With growing pressure to reduce the environmental impacts of buildings, architects and engineers are looking beyond operational performance to the role of structural materials.
The life cycle of structural building products typically starts with the extraction of raw resources such as timber, iron ore, limestone and aggregates. The collection of data starts here, with the tracking of energy use and emissions to air, water and land per unit of resource. Wood’s impacts during this phase are relatively low compared with concrete and steel, which are made from substances that must be mined and heated to extremely high temperatures. (FPInnovations, Synthesis of Research on Wood Products and Greenhouse Gas Impacts, 2010)
A typical concrete mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water, though proportions change to achieve different requirements for strength and flexibility. While most of concrete’s ingredients are themselves manufactured products or mined materials, it’s the cement in concrete that has the highest embodied energy (Environmental Life Cycle Inventory of Portland Cement Concrete, Portland Cement Association). According to the U.S. Energy Information Administration, the cement industry is the most energy-intensive of all manufacturing industries. Cement is also unique in its heavy reliance on coal and petroleum coke.
A major ingredient needed for cement is limestone, which is found in abundance in many places in the world. In most cases, limestone is blasted from surface mines and removed in large blocks to a crusher, mixed with other raw materials, and transferred to a rotating furnace, where it is heated to about 2,700 degrees Fahrenheit in order for the materials to coalesce. The mixture is cooled and ground to fine powder (cement), which is transported to its destination by truck, rail or ship. Fly ash, a byproduct of coal burning, can be substituted for some of the cement, as can a variety of other ingredients, with associated reductions in carbon footprint.
Steel is an alloy consisting mainly of iron and has a carbon content between 0.2 percent and 2.1 percent by weight, depending on grade (NAVEDTRA 1425, Chapter 1). Steel’s main ingredient is iron ore, which must be extracted through open-pit mining and heated to extremely high temperatures. In surface mines, ground is removed from large areas to expose the ore. Ore is then crushed, sorted and transported by train or ship to the blast furnace, where the iron is heated to 3,000 degrees Fahrenheit, usually with charcoal or coke, and charged with the ore and limestone. The molten iron drains off, and iron ingots are formed. This pig iron, as the ingots are called, is the basis for steel.
For both concrete and steel there are environmental consequences from open-pit mining, and from the fossil fuels used to process the raw materials. However, both industries continue making strides to lighten their environmental footprint.
Because of their different manufacturing processes, the use of wood products results in far less carbon emissions than either steel or concrete. While the concrete and steel industries are primarily powered by fossil fuels, many lumber companies use woody biomass (e.g., sawmill residues such as bark and sawdust) to fuel their operations (Dovetail Partners Inc.), which provides information about the impacts and trade-offs of environmental decisions, calls the North American lumber industry 50 to 60 percent energy self-sufficient overall.
Wood grows naturally, is renewable and has advantages from a carbon footprint perspective. It is also durable, adaptable and can have positive impacts on a building’s occupants. However, understanding a material’s impacts at every stage of its life is essential—and LCA studies consistently show that wood has a favorable environmental profile compared with functionally equivalent products made from other materials.
It is worth reiterating that no one material is the best choice for every application. There are trade-offs associated with each, and each has benefits that could outweigh the other material choices based on a project’s design objectives.
To dig deeper into the impacts of the major structural building materials take our Building Materials Matter CEU.