In-service wood preservative test on the Howe truss railway bridge, Waikino

Some of the major components of a Howe truss bridge, built from durable Australian hardwoods, were treated with preservatives applied to the surface and inserted into holes bored into the components. The preservatives included two copper naphthenate formulations, two formulations based on 2(thiocyanomethylthio) benzothiazeole (TCMTB), two boron-based products and an oxine-copper formulation. The application methods, preservative characteristics and preservative costs are described. Adequate surface retentions were achieved with all chemicals although two coats were required for the oxine-copper and one of the copper naphthenate formulations. Although there appeared to be significant differences in retentions and costs the relative benefit of the products will only be determined by a long-term monitoring programme.