Study on dowel bearing behavior parallel to grain of Mongolian Scots Pine modified by sodium silicate

This study focuses on Mongolian Scots pine (Pinus sylvestris var. mongholica), a common tree species in northern China, and employs vacuum-pressure impregnation with a 20% sodium silicate solution to systematically investigate the strengthening effect and underlying behavior of this modification method on the parallel-to-grain dowel bearing performance of the wood. The experimental results show that this treatment significantly improved both the standard compressive strength and the oven-dry relative density of the pine. Regardless of whether the specimens were modified or not, the load–displacement curves of the dowel bearing tests along the grain exhibited four distinct stages: gap compaction, elastic deformation, plastic deformation, and load decline. However, the pressurized impregnation treatment had a pronounced influence on the elastic stiffness, the peak load, and the shape of load decline stage. The modified specimens displayed a larger elastic stiffness, a higher peak load, and a faster post-peak decline. For untreated Scots pine, the dowel bearing strength along the grain remained relatively stable when the steel dowel diameter ranged from 12 mm to 16 mm but decreased when the diameter increased to 18 mm. For the modified specimens, the failure ductility under dowel compression was reduced, but the bearing strength was considerably enhanced, with an increase of 23.59%–61.56%. As the dowel diameter increased, the bearing strength first increased and then decreased, reaching the maximum at a diameter of 14 mm. The current dowel bearing strength formulas in existing timber design codes generally underestimate the values for both untreated and modified wood. By introducing a factor that accounts for the steel dowel diameter, this study proposes a revised expression that significantly improves the calculation accuracy for the parallel-to-grain dowel bearing strength of Scots pine.