Experimental Determination of the Relationship Between the Resistance Micro-Drilling Characteristic and the Density of Spruce Wood at Different Moisture Contents

This study explores the potential of non-destructive methods for diagnosing timber structures, with a primary focus on maximising the capabilities of the resistance-drilling technique. Laboratory tests were performed on spruce wood specimens, the most commonly used construction timber in Central Europe, prepared across a wide range of moisture contents (0–53%) to assess the influence of moisture on resistance-drilling characteristics. The resistance micro-drilling (RM) characteristic was found to be independent of moisture content (coefficient of determination close to zero), confirming that resistance drilling provides stable results under varying in situ moisture conditions. In contrast, wood density and the RM characteristic were strongly correlated, with coefficients of determination of R2 = 0.87 for moisture contents between 0% and 30%, and R2 = 0.90 for the 8–18% moisture range typical of timber in service. Based on these relationships, two linear conversion equations were developed (ρ = 1.685·RM + 183.85 and ρ = 1.982·RM + 135.15, respectively), enabling estimation of spruce wood density directly from RM values. Compressive strength parallel and perpendicular to grain decreased with increasing moisture content up to the fibre saturation point, beyond which the reduction in strength plateaued.