Water absorption of wood polypropylene composite sheet piles and its influence on mechanical properties

The water absorption behavior and durability of extruded wood polypropylene composite (WPC) material used in Z-section sheet piles was investigated. An experimental technique to determine percentage water absorption and its effect on mechanical properties was implemented. Water absorption tests were carried out on specimens cut from flanges and webs of the Z-section by immersion in tap water at 21, 45 and 70 °C. Freeze–thaw resistance of saturated composites was studied by cycling temperatures in the range of 21 to −29 °C. The absorption behavior followed the kinetics of a Fickian diffusion process. This behavior was modeled using an analytical solution based on Fick’s second law of diffusion. Quasi-static bending tests were conducted periodically on specimens immersed in water to assess degradation in flexural properties. A significant decrease in mechanical properties of the water-saturated specimens compared to the dry control was observed. Degradation of flexural modulus was found to be a strong function of water absorption with initial decrement as percentage water absorption increased and finally stabilized as the absorption process approached saturation. However, freeze–thaw cycling did not have any significant effect on mechanical properties of saturated composites. A phenomenological model for moisture intrusion and its effect on mechanical properties was implemented and applied to explain the experimental findings.

► Water absorption process in WPC material follows the kinetics of Fickian diffusion.
► Model based on Fick’s second law of diffusion predicts accurately the absorption process at 21 °C and 70 °C.
► Change in diffusion coefficient with respect to temperature conforms to Arrhenius – type relation.
► Models allow prediction of water absorption at any temperature of interest as a function of time.
► Modulus of elasticity (MOE) decreased by 43% for specimens saturated with water at 21 °C when compared to the control specimens.