Long-term creep behaviour of cross-laminated timber made from fibre-managed Eucalyptus nitens under uncontrolled environmental conditions

Australian fast-grown Eucalyptus plantations have traditionally been used for pulp and woodchip production. However, given their large availability, there is a growing interest among researchers to explore their potential for structural applications as solid wood and engineered wood products. Cross-laminated timber (CLT) provides one option for this resource, which is supported by recent research on its reliable mechanical properties. However, due to its orthogonal arrangements of layers, CLT is more susceptible to time-dependent deformation, which is normally called the creep behaviour. Therefore, the investigation on the creep performance of CLT made from Eucalyptus nitens (E. nitens) becomes necessary. Time-independent factors including relative humidity (RH) and temperature of the surrounding environment, play important roles in affecting the creep behaviour of timber, so these factors were also considered in creep tests in this study. Six E. nitens CLT considering different modulus of elasticity (MOE) grades in different layers, and one strength-class C24 spruce CLT were tested in this study as a control. A non-linear mathematical modelling equation was developed based on the 500-day experimental creep results, which was then used to extrapolate the data to predict the longer-term creep behaviour over 50-year. The E. nitens CLT panels incorporating highest-MOE boards in the transverse layers are expected to have the lowest creep ratio (1.397) after 50 years, followed by the CLT panels with highest-MOE boards in longitudinal direction (1.411). However, the E. nitens CLT panels that incorporate the lowest-MOE boards in both longitudinal and transverse directions are expected to have the highest creep ratio (2.193) after 50 years, surpassed the controlled spruce CLT (1.659). This study revealed how different structural grade (modulus of elasticity) in different lamellas influenced the creep ratios under constant applied loads, as well as the long-term creep performance of CLT under varied relative humidity and temperature.