In recent years, the utilization potential of hardwoods has been a focus of wood research in German-speaking countries. Several building authority approvals for hardwood engineered wood products (EWP) resulted from the research. The hardwood beech (Fagus sylvatica, L.), which is widespread in the DACH region, was mainly studied. Recently, the hardwood birch (Betula pendula, Ehrh. and Betula pubescens, Roth) is gaining interest in Germany, due to its natural regeneration in forest stands that have suffered from climate change. Originally, birch is mainly distributed in the Scandinavian countries, Russia and the Baltic countries and has suitable properties for timber construction. Finger-jointing is an important production step for the performance and economic efficiency of EWP´s. It enables product dimensions far beyond the dimensions of raw wood. Particularly for spruce (Picea abies, L.), the dominant species in timber construction, finger-jointing has been standardised over many years. The adhesive systems and the finger joint geometry have a major influence on the properties of the finger joint. The use of new technologies in wood processing, such as scanner technology, makes it possible to determine and eliminate defects in the wood more and more effectively. Large quantities of hardwoods are available whose strengths can far exceed those of spruce. However, up to now, hardwood EWP´s have rarely been used in the building sector, partly because of uncertainties in their production and high costs. In contrast to softwood, the finger joint is usually the decisive factor for the strength of a hardwood EWP. Previous studies with conventional finger joint profiles carried out that low strengths of finger joints often result from stress concentrations in the base of the finger joint that overlap with the adjacent fingers. One reason for this is the tip gap of structural finger joints, which is prescribed in the standards EN 15497 and EN 14080. The tip gap serves to form a thin bondline and the self-locking of the finger joint. But it also causes a weakening of the lamella cross-section in the base of the finger joint. Furthermore, a prestressing is associated with the joining process of the conventional finger joint, which is visible in the form of cracks during the pressing process or in later use. Hardwoods bring with them new requirements and an adaptation of finger-jointing is required.
In this study, we determined the tensile and bending strength of birch and beech lamellas finger jointed with a conventional
(Standard) and a modified finger joint profile, called cross-grooved finger joint, that was newly developed using the industrial finger-jointing line of our department. We used commercial polyurethane (PUR), melamin-urea-formaldehyde (MUF) and phenol-resorcin-formaldehyde (PRF) adhesive systems to bond the finger joints. The aim of the cross-grooved finger joint is to reduce stress concentrations within the finger joint compared to conventional profiles. The MUF and PRF adhesive systems achieved an increase in the bending and tensile strengths using the cross-grooved finger joint. We identified a high potential of the cross-grooved finger joint. In addition to increased strength, we have observed reduced cracking, and the base of the cross-grooved finger joint could be completely closed. Referring the advantages for structural applications both indoors and outdoors we will further test the cross-grooved finger joint considering more materials, different process parameters and develop an industrial production process.
Keywords: Adhesives, birch, bending strength, finger joint profile, tensile strength
Authors
Hannes Stolze
Wood Biology and Wood Products Department, University of Goettingen, Germany
Holger Militz
Wood Biology and Wood Products Department, University of Goettingen, Germany
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