Engineered wood materials

Engineered wood materials, also known as wood-plastic composites (WPCs), are being investigated by the U.S. Navy for waterfront construction applications primarily because of their superior durability characteristics compared to wood. Durability, however, reaches beyond structural integrity and biodeterioration effects. Fire performance requirements are also critical issues in the acceptance of new combustible construction materials. To address fire performance issues in waterfront construction test methods and protocols were investigated. The objectives of this study were to determine Navy requirements for waterfront component fire resistance, to identify or develop test protocol requirements, and to initiate small sample fire testing based on those requirements. Currently the Navy’s criteria documents defer to NFPA 307, Construction and Fire Protection of Marine Terminals, Piers and Wharves, where component cross-sectional area is a determining factor. Because WPCs allow for hollow cross sections, these criteria will need to be modified. Standard test methods and modifications thereof were assessed for their relevance and potential application to WPCs. In particular, one test method (ASTM E108) was adapted and evaluated for wood-plastic composites, and found to provide good differentiation between various material types.

Engineered wood materials

The proposed fire test method was intended to be used to determine the acceptability of WPCs for outdoor deck applications where it is permissible (by the U.S. model building codes) to use combustible materials. This test method was based on a modification of the ASTM E108 fire test method for roof coverings. The modifications are explained below. In recent work by ASTM D20.20 (and AAMA) a similar fire propagation test method is being developed. The major difference between the method proposed in this report and the method described by D20.20 is the criteria used to determine pass or fail. The modifications to ASTM E108 included positioning of the deck surface (test specimen) horizontally, limiting the test specimen to 102 x 132 cm (40 x 52 inches) of surface area, adding combustible material below the test specimen, and selecting 60 minutes as a minimum test period.

Deck Fire Propagation Test Method

1. Deck Surface – A 2×4 lumber frame is constructed to support the deck boards. The frame shall be 132-cm (52-in) long and 102-cm (40-in) wide with one center support oriented along the 132-cm (52-in) dimension. The deck boards are cut to 102-cm (40-in) lengths and fastened to the lumber frame per the manufacturer’s recommendations. See Figure 1 for construction details.

2. Fire Source – Twenty (20) class C brands shall be placed over a 61 x 61 cm (24 x 24 in) square area, centered along the leading 102-cm (40-in) edge of the deck surface per Figure 1. At least one row of 4 brands shall be placed directly across the gap or joint between deck boards. Airflow across the brands shall be maintained per ASTM E108. The first brand shall be placed in position and all subsequent brands placed at approximate one-minute intervals per Figure 1b. Figure 2 shows a class C burning brand.

3. Kraft Paper – A sheet of 8-kg (18-lb) Kraft paper shall be placed not more than 152 cm (60 in) below the surface of the deck. The paper shall be conditioned to EMC at 21°C (70°F) and 50% RH prior to test.

4. Exposure Time – The brands shall be allowed to burn for a minimum of 40 minutes after the last brand has been placed. Brand placement requires approximately 1 minute per brand (e.g., 20 brands take approximately 20 minutes to place). Total test duration shall be no less than 60 minutes.

A modified ASTM E108 test method was used to determine the acceptability of WPCs for outdoor deck applications. The modified ASTM E108 was evaluated for wood-plastic composites, wood, treated wood, and plastic lumber, and found to provide good differentiation between the various material types. In particular it showed the superior performance of engineered wood when compared with wood, and the potential fire propagation problem with plastic lumber. ASTM D1929 proved that all materials were acceptable for building construction, at least according to the UBC.