Degradation of Wood

Abstract

The mechanical and physical properties of timber can be explained through reference to its anatomical structure and chemical composition. Mechanical properties are strongly anisotropic with maximum strength parallel to the grain. One of the most important attributes of timber, from a structural point of view, is its resistance to effects of heat where its low thermal conductivity, high heat capacity, and resistance to thermal depolymerization below 250°C give advantageous short term behavior in fire situations. Recoverable and irrecoverable creep is also minimal compared with that in metals. Hygroscopicity and dimensional changes, instigated by moisture content, are some of the most problematic features of wood. Wood’s response to stress and moisture content changes maintain a strong negative exponential function below fiber saturation point, made linear by logarithmic transformation of data. Creep deformation remains constant at equilibrium moisture content, but disproportionately increases with cyclic conditions. An interaction effect of moisture content and temperature change on wood properties alters the dependency of the elastic modulus from linearity to nonlinear close to saturation. Wood exhibits reasonable natural resistance to biological degradation and chemical attack given appropriate treatment and choice of the right timber. The biological deterioration agents of fungi, boring insects, marine borers, and fire damage can be mitigated by selection of high density, impermeable, and naturally durable wood, or by impregnation treatments with preservatives incorporating fire-retardants. Chemical surface degradation due to ultraviolet radiation; defibration caused by exposure to acid fumes, acids, and alkalis can be reduced through choice of impermeable timber and application of appropriate surface finishes.