Institute for Transportation

In bridge timbers, wood decay is usually found where water has
accessed the end-grain surfaces. In preservative-treated members, end-grain surfaces are most likely to be those resulting from on-site framing cuts or borings. Because these at-risk surfaces are easy to see, it seems feasible to establish a program where diffusible preservatives are repetitively inserted into those critical areas spatially distributed in a grid and on a schedule that will ensure protection, thereby extending the life of the entire structure. The objective of this study was to determine the vertical and lateral distribution and the post-treatment behavior of injected and inserted borate preservatives in wood exposed to natural wetting in field exposure. During this 1- and 2-year exposure, rain wetting elevated the moisture content of the wood enough to support growth of decay fungi in wood not protected by borates. Point source treatments consisted of either borate solutions or fused borate rods that were injected or inserted, respectively, into predrilled holes. The longitudinal movement of borates applied as either glycol or aqueous solutions was generally greater than that occurring with treatment of borate rods only. Lateral distribution of borates was similar among treatments. In Southern Pine, differences in both vertical and longitudinal movement of borate from the insertion holes were associated with the type of closure used. Results indicate that borates can be included in a maintenance program consisting of time-sequenced treatment of critical regions of wood bridges that are at risk for internal decay. Grids for placement of point sources of diffusible borates in engineered wood structures could be developed on a wood-species-specific basis. Such treatments would complement
the exterior shell of protection provided by the original pressure
treatment and enhance long-term durability.