Durability of Wood Alternatives

Some consumers believe that wood is a non-durable building material while reinforced concrete and steel are durable. In fact, all building materials have limits in terms of durability. It is known that fungi are the major cause of wood deterioration when wood is exposed to constant wetting without preservative treatment, but it is not as widely known that organisms also facilitate deterioration of metal and reinforced concrete infrastructures through production of corrosive chemicals1.  Road salt, sea air and soil salts can also be corrosive to metals while wood is relatively resistant to these chemicals.

Based on a 2001 survey of high-rise envelope performance in the coastal climate of British Columbia sponsored by Canada Mortgage and Housing Corporation and the Homeowner Protection Office of British Columbia, the majority of moisture-related damage was corrosion of concealed metal components including steel studs and fasteners2. This has resulted in problems costing millions of dollars in repairs to high-rises.

Although concrete itself does not corrode or decay, it almost inevitably cracks, and concrete cannot be used structurally without steel reinforcement. Cracking of concrete exposes concealed steel to more moisture, as well as corrosive chemicals, and the moisture and chemicals then initiate more corrosion of the steel components. Then the larger dimension of the corrosion products of steel causes concrete to further crack and spall. Metal corrosion and concrete spalling are causing about 13% of bridges in the USA (about 77,720 bridges) to be classified as structurally deficient, and many of these bridges were built during the last decade3.

Although there is a perception that reinforced concrete and steel resist termite attack and the U.S. IRC (International Residential Code) recognizes steel framing as a method of termite protection, based on large scale investigations in Australia, the contents of steel-framed houses are just as likely to suffer a termite attack as timber-framed houses4,5. Raised wood floors are not only very efficient for mitigating flood damage and controlling other sources of moisture, but also make it very easy for termite inspection in termite zones. Compared to raised wood floors, reinforced concrete slabs make it more difficult for homeowners and inspectors to locate points of termite intrusion since termites usually enter buildings through concrete cracks and service penetrations.

A 2004 survey, conducted in Minneapolis/St. Paul examined building age, building type, structural material and demolition reasons for 227 residential and non-residential buildings6. For non-residential structures, about 66% of demolished wood buildings were over 50 years old, about 66% of concrete buildings were under 50 years old and 90% of steel buildings were under 50. The most common reasons for demolition were related to changing land values, lack of suitability for current use, and lack of maintenance for non-structural components. The ability to expand and modify wood-frame structures easily for new and different purposes may well contribute to the extended service life that these types of buildings enjoy.

While each building material has its own advantages and disadvantages, different building materials do need to work together. Just like steel is always needed to reinforce concrete, modern wood structures cannot be assembled without metal fasteners. Appropriate design, material selection and construction of wood buildings certainly reduce wood deterioration and metal corrosion. Meanwhile, hybrid structures, integrating different structural materials and different building styles, can certainly take full advantage of attributes of different materials.

References:

  1. M. Sanchez-Silva, and David V. Rosowsky. 2008. Biodeterioration of construction materials: State of the art and future challenges. Journal of Materials in Civil Engineering ©ASCE, May 2008.
  2. Homeowner Protection Office, and Canada Mortgage and Housing Corporation. 2001. Study of high-rise envelope performance in the coastal climate of British Columbia. Research report, British Columbia, Canada.
  3. O. Büyüköztürk. 2008. Durability of cementitious materials and multi-material systems. Key-note speech of the 11th International Conference on Durability of Building Materials and Components. 11-14 May 2008, Istanbul, Turkey.
  4. Archicentre Limited. 2006. An analysis of termite damage in Sydney and Melbourne. Special Report.
  5. CSIRO. Student research yields termite map. http://www.csiro.au/resources/Termites.htm
  6. J. O’Connor. 2004. Survey on actual service lives for North America buildings. Presented at Woodframe Housing Durability and Disaster Issues Conference, Las Vegas, October 2004.