By Michael Picco, P.Eng.
The use of natural stone on buildings and paving can be traced back to the beginning of civilization. There are countless buildings, monuments, and structures dating back thousands of years that have stood the test of time and left generations in awe. Some examples of projects built primarily of local limestone include Egypt’s Great Pyramid of Giza, the Mayan temples in Mexico, southern Italy’s round Trulli homes, the Parthenon in Greece, and the Roman Coliseum (which also included local travertine). Many of these great structures were built with no mortar. The stones were simply cut and tightly fitted together and dry stacked—a method of stone installation mimicked in modern-day construction and referred to as ‘dry-stacked stone.’
In many ways, our ancestors in the building industry applied sustainable practices by using the larger quarried blocks for building construction and smaller waste pieces for erosion control and walkways. They also used the gravel to pave small roads from town to town.
When natural stone is properly specified and installed, few cladding materials last as long and perform as well. Not only is it esthetically pleasing, but it is also a low-maintenance, sustainable material.
The greenness of stone
Natural stone’s inherent characteristics make it a green building product—it can be used without any additional finishes or wall coverings, has low maintenance needs, and is highly durable. (This author acknowledges the assistance of Stephanie Vierra, Assoc. AIA, LEED AP [Vierra Design & Education Services LLC], in developing the article’s section on stone and sustainability matters.) Unlike the many cladding materials available on the market requiring extensive manufacturing energy, natural stone is extracted from the earth and processed, slabbed, finished, and cut.
As more emphasis is placed on the whole building design and life cycle assessment, the concept of a product’s embodied energy is an important and relevant measure. Embodied energy is a measure of the carbon dioxide (CO2) emitted from the time raw material is extracted to the point it is installed on the building. This process has been referred to as ‘cradle to gate.’
Since building designs may be using more materials (and/or more carbon-intensive products) to achieve lower energy use, an increasing proportion of the total energy use and carbon emissions for high-performance buildings comes from its materials and products. By taking embodied energy into account, a project team can ensure it is designing for net carbon emission reductions. In the case of natural stone, this could include the CO2 required during quarrying, transport to the plant, energy required for slabbing and fabrication, delivery to the site, and installation. There is still some work being done by various lifecycle analysis (LCA) authorities to clearly define the definitions of what is, and what is not, included in the calculations for various products to ensure a consistent and fair measure is established. However, based on most studies and comparisons, natural stone is consistently rated as one of the building materials having the lowest embodied energy on many scales.
Different materials, different stone
With the advent of many new thin cladding manufactured products introduced to the market, there is a recent trend for designers to specify and detail very large cladding panels. When using products such as porcelain, recycled glass, glass-fibre-reinforced concrete (GFRC), fibre-reinforced polymer panels (FRP), or engineered/manufactured stone, care must be taken by architects and specifiers to make the clear distinction between natural stone and manufactured products. The design approach, attachment detailing, and structure can vary dramatically.
This author has recently seen a trend of substituting an engineered thin cladding product with large, thin natural stone panels, with the erroneous assumption they were basically interchangeable. The variability of natural stone requires a design approach quite different than manufactured products and, thus, has different limitations associated with panel size and connection design.
New natural stones are being introduced to the market on a continual basis. Many are a result of new quarry areas and stones being discovered, and many are a result of new technologies allowing extraction of stones previously impossible to quarry (described later in this article).
Some of these technologies include the use of epoxy injection at the quarry with stones that are heavily flawed and cracked. In other cases, vacuum epoxy injection is employed to stabilize heavily cracked and fissured blocks of stone. For the general consumer, these injections would be undetectable as they would just appear as another vein in the material.
The epoxy in the stone and the amount of veining can have a dramatic effect on the technical properties of the stone. In most cases, these heavily cracked and filled materials are restricted to interior use. Many of the very desirable and heavily veined onyx slabs on the market today are a result of these technologies. In the past, blocks large enough to produce slabs were impossible to extract in one piece.