Adhered exterior stone veneers

March 22, 2002
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Installation methods for veneers fall into two general classifications: adhered and anchored. Thin veneer elements such as tile, thin brick and terra-cotta are often adhered to a solid substrate material, which is in turn attached to the building?s exterior structural wall framing with fasteners. Appropriately sized and spaced fasteners provide effective anchorage of the substrate with the adhered product when the adhesive materials bonding the face materials are properly prepared and applied.

However, when natural stone is used in veneer form using an adhesion installation method, there are a number of issues that will affect the performance of the installation. Building movement is a serious concern. Hard setting stone does not allow it to move independently of the structure. Therefore it must attempt to expand and contract at the same rate as the mortar, steel and other products that the building?s substrate and structural exterior are fabricated from. This will not happen, and the stone may spall or crown at the joints. This will stress or break the bond or it can fracture the stone, as the stone?s natural tendency to expand and contract at a given ratio is constrained by the installation method. Principles of cladding design require independently anchored stones and flexible stone joints 2 to 4 times larger than anticipated building movement to allow for this movement

Joints in the cladding may not be large enough to allow for in-plane drift. In-plane movements can cause cracking of the stone, failure of the fasteners, or both. Jointing material in these installations is normally a cement-based grout, with variable joint widths of approximately 1/16 to 1/4 inch. The joint width for stone panels should be a minimum of 3/8 inch, and the joint material should be a moveable and compactable material, such as caulk.

Exposure to water can deteriorate concealed fasteners. This type of deterioration is not easily detected. Also, water expands by approximately 10% upon freezing, and even modest freezing could be potentially damaging to porous stones. Cladding designers traditionally have used porous stones in greater thicknesses to compensate somewhat for their high absorption values. Stones with high absorption rates are the poorest choices for exterior work, because the water absorption leads to other problems as seen above, especially when the stone is adhered to a structure. The venting that would normally allow the stone to void moisture back to the face in an anchored and vented installation is not present.

Adhesion of the stone may be inadequately designed because the original standard adhesion methodology may not have been designed for unusual loads, such as earthquake forces. When a stone is not completely bonded, there is a void between the stone and the substrate. These voids, especially in moisture environments or where there are temperature variations (heat loss or gain at one face of the stone different than the other face), will tend to cause moisture to collect at those sections of the back face of the stone that are not bonded to the substrate. In addition to showing spots at the face of the stone, the water will cause pressure to build up on the back face of the stone, placing more stress on the bonding material to hold the stone in place.

Thermal hysteresis in genuine marbles is a known factor, and it is an additional concern. Hard setting these stones will tend to increase the effects of thermal hysteresis by increasing the temperature at the exposed face of the stone relative to the rear face. This is opposite to the hysteresis that generally occurs due to normal cycles of building heat gain and loss. If the stone?s setting mortar is sufficiently strong enough to counter the natural crystal movement that the hysteresis requires, the stone may de-laminate into adhered and non-adhered sections, precipitating a section of the stone falling.

Force concentrations, building side sway, and differential vertical movement are not easily calculated into a safety factor within the industry standard safety factor range of 3 to 10, dependent on the stone and the application. Adhering the stone does not allow for these considerations. Hard setting stone can require stones to act as deep beams, especially when the spandrels are fabricated of concrete, which is subject to long-term deflection due to creep.

Very few dimension stones have weights within building code requirements that would allow a stone of 2-cm thickness to be employed. The building code in Los Angeles, CA, (used as a guideline for many other areas of the country) allows installation of stone panels that do not exceed 36 inches in the greatest dimension, or are no more than 720 square inches in total area and/or do not weigh more than 10 pounds per square foot. This building code is not relevant to the practice of adhering stone panels.

Ten pounds per square foot equates to just under 142 pounds per cubic foot at 2 cm thick. ASTM C 97 data for the various dimension stones that have ASTM designations --limestone (ASTM C 568), exterior marble (ASTM C 503), granite (ASTM C 615), quartz-based stone (ASTM C 616), and slate (ASTM C 121) --indicate dry weights. (See Table 1) Additional weight gain from a thorough water soaking leaves only low-density limestone available for consideration in a thickness of 2 cm. All the other stones nature has provided us would have to be furnished in a thickness under 2 cm to conform with the building code.

For example, granite dimension stone has a required minimum weight of 160 pounds per cubic foot, and a maximum permissible water absorbency of 0.40%. By calculating, the result is 160 x .004 = 0.64 pounds additional weight when thoroughly wetted. This equates to a total of 10.54 pounds per square foot at 2-cm thickness, or about 5.4% more weight than the code allows. (2 cm = 0.7874 inches. 12 inches/0.7874=15.24. 160.64/15.24=10.54 pounds per square foot).

The stability of the stone panel is also an issue. A thickness of 1.5 cm would be the next logical lesser thickness choice, because this is a standard stone tile thickness. A panel measuring 36 x 20 inches x 1.5 cm (0.59 inches) would be the maximum size reasonably attainable that conforms to the requirements of the building code. The flatness of both faces of the stone are affected because thinner stones flex more than thicker stones, thus the back of the stone may not be sufficiently flat to properly bond to the adhesion material.

The Marble Institute of America recommends that stones to be adhered, either interior or exterior, be adhered as closely as possible to 100% bonding, and only in projects where the maximum height of the stone is 96 inches off grade, and the stone has a maximum dimension of 24 x 24 inches with a thickness of 5⁄8 inches. Adhesion practice should be limited to this height and stone size.

Table 1 - ASTM Density Standards

* ASTM C 568: Standard specification for limestone dimension stone requires a minimum density of 110 pounds per cubic foot for low-density limestone, 135 pounds per cubic foot for medium-density limestone, and 160 pounds per cubic foot for high-density limestone.

* ASTM C 503: Standard specification for exterior marble dimension stone requires a minimum density of 162 pounds per cubic foot for calcite marble, 175 pounds per cubic foot for dolomite marble, 168 pounds per cubic foot for serpentine, and 144 pounds per cubic foot for travertine.

* ASTM C 615 : Standard specification for granite dimension stone requires a minimum density of 160 pounds per cubic foot.

* ASTM C 616: Standard specification requires a minimum density of 125 pounds per cubic foot quartzitic sandstone, 150 pounds per cubic foot for quartzite, and 160 pounds per cubic foot for sandstone.

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