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Wooden bridges have been an important part of road and rail networks in New Zealand. While wooden structures have largely been replaced by concrete and steel on major arteries, they still have a place where lightweight, easily assembled structures are needed. These timber bridges are also an alternative to other materials in roads which carry relatively low traffic loads. In the last 10 years, there has been a resurgence in the installation of engineer-designed wooden structures. Contrary to the traditional method of construction, these prefabricated components are manufactured in a factory and brought to the job site where they can be assembled quickly with a minimum of cutting and drilling. In this document, we summarise the history and use of timber bridges in New Zealand. Most of the older bridges were constructed using Australian hardwood and native durable New Zealand timber. In the 1950s-1960s, preservative-treated timber particularly glue-laminated bridges were built. Currently, few bridges are built entirely from wood. However, in many smaller bridges, the superstructure and decking are built using treated wood. Using some specific case studies based on our historical service test data, we looked at the durability and maintenance requirements of wooden bridges in New Zealand. [ABSTRACT FROM AUTHOR]

The quest to understand remedial treatment for timber framing began following the 'leaky building crisis' in New Zealand which came to prominence in late 1990s. Several in situ remedial treatment products are used in New Zealand. However, effectiveness of these treatments is still unclear, particularly in situations where not all faces of the timber can be accessed. This research was conducted to establish whether a 'double coat brush-on' or 'double coat brush-on plus injection treatment between studs' system would give the most surface coverage and retention of a commonly used remedial treatment chemical, boron. Results showed that in the 'double coat brush-on' process, the concealed surfaces were left largely untreated resulting in variable preservative retention between components and relatively low overall preservative retention in the multiple stud units. However, the 'brush-on plus injection treatment' application gave much better preservative spread onto concealed surfaces of both vertical and horizontal members. Most samples tested achieved average cross-section boron retention levels of 0.4 % BAE m/m (NZS 3640, H1.2 treatment specification), although the treatment was concentrated in the outer 30 % of the timber, hence not achieving the required full sapwood penetration. Because of the variability associated with the boron injection process, it is recommended that this remediation method should only be used where there is a high degree of confidence that there is no decay present between studs or lintel members. All attempts should be made to remove any decaying timber while repairing leaky buildings. [ABSTRACT FROM AUTHOR]

Traditional benchmark wood durability testing methods such as stake tests take many years to give conclusive results, and in-ground tests do not always indicate the efficacy of preservatives in above-ground situations. To find test methods that would shorten the time required for wood evaluation for above-ground end uses, a series of different types of accelerated durability tests were set up. Five types of test: ground proximity, two types of decking, flat panels and double layer, were reassessed after ten years to determine whether the decay rankings given to the various types of preservative had changed over the extended exposure period. Exposure conditions varied between tests, with ground proximity being close to ground, and the double layer test carried out in very wet conditions, while raised decking and flat panel tests were relatively dry. In all of these tests, the preservative retention was 25% of the normal H3 retention. The results indicated that the ground proximity tests gave the fastest and most reliable results. Flat panels contained the next highest decay rates, followed by ground-level decking, double layer and raised decking. The evaluation and comparison of these five test methods after ten years' field exposure confirmed the trend and relative decay rate that was observed at four-year exposure. The use of a regression model for prediction showed a statistically significant overall relationship between decay scores in 2011 and 2017 (coefficient = 0.14 ± 0.07, d.f. = 345.7, t = 2.038, p = 0.042). When resistance to decay was compared between preservatives, copper-chrome arsenate (CCA)-treated pine and naturally durable spotted gum samples were in better conditions than pine treated with any of the other preservatives. [ABSTRACT FROM AUTHOR]