Wood dimensional changes as consequences of its hygroscopic behavior

Abstract

Wood has a strong hygroscopic behavior, with a huge variation of moisture content and dimension stability caused by modifications of the relative humidity and temperature of the surrounding air. This moisture relationship has an important influence on wood properties and performance. Many of the challenges of using wood as an engineering material arise from changes in moisture content or an abundance of moisture within the wood. Wood is dimensionally stable when moisture content is greater than the fiber saturation point (MCfs). Below MCfs wood changes dimension as it gains moisture (swells) or loses moisture (shrinks), because volume of the cell wall depends on the amount of bound water. With respect to dimensional stability, wood is an anisotropic material. It shrinks (or swells) most in the direction of the annual growth rings (tangentially), about half as much across the rings (radially), and only slightly along the grain (longitudinally). In this work, an experimental program was defined with the aim to evaluate the dimensional stability of three coniferous wood species: maritime pine (Pinus pinaster), scots pine (Pinus sylvestris) and spruce (Picea abies). Comparing the wood species, it is the Spruce (A) that presents higher dimensional changes, while it is the Maritime pine (P) that is more stable under moisture content variation. Tests results allow to conclude that the most important dimensional variation occurs in the first 24 hours.