Extending Wood Pole Life with Groundline Preservatives

For many years, pole owners have recognized the benefits of inspection of in-service wood utility poles and service life extension through the application of remedial treatments.  However, today's economic conditions combined with environmental issues, sustainability objectives, and ever-growing regulatory mandates place an even greater burden on utility management to maximize the safe use of their existing wood poles.

It's impossible to discuss effective remedial treatment programs without an analysis of inspection methods and inspector qualifications.  Suffice it to say that accurate inspection, decay assessment, and treatment selection and application are equally as important to the process, as the performance characteristics of remedial treatment systems themselves.

Remedial treatments are designed to extend service life by supplementing the manufacturer's original treatment.  The depletion of the pole's initial protective system happens over time and may be a result of aging, weathering, migration, or the volatility of the chemical elements.  Combined with the remaining original preservative (typically pentachlorophenol, creosote, or CCA), remedial treatments can provide an effective defense against a wide variety of wood-destroying organisms, thus extending the useful service life many years beyond what is typically expected (see Survivor Curve, Deterioration Zone 3 as referenced by the American Wood Protection Association (AWPA) U1-12).

The All-Important Groundline
Wood poles commonly used in North America are subject to "shell rot" or surface decay below the ground level.  In southern pine poles which compose roughly 85% of the poles in North America, this type of decay is most common.  Western species such as Douglas fir and red cedar are less susceptible to surface decay; however, the sapwood of neither Douglas fir nor western red cedar is naturally resistant to decay.  Therefore, as these poles age they may be subject to surface decay, though at later stages in life when compared to southern pine.

Decay fungi are the most common wood-destroying organisms and they can be found in virtually any environment.  Decay fungi require four elements in order to cause damage: air, water, a favorable temperature, and food (in this case, the wood pole).  These four elements are most prevalent from the groundline to 18 inches below groundline (in most cases, air becomes a limited factor at deeper depths). As a result, this area is highly susceptible to decay.  Since the outer two to three inches of the pole is where approximately 90% of the strength is located, it is vital to protect this part of the pole in order to preserve the pole's strength.

Preserving the durability of the pole plant can be accomplished by in-place treatment with remedial preservatives as part of a cyclical maintenance program.  In Bulletin 1730B-121, the Rural Utilities Service (RUS) recommends an 8 to 12 year cycle based on the decay conditions of the particular environment where the poles are installed.  Excluding remedial treatments from a pole maintenance program leaves owners with an inspection only program.  This "run to failure" strategy can have significant long-term negative impacts on our natural resources, skilled manpower, financial resources, and it increases the pole owner's risk with regard to safety and reliability.

Groundline Preservatives
Externally applied preservatives vary greatly in their chemical make-up, environmental profile, efficacy, penetration into a pole, and their ability to remain in the treatment zone so as to control decay for an extended period of time. Selecting an appropriate remedial treatment strategy can save pole owner's millions of dollars by reducing the number of pole change-outs and reducing the risks associated with pole failures.

An effective external groundline treatment must have the following characteristics: 
  1. The ability of one of the active ingredients to penetrate the outer two to three inches of the pole, at or above threshold levels.  Additional active ingredients can provide increased protection at or near the pole's surface.  Note: The threshold level is the amount of preservative that must be present in order to control decay. 
  2. The active ingredients must display an ability to remain in the designated treatment zone at levels capable of controlling decay and for periods of time consistent with remedial treatment cycles.
  3. The active ingredients must be able to control both soft rot and brown rot decay fungi.
  4. Remedial preservatives with multiple active biocides are preferred as they can provide a broader spectrum of protection against wood destroying organisms.
Active ingredients most commonly used in remedial groundline treatments are various types of copper compounds (such as micronized copper carbonate and copper hydroxide) and various types of borates.  Groundline treatments vary in the way that they are applied to the pole.  Some are available as a brush-on paste application which is then covered with a polyethylene-backed moisture barrier to encourage inward migration of the active ingredients.  Others are available as ready-to-use wrap applications - paste wraps, liquid wraps, and dry wraps.  These wraps include the preservative and the moisture barrier combined.  Preservative systems can incorporate multiple active ingredients that serve different purposes.  In these formulations, one active ingredient, such as boron, typically penetrates deeply while the other active ingredient, such as copper, typically stays closer to the surface.    Preservative systems are found to be much more effective than copper naphthenate alone because of copper naphthenate's limited ability to penetrate wood at threshold levels in a topical treatment. 

Preservative Pastes 
The majority of commercially available pastes are water-bourne and do not use petroleum solvents or carriers.  The consistency of paste makes it easy to apply and helps ensure 100% coverage of the pole surface in the treatment zone with no wasted product and no slumping (the process of the product succumbing to gravity rather than remaining fixed in the treatment zone). 

Preservative Bandages
Bandages, though more expensive than pastes, are viewed by some as easier or less-time consuming to apply, particularly dry bandages.  However, with bandages there is the potential for incomplete coverage of the targeted pole surface due to obstructions or irregular surface resulting from decay removal.  A liquid bandage may be subjected to unequal distribution of material because of the effect of gravity (slumping).  The seaming on segmented bandages can often limit coverage to less than 80% of the treatment zone and excess overlap can cause the preservative to be directed away from the pole.

Investing time to understand what remedial treatment options are available can have long-term financial benefits.  It can also limit environmental exposure and other risks associated with pole ownership.  Pole owner's Engineering, Standards, Risk Management, and Environmental Departments are strongly encouraged to investigate remedial treatment options as they plan their maintenance and sustainability strategies.  Choosing a preservative that will cover 100% of the treatment zone and penetrate and persist into the desired pole depths is paramount to effectively preserving the useful service life of wood poles.  A preservative that can accomplish those objectives and do so with a low toxicity profile and low exposure risk is the ideal choice.