Experimental investigations on the pull-out strength of steel rods bonded in Douglas fir roundwood specimens with polymer concrete

The current market of timber products shows a clearly identifiable dominance of rectangular crosssections in modern timber buildings. To promote roundwood as addition to commonly used wood and wood products, this paper presents experimental investigations serving for axial connectors in roundwood truss structures. Glued-in threaded rods, bonded parallel to grain into Douglas fir (Pseudotsuga menziesii) softwood, represent the basis of this research. To increase the load-carrying capacity and usability of this connection technique in roundwood, a resin-bonded polymer concrete (PC) with an increased bondline thickness and contact surface to timber was investigated. The pullout strength and embedment stiffness of the new modified bonded-in rod connection was studied on Douglas fir roundwood specimen exposed to different service conditions. The study also determined the influence of wood defects and moisture content before testing. The results show a larger single-fastener capacity, also for higher moisture content applications, compared to traditional glued-in rods. The quality of the adhesive bondline, for example bonding length, hardening process and homogeneous adhesive bond could be verified by visual inspection.