Used as a building material for thousands of years, stone's beauty and sense of permanence make it ideal for significant buildings. Within the past 150 years, however, advances in technology and the introduction of new building systems have changed how stone is used. In recent years, thin-stone cladding systems have become increasingly popular.
The unique physical and aesthetic characteristics of stone result from several conditions:
Natural planes of weakness, such as cleavage planes, bedding planes, and riffs, occur as specific stones form.
- The physical properties of an individual stone will also vary depending on whether it is tested wet or dry.
- Stone is not an isotropic material; therefore, its strength will vary depending on the orientation of the load.
- Stone is a heterogeneous material, which also contributes to variability.
Properties of stone that affect its performance are compressive, flexural, shear, and tensile strength; density; abrasion resistance; coefficient of thermal expansion; and modulus of elasticity. Other, frequently overlooked properties can contribute to the premature failure of a stone cladding system: hysteresis (permanent volume change), freeze-thaw weathering, chemical weathering, thermal weathering, permeability, and the effects of stone finishes.
Early thin-stone anchorage systems often failed due to corrosion, lack of expansion joints, formation of ettringite, and stacking of stone.
By the 1960s, the practice of stacking stones between support points was rarely practiced on high-rise buildings. Panels were supported individually to allow for greater movement capabilities. The 1960s also saw the development of alternative systems in which the stone was anchored to a supporting system fabricated offsite, and then the composite system was attached to the structural frame of the building.
One of the most significant factors is tolerances. Tolerances in thin-stone cladding include both fabrication and construction variations. Fabrication tolerances concern length, width, thickness, squareness, and location of kerfs and holes. Tolerances become more significant as the thickness of the panel decreases.
Vertical and horizontal adjustability are achieved through slots or shims. If the system is not properly designed, excessive shimming may occur during installation.
To prevent both local and system failures, it is necessary to consider and accommodate all potential movement within both the cladding system and the structural system. Expansion joints must be properly designed and installed to prevent failures.
One of the most fundamental problems affecting nearly all exterior components of a building is water infiltration. Traditionally, massive exterior walls could holding large volumes of water. Thin-stone-cladding systems rely on relatively thin panels and the sealant between the panels as the primary line of defense against water infiltration. These systems may be somewhat watertight initially but, as the sealant begins to deteriorate, water will reach the underlying substrate and anchorage.
Distress conditions observed in thin-stone cladding include:
An over-compressed sealant joint between the stone panels is typically the result of improperly designed or installed expansion joints that do not accommodate all movements within the system.
- Vertical cracking can also be an indication of failure to properly accommodate expansion, usually suggesting the presence of inclusions, such as shims, within expansion joints.
- Horizontal cracking near the center of the panel is usually the result of a flexural failure of the stone.
- Spalled stone at anchor locations may be the result of corroded steel components or the presence of gypsum in a setting plaster or mortar. Water freezing in anchor locations may also cause spalling.
- Displaced panels may be the result of support components corroding or lateral anchorage failing.
- Bowing of panels may be the result of stacking stresses or hysteresis.
- Cracking may also be the result of discontinuity within the stone, shear stresses, or improper handling during transportation or installation.
- Staining, typically only an aesthetic consideration, may in some instances be a symptom of a greater problem to come.
In a conventional loadbearing system, the failure of an individual component rarely compromised the integrity of the system. In thin-stone systems, however, the support system is frequently a critical structural system. Therefore, the failure of an individual component could result in a localized instability and the loss of adequate support, resulting in pieces coming off a building. Because of the brittle nature of stone and its lack of ductility, a failure can be sudden and unpredictable.
Factors affecting the design of a repair plan range from aesthetic to practical. The easiest and most economical solution is often visually objectionable. Extreme care should be taken in material selection, movement accommodation, and weather protection.
Pinning/supplemental anchorage. Supplemental anchors can be installed in the panels without removing the panel from the facade.
- Patching. A damaged portion can be removed and a new matching piece installed provided that the new piece can be supported properly.
- Alternative anchorage system. If the problem in a particular cladding system is related to anchorage deterioration, it may be desirable to remove the stone panels and reinstall them on a new anchorage system.
- Recladding. When both the anchorage system and the stone have deteriorated or are not adequate for the particular installation, a completely new system may need to be designed and installed.
- Cleaning. Numerous cleaning techniques are available for removing stains from stone. It is critical to determine the cause of the staining, as well as the type of stone to be cleaned.
The use and popularity of thin-stone-cladding systems in the building industry likely will continue at current levels. Many older thin-stone systems have begun to show signs of an aging and outdated design. Inconsistent maintenance, neglect, or normal aging of the envelope have led to a rise in failures.
Newer cladding systems, installed rapidly or using unproven technologies, have failed more quickly than many of the older installation systems. A proper understanding of materials, design, and constructability is crucial to identifying potential sources of failure, as well as appropriate and feasible repair solutions.