Structural clay tile

Structural clay tile (variously called structural tile, hollow tile, or simply building tile) refers to an assortment of burned clay or shale masonry products that were popularly used for structural or nonstructural applications during the late nineteenth century. The tiles are usually hollow with parallel cells, making them popular for floor arches as well as for fireproofing, partition walls, and furring. In the early 1900s, clay tiles became popular for columns and backing for exterior walls. Clay tile is broken into three categories- hard, semi porous, and porous. Each is determined by the amount of time they are fired during manufacturing. The category of the clay tile determines what type of applications it can be used for.

History
Clay (terra cotta) being used as a structural material first emerged in the eighteenth century in Italy. Clay pots lined with plaster of paris were set in concrete for the San Vitale Church. This same technique was seen again in the late eighteenth century Paris with the addition of reinforcements. Frederick Peterson developed a clay tile building method in the 1950s. This technique used a clay tile building method to span iron beams with homemade clay tiles. George H. Johnson later patented a similar method. The Chicago and Boston fires in the 1800s proved bricks fire-resistant qualities and structural clay tile quickly gained acceptance for use in floors and fireproofing iron. In the major cities, structural clay tiles became a widely accepted building material for high-quality buildings. The stringent laws on fireproofing also helped the structural clay tile companies gain popularity. Testing structural clay tiles began after 1910, producing greater compressive strength, allowing for longer floor spans.

Manufacturing Process
The manufacturing process for structural clay tiles has not changed much over the course of time. Clay is kneaded or "pugged" until it achieves sufficient plasticity for molding. The clay is then extruded through a die and cut lengthwise into individual units which are then baked in a kiln. Tile is still classified according to density, glaze, and vitrified (a glassy finish). The strongest and most moisture resistant clay tiles are hard or dense and are burned the longest. They are best suited for heavy structural application where it might be exposed to the weather. This tile was less fireproof and was more prone to crack in intense heat. Semi porous tile was hard burned for moderate strength and could resistant moisture to an extent. Semi porous tile is commonly used in small buildings and houses. Porous tile is made of a mixture of modified clay and ingredients like sawdust or straw that was consumed during the firing. Porous tile is lighter and the most desirable for fireproofing because it could perform better under high heat. The introduction of salt into the kiln while the glazed tile is being fired will produce a glass-like finish over the tile. Vitrified terra cotta is burned at a higher temperature so the material can fuse better. Vitrified terra cotta is extremely hard because it is produced with the densest form of clay.

Uses and Installation
Structural clay tile uses can be broken into three applications: tile floor arches, gravity load-bearing, and shear walls. Non load bearing applications can include partition walls, furring, and fireproof casing for columns. In the late 1800s through the mid 1900s, structural clay tiles were a popular medium in constructing flat-arch floors. Uniquely shaped blocks were manufactured for two general types of flat-arch construction: side construction and end construction. At the end of an arch, a skew tile adjoined to the girder, intermediate tile and a key tile filled in the remainder of the arch. The floor tiles were centered on wooden beams hung by iron hangers. The tiles were usually set in mortar and filled over with concrete to help disperse the weight evenly. With side construction, the thrust of the arch was transmitted by standing the side of one tile block against the side of another. Thrust was transmitted by bearing through the end of blocks oriented end-for-end in end construction. Load test gave demonstrated that end construction is stronger and more efficient that side construction. In the 1890s, a system that combined both types of construction became the most popular form of flat-arch construction. When Portland cement became more reliable, the system of combining clay tiles with reinforced concrete began to grow in popularity. In the composite systems, structural clay tiles were used as centering for reinforced concrete floors or as fillers in reinforced concrete slab floors. After the composite system proved effective, structural clay tile products soon began to emerge. Cylindrical-shaped elements were used to cover columns and shoe tiles were made for girder flanges. Clay tiles cased around beams with steel hangers and mortar were traditionally used as a fireproofing method. Structural clay tile blocks were also used to build load-bearing walls and columns or as backing for exterior walls. Buildings were often single-story when made out of structural clay tiles. The load-bearing elements can be unreinforced with hollow cells or grouted cells or reinforced with grouted cells. Original load-bearing walls were ungrouted and were strengthened by pilaster elements.

Clay tiles were traditionally used in the twentieth century for infill walls in concrete and steel-framed structures, which provided lateral strength and stability. Interior corridor structures were clay tile walls that served as shear walls in the absence of an explicitly designed lateral load system. In load-bearing construction, the wall supports gravity loads and acts as a shear wall for resisting lateral loads. New products were created to compete with an evolving industry. Facing tiles for interior and exterior uses were available by the late 1920s. Matte, glazed, and mottled finishes in a variety of colors became an acceptable substitute for standard bricks. Tiles were created in sizes that were equivalent to two, three, or six bricks and could be color in a similar manner. Ar-ke-tex was introduced in the 1930s and was used for corridors, swimming pools, and several other interior/exterior applications. The National Fireproofing Company of Pittsburgh offered a complete range of tile types by the 1930s and was producing more than one million tons annually. Clay tile elements used in load-bearing applications for walls. Clay tiles used for both load-bearing and non-load-bearing applications were installed like other masonry, with mortar. Clay offers several advantages, the most pertinent being that it is lightweight. The wheeling Tile Company in Fort Myers, Florida created Speedtile, a load-bearing product with a handle to permit faster installation. Composite systems with tile components were replaced after World War II by concrete floor with metal decks. Clay tile blocks remained popular for load-bearing wall construction until they were supplanted by concrete masonry units.

Conservation
Cracking is common in structural clay tile due to the environment and the structure’s movement. Repair techniques should be chosen based on the structures integrity of the tile through visual examination and some testing.

Deterioration
The most common causes of deterioration in structural clay tiles is moisture and freeze-thaw cycles. Physical indications of deterioration include a soft, powdery surface or the presence of efflorescence that is similar to clay brick masonry. Clay tiles can be damaged by structural effects like differential settlements, structural overload, or shrinking and expanding. These usually result in crazing or cracking of the clay tile or the mortar joints and the bowing of wall panels that have been constructed of clay tiles. Visually observing the damage is the foremost method of detecting a problem with a structural clay tile system. Tools for testing brick masonry may also prove acceptable for testing structural clay tiles as well. Observing the pattern of the cracking will help to determine the cause of distress. Step cracking is usually an indicator of differential settlement while diagonal cracks suggest structural overload. Extensive vertical cracks usually indicate restrained expansion or shrinkage. The reason for the crack, both pattern and location, should be studied to determine the exact cause. While visually inspecting the structural clay tile, the in-situ test will help determine the strength of the wall. Tests commonly used on brick construction, such as bond-wrench tests, in plane and out-of-plane shove tests, and in-situ deformability test can also be used on structural clay tile. Masonry codes can be used to determine the strength of the load-bearing tile elements but requires knowledge of the strengths of the materials used, like the clay tile, mortar, and grout. Materials testing should be done for each member of the structure or estimated strengths based on the type of material should be determined in order to understand the structural make-up of the material. The latter of the test is less reliable than the in-situ strength test because there is no definitive information when guessing is involved.

Conservation Techniques
Patching or repointing is appropriate in replacing minimally damaged areas, as is selective replacement of individual elements. Filling individual hollow elements with grout may be necessary when distress is caused by structural overloading but will require evaluation due to the grout filling that attributes to additional dead loading. When experiencing a large amount of mild to moderate distress, solidly grouting existing hollow pilasters or wall panels may be a possible solution. Grouted cells need to be reinforced if possible and the additional dead load must be evaluated. Strengthening the supporting structural elements or foundations might be necessary if the new dead load is significant.

Replacement
If the distressed tile is of large quantity, replacement should be considered. Wall systems can be replaced by similar materials when a new tile system has adequate design capacity. Ordinary clay tile blocks are commonly produced today and can be grouted and reinforced to achieve higher design strength. More decorative or specialty blocks may require custom production that may prove difficult. If structural strength cannot be achieved by the addition of new tiles, stronger medias may be required, such as reinforced concrete. Historic features that require preservation could be supported by new structural systems or by independent systems created specifically to support them. Replacing the structural clay tile while maintaining the historic fabric is possible but usually expensive. Structural clay tile blocks created for floor systems are no longer manufactured, making it impossible to replace clay tile arch floor systems. Also, artisans that are skilled in laying this type of flooring system are no longer in existence. Damaged tiles are, instead, grouted soli and left in place. If the decay is extreme, the tile arch is removed entirely and a new structural floor of a different variety is installed in its place.

Standards and specifications

 * ASTM C67 - 09 Standard Test Methods for Sampling and Testing Brick and Structural Clay Tile
 * ASTM C126 - 09 Standard Specification for Ceramic Glazed Structural Clay Facing Tile, Facing Brick, and Solid Masonry Units