Last month’s Tile Q&A provided a general overview of the recently-revised ANSI A137.1 ceramic tile standard, which outlines the test methods that are used to measure various aspects of tile performance, dimensional characteristics, and the minimum requirements for tile to be labeled A137.1 compliant. In this article, we will look more specifically at some of the laboratory tests and corresponding requirements and ratings that relate to application suitability for glazed tiles, as defined in A137.1.
How much traffic can (should) glazed floor tile withstand?To determine this, a tile’s glaze is tested for resistance to abrasion per the ASTM C1027 test method. This test involves subjecting the glaze to a cocktail of swirling abrasives including chrome steel balls of various diameters, 80-grit aluminum oxide, and demineralized water. At the required intervals, a test tile is visually inspected for wear or abrasion. The point at which wear is detected determines the tile’s wear classification. For example, the tile cannot show wear before 150 revolutions to be classified as Class I and cannot show wear before 2,100 revolutions to be classified as Class IV. Tiles that do show wear before 150 revolutions are classified as Class 0. To be classified as Class V, the highest category, the tile cannot show wear at 12,000 revolutions, and it must also pass a staining test wherein the abraded tile can be thoroughly cleaned after being subjected to the staining agents.
The A137.1 standard provides the following guidelines for application suitability based on a tile’s glaze wear classification:
- Class O - Not recommended for floors: should not be exposed to wear/traffic or aggressive maintenance practices and is generally used on walls.
- Class I - Light residential: may withstand soft-soled foot traffic, provided that dirt and/or other abrasives are not present on the wear surface; should not be used in areas with direct access to the outside or large amounts of foot traffic.
- Class II - Residential: may withstand soft-soled and some normal foot traffic provided the dirt and/or other abrasives are kept to a minimum; should not be used in areas with direct access to the outside, large amounts of foot traffic, or areas exposed to abnormal footwear.
- Class III - Heavy residential/light commercial: may withstand normal footwear and regular traffic, with some dirt and/or other abrasives present in limited quantities. Tile in this class may be used in light commercial installations with limited foot traffic and with no direct access to the outside. Examples: residential kitchens and hallways, with limited traffic from the outside.
- Class IV - Commercial: may withstand heavier amounts of traffic with greater amounts of dirt and/or other abrasives. Examples: commercial kitchens and areas with regular traffic from the outside.
- Class V - Heavy commercial: may withstand constant foot traffic with larger amounts of dirt and/or other abrasives. Examples: airports, malls, and other commercial walkways subject to high volumes of foot traffic and constant traffic from the outside.
What is resistance to crazing?Typically, the ceramic engineer designs a tile so that, during the cooling period after firing, the tile body contracts slightly more than the glaze, putting the glaze in state of compression. This compatibility of a tile glaze and a tile body is called the “glaze fit.” When the fit is not good and the glaze contracts more than the body, crazing (glaze cracking) can occur. If you ever noticed a fine web of tiny lines in older china, this is a good example of crazing, which can occur whenever a glaze and body don’t expand and contract similarly.
If this only happened during the cooling period after firing, a test for crazing resistance would probably not be necessary - a tile manufacturer would not ship such tiles. However, crazing can occur long after production (and installation). This is why all glazed tiles must pass the ASTM C424 test for resistance to crazing. In this test, glazed tiles are subjected to steam pressure followed by rapid cooling and release of pressure. In order to pass this test, per A137.1, glazed tiles must withstand at least 150 psi of steam pressure and the subsequent cooling without crazing.
What is thermal shock resistance and resistance to freeze/thaw cycling?Thermal shock resistance refers to a tile’s ability to withstand rapid or severe temperature changes. When tiles cannot withstand thermal shock, cracking or spalling of the body or glaze could occur. To test this property, glazed tiles are placed alternately in a water bath held at 15 ± 5°C (59 ± 9°F) and an oven held at 145 ± 5°C (293 ± 9°F). The tile body and the glaze are inspected for damage and all glazed tiles must withstand 10 cycles of this, per A137.1. This test most closely simulates the thermal shock that an exterior installation in warmer conditions would experience.
For tiles that will be subjected to cycles of freezing and thawing, a different test is used. Saturated tiles are alternately placed in a freezer and a room temperature water bath. The tiles must show no evidence of damage after 15 cycles. Typical damage, for tiles that do not pass, also includes cracking or spalling of the body or glaze. Because the majority of tile produced will not be exposed to freeze/thaw cycling, only tiles that may be exposed to such conditions are required to pass this test, per A137.1.
Next month, this series will conclude with a discussion of the dimensional requirements for the various tile types.