The Materials Desk: 4 Ways to Replace Terra Cotta
October 9, 2018
By: Joseph Nevins, Lauren Printz, and Thomas Pentoney
Terra cotta is one of the oldest and most popular materials in architecture. Over the centuries, terra cotta has been used to create decorative building elements around the world, in Europe through the Renaissance and in America after the late 19th Century. Terra cotta is a light-weight, clay-based ceramic material that is shaped, molded, most often glazed, and then fired. Due to the variable nature of finishes and glazing, terra cotta was often used as a replacement for the more expensive marble and limestone. Later in its use, architects used terra cotta not as a replication material, but as its own unique material celebrating its plastic and finish properties with brightly colored installations.
Decorative terra cotta column capital at 108 Leonard Street.
Despite its impressive fire-resistant qualities, terra cotta’s brittleness, inability to withstand impact damage, and susceptibility to deterioration from internal stresses caused by water absorption make terra cotta difficult to use and maintain in our northern climate. Internal damage to terra cotta is typically caused by the corrosion and expansion of deteriorating steel, which is in turn caused by water infiltration from failed joints, water shedding systems and roofs. Combined with the freeze/thaw cycles, that we have in New York, terra cotta is subject to stresses that were not part of the original design of the building and can be ascribed more to inadequate maintenance than a failure of the material itself. Glazed terra cotta is also subject to spalling over time, especially in climates with many freeze/thaw cycles per year.
Deteriorated steel anchors and support in terra cotta architectural element.
Cracked terra cotta architectural element.
Many variables need to be considered when replacing damaged and deteriorated terra cotta elements in buildings. While in-kind replacement is preferred, this option can be costly and time consuming. Ultimately, the choice of a replacement material will depend on the building’s existing conditions, the extent of deterioration, project budget, timeframe, and landmark status.
Below is a comparison of four materials that can be used to replace terra cotta. Each option has differences in materials, properties, fabrication, availability, installation methods, and cost.
Terra cotta architectural elements in the façade of 6 Harrison Street.
1. Terra Cotta
Replacing severely deteriorated units with new terra cotta will provide the best visual match to the original. Terra cotta replacement units will also perform very similarly to existing units in physical properties such as expansion/contraction and compressive strength. Another advantage of using terra cotta is that the new units will use the same general types of attachment. However, producing new terra cotta units is a labor intensive and expensive process, often requiring a long lead time. Each distinctive unit type requires its own unique model and mold, and each mold can only be used a limited number of times as the molds are subject to wear during the fabrication process. Additionally, only two major manufacturers of terra cotta currently serve the New York metropolitan market (smaller producers of terra cotta are available, although we cannot verify their quality or capacity).
Terra cotta replacement units at 400 Madison Avenue.
2. Glass Fiber Reinforced Concrete (GFRC)
GFRC is a common building material and readily available. It is a composite material made of cement, fine sand, polymer, water, alkali-resistant (AR) glass fibers, and other admixtures. Similar to terra cotta, molds are required for GFRC unit fabrication. GFRC has a shorter lead time than terra cotta; however, as a concrete product, it requires time to cure. While GFRC provides a close visual match to terra cotta, it will weather differently than the adjacent terra cotta units. Consideration must be given to supporting masonry loads above, as GFRC is non-structural. The introduction of GFRC will require further investigation and engineering to ensure adequate support is provided to existing building elements.
GFRC replacement architectural unit at 39 West 37th Street.
3. Architectural Fiberglass
Fiberglass is a glass fiber reinforced plastic consisting of a resin matrix, glass fiber reinforcements, and additives. Architectural fiberglass offers a light-weight alternative replacement material that can mimic the look of historic terra cotta. It can be produced in large prefabricated sections that make the hauling, delivery, and installation of fiberglass both quicker and easier than other alternative materials. In addition, it requires less anchorage and support because it is light-weight. For these reasons fiberglass is particularly well suited for cornice replication.
On the other hand, it is brittle, fragile, and non-loadbearing; because of these properties, fiberglass cannot be installed in place of terra cotta as originally designed since terra cotta often supports masonry loads above. Further investigation and engineering is required to ensure adequate support is provided to existing building elements.
Existing metal cornice at 6 Harrison Street.
Replacement fiberglass cornice at 6 Harrison Street.
4. Cast Stone
Cast stone is a mixture of cement, sand, crushed stone or gravel, and color pigments that can also be used to replace terra cotta. However, there are several considerations when using this material: for highly glazed units, obtaining a color match is difficult and the color on the cast stone will likely weather differently over time than terra cotta. In addition, individual cast stone units are heavier than terra cotta, which must be accounted for when designing, delivering, and installing. Several advantages to utilizing cast stone is that the molds can be pulled right off the pieces on the field without creating a model due to the low shrinkage that occurs during fabrication. Well produced cast stone units have a long service life. Anchors can also be cast into the replacement for mounting.
Decorative terra cotta column capital, prior to replacement, at 6 Harrison Street.
Decorative terra cotta column capital, after replacement with cast stone, at 6 Harrison Street.
Material Qualities at a Glance
About The Materials Desk
The Materials Desk is a recurring section in HLZAE Newz featuring an in-depth discussion of a specific architectural or engineering material.
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