Dual borate and copper naphthenate treatment of bridge timbers – potential cost savings by various performance enhancements

Dual-treatment technology combining diffusible preservatives with oil-borne preservatives, widely used for crossties in the USA, has now also been commercialized with bridge ties/timbers. In order to understand the implications of these changes, the historic service life of creosote-treated bridge timbers in northern and southeastern USA were considered as well as field-test data for both creosote and copper naphthenate. These were used to estimate potential future service life. Estimates on life expectancy with added borates were also made from published data on performance. Cost-benefit analysis based on creosote and copper naphthenate costs as well as assumptions made from field-test efficacy data suggest cost savings of up to $20 per timber per year of additional service. Service life extension and the resulting cost savings could be achieved in a number of ways: change preservative from creosote to copper naphthenate; increase active ingredient retention; and/or add dual-treatment protection. A preservative change from creosote to copper naphthenate would be the simplest and lowest cost way of increasing service life of bridge timbers, with potential savings to both treater and railroad. An increase in copper retention could also give significant life extension, could be carried out at little additional cost and without increasing bleeding. The addition of borate to protect the heartwood also provides significant assumed increase bridge tie life, and can be used with either creosote or copper naphthenate treatments.