Fire Retardant Treated Wood can improve the reaction to fire performance of wood products to the extent that they can meet the requirements of Class B according to the European system, which is the highest possible fire class for combustible construction products. This means that greater use can be made of wood in visible positions in buildings, both as a cladding for interior walls and ceilings and as an external cladding, e.g. for building facades. However, the long-term fire performance of the treated wood products needs to be verified, particularly if used outdoors. In addition, inspection systems to ensure correct practical use at construction sites are needed.
Fire retardant treated wood
The combustibility of wood can be influenced by chemical means using substances known as fire retardants: substances that affect properties of the wood such as time to ignition, the spread of flame and heat and smoke evolution. Fire retardants play a part mainly in the early stages of a fire, i.e. up until flashover in, for example, a room. The time to flashover can be extended and, in certain cases, flashover can be totally prevented. Once a fire is fully developed, the presence of fire retardants plays little part. Wood cannot be made totally non-combustible: in a sufficiently fierce fire, even fire retardant treated wood may burn.
It is relatively easy to achieve a high fire performance class of wood materials using fire retardants: the difficulty lies in at the same time retaining the other good properties of the wood. Large quantities of fire retardant are generally needed in order to achieve a required level of fire protection, which can affect the other properties of the wood. The most important of these is its long-term durability.
Long-term durability of the fire properties
There are two cases that affect the long-term durability:
- The risk of exposure to high moisture ratios and precipitation of fire-retardant chemicals on the surface of the product, which can also occur in indoor-use applications.
- The risk of degraded fire performance properties due to leaching of the fire-retardant chemicals. This risk is greatest in outdoor application, and presents the most severe challenge to development of new fire retardant wood products for outdoor use.
A new European technical specification, CEN/TS 15912, defining the durability of the fire performance (DRF) in end-use applications, has been developed in order to provide potential users with guidance for choosing suitable fire retardant treated wood products and to encourage manufacturers to produce competitive products: see Table 1.
Fire-retardant treated wood products with documented properties
Many different types of fire-retardant treated wood are on the market, but properties can vary widely from one manufacturer to another. Products can be roughly divided into two categories:
- Industrially manufactured products, which may be impregnated or surface-treated
- Products intended for surface treatment at construction sites.
Four types of improved fire performance can be obtained for wood products:
- Higher reaction to fire class, e.g. Class B-s1,d0.
- Cladding with fire protection ability Class K210/B-s1,d0.
- Multi-storey facades after testing in accordance with the Swedish façade fire test SP Fire 105.
- Higher fire resistance, e.g. REI 30 of wall and floor elements (mainly through surface treatment with intumescent paints).
Details of fire-retardant treated wood products with documented properties can be found on www.brandskyddattra.info, which is updated regularly and includes translations to several languages. The site presents wood products having both fire performance class and durability class for end-use applications, as shown in Table 2. At the time of writing (summer 2013) only three products have been registered, of which one has complete documentation.
Outdoor application (durability class DRF EXT) normally requires ordinary painting on top, intended to work in conjunction with the fire-retardant-treated wood product. Products to be presented on the website must be supported by test documentation for:
- Fire class (in accordance with EN, IMO and/or SP Fire 105)
- Durability class (in accordance with CEN/TS 15912 or NT Fire 054) which include testing of:
– Moisture resistance
– Outdoor durability
In addition, information on CE-marking, environmental aspects, application etc. are desirable: see Table 2.
Inspection systems for practical use in buildings
Inspection systems for these products are defined in product standards for those cases for which CE-marking of fire retardant treated product s is possible, or in associated technical approval documents, e.g. from Sitac, although such marking systems are often lacking. The industry association, NBT (Nordiskt Brandskyddat Trä/Nordic Fire Retardant Wood), has therefore developed appropriate inspection systems for determining the fire performance characteristics of fire-retardant treated wood products. The NBT system includes:
- Industrially manufactured products should be subjected to an inspection regime showing that the products comply with the relevant fire performance class. A suitable inspection system for this is CE-marking, with the fire-retardant treated wood products required to fulfil the highest compliance requirements, which includes surveillance-monitored manufacturing quality control.
- Fire protection applied by surface treatment at the construction site must fulfil the same requirements as industrially manufactured products. However, the inspection systems are different, and the surface to receive the fire retardant treatment is very important. At present, fire protection by means of surface treatment is used primarily indoors. Any inspection system needs to ensure that the correct quantity of fire retardant is supplied in order to achieve the necessary fire performance class.
NBT proposes the following inspection controls at site:
- Fire-retardant paint (film-forming). Inspection and measurement performed at site, with the thickness of the fire retardant paint determined by drilling a small hole in the paint layer.
- Fire-retardant liquids (non-film-forming). Inspection performed by sampling at site, followed by laboratory testing. Two alternative methods can be used:
– Small-scale fire testing, e.g. in accordance with ISO 5660 (the cone calorimeter).
– Measurement of active substance. Chemical analysis of a surface sample to determine the content of active fire-retardant substances. This method assumes that the producer has stated the quantity of active substance needed to achieve the claimed fire performance class.
A suitable inspection system would be as in ETAG 028, which was published in June 2013.
Fire resistance properties can be tested as in accordance, for example, with ETAG 018-2 applied to wood building elements.
New guidelines for reaction to fire testing of wood products
Wood products have predictable and well-known fire behaviour properties. Products thicker than about 10 mm normally meet the requirements of Class D, but this can be affected by density, the presence of joints, the material behind the wood, airgaps behind the wood and surface profiles. CWFT (Classification Without Further Testing) cannot be employed for fire-retardant treated wood products.
A European guidance document has been prepared to describe how fire-retardant treated wood products can be grouped and tested in a strategic manner in order to include as many products (e.g. types of wood) as possible. The guidance has been published as a Position Paper by the institutes that have been notified to the EU Commission for the Construction Products Directive. A main principle of the guidance is that one product must be comprehensively tested, with other variants, such as the type of wood, being tested by means of individual tests. If the results are compatible, several types of wood can be included in the classification. If not, complete fire testing is required.
Nordic Fire Retardant Wood
NBT – the Nordic Fire Retardant Wood association – was established in March 2011, replacing an earlier network which was active during the period 2008-2010. NBT’s objectives include:
- Disseminating information and knowledge of fire-retardant treated wood to interested parties on the market and to the public in the Nordic countries.
- Pressing for harmonisation of use and information on fire-retardant treated wood in the Nordic countries.
- Providing an impartial platform for the exchange of information and knowledge.
- Pressing for products, processes, suppliers and the use of fire-retardant treated wood to be quality-assured.
The Association’s members are producers, suppliers and resellers, as well as users (architects, contractors, fire consultants etc.). Membership is open to physical and legal persons who support the objective of the Association and its ethical roles. The Association’s secretariat is run by SP Wood Technology.
The Association’s website is www.brandskyddattra.info, from where information on fire-retardant treated wood, properties, application areas, building design fire protection requirements and approved products on the market can be obtained. The Association’s objective is to disseminate and upgrade information on, and knowledge of, fire-retardant treated wood so that parties such as architects and designers can identify and select products meeting their desired requirements. The Association also assists development of new quality management systems for both fire classes and new durability classes for fire performance in end-use applications, particularly for use outdoors and for on-site inspection systems.
a) Must be fulfilled by material produced by the same manufacturing process and with the same retention as for the achieved fire classification.
b) For INT 1 at (70 ± 5) % RH and (25 ± 2) °C and for INT 2 at (90 ± 5) % RH and (27 ± 2) °C.
c) Criteria for fire performance after weather exposure, as tested by the cone calorimeter method, ISO 5660.
d) Fire Class B products: (Rate of Heat Release) RHR30s ave ≤ 150 kW/m2 for 600 s testing and (Total Heat Release) THR600s ≤ 20 % increase in comparison to test results before weather exposure.
e) Fire Class C products: (Rate of Heat Release) RHR30s ave ≤ 220 kW/m2 for 600 s testing and (Total Heat Release) THR600s ≤ 20 % increase in comparison to test results before weather exposure.
f) DRF Class EXT is valid only for the specific type of surface treatment that has been tested.
Article published courtsey of SP Fire Technology, Norway.
