Experimental study on the long-term creep behavior of cross-laminated timber under axial compression used for mass timber structural systems

Comprehending the creep behavior of cross-laminated timber (CLT) is the basis for evaluating the long-term performance of CLT structural systems (e.g., post-tensioned CLT shear wall structures). In the study, short-term compressive tests were conducted on CLT material specimens to obtain their compressive strength. Then, long-term creep tests were conducted on 10 subgroups of CLT blocks to obtain their creep behavior under axial compression. Ambient temperature and relative humidity, as well as moisture content, environmental strain and the creep strain within the CLT specimens were monitored continuously over 330 days. The variations of both the relative humidity and the moisture content were analyzed, and the parameters related to the moisture content of CLT were calculated. For the CLT subjected to in-plane and out-of-plane compression, both the coefficient of environmental deformation and the creep coefficient were calculated. The effects of the stress levels, the section geometry, and the number of CLT layers on the CLT creep behavior were analyzed. The common creep models of timber were calibrated by using the normal and the weighted nonlinear regression methods, and the creep coefficients of CLT over the 50-year service life were estimated by using the calibrated models. It is found the transverse environmental strain of CLT along its out-of-plane direction is significantly higher than the in-plane environmental strain of CLT along its major strength direction. When subjected to a sustained constant stress, the CLT with a less section geometry is prone to larger creep deformation. Besides, compared to the 3-layer CLT, the 5-layer CLT is more prone to the mechano-sorptive creep along its in-plane major strength direction. For the 5-layer 100-mm-thickness CLT floors, their creep coefficient over 50 years ranges from 6.00 to 7.70. By contrast, for the 3-layer 105-mm-thickness CLT wall panels and 5-layer 175-mm-thickness CLT wall panels, their creep coefficients over 50 years are suggested as 1.19 and 1.42, respectively.