• Recommended Guidelines
  
  • Selecting a Wood Preservative
    
    • Wood Species
      
    • Types of Structures
      
    • Ecosystem Sensitivities
      
  • Purchasing Treated Wood Products
    
    • Treatments
      
    • Best Management Practice
      

The following guidelines can help field employees select wood preservatives that are most appropriate for their projects (figure 6).

The type of preservative that is most appropriate depends on the species of wood being treated, the type of structure, the cost, the availability of treated wood, and the specific area where the wood will be used.

Wood Species

Generally, hem-fir (hemlock and fir) and southern pine can be treated adequately with any of the commercial wood preservatives, although copper napthenate has not been standardized for use with hem-fir. CCA is not recommended for treatment of Douglas-fir, which is more readily treated with oil-type or ammoniacal preservatives. CCA is not recommended for treating hardwoods that will contact the ground.

Figure 6—A viewing platform in the Garden of the Gods
in the Shawnee National Forest, IL.

Types of Structures

Although appearance is not a major concern in many applications, wood treated with ammoniacal copper preservatives (ACZA, ACQ-B) may not be colored as uniformly as wood treated with other waterborne preservatives. However, ammoniacal preservatives allow better penetration and treatment of larger material, such as pilings and timbers—especially those made from Douglas-fir.

Often, large glue-laminated timbers (those used in bridge supports, for instance) are treated with oil-type preservatives, such as pentachlorophenol and copper naphthenate to reduce problems with checking and cracking. Laminated members, except for those that are small and straight, should not be treated with waterborne preservatives. The characteristic odor and appearance of oilborne preservatives may make them less desirable in areas of frequent human contact, such as handrails or decks.

In marine construction, or construction in areas with brackish water, such as ocean estuaries, only creosote, CCA, and ACZA are approved for use. In some areas south of a line connecting San Francisco and Virginia, dual treatments of creosote and one of the waterborne treatments may be needed to prevent attack by the different types of marine borers.

Ecosystem Sensitivities

Although largely undocumented, some preservatives may be more appropriate than others in sensitive ecosystems (figure 7). For example, CCA has a much lower copper content than other waterborne preservatives (except the borates). Although there is no evidence at this time to suggest that any of the wood preservatives leach enough copper to harm terrestrial or freshwater ecosystems, CCA may pose less of a threat to aquatic ecosystems than preservatives with more copper.

Similarly, preservatives without arsenic may pose less of a threat to mammals (including construction workers) than those that do contain arsenic. Once again, there is no evidence that wood preservatives containing arsenic harm people or other mammals if they are used as intended.

Wood treated with oilborne preservatives often produces an oily surface sheen when installed in stagnant freshwater environments. This may be unacceptable in some situations. Waterborne preservatives may be more appropriate when the treated wood will have extensive contact with freshwater.

As more information is obtained about the leaching rates and biological impacts of treated wood, it will be possible to make more informed decisions about the appropriate use of wood preservatives in sensitive ecosystems. Managers can use computer models at the Western Wood Preservers Institute Web site (http://www.wwpinstitute.org) to help them make decisions about the use of treated wood in aquatic ecosystems.

Figure 7—A boardwalk on the Trail of Blue Ice
in the Chugach National Forest, AK.

Regardless of the type of preservative, the wood should be treated properly and allowed to fix before it is placed in service.

Treatments

The AWPA Commodity (C) Standards have been replaced by the Use Category System (UCS) Standards U1 and T1. The new basic standard for specifying treated wood products is U1. The new basic standard with all of the requirements for producing treated wood is T1. The UCS was developed as a format revision of the Commodity (C) Standards. The UCS contains descriptions of the service conditions for use categories, a guide to commodity specifications for the end uses of treated wood, a list of AWPA standardized preservatives, and various commodity specifications relating to specific product types, such as sawn products, utility poles, pilings, or fire-retardant applications. The UCS helps specifiers and users select the appropriate treatment.

Simplicity is the primary advantage of the UCS. All treated wood products can be placed into one of five use categories, based on the hazard of biodeterioration and expected product performance. Products treated with fire retardants are placed in their own use category class.

With changes taking place rapidly in the wood-treatment industry, it is more important than ever to ensure that wood is treated to standard specifications. The Commerce American Lumber Standard Committee, Icorporated (ALSC) accredits third-party inspection agencies for treated wood products. Updated lists of accredited agencies can be found on the ALSC Web site (http://www.alsc.org). The easiest way to know whether wood has been treated to standard specifications is to look for a quality mark or the symbol of an ALSC-accredited agency on the front or back of the end tag. Avoid wood that is purported to be treated to refusal (treated until it will no longer absorb preservative).

Incising is a technique that increases preservative penetration and retention in species such as Douglas-fir. Douglas-fir should be incised regardless of the type of preservative used. Incising is especially important in larger material, such as timbers, that contact the ground. Smaller material that is not in contact with ground or water, such as decking, may perform adequately in some applications without incising. Although the AWPA Standards do not require southern pine to be incised, incising is beneficial when larger dimension material is treated. Larger pieces of material often have at least one face with exposed heartwood that preservatives have a hard time penetrating. The availability of incised southern pine is extremely limited. Table 3 summarizes the use categories for treated wood, while table 4 summarizes the standard preservative formulations and retentions for typical Forest Service applications.

Best Management Practices

The active ingredients of various waterborne wood preservatives (copper, chromium, arsenic, and zinc) are water soluble in the treating solution, but resist leaching when placed into the wood. This resistance to leaching is a result of chemical fixation reactions that render the toxic ingredients insoluble in water. The mechanism and requirements for these fixation reactions differ, depending on the type of wood preservative.

For each type of preservative, some reactions occur very rapidly during pressure treatment, while others may take days or even weeks, depending on storage and processing after treatment. If the treated wood is placed in service before these fixation reactions have been completed, the initial release of preservative into the environment may be much greater than when the wood has been conditioned properly.

Concerns about inadequate fixation have led Canada and European countries to develop standards or guidelines for fixing treated wood. Although oil-type preservatives do not undergo fixation reactions, the amount of environmental release still depends on treatment practices. With oil-type preservatives, preservative that is bleeding or oozing out of the treated wood is a particular concern. This problem may be apparent immediately after treatment. Such members should not be used in bridges or other aquatic applications. In other cases, the problem may not become obvious until after the product has been exposed to direct sunlight. This problem can be minimized by using treatment practices that remove excess preservative from the wood.

Best Management Practices (BMPs) standards are being developed to ensure that treated wood is produced in a way that will minimize environmental concerns and human health concerns (Pilon 2002). The Western Wood Preservers Institute (WWPI) has developed guidelines for treated wood used in aquatic environments (Western Wood Preservers Institute 1996).

Purchasers should specify and require assurance that the material they buy has been produced in compliance with Best Management Practices for the Use of Treated Wood in Aquatic Environments, USA version, revised January 1996, a publication of the Western Wood Preservers Institute and the Canadian Institute for Treated Wood. Publication of a new edition is scheduled for the end of 2006. Although these practices have not yet been adopted by the industry in areas outside the West Coast, purchasers in other areas can require that these practices be followed. Commercial wood treatment firms are responsible for meeting conditions that ensure fixation and minimize bleeding of preservatives, but persons buying treated wood should make sure that the firms have done so.

The risk of chemical exposure from wood treated with CCA is minimized after chemical fixation reactions lock the chemical in the wood. The treating solution contains hexavalent chromium, but the chromium reduces to the less toxic trivalent state within the wood. This process of chromium reduction also is critical in fixing the arsenic and copper in the wood. Wood treated with CCA should not be exposed to precipitation or other sources of environmental moisture until the fixation process is complete or nearly complete. The rate of fixation depends on temperature, taking only a few hours at 150 degrees Fahrenheit (66 degrees Celsius) but weeks or even months at temperatures below 60 degrees Fahrenheit (16 degrees Celsius). Some treatment facilities use kilns, steam, or hot-water baths to accelerate fixation.

The BMP for CCA stipulates that the wood should be air seasoned, kiln dried, steamed, or subjected to a hot-water bath after treatment. It should be evaluated with the AWPA chromotropic acid test to determine whether fixation is complete (AWPA Standard A3-11, 2005). There is some concern in the treatment industry that the chromotropic acid test may be overly conservative because it requires more than 99.5 percent of the chromium to be reduced to the trivalent form. However, the chromotropic acid test is the only standardized test available now.

The key to achieving stabilization with these preservatives is to allow ammonia to volatilize. This can be accomplished by air or kiln drying. The BMPs require a minimum of 3 weeks of air drying at temperatures higher than 60 degrees Fahrenheit (16 degrees Celsius). The drying time can be reduced to 1 week if the material is conditioned in the treatment cylinder. At lower temperatures, kiln drying or heat is required to complete fixation. There is no commonly used method to determine the degree of stabilization in wood treated with ACZA or ACQ-B, although wood that has been thoroughly dried is acceptable. If the wood has a strong ammonia odor, fixation is not complete.

Although these newer formulations are not in the current BMPs, they will be included in the 2006 revisions. Proper handling and conditioning of the wood after treatment helps minimize leaching and potential environmental impacts. Amine (and ammonia in some cases) keeps copper soluble in these treatment solutions. The mechanism of copper's reaction in the wood is not completely understood, but appears to be strongly influenced by time, temperature, and retention levels.

Copper stabilization in the copper azole formulations is extremely rapid (within 24 hours) at the lowest retention—0.10 pounds per cubic foot (6.4 kilograms per cubic meter)—but slows considerably at higher retentions unless the material is heated to accelerate fixation. As a general rule, wood that has thoroughly dried after treatment is properly stabilized.

The BMPs for pentachlorophenol treatment stress thorough drying of the wood before treatment and the use of an empty-cell process. In an empty-cell process, the air pressure is applied to the wood before the preservative is introduced to the treatment cylinder. After the pressure period, a final vacuum should be used, as well as a final steaming or an expansion bath similar to that described for the creosote treatments.

For creosote, the BMPs stipulate use of an expansion bath and final steaming period at the end of the charge.

• Expansion Bath—Following the pressure period, the creosote should be heated 10 to 20 degrees Fahrenheit (-12 to -7 degrees Celsius) above the press temperatures for at least 1 hour. Creosote should be pumped back to storage and a minimum vacuum of 24 inches of mercury (-81kPa) should be applied for at least 2 hours.
  
  
• Steaming—After the pressure period and once the creosote has been pumped back to the storage tank, a vacuum of not less than 22 inches of mercury (-74 kPa) shall be applied for at least 2 hours to recover excess preservative. Release the vacuum back to atmospheric pressure and steam for a 2-hour period for lumber and timbers and 3 hours for pilings. The maximum temperature during this process shall not exceed 240 degrees Fahrenheit (116 degrees Celsius). Apply a second vacuum of not less than 22 inches of mercury (-74 kPa) for a minimum of 4 hours.

The BMPs for copper napthenate are similar to those for creosote and pentachlorophenol. The recommended treatment practices for treatment in heavy oil include using an expansion bath and/or final steaming, similar to that described for creosote. When No. 2 fuel oil is used as the solvent, the BMPs recommend using a final vacuum for at least 1 hour.

Buyers can take steps to ensure that wood will be treated according to the BMPs described above. Proper fixation may take time. Material should be ordered well before it is needed so that the treatment firm can hold the wood while it fixes. If buyers order wood in advance, they may be able to store it under cover, allowing further drying and fixation. In general, allowing the material to air dry before it is used is a good practice for ensuring fixation, minimizing leaching, and reducing risk to construction personnel. With all preservatives, inspect the wood for surface residue. Wood with excess surface residue should not be placed in service.