Effects of Protective Covering Configurations on the Fire Behavior and Charring Characteristics of Korean Larch Cross-Laminated Timber Walls

Protective coverings are an effective means of preventing direct fire exposure and delaying the charring of cross-laminated timber (CLT). This study evaluated the effects of practical covering configurations on the fire behavior and charring characteristics of four Korean larch CLT wall assemblies through 120-min fire resistance tests. The specimens varied in fire-resistant gypsum board thickness, joint arrangement, fastening details, and insulation conditions. Fire-induced damage, internal temperatures, and post-fire charring characteristics were evaluated. Specimens with non-staggered gypsum board joints exhibited cracking and detachment of the protective covering, resulting in localized flame penetration, with a maximum measured localized charring depth of 80 mm in CW-WS-G-1 and a measured localized charring depth of 73 mm in CW-WS-G-2. In contrast, specimens with staggered joints and improved fastening details largely maintained covering integrity. Only minor surface charring was observed in CW-WS-G-3, while no distinct charring occurred in CW-WS-W-1. The highest temperature recorded by the installed thermocouples was 225.2 °C in CW-WS-G-1, whereas the highest recorded temperatures in all other specimens remained below 100 °C. Nevertheless, localized charring occurred in CW-WS-G-2 despite its low internal temperature, indicating that temperature measurements alone may not adequately capture localized fire damage. These results indicate that, within the tested configurations, covering integrity associated with the combined joint arrangement and fastening details played an important role in limiting localized flame penetration and charring.