Terrazzo

Fleur de lis patterned terrazzo floor

Terrazzo is a type of flooring made from hard stone pieces embedded in cement. It is best known for being easy to maintain as well as its resiliency. It can be made into a variety of colored patterns using divider strips. Though it was introduced in the 1890s, it did not reach popularity until early 1920. Terrazzo is still a competitive flooring material today thanks to the development of things like epoxy cement.

History

Terrazzo is derived from mosaics that were applied to waters and floors for decorative purposes. Mosaics were originally developed in Alexandria, Egypt and were traditionally formed by hand-setting small pieces of ceramic or stone into a decorative pattern with a mortar base. Romans further developed this technique by labeling the mosaics either opus museum, glass mosaics used for walls, or lithostrotum, which was used for flooring. Floor mosaics were later broken into three subgroups: opus tesselatum, opus vermiculatum, and opus sectile. Opus tesselatum and vermiculatum both involved similar methods of hand-laying broken marble or travertine pieces into a cement base to form pictures while opus sectile used cut marble pieces were set within larger marble sections. Eventually, the three were combined, adding decorative mosaic works were laid between larger marble slabs. Ultimately, these methods were blended until they lost their definition. After Rome fell, the mosaic tradition carried on in the early Christian churches as well as in Venice, where the techniques are still used heavily today. Eighteenth century Venice produced a crude form of mosaic flooring that is similar to the terrazzo technique used today. Pavimento alla Veneziana, or Venetian pavement, was developed in the early 20th century. Venetian pavement used marble fragments that were no longer than 2 inches and laid them closely together in a cement mixture. The use of irregular marble pieces was a less expensive way of creating a similar effect and was known as seminato. 3 or 4 inch pieces were dropped into the cement base first, followed by pieces that were ½ inch long, and finished with ¼ inch pieces to fill any visible gaps. The slab was rolled until the pieces settled into the cement base. It was given time to cure and then polished. Both Venetian pavement and seminato became known as the genetic term terrazzo. Terrazzo first came to the United States in the 1890s. The first installations were prone to cracking which made the process slow to gain popularity.

Manufacturing Process

Designs are usually picked from pre-existing terrazzo samples that will outline the type of marble chip, the cement mix, and what method it to be used. Sample blocks are available from the manufacturer and are used after the contractor has bid the job to help guide them through installation. The contractor prepares drawings and a sample. Traditional terrazzo is made from 70 percent of more of marble chips and 30 percent of less of Portland cement over a concrete vase. Color can be added to the Portland cement. Resinous terrazzo toppings can be made by mixing equal parts of marble chips and synthetic binder. Decorative chips should be chosen based on their color and strength. Other hard stones can be used in the terrazzo mixture. Chips are graded based on size; 1 meaning 1/8 to ¼ inches to 8 meaning 1 to 1 1/8 inches. It is common for the mixture to have equal parts of a variety of sizes. Divider strips can be installed to localize cracking, divide different colors, or provide guides for construction. These strips also help with the uniformity of the topping thickness and helps create workable panels. If the topping is Portland cement, the divider strip should be located over beams and should be no more than 10 feet apart for large areas.

Uses and Installation

In the United States, terrazzo became the flooring project of choice in the 1920s, replacing the more difficult to lay mosaics. Terrazzo is smooth and can be made into a curvy line, which made it ideal during the Art Deco period. Using the unbounded method meant more stability while the marble and pigmented cement meant for design opportunities for flooring. Technological advances with terrazzo made the material more economical and durable. Originally, terrazzo floors were laid in one massive slab, which lead to cracking in large office towers. L. Del Turco and Brothers Company was the first to introduce a method for subdividing the slab with brass divider strips. Before, slabs were subdivided using small mosaic tile. Brass, copper, nickel silver, and zinc became some of the most popular strip metals used to prevent cracking. These strips were to be placed at regular intervals and could be curved or geometric to help make the pattern. The introduction of the electric grinding machine also offered new advances in the creation of terrazzo floors. Before the grinding machine, workers would have to manually grind down the pieces with a clamp and a pumice-like stone. The machine made the process quicker, more precise, and a little less expensive. Before too long, terrazzo had become the flooring of choice for public buildings and apartments and was explored for its industrial potential. This flooring style was called mosaic-terrazzo until the early 1930s, when the name was shortened to terrazzo. The National Terrazzo and Mosaic Association was founded in 1931 to develop recommended terrazzo practices as well as report completed installations. Eventually, terrazzo was developed for more than flooring. It could be precast into several architectural uses including murals. Cover moldings and wainscoting became some of the more popular architectural uses for terrazzo. Terrazzo had become a common sight in schools, airports, and many other public buildings. There are two different types of installation processes used: bonded and unbounded. Bonded systems are subdived into four classes: bonded underbed, monolithic, chemically bonded, and thin-set. For each, the topping is applied and allowed to cure, then the surface is ground using machines. The bonded underbed method uses a structural slab soaked in water. Then, a slurry coat of Portland cement and water is applied and a mortar underbed is poured over the slab. Dividing strips are inserted in the underbed and followed by a layer of Portland cement over the top. The top is cured and then ground and finished. Monolithic bonding systems are made by pouring Portland cement terrazzo topping onto a structural slab with no underbed. Divider strips can be used to separate the terrazzo colors and provide control joints. This system is more likely to crack. The chemical bonded system is similar to the monolithic system except that a bonding agent is applied to the concrete before the topping is applied. Unbonded installation is known as the sand cushion method . The mortar underbed and the terrazzo topping are separated from the slab with a bond-breaking film over a bed of sand. The mortar underbed is usually reinforced with wire and at least 1 7/8 inches thick. Any movement in the slab is absorbed by the sand bed. Expansion in the underbed or topping is usually covered by the divider strips, which are most effective in this system.

Conservation

It is highly unlikely that a terrazzo floor will fail except for failure on the structural support system. Maintenance, material quality, and installation quality can severely affect the support system.

Deterioration

Cracking is likely to occur from movement of the structural system and can occur regardless of installation. Differential settlement or expansion and contraction of the structural system are also common causes of cracking in terrazzo flooring. Once the floor has cracked, it is likely to spall along the crack especially near the control joints. The use of metal divider strips in sand bed systems is meant to provide a fault line and help hide cracks. The sand cushion is meant to act as a fault zone so that the structure can move without disturbing the terrazzo topping. If movement were to happen, it is likely to be controlled. Bonded and monolithic installations are likely to show cracks regardless of the location of the divider strips and may need more frequent repairs. Acids and caustic cleaners, as well as floor stripping material, are likely to cause pitting in terrazzo floors. These materials may also cause the bonding agent to disintegrate as well as some of the aggregate. Engine oils and transmission fluids may cause a ghost-like pattern if they are permitted to seep into the floor. Ink, ketchup, mustard, and some waxes may also cause staining problems. Some of the blue and red marbles or elastic polymer setting beds can be affected by ultraviolet light and may prove inappropriate for outside applications. Terrazzo aggregates have a high abrasion resistance and low porosity. Abrasion is likely to wear down terrazzo surfaces, especially in high-traffic areas like front entrances and steps by railings. The structural system may not be designed to carry the weight of the terrazzo flooring and can cause the topping to crack. Bonded installations are likely to have this kind of problem. Improper installation is likely to cause failure and is easily spotted immediately after the topping has been polished. The floor should not be loaded before it has had time to completely cure. Drying accelerants may help the floor dry quicker but is not ideal for the topping material as it is likely to cause delamination.

Conservation Techniques

A visual inspection is the first step in assessing a terrazzo floor. The investigator should look for evidence or structural movement. Maintenance history should be investigated and the assistance of an experienced terrazzo floor installer may prove beneficial. Destructive testing can show if the bonding agent is failing and structural engineers can establish the quality of the subfloor. Structural engineers should be capable of telling if the subfloor is compatible with the terrazzo floor and can perform impact and load testing. Destructive testing may be necessary to fully understand why the floor is failing. Core samples should be taken for laboratory analysis which will evaluate the topping, mortar bed, and subflooring. Sweeping the floor, with a compound that does not contain oil or sand, is recommended for general maintenance of portland cement-based terrazzo. Mopping can be done to remove dirt but should be done with a non-ionic neutral detergent. All cleaning supplies should be cleaned and the floor permitted to dry before rinsing to avoid dirt absorption. All purpose and harsh cleaners, soap, and waxes should be avoided. Also, acids and alkalis should be avoided. Roughly 70 percent of the marble chip is portland cement terrazzo is exposed at the surface. After polishing and new installation, the cement binder, which is porous, may require a penetrating solvent-type sealer immediately. Water-based sealers should be applied periodically to prolong hydration of the binder. Resinous epoxies do not require this sealer; they require a surface sealer instead. Stains are common in terrazzo floors because their abrasion resistance makes them popular in high-traffic areas. The stain must first be identified before treatment begins. The stain may be removable using a dissolving agents or absorbing the material with a poultice. If the stain is not first identified, the treatment may actually cause the stain to set, permanently damaging the portland cement pigment. Chemicals should be a last alternative when less aggressive methods prove unsuccessful. Water based stains should be easily removed with water, while grease can be removed with a cotton cloth and a little soap (being careful to blot and not rub). Inks have varied composition and therefore have a variety of removal methods. Writing ink can usually be removed in a solution of sodium perborate in hot water. Structural systems under terrazzo floors should be stabilized before any repairs take place. Small damaged areas of terrazzo can be repaired by removing damaged areas of the terrazzo topping and replacing the original void. A matrix patch of sorts should be added to the subsurface, making sure that it matches the existing color, then the marble pieces are pressed into the cement matrix. The piece should be grinded to a smooth finish after the patch has had time to cure.

Replacement

When large terrazzo squares require replacement, cutting out the deteriorated pieces may be required. The structural surface should be examined to ensure compatibility with the replacement piece. It is suggested that replacing the entire area within a divider strip should be done to create uniformity in the section. When replacing an entire floor, it is important to note the location of the metal divider strips so that the design can be properly recreated. Original marble chips and binder should be investigated to develop specifications for new pieces. Physical data can also be compared with original specifications. Marble chips should match both the color and grade of original mix and may prove extremely challenging. Combining mixtures to achieve a similar color or shape pattern is possible. The topping is then mixed and recorded for duplication in the installation. Cracked flooring may need expansion joints to control the cracking. Installation type should depend on the structural subfloor and the depth available for the floor. Unbonded floors of 2 ½ inches deep should offer the best result, followed by a bonded installation. Mixing the bonded and unbounded flooring may be possible in repairs if they are separated by control joints and the subflooring is adequate. Terrazzo surfaces are unlikely to match the existing terrazzo unless the existing surface can be resurfaced and the new material formulated to match. The National Terrazzo and Mosaic Association should be consulted for grinding, buffing, and sealing practices.

Bibliography

• Botting, T., J. Guidry, M. Schmidt, C. Stevens and M. Striegel. "Effects of Vitrification on Traditional Terrazzo." APT Bulletin, Vol. 38, No. 4 (2007), pp. 33-42.
• Johnson, W. C. "Terrazzo." Twentieth-century Building Materials: History and Conservation. By Thomas C. Jester. New York: McGraw-Hill, 1995. 234-38. Print.

Examples

Terrazzo floor in Severance Hall.   Cement terrazzo floor.