Impact of preservative retention levels on the mechanical strength of construction-grade lumber

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Abstract

Wood preservation as a discipline occupies the intersection of materials science, chemistry, and structural engineering, yet the relationship between preservative retention and the mechanical integrity of treated lumber remains incompletely characterised for tropical hardwood species used in Indonesian construction. This study investigated the effect of five chromated copper arsenate (CCA) retention levels 4, 8, 12, 16, and 20 kg/m³ on the modulus of elasticity (MOE), modulus of rupture (MOR), compressive strength parallel to grain, and Janka hardness of construction-grade Shorea spp. (meranti) and Tectona grandis (teak) lumber. Untreated matched specimens served as controls. Boards were vacuum-pressure impregnated at 800 kPa for 90 minutes, reconditioned to 12% moisture content, and machined to ASTM D143 specimen dimensions. Static bending, compression, and hardness tests were conducted on a Shimadzu AG-X 100 kN universal testing machine. At 4 and 8 kg/m³, neither species showed a statistically significant reduction in any strength property relative to untreated controls (p>0.05). At 12 kg/m³, MOR decreased by 8.5% in meranti (p = 0.032) and 5.9% in teak (p = 0.044). At 16 and 20 kg/m³, all four strength properties declined significantly in both species, with MOR reductions reaching 21.4% in meranti and 15.1% in teak at the highest retention level. MOE losses followed a similar trend but were less pronounced, declining by a maximum of 14.2% in meranti at 20 kg/m³. Compressive strength and Janka hardness were less sensitive to retention, with maximum reductions of 11.8% and 8.4%, respectively. Two-way ANOVA confirmed a significant retention × species interaction for MOR (p

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