In 1996 an extensive field trial program was set up in order to obtain more data on wood treated with chromium and arsenic free preservatives in Use Class 3 retention. Apart from durability in and above ground, other features were also assessed, e.g. discoloration by staining fungi, depletion of active ingredients, weathering and corrosion of fasteners in con¬tact with the treated wood. Wood specimens were treated according to NWPC class AB (UC3 retention) in commercial treating plants with the fol¬lowing type of preservatives: CCA (ref), ACQ, Copper-azole, Copper-betain, Copper-HDO and two metal-free formulations based on propiconazole and tebuconazole (+ cypermethrin), respectively. Wood treated with CCA in UC4 retention and heartwood of pine and larch were also included in the trial. In addition to testing according to EN 252 and ENV 12037 (lap-joint), testing was also carried out according to methods specifically designed for this trial, e.g. a mini-deck trial, “Johansson-lap joint” and a corrosion test. The results show that pine sapwood controls have failed in all set-ups. Pine heartwood reference has failed in ground and is severely decayed in the above ground tests. Also the two metal-free preservatives have failed in ground and are severely decayed above ground. Copper-betain performs slightly poorer than the other copper-organic preservatives. ACQ, Copper-azole and Copper-HDO preservatives in UC3 retention level performs similar to CCA in UC3 retention (NWPC Class AB) in the above ground set-ups. However, ACQ performs better than CCA Class AB in the lap-joint trial but equal in the mini-deck set-up, whereas Copper-azole performs poorer than CCA Class AB in the lap-joint trial but better in the mini-deck. Metal-free preservative-treated boards coated with an acrylic paint perform similar to Copper-organic preservative-treated boards concerning decay. The decay index values for all materials after 13 years above ground are in the same range as the decay index values after four years in-ground.

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