A long-term field trial was established in June 1996 to evaluate the performance of preservative-treated Pinus radiata in above-ground, weather-exposed decking applications, with particular emphasis on the role of preservative penetration into heartwood. The study, first documented by Gardner et al. (1997), compared treated and untreated heartwood and sapwood at two sites representing different decay hazards: Beerburrum, Queensland (Scheffer’s Index ≈ 42; moderate hazard), and Coopernook, New South Wales (Scheffer’s Index ≈ 65; high hazard).
Boards (1000 × 90 × 45 mm) were treated with one of four preservative systems: Protim 235WR (tri-n-butyl tin naphthenate and permethrin), Protim 70WR (copper naphthenate and permethrin), Tanalith O (CCA), and ACQ 2100 Type B (copper oxide and didecyldimethyl ammonium chloride). Decks were constructed in accordance with draft specifications of the Protocols for Assessment of Wood Preservatives (AWPC, 1997), with 12 boards per deck. To examine potential treatment vulnerabilities, half the boards were drilled before treatment and half after treatment. In addition, half of the decks were covered with shade cloth to moderate drying.
The Beerburrum site was destroyed by a bushfire in 2000. The Coopernook decks remained in service for 28 years and were harvested in early 2024. Individual boards were assessed and the decks reconstructed on 100 × 100 mm CCA (H4) treated pine bearers, incorporating both treated (H3) and untreated pine palings to encourage biological pressure. Decks were reinstalled at the Queensland Department of Primary Industries Maroochy Research Facility, Nambour, in August 2025.
Over time, dense vegetation grew through and around the decks at the Coopernook site due to minimal site maintenance, creating persistently wet conditions more severe than those prescribed for Hazard Class H3 exposures. These conditions likely increased decay hazard beyond above-ground expectations.
After 28 years, CCA-treated boards performed best, with 163 boards remaining, followed by ACQ (133 boards). Protim 70WR had only 15 surviving boards, while Protim 235WR (TBTN and permethrin) effectively failed, with only three boards remaining. For both ACQ and CCA, a higher proportion of surviving boards comprised 100% heartwood, suggesting that heartwood presence contributed to improved long-term performance, although heartwood penetration levels were unknown.
All surviving boards exhibited severe surface weathering. Although many remained biologically serviceable, aesthetic degradation and checking would likely necessitate replacement in practice. The long intervals between assessments highlight the challenges of modelling deterioration in multi-decade trials and emphasise the importance of sustained frameworks to support long-term durability research.