avoid unsightly
appearance?
-
Are there standards for color stability/durability?
-
Have shop drawings been made for each piece?
-
Have samples been matched for color and texture?
-
Are fabricators/installers experienced?
-
Do codes restrict use of FRP?
Epoxies (Epoxy Concretes, Polymer Concretes):
Material: Epoxy is a resinous two-part thermosetting material
used as a consolidant, an adhesive, a patching compound, and
as a molding resin. It can repair damaged material or
recreate lost features. The resins which are poured into
molds are usually mixed with fillers such as sand, or glass
spheres, to lighten the mix and modify their
expansion/contraction properties. When mixed with aggregates,
such as sand or stone chips, they are often called epoxy
concrete or polymer concrete, which is a misnomer as there
are no cementitious materials contained within the mix.
Epoxies are vapor impermeable, which makes detailing of the
new elements extremely important so as to avoid trapping
moisture behind the replacement material. It can be used with
wood, stone, terra cotta, and various metals.
Application: Epoxy is one of the most versatile of the new
materials. lt can be used to bind together broken fragments
of terra cotta; to build up or infill missing sections of
ornamental metal; or to cast missing elements of wooden
ornaments. Small cast elements can be attached to existing
materials or entire new features can be cast. The resins are
poured into molds and due to the rapid setting of the
material and the need to avoid cracking, the molded units are
generally small or hollow inside. Multiple molds can be
combined for larger elements. With special rods, the epoxies
can be structurally reinforced. Examples of epoxy replacement
pieces include: finials, sculptural details, small column
capitals, and medallions.
Advantages:
-
can be used for repair/replacement
-
lightweight, easily installed
-
good casting ability; molds can be taken from building
material can be sanded and carved.
-
color and ultraviolet screening can be added; takes paint
well
-
durable, rot and fungus resistant
Disadvantages:
-
materials are flammable and generate heat as they cure and
may be toxic when burned
-
toxic materials require special protection for operator
and adequate ventilation while curing
-
material may be subject to ultraviolet deterioration
unless coated or filters added rigidity of material
-
often must be modified with fillers to match expansion
coefficients
-
vapor impermeable
Checklist:
-
Are historic materials available for molds, or for
splicing-in as a repair option?
-
Has the epoxy resin been formulated within the
expansion/contraction coefficients of adjacent materials?
-
Have samples been matched for color/finish?
-
Are fabricators/installers experienced?
-
Is there a sound substrate of material to avoid
deterioration behind new material?
-
Are there performance standards?
Further Reading
Berryman, Nancy D.; Susan M. Tindal, Terra-Cotta;
Preservation of an Historic Material. Chicago: Landmarks
Preservation Council of Illinois, 1984.
Brookes, A.J., Cladding of Buildings. New York: Longman Inc.,
1983.
Fisher, Thomas, "The Sincerest Form of Flattery," Progressive
Architecture (Nov. 1985).
Gayle Margot; David W. Look, AIA; John G. Waite, Metals in
America's Historic Buildings: Uses and Preservation
Treatments. Washington, D.C.: Preservation Assistance
Division, National Park Service, U.S. Department of the
Interior, 1980.
Historic Building Facades. New York: New York Landmarks
Conservancy, 1986.
Hornbostel, Caleb, Construction Materials: Types, Uses and
Applications, New York: John Wiley and Sons, Inc., 1978.
Lynch, Michael F; William J. Higgins, The Maintenance and
Repair of Architectural Sandstone, New York Landmarks
Conservancy, 1982.
National Park Service, Rocky Mountain Regional Office,
Preservation Briefs 12: The Preservation of Historic
Pigmented Structural Glass. Washington, D.C.: Preservation
Assistance Division, National Park Service, U.S. Department
of the Interior, 1984.
Phillips, Morgan and Judith Selwyn, Epoxies for Wood Repairs
in Historic Buildings. Washington, D.C.: Preservation
Assistance Division, National Park Service, U.S. Department
of the Interior, 1978.
Phillips, Morgan W., The MorseLibby Mansion: A Report on
Restoration Work. Washington, D.C.: Preservation Assistance
Division, National Park Service, U.S. Department of the
Interior, 1977.
Tiller, deTeel Patterson, Preservation Briefs 7: The
Preservation of Historic Glazed Architectural Terra-Cotta.
Washington, D.C.: Preservation Assistance Division, National
Park Service, U.S. Department of the Interior, 1979.
Acknowledgements
The author gratefully acknowledges the invaluable assistance
of coworker Michael Auer in editing this manuscript. The
following individuals are to be thanked for their technical
assistance: Mary Oehrlein A.I.A., Washington, D.C.; John G.
Waite, Albany, NY: Hyman Myers, R.A., Philadelphia, PA,
Thomas Fisher, Stamford, CT; Harrison Goodall, Kinnelon, NJ.
In addition, the staff of Preservation Assistance Division,
the cultural resources staff of the National Park Service
Regional Offices, and Stan Graves, on behalf of the National
Conference of State Historic Preservation Officers, provided
useful comments that were incorporated into the manuscript.
Washington, D.C. September, 1988.
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