Accoya takes on the elements in the ultimate test of durability
When wood is to be used outdoors in contact with the ground or fresh water and exposed to prolonged wetting and temperature changes, its durability is essential to the long-term performance and safety of the project it is part of.
The modified wood brand Accoya is increasingly being specified for construction projects that call for a proven BS EN 335 Use Class 4 (UC4) durability performance. In this WPA blog, we take a look at what lies behind the ability of Accoya to take on the elements and earn its reputation as robust option for these applications.
Developed to perform
Accoya is the brand of acetylated wood.
Acetylation is the process used to enhance the properties of wood. The process reduces how much water the cells in the wood can absorb. It does this without adding toxic chemicals to the final product, but by boosting the concentration of ‘acetyl groups’ – which are already found naturally in trees and their wood.
More specifically, the process uses acetic anhydride to change hydroxyl groups (which like water) into acetyl groups (which don’t like water), resulting in improved performance characteristics without introducing harmful or environmentally unfriendly additives.
Altogether, this means the wood shrinks and swells much less when wet, and is much less susceptible to decay.
Accoya took the joinery industry by storm when hit the market in 2007, causing a renaissance in demand for wood windows and doors. However, the scientists behind its development always knew it had potential for use in more challenging exposure environments. As such, the underlying acetylated wood technology was subject to field exposure testing many years before the Accoya brand was launched.
Freshwater durability trials in the polders of Holland
Two long-term trials in the Flevopolder, near Almere in Holland, were installed in 1995 and 2000 to assess acetylated wood performance in canal-bank linings.
Accoya wood canal lining - H2H Workum, the Netherlands
The trials were established by independent wood durability specialists at the SHR institute in the Netherlands, and detailed inspections undertaken after 15 and 20 years. The inspections assessed visible decay, fungal degradation and material integrity, with particular attention to the waterline.
The exposure regime was particularly severe because the critical zone at the waterline combines three drivers of biological attack: high moisture availability, oxygen and biologically active soil or sediment. This creates a more aggressive environment than typical UC4 and above-ground Use Class 3 exterior applications and provides a rigorous test of sustained wetting durability.
Accoya wood canal lining
The test programme compared acetylated wood with untreated control material, preservative-impregnated wood and the naturally durable hardwood – the traditional material for such applications.
The SHR Institute concluded that, in fresh-water applications, acetylated wood can deliver durability comparable with highly durable hardwoods and extended service life preservative impregnated wood, supporting its use in UC4 application where long-term resistance to moisture-driven decay is required.
Marine applications – the ultimate durability test
Spurred on by the success of the UC4 durability test results the R&D technicians at Accoya turned their attention to Use Class 5 (UC5) marine applications. This is the ultimate test of durability where wood is exposed to salt water, tidal action, marine organisms and a higher risk of biodeterioration, including attack from marine borers.
Accoya wood fenders in the Thames Estuary
In 2020, the Environment Agency trialled an Accoya fender at Duke Shore Wharf in London as part of the Thames Estuary flood risk management asset programme. The requirement was for a material capable of maintaining dimensional stability, mechanical integrity and durability in a brackish, tidal environment, while also reducing reliance on tropical hardwood and avoiding chemicals or plastic-based additives that could compromise aquatic habitats.
Following two years of positive performance at Duke Shore Wharf, a further nine Accoya fenders were commissioned for Deptford Walls – near the Cutty Sark in Greenwich - and installed in September 2022. Subsequent annual inspections have indicated continued stability and durability, with the material retaining its intended function in service.
Accoya wood fenders in the Thames Estuary
The Thames Estuary project provides a severe real-world test environment, combining both saline and fresh-water influences, tidal movement, mechanical impact exposure, surface colonisation and active marine organisms. Performance in this environment is consistent with wider sea-trial data for Accoya, including test programmes conducted for up to 13 years in locations ranging from the Pacific Ocean to the North Sea.
Accoya offers designers UC4 material options
The very positive evidence from the UC5 fender installations reinforces the technical case for using Accoya in UC4 applications. By reducing moisture uptake at cell-wall level and resisting the biological mechanisms that drives the fungal decay of wood, Accoya offers architects and engineers a durable, dimensionally stable, low environmental impact wood option for ground-contact, fresh-water and other high-moisture infrastructure applications.
® Accoya is a registered trademark of Accsys Technologies PLC
Visit www.accoya.com for more information.