The first demo case “Firewood shed” based on wood-plastic composites produced from construction and demolition (C&D) wood and mineral wool waste and recycled plastic has been constructed and installed in the forest area at the Evo national park in South of Finland. This demo, prepared commonly by HISER Partners, is the very first public structural construction ever built from WPCs in Finland. 

The Forestry Department of Finnish State “Metsähallitus” ordered a new firewood shed in order to improve storage conditions of firewood. KS Laatuenergia Ltd collected and supplied C&D wood and mineral wool fractions and Tiihonen Ismo processed those fractions to the suitable form needed for the manufacturing of WPCs. Conenor Ltd. was the responsible partner for developing optimal WPC-material formulations and producing the WPC panels and blanks needed in the shed. The shed design was carried-out by company Raseko. VTT supported the development work by analyzing the mechanical properties of the WPC-materials based on different material formulations.

Unsorted and pre-crushed C&D wood fraction including clean wood, painted wood and particle and medium density fibre board pieces (size below 80 mm), were cleaned with magnetic and air separators and refined to a particle size below 4 mm by the use of a hammer mill in two processing steps. For the C&D stone wool fraction only one cleaning and refining step was enough to achieve the sufficient purity level and particle size requirement. The processed wood and stone wool fractions and plastics were mixed to agglomerates (Figure 1) by a high speed mixer. Recycled high-density polyethylene (HDPE) was used in the production of roof panels and a mixture of recycled and virgin polypropylene (PP) in the production of planks. Additionally, selected processing additives were dosed in the mixing phase. The agglomerates were extruded into panels and planks (Figure 1) using a conical rotory Conex®-extruder.

Figure 1. Agglomerates (upper), roof panel (middle) and planks (lower).

The following cross-sections of the three different WPC-components were used:

• Solid roof panels 400 x 10 mm (no additional roof cover material was used in shed)
• Hollow wall planks 120 x 28 mm
• Solid structural planks 120 x 30 mm

Prior to delivery, 2 or 3 or 4 structural planks were fixed together at the Conenor site by screws and bolts chosen based on the needed load bearing capacity of supporting element (Figure 2).

 Figure 2. Supporting elements based on 2 or 3 or 4 structural planks.

Installation work was professionally carried out by personnel of Metsähallitus and supported by Tiihonen and Conenor, although all partners and stakeholders involved lacked previous experience working with these materials in structural constructions. Metsähallitus highly welcomed the use of the recycled materials in their facilities and was pleased with the outcome as the appealing aesthetics of the structure fits well in the nature. The dark grey color was chosen to be similar as found in deadwood in Finnish forest. The installed firewood shed is presented in Figure 3.

Figure 3. Installed firewood shed at the Evo national park.

The demonstration proves that the WPCs made from recycled materials are very attractive alternative materials suitable in small structures e.g. sheds, shelters, storages and other utilities in remote forest areas  and thus promoting the circular economy and reducing the CO2 footprint.