Failure is defined by the MIA as any condition that results in structural damage to the stone. Although the Marble Institute of America (MIA) recommends that commercial stone floors be a minimum of 3/4 inch thick (2 cm), many installations are ongoing using thinner modular stone tiles. These pavers are normally 3/8 inch thick (1 cm) and of a size ranging from 12 x 12 to 24 x 24 inches (30.5 x 30.5 to 61 x 61 cm). The selected installation method is usually TCA F111, F112, or F113. F111 is for substrates subject to deflection, while F112 and F113 are for use where no bending stresses occur.

Some project specifications require these thin stone pavers to conform to the ASTM C-99 Compressive Strength, ASTM C 170 Modulus of Rupture, and ASTM C 880 Flexural strength test results published for the particular stone variety being used. When the stone fails, due to reasons discussed below, the stone contractor is held responsible, because the stone did not perform in accordance with the minimum ASTM requirements published for that stone variety.

It is the position of the Marble Institute of America that stones under 1 1/4 inch thick, when used for paving, do not furnish the finished floor any structural qualities other than abrasive resistance. The flexural, compressive, and breaking strengths these thin stones have will not materially improve the engineering quality of the designed surface. Thin modular stone tile is furnished for its aesthetic appearance, and to supply abrasive resistance only. ASTM C99 (Standard Test Method for Modulus of Rupture of Dimension Stone), ASTM C170 (Standard Test Method for Compressive Strength of Dimension Stone), and ASTM C 880 (Standard Test Method For Flexural Strength of Dimension Stone) test results are not suitable to use for horizontal (floor) applications where the thickness of the stone tile being employed is under 1 1/4 inches.

When using method F113, the TCA recommends (page 9, 2001 Handbook for Ceramic Tile Installation) a methodology which provides for full support of corners and edges of the stone tile to the thin-set. This support is critical to the success of a thin stone paving installation. The MIA method is different then the TCA method, but the result should be similar. The MIA recommends that thin stone tiles be installed with as close as possible to 100% contact. If you need to use a notched trowel in order to obtain uniform depth of coverage, go back over the area with the smooth side of the trowel. The MIA method requires stone tiles be back-buttered. Although both methods will perform equally, the MIA believes its method is superior, because the TCA method requires movement of the stone across the ridges formed by the notched trowel, in effect removing the ridges. Unfortunately, in practice, this requirement is seldom adhered to.

When the floor is to be subjected to wheeled traffic, the prudent installer will ask the project engineer to supply point loading data, and that information should be relayed to the technical department of the thin-set manufacturer who is supplying the product to determine that the thin-set will support the required anticipated loads. After failure to properly install the stone, failure of the thin-set or mortar bed to support the load is the principal reason for these stone failures. The point load of a four-wheel cart with a light load (under 300 pounds), will cause many thin-set mortars to crumble when the ridges from the notched trowel are left.

Poorly constructed and poorly compacted mortar beds can lead to failure when method F 111 or F112 is employed. Either the sand-to-mortar ratio is not correct, or the mortar has not been properly blended. In failures examined by the MIA, laboratory analysis of the mortar beds has shown sand-to-Portland ratios as high as 12 to 1 due to poor mixing.

Other failures occur in floors subjected to high loads when mortar shifts to fill voids remaining from poor compaction of the mortar bed. This in turn leaves the stone tile partially unsupported, and it then cracks under the load.

Installing thin stone floors in shopping malls and other projects where carts and scissor lifts are used is not a slam-dunk. These projects require sophisticated and careful installers; a high degree of attention to the installation method; proper blending of installation materials, and the owner's knowledge that the stone will perform only its function of abrasion resistance and good looks.