Historical Architectural Terra Cotta Replacement with GFRC (Glass Fiber Reinforced Concrete) or GFRP (Glass Fiberglass Reinforced Polymer)
Terra Cotta Replacement Table of Contents
What is Architectural Terra Cotta? |
Replacement Choices |
Selecting the Supplier for Terra Cotta Replacement |
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How the process works |
Architectural Elements |
Technical Data |
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Additional Reading |
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What is Architectural Terra Cotta
Terra Cotta is from the Italian for "baked earth". Terra-cotta elements became popular for architectural elements between the late 1800s through the 1930s. Architects such as Louis Sullivan, Cass Gilbert, and Daniel H. Burnham and others produced several terra-cotta clad masterpieces of American architecture.
To produce architectural terra-cotta, clay was mixed with sand or crushed pottery. The mixture was then packed into molds or carved. Once the terra-cotta had air dried it was fired. Terra-cotta was then glazed. In other words, terra cotta was an enriched molded clay brick. Terra-cotta was usually cast in hollow blocks which were open to the back, like boxes. The terra cotta was molded with internal stiffeners of clay called webbing. The webbing strengthened the load-bearing capacity of the hollow terra-cotta block.
Architectural terra-cotta was generally finished with a glaze. The terra cotta glaze provided color, protected the clay, and imitated different materials such as stone. The terra-cotta glaze had rich color a hard surface. Glazing terra cotta offered virtually unlimited color to the designer.
Today many of these terra-cotta clad buildings have serious deterioration problems. Sources of new glazed architectural terra cotta are limited.
Replacement material choices
Depending on the application, the location and the degree of deterioration, terra cotta will either need to be repaired or replaced. Replacement choices are either FRP (fiberglass) or GFRC (glass fiber reinforced concrete).
NOTES ON REPLACEMENT:
top- When properly preformed, GFRC or FRP replacements of terra cotta should be virtually indistinguishable from the original material.
- When replacing glazed architectural terra-cotta, all of the original deteriorated material should be completely removed. Do not try to replace a half block or part of a piece.
- Anchorage should be concealed.
- Visual compatibility should be a major consideration when choosing a replacement material.
- Re-anchoring deteriorated terra cotta units is practically impossible due to the anchorage method of steel straps, mortar and interlocking courses. If the terra-cotta in question is loose, deteriorated, or its structural integrity is in question, it is best removed and replaced.
- Always keep in mind the potential liability of a poorly done job. A piece of terra cotta can become lethal when it falls.
GFRC (glass fiber reinforced concrete) for Terra-Cotta Restoration and Replacement
For over 25 years, the Stromberg Company has pioneered in the development of GFRC replacement pieces for terra-cotta. The use of Stromberg GFRC as a replacement for architectural terra-cotta offers several advantages
- Stromberg GFRC is lightweight and easier to handle than terra cotta
- Stromberg GFRC is faster to erect than terra cotta
- Stromberg GFRC reduces the loads on the building structure
- Stromberg GFRC reproduces fine detail and texture and can be molded in complex shapes
- Stromberg GFRC is environmentally friendly
- Stromberg GFRC is low maintenance
GFRC (glass fiber reinforced concrete) offers great benefits in replacing glazed architectural terra-cotta.
Benefits of GFRC:
- Anchoring provisions are included in casting
- Color compatible
- Non combustible material, GFRC will not burn
- Light weight
- Durable and long lasting
- Suitable for seismic zones
- Fast production times
- What is GFRC?
- GFRC (glass fiber reinforced concrete) is concrete that uses glass fibers for reinforcement instead of steel. It is typically cast in a thin section of around 1/2" to 3/4". Since the fibers cannot rust like steel, there is no need for a protective concrete cover thickness to prevent rusting. With the thin, hollow construction of GFRC products, they can weigh a fraction of the weight of traditional precast concrete or terra cotta.
- How strong is GFRC?
- As an engineered material, the properties of GFRC can vary depending upon mix design, glass content and production methods. Glass fiber used in quality GFRC has a higher tensile strength than steel. As a general rule, the higher the fiber content, the higher the strength. A typical GFRC mix with 5% glass fiber has a flexural modulus of elasticity of around 1.5 to 2.9 x 10 6 and compressive strength of 6,000 to 8,000 psi.
- How durable is GFRC?
- GFRC has been tested both by accelerated aging tests in the laboratory and in real life installations. GFRC can be expected to last as long as pre-cast concrete. In many environments, as when exposed to salt spray or high moisture, the GFRC can be expected to perform better, as there is no steel reinforcement to corrode. Since the surface of GFRC is a Portland concrete, it weathers much as a quality architectural pre-cast concrete would.
- How is GFRC attached?
- Most GFRC can be divided into two categories: with internal frame and without. GFRC with an internal frame is for larger pieces and panels. The internal frame allows for what would have been multiple pieces of terra-cotta , to be installed in one large unit. GFRC without the internal frame is generally set one piece at a time, much the way terra-cotta would have been. This is useful in cases where not all of the terra-cotta needs to be replaced. How do you make the GFRC (glass fiber reinforced concrete) match the existing terra-cotta elements or match terra-cotta elements that no longer exist? In some cases a historic structure has pieces on it that we can take mold from, to exactly replicate the existing terra-cotta In other cases all that's available our old photographs or drawings of terra-cotta architectural elements that no longer exist. In these instances our sculptors and model makers re-sculpt the piece from the historical images.
FRP (Fiberglass Reinforced Polymer)
FRP Fiberglass Reinforced Polymer replacement is an excellent alternative, particularly when rich and elaborate ornamentation has to be duplicated. Molds are taken from original intact pieces or models are produced from photos and dimensioned drawings. Several pieces can be combined into one large unit.
Benefits of FRP:
- Anchoring provisions are included in casting.
- Color compatible
- Class A fire rated
- Light weight
- Durable and long lasting
- High ratio of strength to weight
- Suitable for seismic zones
- Faster production times
- Resistant to water, acid rain, rot, rust and corrosion
Replacement of architectural terra-cotta with fiberglass offers several advantages. Fiberglass has the ability to duplicate intricate and elaborate ornamentation, fiberglass is lightweight and easier to install a terra-cotta, Stromberg fiberglass is durable and weather resistant is particularly suitable for seismic zones and hurricane areas.
- What is FRP?
- FRP stands for fiberglass reinforced polymer. Most people are familiar with fiberglass boats, yachts and Corvette bodies. In the last 40 years, fiberglass has proven itself to be strong, durable and waterproof. To produce fiberglass, special polymer resins combine with strands of glass fiber.
- How strong is FRP?
- FRP is an engineered material and its strength can be designed to meet specific needs. By increasing fiberglass content the strength of the FRP is increased. As a general rule Stromberg FRP for terra-cotta replacement as about 60 times the impact resistance and tensile strength of architectural terra-cotta.
- How durable is FRP?
- Stromberg FRP (fiberglass) has been used around the world. Fiberglass columns, domes and architectural details from Stromberg have withstood hurricanes, blizzards and earthquakes as well as tropical sun, seawater (even submerged!) and ground contact. FRP does not rot, harbor termites or rust.
- What about glazes and colors?
- Stromberg has years of experience in replicating terra-cotta glazes in GFRC (glass fiber reinforced concrete) and FRP (Fiberglass). We would be happy to help you with your restoration and preservation project.
Selecting the Supplier for Terra Cotta Replacement or Restoration
So how do you go about selecting the best source for replacement elements for architectural terra cotta?
- Experience
- With projects around the world and across the USA no one can offer the experience and expertise in historical restoration and replacement that Stromberg can. For 30 years, we have been producing restoration products and with our continuous improvement process we learn with every project. Put our experience to work for you.
- Capacity
- When schedule is important, you want a company with the capacity to get your project done. No one can offer you more capacity for production. Large jobs are no problem.
- Options
- We offer a full range of expertise in GFRC, FRP and terra cotta. We can recommend the best material for your project and can help develop the best combination of approaches to meet your needs.
- Customer satisfaction
- Over 80% of our business is repeat customers. We encourage you to speak with our clients.
Give us a call or email. Let us prove what we can do for you.
Architectural Terra Cotta Services we provide:
- Custom CAD drawings and renderings from historical documents
- Art molds from existing terra cotta pieces
- Custom sculpting and recreation of missing pieces
- Site surveys, testing , inventory and consulting
- Engineering of terra cotta replacement connection methods and means
- Color matching and texture matching of terra cotta glazes
- Reproduction of terra cotta in GFRC or FRP
- Turn key replacement
What about quality?
Stromberg has worked on some of the nations leading historic landmarks. Stromberg Historic is part of the Stromberg family of companies, with over a quarter century and work on some of America's finest architectural treasures.
Clients and commissions include:
- The White House
- The Smithsonian
- The Pentagon
- West Point
- Yale
- St. Peter's
- Palaces, five-star hotels and historical treasures throughout the world
How the Process Works
- Terra Cotta Survey
- Upon request, a survey is performed of the existing terra cotta. Surveys may include:
- Historical Research for documents to determine the original condition of the terra cotta.
- Inventory of existing terra cotta installed
- Survey of condition of existing terra cotta, sealant condition, flashing, anchorage. Each piece of terra cotta may be sound tested by tapping with a rubber mallet to determine if it is sound.
- Survey of missing terra cotta
- Shop Drawings
- Detailed shop drawings in CAD format are produced that detail each piece of terra cotta, its location, and setting plans. These drawings include sections, connection details, jointing and hoisting. These shop drawings are submitted, along with material samples to the architect for approval.
- Models and Molds
- Models are created from the drawings and molds produced from those models. If original pieces still exist, then the molds are produced from the original pieces.
- Casting and Finishing
- The GFRC is cast into the molds and compacted. The GFRC is a mixture of sprayed in cement, fine aggregate, polymers and additives along with glass fibers. The fiberglass reinforced concrete is allowed to harden in the mold, and is then removed and sanded or finished. Framing can be incorporated into the fiberglass cornice if necessary. To match the original terra cotta, the color is determined by cast in pigments and the aggregates, as well as glazes.
- Installation
- The replacement elements are then installed. A range of stainless steel and other non corroding straps, anchors and attachments are available. The new elements can be designed to be installed one piece at a time, or as multiple elements preassembled and erected in large lengths or panels.
Typical Architectural Terra Cotta Elements and their Replacements
Parapet caps and coping
A typical terra-cotta parapet cap or coping.
Because the limitations of the manufacturing process for terra-cotta, it was not possible to produce large pieces. This required numerous joints which allowed the possibility of water infiltration through the top of the parapet. In replacing this deteriorated parapet cap the three individual terra-cotta units were cast as one single piece of GFRC with "false joints" that were then pointed with mortar.
Styles of Terra Cotta coping
Terra Cotta coping style "A"
Terra Cotta coping style "B"
Terra Cotta coping style "C"
Terra Cotta coping style "F"
Terra Cotta coping style "G
Terra Cotta coping style "H"
Terra Cotta coping style "I"
Terra Cotta coping style "J"
A typical section of architectural terra-cotta parapet balustrade.
The individual terra-cotta balusters were produced as two-piece units. The top rail was supported by piece of one or each inch and half flat steel bar and was anchored inside the piers. The top rail and bottom rail was produced in numerous short segments which were pinned together with steel pins and mortar. In restoring the terra-cotta balustrade, complete balusters sections can be produced in GFRC or FRP and with sections as long as 12 to 14 feet.
Component parts of a Stromberg balustrade system. These are custom made to match the existing terra cotta railing (or are reproduced from drawings or old photos to match the railing that once existed).
Example of new Stromberg Fiber Cotta parapet railing and Corinthian column capitals.
Stromberg cast parapet railing.
Terra Cotta Parapet Railing
Threaded rod is used to attach the terra cotta elements to the structure. The replacement of this type of railing is somewhat easier than in fully mortared systems, since the deteriorated terra cotta may be unbolted and removed
Gothic type of terra cotta parapet railing
Available in GFRC or FRP. Call for details
Terra Cotta Restoration: Parapet Rail
This example of a terra cotta railing with a metal brace is fairly typical of many roof top terra cotta parapet rails. This was especially common where no piers were in place to run the rails into. The braces can become rusted and corroded and the terra cotta may deteriorate from exposure and water infiltration.
Columns and Column Capitals in Terra Cotta and GFRC
Architectural terra-cotta ionic column capital.
This drawing shows the typical method that was used to produce terra-cotta columns. The column shaft is divided into numerous sections with joints running horizontally and vertically. Each section was then pinned to the one adjacent to it and the interior core was filled with mortar and rubble. Because of this type of construction, removing one particular piece of deteriorated terra-cotta becomes very difficult. Another concern is the steel column used as a support for the decorative terra-cotta column cladding. At the time most architectural terra-cotta work was being done, this structural steel was typically not properly protected from rust and corrosion. In any terra-cotta restoration project, if the terra-cotta columns are not being completely removed and replaced along with the structural steel supports, it's a good idea to at least check the steel to ensure that it is not rusted and to ensure it is still adequate to support the loads.
Ionic column capital. Preparation of the model.
The Ionic fluted columns for the First Presbyterian Church were created from old photographs of the columns. Color and texture match the weathered appearance of the existing terra cotta and stone
Terra Cotta Cornice and GFRC Cornice
Terra Cotta Cornice
Typical Section through a large architectural terra cotta cornice with integral gutter. This illustrates some of the challenges of architectural terra cotta replacement. The myriad steel straps and interlocking nature of the assembly, makes removing a single piece of terra cotta extremely difficult. Often a deteriorated piece can be cut out and a new GFRC or FRP replacement unit installed with dowels and epoxy. If this is not possible, the entire cornice may need to be replaced.
Terra Cotta Cornice RCP.
Reflected ceiling view of an Architectural Terra Cotta Cornice.
Section through a historic terra cotta cornice
The mortar infill and unprotected steel of terra cotta assemblies can lead to problems as the steel rusts from moisture that penetrates the cornice and is trapped inside. As the steel rusts it swells, which leads to cracking and further deterioration. The wood sleepers create an area of expansion and contraction as they swell and shrink. This steel was typically not properly protected from rust and corrosion. In any terra-cotta restoration project, if the terra-cotta columns are not being completely removed and replaced along with the structural steel supports, it's a good idea to at least check the steel to ensure that it is not rusted and to ensure it is still adequate to support the loads.
GFRC cornice as Terra Cotta Replacement
While every cornice is slightly different, this GFRC cornice is a good example of the simplification of pieces that are possible with the use of Stromberg composites for terra-cotta replacement. At 6' in height, the 10 types of terra cotta profiles that would have been required to produce the cornice profile, and the 50 to 120 individual units that would have gone into this 10' length of cornice in architectural terra cotta, have been replaced by a 1 piece unit of GFRC, internally framed with galvanized steel studs and stainless steel anchors and ready to hoist and install as one unit.
GFRC Cornice Pre fitting
Multiple sections of cast GFRC (glass fiber reinforced concrete) cornice being pre fitted at the factory to ensure proper match of all the profiles. Note the lapped end detail at the foreground of the photo. This allows each unit to interlock with the next.
Joint Alignment of GFRC Cornice
Typical joint before sealant is applied.
Replacement Cornice pieces in FRP for historical restoration.
They are being checked at the factory.
Architectural Terra Cotta Gargoyles and Figures
Terra cotta gargoyles anchored by steel angles and bronze rods hooked to the angle. Replacements anchored by stainless steel or galvanized steel frames and bolted or epoxied to existing structure.
Close-up of new Gargoyle by Stromberg. Gargoyles for restoration may be cast from existing gargoyles or sculpted from old drawings or photos.
Because of the exposure to the elements, the Gargoyles on historic structures are often the first pieces of terra cotta to deteriorate. The use of raw steel angles to support them is usually the cause of the failure.
Producing Project Specific Specifications for Terra Cotta Replacement
Specifications for Terra Cotta replacement are available as word document or PDF. Call or email us and we will be happy to send them. If you need help with project specific specifications, we are available to help.