Small scale tests on the performance of adhesives used in cross laminated timber (CLT) at elevated temperatures

Abstract

This study is part of a research project examining fire risk in cross laminated timber (CLT) buildings. “Delamination”, a major concern for CLT fire safety, can occur when the adhesive bondline fails in the panel before it chars which can lead to additional fuel for fire regrowth. Understanding the mechanical strength and stiffness of the adhesive bond under elevated temperature is one of the fundamental steps towards better understanding delamination. In this study, the material properties of wood and wood adhesive bonds are studied at various temperatures levels below the char temperature of wood (300 °C) to characterize the loss in strength and stiffness as a function of temperature. Four adhesives, three of which are commonly used in CLT production, were studied: two formulations made from polyurethane, one made from melamine formaldehyde, and one made from phenol-resorcinol formaldehyde. Tests were performed in a half-lap shear joint and compared against a specimen with identical geometry made from a solid piece of wood with no joint. The phenol-resorcinol formaldehyde joint had approximately the same shear strength as solid wood at all temperatures, whereas the other adhesives had noticeable reduction in strength at high temperatures. The method presented herein is unique among other CLT adhesive tests as it measures the performance of the adhesive under heat and load.