Charring rate of Korean larch (Larix kaempferi) CLT wall panels: Effect of adhesive type and applicability of fire design models

ABSTRACT This study evaluates the charring behavior of cross-laminated timber (CLT) wall panels manufactured from Korean larch ( Larix kaempferi ) and compares the measured charring rates with four fire design standards: KDS 41 50 50 (Korea, 2026 draft), EN 1995-1-2:2004, EN 1995-1-2:2025 (second-generation Eurocode 5), and AWC FDS 2024. Three CLT wall specimens were tested under non-load-bearing, single-sided ISO 834 standard fire conditions: a 5-ply specimen (150 mm thick, unprotected) and a 3-ply specimen (90 mm thick, protected with 12.5 mm gypsum plasterboard), both bonded with a heat-resistant epoxy adhesive and exposed for 120 min, and a 4-ply specimen (112 mm thick) bonded with a two-component polyurethane (2c-PUR) adhesive and exposed for 90 min. The data were supplemented with published results for Korean larch CLT bonded with phenol-resorcinol formaldehyde (PRF) and one-component polyurethane (1c-PUR) adhesives. Three adhesive performance groups emerged: heat-resistant adhesives (heat-resistant epoxy and PRF) yielded cumulative charring rates of 0.50–0.67 mm/min, the 2c-PUR specimen 0.84 mm/min, and the 1c-PUR specimen 0.94 mm/min, approximately 1.3–1.7 and 1.4–1.9 times the heat-resistant group, respectively. The AWC FDS nonlinear model for bond-line-intact CLT agreed most closely with the heat-resistant data (within ±20%), whereas the EN 2025 phase-based model (orientation-dependent k 3 ) most closely matched the 2c-PUR specimen (−0.5%) and gave the best available prediction for the more severely delaminating 1c-PUR specimen (−13%). Adhesive type is the dominant factor governing CLT charring rate, and the three-group classification offers an experimentally grounded basis for adhesive-differentiated charring provisions in fire design standards.