A flexible adhesive layer to strengthen glulam beams

Glulam is manufactured by gluing graded timber boards on top of each other with standard, stiff adhesives to form a beam. The research presented here is concerned with the use of a single flexible adhesive layer at a certain position of the beam cross section instead of the standard, stiff adhesive. In a first step, fundamental theoretical considerations for a favourable redistribution of the stresses over the cross section of the beam are presented and the position of the flexible adhesive layer is optimized in order to obtain the highest bending resistance of the total beam. Further calculations help to determine the requisite mechanical properties of the adhesive layer. They are communicated to the adhesive manufacturer as target values for the development of new adhesives. The industrial partner was able to produce several adhesive layers with the required mechanical properties. There was a need to select a limited number of the adhesive layers for further tests. The selection was done based on creep and delamination tests. Finally, several beams were manufactured with three selected adhesive layers. Bending tests were carried out to verify the theoretical predictions. The beams manufactured with two of the selected adhesives did not perform well on the large scale as compared to the small scale tests. One of the adhesive layers, however, gave completely satisfactory results: The beams manufactured with this adhesive layer exhibited much higher strength than the control beams and thus confirmed the theoretical expectations. The authors envision a new strengthening technique for glulam beams with this simple technique of using a single, flexible adhesive layer in the beam cross section.