PUFJ (PolyUrethane Flexible Joints) as an innovative polyurethane system for structural and non-structural bonding of timber elements

PolyUrethane Flexible Joints (PUFJ) as an innovative adhesive system has been developed by a team of researchers from the Cracow University of Technology (CUT) for several years. Initially, the research focused mainly on joining masonry and reinforced concrete elements. Due to environmental concerns and climate changes, further advancement of the PUFJ system was focused on the technology development based on connecting natural materials such as wood and wood-based materials. In 2019, the cooperation of the CUT with a private company resulted in the implementation of the PUFJ system for the construction of innovative connections in a prefabricated timber frame construction of a family house near Cracow. Currently, further research are carried out focused on the complex understanding of behaviour of adhesive bond used in structural and non-structural timber bonding, taking into account different species of wood (pine, spruce, beech, douglas fir) and a different thickness of the flexible adhesive layer (from 1 mm to 30 mm). The article contains the properties of the PUFJ system, describes variants of the available applications (wet application in form of liquid substance curing in time or dry application as a prefabricated bonding layers glued to the elements with a thin adhesive layer). Examples of double-lap shear connections and laminated beams made of douglas fir wood of C24 class and Sika®PS polyurethane flexible layer are presented. All specimens were constructed with three various flexible adhesive thicknesses of 1, 2 and 4 mm. The influence of elevated temperature on the load capacity of the connection and on the final deflection was tested. The tests were conducted at four temperature levels (20, 40, 60 and 80°C).