Wood is by far the best structural material in terms of environmental impact, as shown by a number of LCA studies. This environmental impact superiority can be improved if the longevity of the commodity is extended. The main durability concern in large cross-section treated materials is the susceptibility of the heartwood to decay, as it is not typically treatable using traditional methods. This can lead to premature failure, for example of Douglas-fir, pine and spruce poles, oak and hickory crossties (sleepers) and pine bridge ties. Traditionally these issues have been managed by high cost remedial treatments, or simply by accepting the short service life and the required product replacement costs. Research over many decades has led to the growing commercial practice of dual treatment. Dual treatment is defined here as combining two (dual) preservative systems, a water-diffusible system and an oil-borne pressure-impregnated system being applied to the same piece of wood. This is often done as separate treatments at different times (‘two-step’); in this case it is essentially the same as using an oil-borne preservative with a remedial diffusible treatment (typically a sodium borate salt), but has the potential benefit of preventing decay before it happens. Adding borates to railroad crossties prior to creosote treatment has been shown to provide significant benefit to crossties (Amburgey et al. 2003, Anon. 2010) and utility poles (Dickinson and Murphy 1991). Currently, more than 50 % of the 22 million wood ties produced annually in the United States are dual treated with an initial diffusible borate dip treatment. The multi-billion-dollar potential savings possible with this approach, especially in the high-hazard southeast of the USA, have been documented (ZetaTech 2011). The inclusion of borate has also been used to justify reducing typical copper naphthenate or creosote retention specifications from 0.96 kg·m-3 as copper and 128 kg·m-3 to 0.64 kg·m-3 as copper and 96 kg·m-3 respectively This means that the cost of the borate treatment is more than paid for with the copper naphthenate or creosote reduction (savings). Treatment of bridge timbers has now also been commercialized using a further modification of this approach, with even higher unit cost savings (Lloyd et al. 2017) and commercial testing is now underway in utility poles.

Download document (1 Mb)
free for the members of IRG. Available if purchased.

Purchase this document