The present and future of wood products with improved fire performance

This section contains following topics:

1. Present situation

In many applications, ordinary wood cannot be used due to fire risks and restrictions in national building codes [1]. The main advantage of wood based products with improved fire performance is that they extend the usability of wood. High fire performance wood products can meet the requirements of class B in the European classification system for construction products, whereas ordinary wood products typically fall into class D.

At present, the level of development of wood products with improved fire performance is not high enough for their extensive utilization. The main problem is the long-term durability of fire retardant treatments in exterior applications where weather exposure may leach out the fire retardant chemicals. In interior use, the most important durability issue is the permanence of the aesthetic appearance that cannot always be predicted or guaranteed. Many traditional fire retardants are hygroscopic that might cause e.g. salt crystallization on surfaces also at interior applications [2].

Even though some wood products with improved fire performance exhibit excellent fire properties, examples of products with hardly any benefit compared to ordinary wood also exist. The main reason is the vast range of variation related to the manufacture of fire retarded wood products: the properties of wood can vary notably even within a species, e.g. the permeability that mainly influences the possibility to impregnate the wood. The product properties can therefore vary from a batch to another [3]. Another problem is the selection of an unsuitable product for a certain application.

The financial aspects of high fire performance wood products are greatly affected by general trends in the society and the resulting needs at present and in the future. The branch of activity is highly dependent on the regulations set by the authorities. If ordinary wood meets the requirements, or if there are no requirements for a certain application, wood with improved fire performance is rarely selected. In most cases, the additional costs outweigh the fire safety benefits.

2. Prospects - New possibilities

2.1. General trends

The needs to prevent fire risks and reduce the number of fire fatalities empower the market outlook of high fire performance wood products. Wood products with improved fire performance provide a continuous means to decrease the risk of initiation of a fire. This improves the fire safety especially for people with limited physical or mental capacity, e.g. the elderly and the handicapped.

In addition to societal fire safety aspects, increasing need to prepare for fires can be seen in the private sector. In this case, the willingness to improve the fire safety level is not based on the regulations but other needs, for example protecting the corporate image or reducing insurance premiums.

The current trend of fire safety systems is to proceed from prescriptive criteria towards performance-based approach. This development will probably facilitate the use of high fire performance wood products. Performance-based fire design increases the freedom of architects, designers and constructors to choose materials and structures, as far as the solutions meet the fire safety objectives defined for the application. Therefore, high fire performance wood products have equal opportunities with other construction products.

2.2. Market aspects

The fire performance of internal surfaces is regulated in most countries. The most stringent requirements are usually set for corridors and exits to ensure safe egress in case of fire. Extensive use of wood products as surface linings necessitates therefore a high-quality fire retardant treatment and offer market potential for high fire performance wood products, because wood is often preferred as a furnishing material for aesthetic reasons. Furthermore, the price of wall and ceiling linings is seldom the main issue in public premises and office buildings of large companies.

Another demanding market possibility for high fire performance wood products is external claddings, possibly combined with a wooden frame in three to five-storey buildings. This kind of buildings are increasingly allowable in many European countries as a result of performance-based fire codes. However, the durability of fire retardant wood has to be demonstrated for such a demanding application.

The currently available methods for improving the fire performance of wood require continuous development in order to maintain and expand their market share. New, innovative products might open whole new market areas. One of the new fire retardant solutions with a considerable market potential could be a combination of a fire retardant and a wood protective liquid. Such a product would be useful in practically any exterior application where the possible contribution to fire spread of wood must be taken into account.

To increase the efficient use of timber resources, forest products industry has developed technologies for converting low-quality wood into high-grade structural products, e.g. different types of wood-based panels and structural elements. These products are currently not much manufactured as fire retarded, but new solutions could be created by introducing the benefits of high fire performance. Such composite wood products are also much easier to treat than solid wood products.

Market prospects for high fire performance wood products can be found in regions where earthquakes are a considerable risk factor. Due to their flexibility, wooden structures tolerate vibrations caused by earthquakes notably better than concrete and other rigid materials. However, ordinary wood products can spread fire ignited as a result of an earthquake, thus increasing the damage. This risk can be reduced using wood products with improved fire properties.

2.3. New service classes for different end use applications

A new Nordic system with service class requirements for different end uses, as short term use, interior and exterior use has been developed in order to guide the potential users to find suitable FRT wood products [4, 5, 6, 7], see Table 1. It is intended to supplement existing requirements for the fire performance in national building codes. Suitable procedures for test verification and performance criteria are presented [8, 9]. The supplementary service classes are needed in order to form a basis for new and reliable wood products with improved fire performance.

Table 1. Draft requirements for service classes of FR wood products in different end uses.

A new Nordic labelling system has also been developed with symbols for the different service classes.

The new system has been developed upon Nordic initiatives from industry and research. It consists of both a control system for the durability properties of FR wood and experience of suitable test procedures. The system is based on existing systems in the UK and USA, see Table 2.

A Nordic project (NICe 04150) to agree and approve a standard for the service classification system is underway during 2005 [10]. It might result in some changes in terminology, requirements and labelling.

The service class system offers possibilities to introduce a quality mark for fire retardant wood products in addition to the basic fire properties. It has the potential of increasing the reliability of the products among different actors on the market.

Table 2. Types of FRT wood based on durability at humid and exterior conditions.

2.4. Control of other performance properties

Several other properties of FR wood products may be influenced and changed over time. Among them corrosion, mould growth, paint or glue adhesion and mechanical strength are probably most important.

The corrosion on metallic fasteners in FR wood may be enhanced since the fire retardants often are either acidic or alkaline, which both might cause corrosion on metals during the service life of the wood product [11].

Mould growth has been observed in several cases even at moderate relative humidities, but is seldom reported in literature [12]. The growth has mainly been associated with some fire retardants containing phosphorous and nitrogen and is in fact not surprising since both these substances are fertilisers and nutrition for micro-organisms. However, it is important to differentiate between non-destructive surface mould and true fungal decay of the wood material.

The adhesion to paint layers or glue lines may be influenced by the fire retardant treatments. This is usually solved during the product development.

Mechanical strength of FR wood may be decreased [13, 14] and special test methods have been developed in the USA [15, 16]. In Europe, FR wood is mainly used in applications where the mechanical strength is not critical.

It is most essential to be able overcome the negative effects of some fire retardants on other wood properties in order to increase the reliability and quality of fire retardant wood products.

3. Threats

High fire performance wood products in construction encounter two main problems related to legislation. Firstly, the division of construction products to combustible and non-combustible materials can prevail for a long time despite the fact that performance-based fire codes are becoming more common. Secondly, new regulations will be set for fire retardants, because the stipulations for the trade and use of chemicals are becoming stricter.

The trade of wood products with improved fire properties is to some extent vulnerable to fires in buildings where such products have been used, even though the ignition of fire and the damages were not directly related to wood. There are two reasons for this problem. Firstly, doubts are constantly presented on the safety and usability of wood products. Satisfactory fire safety properties of wood products are not considered possible because wood is known as a good fuel. Therefore, wood is not regarded as a suitable construction material for many applications subject to fire safety regulations. Secondly, the history has shown that inappropriate use of wooden structures and claddings can cause severe fire safety risks. Even though properly used high fire performance wood products increase the safety level against fires and in fire situations, problems can arise as results of inappropriate use and low-quality products.

For the determination of properties other than fire performance, only a few methods and standards exist for wood products with improved fire behaviour. The lack of generally approved European methods for determining the product properties hinder the trade of high fire performance wood products. If such methods are not developed, the growth of the area of business can be prevented or at least delayed.

The overall threat is that fire retardant wood will remain to be considered as an unreliable and non-high-quality construction product.