Timber bond durability

High initial bond strengths are relatively easy to achieve with the type of adhesives commonly used in the rehabilitation and repair of modem and historic timber structures. However, maintaining good bond durability in some situations is comparatively more difficult with these adhesives. For instance, while adhesives such as phenolic, resorcinolic, and aminoplastic resins produce durable bonds in EN 1995-1-1 [4] service classes 1, 2 and 3, the typically used on-site polymerized adhesives do not form bonds of adequate durability in service class 3, when bonding some preservative treated timbers, in situations in which the adhesive is bonded to dense hardwoods, or when bonding wood to non-wood materials, such as FRP profiles, steel rods, etc. 

Many examples of bonded wood constructions have been known from ancient times for their usefulness in the building of boats and ships. However, the important technology of combining small pieces of wood to form large timber sections, with the strength and durability of intact lumber, is certainly a more recent product of the last forty years. The early stages of this modem development have been reviewed by Selbo and Knauss (1). 

This article discusses briefly the use of adhesives on the construction site in the context of structural repair and reinforcement the requirements and practical dilficulties in the work on site with regtirds to the strength and durability of the rehabilitated timber structure and the consequent need for queility control. It also highlights the characteristics and requirements that must be fidfilled by structural adhesives and reinforcing materials factors affecting performance and durability of bonded joints tmd ways to improve adhesion and durability. Finally, it points out some research needs and future developments identified by the authors. 

However, if safety is at stake, these approaches are not at all suitable due to the high variability in strength and durability regain provided by such an approach [13]. Not only will the adhesive most likely fail to reach the (closed) very tip of the fissure, but also this type of bonded joint itself is very prone to delamination due to timber dimensional variations. 

Another chemical treatment that has resurged recently due to improvements made in its formulation is the hydroxymethylated resorcinol (HMR). HMR has been used successfully with several timbers and adhesives, to promote the exterior durability of their bonded joints [65, 71]. Consequently, this technique seems ready for industrial application, at least for the species and adhesives tested. Nevertheless, studies to clarify some aspects of its action mechanism are still needed [20]. 

Custodio J, Broughton J, and Cruz H. A review of factors influencing the durability of structural bonded timber joints. IntemationalJoumal of Adhesion and Adhesives, 29(2), 2009, pp. 173-185. 

Key words on-site bonded composite systems, repair, reinforcement and seismic retrofit, timber and concrete structures, limitations and requirements, performance and durability. 

This section focuses very briefly on the most relevant of the aforementioned factors, thus providing only a general understanding of the factors that influence the durability of bonded timber and concrete joints. The durability of APC materials has already been discussed in the previous chapters so is not dealt with here. The durability of the substrates, timber and concrete, is not also dealt with here the reader can obtain information on this topic from the relevant sub-section of Section 22.6. 

Temperature is an important factor in the durability of structural adhesive joints, since it can affect the creep, fatigue and fire performance of adhesive bonded joints. Well-designed and well-made joints with any of the normally structural adhesives should retain their mechanical properties indefinitely if the substrate moisture content is kept low (e.g., if timber moisture content stays below approximately 15%) and if the temperature remains within the range of human comfort. However, when adhesives are exposed either... 

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