**Miha Humar 1,\*, Davor Kržišnik 1, Boštjan Lesar <sup>1</sup> and Christian Brischke <sup>2</sup>**


Received: 31 August 2019; Accepted: 10 October 2019; Published: 14 October 2019

**Abstract:** Wood is one of the most important construction materials, and its use in building applications has increased in recent decades. In order to enable even more extensive and reliable use of wood, we need to understand the factors affecting wood's service life. A new concept for characterizing the durability of wood-based materials and for predicting the service life of wood has recently been proposed, based on material-inherent protective properties, moisture performance, and the climateand design-induced exposure dose of wooden structures. This approach was validated on the decking of a model house in Ljubljana that was constructed in October 2013. The decay and moisture content of decking elements were regularly monitored. In addition, the resistance dose DRd, as the product of the critical dose Dcrit, and two factors taking into account the wetting ability of wood (kwa) and its inherent durability (kinh), were determined in the laboratory. DRd correlated well with the decay rates of the decking of the model house. Furthermore, the positive effect of thermal modification and water-repellent treatments on the outdoor performance of the examined materials was evident, as well as the synergistic effects between moisture performance and inherent durability.

**Keywords:** decay; decking; inherent durability; moisture performance; resistance model; service life

#### **1. Introduction**

Wood is one of the most important building materials. It is frequently used outdoors, where it is exposed to weathering and degradation. In Europe, wood-degrading fungi are the predominant reason for failures of wood used in outdoor applications [1,2]. Various solutions are used to prevent fungal decay and to achieve the desired service life, namely the use of biocides, wood modification, proper design and the use of domestic or imported durable wood species [3]. More recently, consumers are avoiding tropical wood species, so the importance of domestic wood species is increasing [4]. Unfortunately, the majority of European wood species do not provide a sufficiently high durability [5]. Particular emphasis is therefore placed on the utilization of domestic wood species [4].

Service life prediction of wooden objects is challenging, because the time during which a particular wooden structure will fulfil its function depends on a variety of factors, including the wood material used, the protection applied and various climate-related parameters [6]. In addition to the material-inherent durability, the moisture and temperature conditions inside the wood, i.e., the material climate, are the most important factors influencing the ability of fungi to decompose wood [2,7,8]. These two factors are influenced by the design of the construction, the exposure conditions and local climatic conditions (microclimate).

Building information modelling (BIM) software packages nowadays require information about service life and maintenance intervals for key materials used in the planning phase [9]. This information is required by the European Construction Products Regulations [10] and is needed for performance-based design [11]. The Eurocode [12] provides indicative design working lives of 10 years for temporary structures, 50 years for building structures and 100 years for monumental building structures and bridges. These values are set for objects regardless of the materials used. The expected service life of wooden structures under the given exposure conditions is a key parameter in the selection of materials for construction. Unfortunately, the current European standardization system provides neither information about expected service lives nor a methodology for the service life prediction of wood-based materials and components.

A new concept for characterizing the durability of wood-based materials and for predicting the service life of wood was recently proposed by Meyer-Veltrup et al. [13,14], based on the material-inherent protective properties, the moisture performance and the climate- and design-induced exposure dose of wooden structures. This approach has been successfully applied to untreated wood [15–17]. In this study, this modelling approach will be expanded to variously modified and preservative-treated woods.

#### **2. Materials and Methods**

#### *2.1. Materials*

This study investigated the performance of 19 different wood species and wood-based materials used in a decking application (Table 1). The selected materials were eight untreated wood species, Norway spruce (*Picea abies* (L.) H. Karst), European larch heartwood (*Larix decidua* Mill.), European beech (*Fagus sylvatica* L.), European ash (*Fraxinus excelsior* L.), Scots pine heartwood and sapwood (*Pinus sylvestris* L.), sweet chestnut (*Castanea sativa* Mill.) and European oak heartwood (*Quercus* sp.), and 11 materials that had been treated or modified in different ways. Although the authors are aware that not all these materials are traditionally used in decking, we have included them as reference wood species. In addition, the objective of this paper was not only to determine the performance of decking per se, but to validate the model [13]. In order to address this objective, materials of various durability have to be investigated.

Thermal modification (TM) was performed according to a commercial process (Silvapro®, Silvaprodukt, Ljubljana, Slovenia), with an initial vacuum in the first step of the treatment [18,19]. The modification was performed for 3 h at the target temperature (ranging between 210 ◦C and 230 ◦C, depending on the wood species). Impregnation was performed with a commercial copper–ethanolamine solution Silvanolin® (Silvaprodukt, Ljubljana, Slovenia), which consisted of copper, ethanolamine, boric acid and quaternary ammonium compounds [20]. The concentration of active ingredients and consequent retention met use class 3 (UC 3) [21] requirements. Impregnation was performed according to the full cell process in a laboratory impregnation setup. It consisted of 30 min vacuum (80 kPa), 180 min pressure (1 MPa) and 20 min vacuum (80 kPa). The same procedure was applied for the impregnation of wood with 5% commercially available natural wax dispersion with a solid content up to 50% by weight (Montax 50, Romonta, Germany) [22]. The acrylic surface coating Silvanol® Lazura B (Silvaprodukt, Ljubljana, Slovenia) was manually applied on the wood by brushing in two layers, with a 24-h drying time between them.


Nineteen different investigated wood species and wood-based materials.

> **Table 1.**
