Creep of thixotropic adhesives in bonded-in timber connections as a function of temperature and humidity

Thixotropic, shear-thinning epoxy adhesives are the optimum choice for in situ assembly of timber structural joints under ambient conditions. These adhesives are suitable for injection into roof members and under bridge decks where gravity is a problem for conventional adhesives. However these ambient cure adhesives possess relatively low glass transition temperatures (Tg) and there are concerns associated with their stability under creep loads in conditions of high temperature and humidity. These concerns have been addressed in a paper presented at WCTE 2010 and in a subsequent journal publication where creep data generated in a dynamic mechanical thermal analyser (DMTA) has demonstrated the stability of these adhesives above Tg. In this paper results of creep tests on bonded-in connections, at a design shear stress of 2N/mm2 at the rod to adhesive interface, arc presented at temperatures of 20,30, 40 and SOX with levels of relative humidity (RH) of 65,75,85 and 95%. Creep experiments were conducted in a walk- in environmental chamber and creep was followed with laser displacement sensors. At intermediate combinations of temperature and RH level (e.g. 30°C, 75%RH), secondary creep was observed up to a limit. Under extreme conditions of high temperature and RH tertiary creep to failure occurred.