Wood Preservative Solutions for Creative and Sustainable Bridge Design and Construction
Ted John LaDOUX
Western Wood Preservers Institute. Washington 98684, USA
Abstract：
Timber bridges can provide an economical alternative to concrete and steel structures. particularly in rural areas where vehicle traffic is light to moderate. The wooden components of bridges have historically been preserved with either an oil type or waterborne preservative system to protect the wood from decay in order to maintain required performance standards for an extended period of time. The focus of this paper is to describe some of the key preservatives, research and case studies that support use of preserved wood, and some important steps to follow for the appropriate and safe use of preserved wood when the planned application will be in or over aquatic and wetland environments. A wealth of scientific information has been collected and analyzed that clearly suggests the use of preserved wood does not present a significant adverse effect on aquatic and wetland environments. This conclusion is based on two decades of empirical research and case study evaluating the environmental fate and effects of preserved wood, level of migration of contaminates into aquatic and marine environments. and the preserved wood environment. This is particularly true when risks are properly assessed on a project site. the appropriate preservative is selected and the wood is preserved to the Western Wood Preservers Institutersquo;s BMPs(best management practices),along with properly installing and maintaining the preserved materia1.To assist with the assessment process.peer.reviewed risk assessment models for l1 commonly used preservatives have been developed that provide for streamlined data entry by users and allow for evaluation of a structure above and below water. A companion preliminary screening level assessment tools also available. When these measures are properly utilized engineers, biologists and other responsible officials can be confident that the service life of the preserved wood components will more than likely meet the required performance standards in an environmentally safe manner for up to 50 or more years on a majority of timber bridge projects.
Key words：Waterborne wood preservative, oil-type wood preservative, environmental risk assessment, best management practices
- Introduction
Why should you use preservative wood? Based on the fact wood degrades via non—living or living agents, or both at the same time, when wood is left unprotected from these agents there will be a significantly shorter life expectancy. This will continue to be a significant concern when using wood and for this very concern, preserved wood plays a key role in protecting wood from degradation while significantly extending the life expectancy. As a case study, the findings of 3 0 timber bridges that were inspected in the Pacific west ranging in age from 31 to 78 years were found to still be meeting or exceeding the required performance standards. Based on their individual conditions. these bridges were expected to satisfactorily perform for an additional 25-50 years. These inspections were part of a US Federal Highway Administration and US Department of Agriculture, Forest Service Forest Products Laboratory national timber bridge survey project conducted between 20l1 and 2013[1].In another cooperative study between the US Federal Highway Administration, the USDA Forest Products Laboratory and Aquatic Environmental Sciences. the environmental risks associated with timber bridges preserved with creosote, pentachlorophenol and chromated copper arsenate under worst case conditions were evaluated. The study findings determined that no adverse biological effects were observed in either the invertebrate community or in laboratory bioassays at any of the bridges evaluated.
Researchers concluded the study results suggested that there would be minimal environmental risks associated with the level of preservatives lost from timber bridges. Researchers further concluded that many environmental risks could be minimized or eliminated by utilizing best management practices for construction and maintenance[2].Knowing the environmental benefits of using preserved wood compared to alternative materials is also an important factor to consider when determining the type of material to use on a structure. This determination was made in a series of quantitative evaluations conducted by the Treated Wood Council on the environmental impacts associated with the national production, use and disposition of various preserved wood products compared to alternative materials.
As an example, a LCA(1ife cycle assessment)on ACQ(alkali copper quat)·treated lumber found that seven out of eight key environmental impact indicators required less total energy and less fossil fuel, had lower overall environmental impacts. and when reused for energy recovery in permitted facilities with appropriate emission controls, there would be further reduction of greenhouse gas levels in the atmosphere. The evaluation was conducted using LCA methodologies and followed IS0 1 4044 standards ,as well as being peer—reviewed. A link to the summary of the ACQ. treated lumber LCA can be found in Ref [3].
The use of wood preservatives is highly regulated by the US EPA(Environmental Protection Agency)under the FIFRA(Federal Insecticide, Fungicide and Rodenticide Act).All wood preservatives are required to undergo a rigorous registration and re-registration process. The EPA considers wood preservative systems as antimicrobial pesticides and requires that the pesticides must be supported with thorough scientific review and analysis, as well as show they can be used without causing undue adverse effects to human health or the environment. Under federal
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