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In the early twentieth century, Frank Lloyd Wright was a pioneer in the use of concrete in modern architecture. Concrete is a significant component of construction in nearly all the buildings that comprise the UNESCO World Heritage site The 20^th-Century Architecture of Frank Lloyd Wright. At Unity Temple (1905) and the Guggenheim Museum (beginning 1943) it is the primary component. In both buildings, the shapes Wright created out of concrete give those buildings their familiar images.

Wright used concrete in some application on almost all of his built work, so understanding the characteristics of concrete is critical to the preservation of his work. These applications include:

• “Cast-in-place” structural components of some of his most acclaimed buildings, as well the more utilitarian basement walls and floor slabs of his early and Prairie-period houses.
• “Pre-cast” units, such as the ornament of Hollyhock House (1919) and other buildings, or the Textile Blocks Wright used from the 1920s on, to construct a number of buildings.
• Floor slabs of the Usonian houses.

This article will give overviews of concrete’s characteristics and Wright’s uses of concrete. For a general review of historic concrete, we recommend the National Park Service’s Preservation Brief #15. As the brief notes, “While early twentieth century proponents of modern concrete often considered it [concrete] to be permanent, it is, like all materials, subject to deterioration.” The article will conclude with basic considerations for the maintenance and restoration of concrete.

Concrete is a mixture of Portland cement, sand, crushed rock or gravel, and potable water. A chemical reaction between Portland cement and water combine to form crystals as the concrete “cures,” the process that occurs as the concrete hardens.  These crystals bind together the aggregates making the strong material we call concrete. Varying the proportions of the ingredients of the concrete mix will alter characteristics such as strength, texture and color. This ability to vary its characteristics is part of what gives concrete its versatility, making it appropriate for both rough structural work and fine finished surfaces.

While concrete alone has great “compressive” strength, allowing it to withstand significant squeezing pressure such as that created by gravity or heavy direct loads, the material has very little “tensile” strength and will fail if it is subject to any sort of bending or twisting force. For this reason, concrete is typically combined with another material that can withstand tensile forces. That material is usually some form of steel reinforcing. This can take the form of a network of individual steel reinforcing bars (commonly known as rebar) or steel mesh of varying sizes and weights. Heavier versions of this mesh may be referred to as “welded-wire fabric,” or WWF. Even a concrete slab poured on a prepared, flat surface is subject to settlement and other forces, therefore it too must be reinforced. In Wright’s later work WWF was typically used for this purpose.

When concrete is cast, it starts as a “slurry,” an unstable mixture of solids and liquid that needs to be held in place by formwork while it cures. Typically, concrete is expected to reach its intended strength after twenty-eight days of curing. Because concrete is created by “hydration,” where water and cement become a crystal during the curing process, it must be kept damp while it cures. If concrete is allowed to dry out during this process, it cures too quickly, which reduces its strength. Slabs are particularly prone to premature curing if poured during hot or windy weather. They must be protected from elements such as sunlight and wind that will accelerate curing. Regular spraying of the slab with a mist of water will also slow curing until the slab has hardened enough to be protected with moisture-retaining covers.

Cold weather also has an effect on hydration. At low temperatures, curing proceeds at a slower rate. If newly-poured concrete is exposed to sub-freezing temperatures, the curing process stops completely until temperatures rise above freezing. Because hydration creates heat, it is possible to insulate concrete in cold weather, so that heat is retained to keep the curing concrete above freezing. This practice should be carefully monitored.

Concrete can be installed in various ways depending on its use. In Wright’s work, concrete was constructed in two ways, either “cast-in place” as a slab, or in formwork, or “pre-cast,” where components are cast in forms (either off-site or elsewhere on site) and then arranged in their final location. What follows is a description of the types of concrete most used by Wright, along with some significant examples.

Wright’s use of concrete was daring. At Unity Temple he was, as noted, a pioneer in the use of the material for the finished exterior of a non-utilitarian building. At the Johnson Wax Administration Building (1936), Wright’s act of proving the strength of his “dendriform” columns by overloading them with sandbags became a famous piece of theater. At Fallingwater (1935), the prominence of the slabs that cantilever over Mill Run have over-shadowed the elegance of the folded slabs of concrete that appear to lightly curve up to the property’s guest house, held up by the thinnest of metal supports.

Each of these structures was built by using formwork to hold the curing concrete in place until it had reached sufficient strength to support its own weight. Formwork can be made of rough boards for less visible components, such as basement walls. Some designers use the form material to give character to the surface of the finished concrete. Unless concrete was to be painted, which many of his famous later buildings were, Wright typically allowed the components of the concrete mixture themselves to provide that character.

When the concrete is intended to be a finished exterior, like at Unity Temple, care is necessary when pouring the concrete mixture, because the surface created by the forms is not visible when that mixture is being poured.  Air pockets and other imperfections must be avoided. The amount of water in a mixture can affect the quality of the pour. Installers prefer a wetter mixture, as this eases the process of pouring into (sometimes complex) forms, filled with rebar. Additional water can affect the strength and surface qualities of the finished concrete, though. Typically, water should be no more that 60 percent of the weight of the cement. Today, admixtures that will increase the mixture’s workability can be included in the mix. While such admixtures were not in use when Wright’s early work was being created, they may be of use in concrete restoration. As with any such work, a qualified structural engineer and/or architect should be consulted before attempting such restoration.

When the concrete was to be exposed, Wright often preferred a mix to be as dry as possible. This was particularly true with his Textile Block, where a dry mix promoted fast curing on site and also imparted a character-giving texture to the blocks. An overly dry mixture can create a “thirsty” concrete that will allow moisture to easily enter the concrete. This will be discussed further in the section on Textile Block below.

The placement of rebar in a reinforced concrete member is an important factor in determining its strength. For instance, on a simple concrete beam, which is supported at two points, the greatest tensile forces are at the bottom of the middle of the beam. Because reinforcing is specifically intended to counteract tensile forces, placement of rebar in this location is the appropriate course. With a cantilevered beam, which is supported only on one end, the opposite is necessary. Rebar should be placed at the top of the beam because that is where the tensile (stretching) forces occur. Today’s engineers typically specify a robust rebar arrangement to support new concrete so that various loads can be applied from many directions.

The amount of concrete that covers the rebar is also critical for the long-term viability of a concrete member. Rebar that is too close to the surface of the concrete is susceptible to moisture infiltration. This can cause the rebar to corrode and expand, pushing the surface concrete outward. This is known as spalling, and it can lead to portions of the surface concrete falling away from the member and often exposing the rebar itself. This exacerbates the problem and accelerate the rate of deterioration.

While much of Wright’s later concrete work was painted, the painting of concrete (or any form of masonry, such as concrete block or brick) must be approached with care. Paint or other finishes can possibly trap moisture within the concrete, leading to deterioration of both the concrete and its reinforcing.

Desert Masonry

In 1938, for his winter quarters at Taliesin West in Scottsdale, Arizona, Wright developed a variation on traditional cast-in-place masonry, which has been referred to as Desert Masonry. Wright differentiated this building system from traditional concrete by specifying it under masonry instead of concrete. Nonetheless, the construction process shares several aspects with traditional concrete. Construction was started by the creation of formwork, just as in traditional concrete. After that, large, flat-faced boulders were carefully placed in the formwork, with the flat side facing out. A relatively dry concrete mix was then poured around the boulders. In order to keep the concrete from running down the outside face of the boulders, small rocks or rolled newspaper was placed at the top of the boulders. The result created massive, intentionally primitive-looking shapes that often contrasted with other more finished materials, such as wood, glass or (at Taliesin West) canvas roofs.

Other extant examples of the use of Desert Masonry include the Berger House (1950), the Austin House (1951), the Boomer House (1952) and Pilgrim Congregational Church (1958).

Concrete Slabs-on-Grade

Beginning with the first Jacobs House in 1936, Wright’s use of floor slabs poured directly on the ground became a trademark of his Usonian houses. They were particularly striking because they were usually colored with an integral red pigment. The topic of the pigment will be covered in a future article. Here we will focus on the construction of the slab itself.

Usonian floor stabs were typically poured on a gravel bed, five inches or five-and-one-half-inches deep. In most houses, piping of cast-iron, steel or copper was placed in the gravel to carry the hot water of a radiant system that heated the house. All plumbing and electrical conduit would be installed before the (typically) three-and-one-half inches deep slab was poured on top of the gravel. As described above, the slab was reinforced with some sort of metal mesh. A six-inch by six-inch WWF was often specified for the Usonians. In the 1950 Sweeton House, the contractor appears to have poured the slab in two horizontal layers. The upper layer, which is no more than one-inch thick was made with finer aggregate, allowing for a smoother top finish. This layering was not called for in the specifications or drawings. Further research will be necessary to determine how common a practice this was.

Today, the undersides of slabs-on-grade are insulated and vapor barriers are installed to prevent the loss of heat into the earth and the rising of moisture up through the slab. Wright did not specify either of these for his Usonian slabs, so when the radiant heating system is not running, condensation can occur on the floor. Similarly, efflorescence can develop as salts rise through the floor along with vapor.

Ornament

In the mid-1910s, Wright designed several buildings with pre-cast concrete ornament. His most elaborate and extensive use of this type of ornament was in Midway Gardens (1913), where the concrete ornament included the famous “Sprite” design. Remaining buildings that include a significant amount of pre-cast concrete ornament are the German Warehouse (1915), the Bogk House (1916) and Hollyhock House. Later in his career, Wright used pre-cast concrete in a similar fashion for the small windows along the front of the Zimmerman House (1950). For these buildings, the concrete ornament would have been fabricated off-site, something like terra cotta ornament, of which Wright’s mentor, Louis Sullivan, was a master. Off-site production provides a much more controlled fabrication process, allowing much greater quality and detail in the finished product.

Similar to terra cotta, pre-cast concrete components can be subject to certain concerns over time. If the component is held in place by metal attachment, or metal reinforcing was installed in the component to control shrinkage during the curing process, moisture may cause the metal to corrode and expand. This is particularly true at exposed areas like a roof parapet. Unlike glazed terra cotta, unfinished pre-cast concrete ornament itself can be subject to deterioration over time.

Textile Block

In California in the mid-1920s, Wright began to experiment with the use of block that was pre-cast on site. The first house using this type of block was the Millard House in 1923. While the blocks at this house were laid up in a bed of mortar in a relatively traditional manner, the Millard House heralded a new type of building using blocks cast on site. At the Storer House, also of 1923, the blocks were “woven” together by a network of steel rods, hence the name “Textile Block.” The rods were placed in channels in the perimeter of the blocks. These channels were then filled with grout that held the rods in place and the rods in turn held the blocks together – at least in theory. Just like pouring concrete into a rebar-filled form, when the grout was poured into the channels between the blocks, it was not uncommon for voids to be created in spaces the stiff grout did not reach. Challenges were created by the use of on-site materials such as sand. These materials were not always free from contaminants that could undermine the strength of the finished block. Additionally Wright specified as dry a concrete mix as possible, hoping to expedite on-site production of the blocks. The dry mix also gave a texture to the blocks that appealed to Wright. This dry mix could lead to a concrete that could easily absorb water. This absorbed water, of course, led to corrosion of the metal rods that bound the blocks together.

Wright would continue to refine the use of this type of construction for the rest of his career, even developing new block shapes that would provide integral windows, fascias, and roof blocks. These later adaptations were used in houses called “Usonian Automatics.” The block system used was known as “Textile Block.” Preservation of Textile Block and Usonian Automatic block has become one of the greatest technical challenges in the preservation of Wright’s work. Jeffrey Chusid wrote an excellent account of efforts to restore the Freeman House (1924) in his book Saving Wright, and the current owner of the house discusses the challenge he has taken on in the Summer 2023 Frank Lloyd Wright Quarterly.

While historic buildings sometimes appear to continue to serve their purpose effortlessly, we know that they require ongoing maintenance (and sometimes extensive restoration) in order to fulfill their intended function. The National Park Service Preservation Brief referenced above provides a number of tips for preserving historic concrete.

As with most materials, inappropriate infiltration of moisture is a chief cause of concrete deterioration. Regular general maintenance of a building can be the best way to deter this deterioration. Maintaining roof parapets and sealant joints, and avoiding the use of deicing salts near vulnerable concrete, are all ways to prevent damage – as well as being important to the general preservation of a structure.

Surface applied coatings and penetrating sealers (as opposed to sealants, which fill cracks or joints) should be avoided. As the brief notes:

Film-forming coatings are often inappropriate for use on a historic structure unless the structure was coated historically [such as the painted finishes of Fallingwater, the Guggenheim Museum and other later work by Wright]. Film-forming coatings will often change the color and appearance of a surface and higher build coatings can also mask architectural finishes and ornamental details…. Laboratory and field testing is recommended prior to application of a protective system or treatment on any concrete structure; testing is even more critical for historic structures because such treatments are not reversible. As with other repairs, trial samples are important to evaluate the effectiveness of the treatment and to determine whether it will harm the concrete or affect its surface [emphasis added].

Finding ways to divert moisture from areas prone to moisture infiltration is the best initial line of investigation when dealing with this issue.

Wright-designed buildings can have issues specific to each design. Like many architects of the twentieth century, Wright often worked with materials that had limited track records. The exciting possibilities provided by concrete, glass and other materials encouraged these architects to stretch their capabilities. Wright’s use of cantilevers has led to a need for significant intervention at more than one site. At Fallingwater, steel cables were inserted in the structure to counteract the deflection of the house’s balcony decks. At the Tonkens House (a 1954 Usonian Automatic) investigation of a deteriorating roof led to the realization that the house’s roof-block slab needed serious reinforcement. This was accomplished by inserting a grid of reinforcement on the roof slab between the roof blocks and covering the slab and new reinforcement with a thin layer of high-strength grout, itself reinforced with a fine carbon-fiber mesh, integrating the tensile strength of the additional reinforcement within the existing concrete slab.

Not all preservation work on concrete need be as extensive as that described above. As with all preservation, creating a preservation and maintenance plan is critical in helping to avoid serious issues later. Developing relationships with professionals, including an architect, engineer, and contractor, with significant experience working with historic concrete is also key to a successful preservation effort.