Properties of Mixed Species/Densities Cross Laminated Timber made of Rubberwood and Coconut Wood

Abstract

The utilization of rubberwood (Hevea brasiliensis) and coconut wood (Cocos nucifera L.), the essential economic crops in Thailand and tropical countries, was proposed for manufacturing mixed species/density cross-laminated timber (CLT) for building construction. Six 3-layer CLT configurations, which are composed of either medium-density (600 – 799 kg/m3 ) rubberwood (MRB) or coconut wood (MCC) or high-density (800 – 999 kg/m3 ) coconut wood (HCC) laminations, were determined considering the mechanical properties and material costs. The outer layers of the control MCC CLT were replaced with either MRB or HCC to improve its mechanical properties, while either outer or core layers of the control MRB CLT were replaced with HCC to reduce its material cost. The material properties of the three wood types and the six CLT configurations were examined. The densities of the produced CLTs were not affected by the chosen manufacturing parameters showing a strong correlation to the lamination's density. From the bonding performance perspective, the mixed-species approach significantly increased the average wood failure percentage of the control MRB CLT. However, only the control MCC CLT achieved the average wood failure percentage greater than 80%, as required in North America's CLT standard. The compressive strength properties of the CLTs in their major strength directions, σclt, were governed by the outer laminations' parallel-to-grain compressive strength. Unlike softwood CLTs, neglecting the load sharing contribution of the core layer in the σclt estimation resulted in 15% underestimation. Rolling shear strength, τrs, was determined by the core laminations regardless of the CLT layups. MRB achieved the highest τrs followed by HCC and MCC, and all values were significantly larger than the common softwood used in CLT production. The results imply that the mixed species/densities approaches can effectively improve the mechanical properties of the coconut wood CLT and reduce the material cost of rubberwood CLT without compromising structural performance.