Comparative evaluation of non-destructive and CT-derived indicators for stiffness assessment of reclaimed Norway spruce timber

Abstract Reliable evaluation of mechanical performance is a central constraint for structural reuse of reclaimed timber. This study provides a controlled comparative assessment of non-destructive indicators for piece-wise characterisation of reclaimed Norway spruce elements intended for reuse as glulam lamellas. Visual assessment, longitudinal dynamic excitation, and CT-derived modelling indicators were evaluated against global and zone-wise flatwise bending tests on 56 reclaimed beams from a single material stream with known species, cross-section, and initial strength class. Dynamic excitation provided the most reliable single predictor of global bending stiffness at specimen level. Visual assessment showed limited discriminatory power for this comparatively homogeneous material. CT-derived indicators based on density and orientation fields reproduced principal intra-member stiffness variations associated with knots and fibre disturbance and showed predictive capability of similar order to established mechanical non-destructive indicators. Systematic differences between methods were primarily attributable to moisture-state uncertainty, modelling assumptions, and the global nature of the mechanical reference tests. Strength-related results, based on a limited destructive subset, are interpreted as exploratory. Within these bounds, the study establishes a comparative reference dataset for multi-modal evaluation of reclaimed structural timber and demonstrates how conventional and volumetric indicators complement each other in resolving global stiffness and intra-member variability relevant to allocation in engineered wood products.