Utility poles are an integral part of critical grid infrastructure in every country across the globe. Most utility poles are still made from wood, pressure treated with a preservative such as Pentachlorophenol, CCA, Copper Naphthenate, ACQ, etc. to protect against rot, fungi, and insects.
Knowing the relative effectiveness of wood preservative or barrier system prior to setting pole specifications, is vital in maintaining a resilient grid. Basing pole specification decisions on the correct test method is critical, especially when other potential issues such as juvenile wood entering the supply chain can also impact pole strength.
System Hardening and Vegetation Management are topics discussed in the educational track with the same name at DISTRIBUTECH International, set for February 26-29, 2024 in Orlando, Florida.
Conditions for Decay
Conditions for wood decay are ideal at the critical ground line section of the pole. It is at this point that low-level decay can start in the outer “ground contact” part of the pole after only a few years in service on poles protected with partial protection systems such as wood preservatives or partial barrier sleeve systems.
Standard Decay Testing – Visual or Mass Loss Measurements
The vast majority of standard decay tests used by preservative manufacturers and pole producers, such as AWPA E7, Standard Method of Evaluating Wood Preservatives by Field Tests with Stakes, European Standard EN252: 2014, or DD ENV 807: 2001, rely on either visual assessment of the decay on test samples or mass loss measurement (removal of outer softened wood) to assess the impact of decay over time.
Limitations of Standard Decay Testing
Whilst indicative, these assessments can bear a limited relationship to the far more critical effect of decay on a pole’s strength over time. This is because different wood-destroying fungi attack different elements in the wood in different ways. For instance, common brown rot (Basidiomycota) destroys the carbohydrate elements of the wood but leaves the more rigid lignin of the cell walls intact.
In practice, this means that wood can appear to be in a relatively good condition, with perhaps some slight discoloration and a slight change in surface texture due to decay, and be firm to the touch, yet it can have lost significant strength as the fungi consume one element of the wood whilst leaving the more rigid supporting structure intact.
Modulus Of Rupture (MOR) Mechanical Testing
A far more accurate and relevant method for assessing the comparative effectiveness of wood preservatives and barrier products’ ability to maintain pole strength over time is to use modulus of rupture (MOR) mechanical testing. In this, test samples, which have been exposed to decaying organisms, are subjected to mechanical loading between two points until they break, giving a true measure of ultimate strength.
For example, the table below summarises the results of an independent UK utility-sponsored decay test. Whilst this test is not a standard test, it is far more relevant for utilities as it measures the critical loss of strength due to decay. In this test, a range of different wood preservatives along with partial and full ground line barrier sleeves have been tested. This accelerated test (equivalent to 40 years of service) included both visual and modulus of rupture testing, thus allowing an easy comparison of visual assessment and MOR strength test methods.
Summarised results of the APPEAL Test, showing Strength (MOR) after 48 months
| Preservative Treatment | Visual assessment after 48 months | Strength (MOR) after 48 months test (equivalent to 40yrs) | |
| Preservative Treated Only | Preservative Treated With Polesaver Sleeve | ||
| Creosote | No visual decay | 84% | 115% |
| RVP | No visual decay | 82% | 100% |
| TS | No visual decay | 76% | 99% |
As can be seen, the summary of the visual assessment of the samples treated with wood preservative was positive with no decay observed. Yet when these same samples were subjected to MOR testing, they all showed significant loss of strength due to (invisible) low-level decay.
By contrast, the test samples protected with full barrier sleeves gave excellent results on the MOR test with no loss of strength after a correlated 40 years in service, clearly outstripping the performance of all the wood preservatives by a good margin, making this kind of technology a contender for government grid hardening funding.
In Conclusion
From the above, it is clear that relying on visual decay assessment of decay either on test or in service is not always a reliable method for evaluating pole strength or different treatment or protection options, especially when grid hardening is a high priority and should be taken into account when setting wooden pole standards.
[1] https://public.wmo.int/en/media/news/extreme-weather-new-norm
[2] Freedom Group – Network Innovation Allowance (NIA) – Project APPEAL. Environmentally Acceptable Wood Pole Pre-treatment Alternatives to Creosote: Review of Potential Products and Initial Efficacy Tests, Project Report 5 (of 6)
