Photos courtesy of ProWood (left) and ICC (right)

A fire test using ASTM E-119/UL 263 at the end of 2 hours is reviewed against the test criteria to achieve a fire-resistance rating. For ASTM E84 testing, a Steiner Tunnel is used (shown on the right).

Construction Fire Ratings

In order to determine the fire-resistance ratings of materials and assemblies used in all construction types, independent fire testing is required based on accepted standards. There are two primary types of tests that the codes establish criteria for in order to be considered code-compliant.

• Materials or Assemblies: The most often cited test is ASTM E-119/UL 263, Standard Test Methods for Fire Tests of Building Construction and Materials. These test methods are intended to evaluate the duration that tested specimens contain a fire, retain their structural integrity, or exhibit both properties during a predetermined test exposure. An independent testing laboratory uses a specified assembly or component (i.e. wall, floor, ceiling, beam, column) and secures it to a testing furnace. A fire is started according to the testing protocols with a time clock recording the condition at the desired times (i.e., 1-hour, 2-hour, etc.). To pass the test, the wall cannot have burned through the tested surface, cannot exceed a specified increase in temperature (250 degrees F) on the exterior side, and cannot fail structurally due to the load being applied.
• Surface Effects: The codes are also concerned with the effects of fire on the surfaces of materials. ASTM E-84/ UL 723 Standard Test Method for Surface Burning Characteristics of Building Materials documents two fundamental things: First is to measure a flame spread index (FSI) using the required Steiner Tunnel test method. In order to meet code requirements, the product must be exposed to testing for 10 minutes with an FSI of 25 or less. Secondly, smoke development is measured at the 10-minute mark of the test. This uses a photometer to measure the concentration of smoke the sample emits as it burns. A smoke development index of 450 or less is required by the code for fire-retardant-treated wood.

Note that the test sample in this case is 2 feet wide x 24 feet long while the tunnel width is 17 inches so the edges of the sample sit on 3.5-inch-wide concrete. Metal batten strips are used perpendicular to the length on the top side of the sample to connect pieces of lumber or plywood that contain a 1/8-inch longitudinal gap down the center length of the sample. Each material and species combination is tested separately. In order to demonstrate a passing conformance, 2 of 3 tests must pass. The third test can be waived if the first two pass with consistent results.

• Extended Test: Flame Spread Index and Smoke Development are both first calculated at the initial 10-minute marks as noted above, but then the test needs to continue for an additional 20 minutes. In this extended 30-minute ASTM E84 test, the Flame Front is measured which is defined as the furthest distance the flame spreads across the surface of the sample. To comply with the code, the flame front cannot progress more than 10.5 feet at the 30-minute mark. The initial burner flame is 4.5 feet so that flame cannot extend more than an additional 6 feet at the 30-minute mark. (IBC 2303.2 Fire-Retardant-Treated Wood). The 30-minute E84 test is just one of many tests required in AC66 (Acceptance Criteria for Fire-Retardant-Treated Wood) in order to obtain an ICC-ES Code Compliant Report. Other testing evaluates strength properties and hygroscopicity.

Note that there is a new update in the 2021 version of the IBC related to ASTM E84/|UL 723 testing. It involves a change to the tests for plywood by requiring a horizontal rip cut that must be exposed during the test. For pressure-treated FRTW products, that should not be a problem since the treatment impregnates the wood. However, topical treatments are on the surface only and may not have the capability to meet this additional testing requirement–meaning it is not deemed as safe as the pressure-treated method.

In order to be code-compliant, construction materials, including FRTW, and assemblies must be tested and shown to meet minimum levels of performance. For example, UL has different category designations with the highest being FR-S, signifying minimal flame spread and smoke development. The tested products must then be labeled according to their level of performance. It is incumbent upon the manufacturer to ensure their products meet the testing requirements and then to label their products appropriately according to the requirements of IBC 2303.2.4 – Labeling.

Image courtesy of ProWood

Wood building products are inherently more environmentally friendly than concrete or steel.

UNDERSTANDING FIRE-RETARDANT-TREATED WOOD

Low-rise construction is often based on wood framing and sheathing, including residential, light commercial, and mixed-use buildings. This is due to several factors. First, it is often the most economical choice that is easy to work with using well-known carpentry tools and methods. Second, wood is still one of the most environmentally friendly and energy/carbon-saving materials in construction. Finally, updates to the International Building Code (IBC) have recognized that wood members with larger mass or with fire-retardant treatment can be as safe or even safer than steel framing, which can lose its integrity during fires.

In light of all of the above, fire-retardant pressure-treated wood (FRTW) in the form of dimensional lumber and plywood or sheathing are now readily available which are code-compliant solutions for exterior and interior bearing walls, floor, and roofing assemblies. More specifically, FRTW is wood impregnated with chemicals during manufacture that has been tested under ASTM E-119/UL 263 and ASTM E84/UL 723 and meets the code required performance. While still considered a combustible material, code-acceptable FRTW can be used for 1-hour and 2-hour assemblies, does not support combustion, and its burning rate is appropriately limited when the flame is applied.

The greatest advantage of FRTW is its accepted use in a wide variety of building and construction types. It is permitted for use in types III, IV, and V construction for load-bearing walls, roofs, and floors within the specific parameters of the codes. It can also be used for non-structural items in Type I and II construction such as certain non-bearing partitions and walls plus most roof construction including girders, trusses, framing, decking, and some balconies, porches, decks, and exterior stairways.

Wood and the Environment

Wood is a natural, renewable resource that can be managed and harvested in a sustainable and responsible manner. That means it is a desirable and logical choice for construction whenever it can be used, particularly since it is often more economical than other alternatives such as steel and concrete. In the broader environmental sense, it has a much lower carbon footprint than steel or concrete construction. The natural process of photosynthesis while a tree is alive and growing is based on absorbing carbon dioxide from the atmosphere and storing (sequestering) it in the tree. As such, wood is considered by many to be the only truly carbon-neutral construction material in use. Steel and concrete require considerable amounts of fossil-fuel-based energy to extract, process, manufacture, and transport thus resulting in very high carbon footprints (i.e., very high levels of embodied carbon) for those building products. This has led to the growing popularity of wood construction in buildings.

Wood is also seen as a healthy and natural material, free from the concerns of other building materials that are synthetic or can contain harmful substances. In the case of FRTW, some manufacturers provide certifications related to green or sustainable standards, such as UL Greenguard Gold for low chemical emissions. Furthermore, wood does not conduct electricity, so it has no electrostatic charge associated with it. It also conducts less heat compared to concrete, steel, and aluminum, thus helping reduce energy consumption in buildings if designed into assemblies properly.

Producing FRTW

Fire-retardant treated wood is first and foremost a wood product with all the features and benefits of other wood construction products. Hence, the process of creating FRTW starts with obtaining untreated wood (dimensional lumber or plywood), typically made from a variety of Spruce, Pine, or Fir species. Note, however, that material and species cannot be mixed in a single treatment procedure. This wood then undergoes three primary steps, as follows.

• Treatment of Products: The untreated wood is impregnated with a blend of phosphate powder (the salt of phosphorus), boric acid powder, and water (for dilution). These are chosen because phosphorous is a common fire retardant used in other applications, too. When subjected to fire, the carbon dioxide reacts with these chemicals to create a char on the wood which insulates and suppresses the spread of fire and the development of smoke. When the treatment of the wood is done using a pressure process (Per IBC 2303.2.1), it must be performed in closed vessels under pressures not less than 50 pounds per square inch gauge (psig) (345 kPa). Remember that topical applications are also used by some manufacturers instead of the pressure treated process, but they don’t produce the same fire-retardant results. Therefore, such topical or coated products should be investigated for appropriateness and code compliance before specifying. Either way, fire-treated wood often contains a red/orange tinted dye for easy identification so it can be distinguished from non-treated wood on a construction jobsite. Certificates of Treatment are often issued for specific lots of material upon request or if specified.

It is typical for manufacturers of FRTW to incorporate a strict quality control process during all steps of the treatment. This can include a solution analysis of the mixed chemicals based on samples taken directly from mixing tanks and analyzed by a hydrometer. In some cases, chemicals to inhibit mold growth are added, so those are subjected to quality control as well. After treatment, lumber is drilled to verify the impregnation of the treatment based on 1/8-inch depth penetration and application of a two-part phosphorous indicator. Finally, documentation of quality control findings is recorded on a batch-by-batch basis for third-party inspection when requested.

• Kiln Drying After Treatment: The treated wood is moved from the pressure vessels to a kiln to be dried. It must be stickered to track each batch prior to entering the kiln and de-stickered after drying. Fire-retardant lumber must achieve a specified moisture content after treatment per the code. For lumber, this moisture content is not to exceed 19 percent, and for structural wood sheathing (plywood) not to exceed 15 percent. (IBC 2303.2.8) Spacers are placed between the wood to maximize airflow and proper drying. Fire-treated wood remains in the kiln until the specified moisture content is achieved.
• Protective Storage and Handling: In order to keep the fire-resistance treatment intact and to avoid any further absorption of moisture, the finished products are wrapped with protective wrappings. Stacks are separated by blocking for proper ventilation. The finished products are then stored and prepared for distribution to suppliers or project sites.