This study conducted a final inspection of the in-ground natural durability field trial at five sites in Australia. In 1968-69, 77 timber species were installed.
The best performing timber at each test site was CCA-treated Pinus radiata. All but one replicate was still serviceable after 33 to 36 years exposure. The next best performing timber at each test site was creosote treated P. radiata.
Timber species with greatest natural durability in the outer heartwood were raspberry jam (Acacia acuminata), red box (Eucalyptus polyanthemos),wandoo (E. wandoo), tallowwood (E. microcorys), bull oak (Allocasuarina luehmannii), grey ironbark (E. paniculata), yellow box (E. melliodora), Gympie messmate (E. cloeziana), grey box (E. moluccana) and white mahogany (E. acmenoides).
Reference Number:
PN04.1004
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Executive Summary
Objective
The objective of this study was to conduct a final inspection of the in-ground natural durability field trial exposed at five sites in Australia. Originally, 77 timber species were installed.
Key Results
- The best performing timber at each test site was CCA-treated Pinus radiata. All but one replicate (from Innisfail) was still serviceable after 33 to 36 years exposure. The test stakes had been treated to a retention of 12 kg CCA salt/m3.
- The next best performing timber at each test site was K.55 creosote treated P. radiata. The test stakes had been treated to a retention of 175-210 kg K.55 creosote/m3.
- Timber species with greatest natural durability in the outer heartwood were raspberry jam (Acacia acuminata), red box (Eucalyptus polyanthemos), wandoo (E. wandoo), tallowwood (E. microcorys), bull oak (Allocasuarina luehmannii), grey ironbark (E. paniculata), yellow box (E. melliodora), Gympie messmate (E. cloeziana), grey box (E. moluccana) and white mahogany (E. acmenoides).Based upon these results, revised natural durability ratings in Australian Standard 5604 are suggested for some species.
Application of Results
The trial demonstrates that CCA and creosote treated timbers can perform better than the best of the naturally durable timbers. Nevertheless, many untreated timbers will provide useful service lives, and this trial provides the best comparison available in Australia of 77 species. Based on these results, the natural durability ratings given in Australian Standard 5604 should be revised for some species. Future research that aims to produce an organic wood preservative would do well to examine the extractives of the most naturally durable timber species identified in this project.
INTRODUCTION
In 1968-69, Australia’s largest in-ground natural durability trial was established over a range of climatic conditions at five test sites. The progress of this trial has been reported in a series of articles. The latest inspection before this report was in 1999 after 31 years exposure. Interest in natural durability is growing, partly due to concerns about the chemicals used to make nondurable timbers durable. An Australian Standard devoted to the subject was produced in 2003. It broadens the traditional information available on in-ground natural durability, to other end uses such as termite resistance indoors (H2) and outside above ground natural durability (H3). Plantation species with higher natural durability generally attract higher prices, and may be easier to treat according to Australian Standards when only the sapwood needs to be penetrated due to the presence of naturally durable heartwood. Some of the species examined in this report are no longer available in commercial quantities. However, the search for more environmentally friendly organic wood preservatives also benefits from a greater understanding of those timbers found by evolution to be durable and the chemicals that confer that durability. The in-ground natural durability trial was removed from exposure after 33-36 years, as some sites were to be redeveloped or closed. This report provides the final inspection results.
RECOMMENDATIONS AND CONCLUSIONS
After 33-36 years of exposure in ground, the best performing timbers were preservative treated. CCAtreated Pinus radiata performed best, with all but one replicate still serviceable at the five test sites. The next best performing timber was K.55 creosote treated P. radiata. The best performing naturally durable timbers were raspberry jam (Acacia acuminata), red box (Eucalyptus polyanthemos), wandoo (E. wandoo), tallowwood (E. microcorys), bull oak (Allocasuarina luehmannii), grey ironbark (E. paniculata), yellow box (E. melliodora), Gympie messmate (E. cloeziana), grey box (E. moluccana) and white mahogany (E. acmenoides). The in-ground natural durability ratings given in AS 5604 for some species should be reconsidered based on these results.
The surviving test stakes are currently stored at the Clayton laboratory. While this trial has ended, future work should include the chemical analysis of the CCA-treated P. radiata stakes, to determine the extent of leaching.
RESULTS AND DISCUSSION
The median ratings for each timber species at each of the five test sites after more than 33 years exposure are shown in Table 1. During inspections, test stakes were rated separately for decay and termite attack on a scale of 8-0, where 8 is sound and 0 destroyed. However, in Table 1, the worst rating from either biodeteriogen was used in the calculation of the median. The specimen life for each replicate stake was taken as midway between the inspection when the specimen first rated 3, and the previous inspection (when it rated 4 or higher).
