A shear test for structural adhesives used in the consolidation of old timber

A test method was developed specifically for structural adhesives used in thick joints. It is based on compressive shear loading of glued joints (GJ) and solid wood (SW) specimens, before and after a suitable accelerated ageing cycle. The evaluation of results is based on the ratios of wet strength to dry strength and on the comparison of Gl specimens to SW specimens which are prepared from the same batch of wood, in order to minimise the influence of the variability in wood characteristics. On a group of 10 epoxy-based adhesives, specifically marketed for the consolidation of old timber structures, the method has proved to be effective to differentiate the performances of each product. Among the adhesives tested, only two types combine a dry strength of the GT which is equal or higher than that of SW and a wet strength which is >50% of the dry strength. Three other types have a wet shear strength >40% of the dry strength. Three types showed significantly lower performances when wet, and two types delaminated completely during the accelerated ageing cycle. The test method has been proposed for standardisation.