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Glued-in rods (GiR) are a successful class of structura bonded joints for timber engineering applications. However, they suffer from two limitations: relatively slow curing processes, and requirements related to minimum temperatures for the bonding process. Both, to some extent, limit their wider application, in particular on site. Inductive heating was investigated as a method to overcome both drawbacks by speeding up the process, and enabling it under low temperatures. To verify that, a series of four commercially available adhesives was used to manufacture GiR with beech LVL and Ø16 threaded rods embedded by 100 mm, which were then cured under a variety of conditions. Firstly, cold-curing according to the prescriptions of the technical data sheets and inductively, both at +23°C; secondly, inductively at +5°C; and thirdly, at – 10°C. Numerical modelling complementing these studies allowed for insights related to temperature distributions and curing progress otherwise inaccessible through purely experimental means. The results of this study clearly show that, if selecting appropriate adhesives, curing of glued-in rods is not only accelerated from several days down to a few minutes, but that the process can also be carried at much lower temperatures than those prescribed by the technical datasheets. Based thereupon, a completely new range of practical applications, in which gluing of rods can potentially be made independent of ambient temperatures on-site, has been unleashed. [ABSTRACT FROM AUTHOR]

Accelerated curing of glued-in rods (GiR) has been demonstrated in recent papers to represent an interesting option to shorten dramatically the process time. It has been shown that once heated up within minutes, GiR require meaningfully longer to cool down; it is posited that the cooling phase significantly contributes to the curing of the adhesive. This hypothesis has been experimentally tested in the study presented herein on GiR consisting of M16-8.8 threaded steel rods bonded into blocks of laminated veneer lumber (LVL) with a cross section of 120 × 120 mm2 considering four different two-component adhesives (one polyurethane, and three epoxies). Accelerated curing was achieved by transient heating regimes, in which temperatures were raised to different peak temperatures, followed by their cooling. Results indicate that, depending upon the adhesive, transient heating has, for some adhesives, the potential to achieve joint capacities comparable to cold-cured reference GiR, but that is not true for, in particular, 2K polyurethanes. Comparisons between experimental data and results obtained through the characterisation of the curing kinetics pointed out the complex relationship between the development of joint strength and that of chemical conversion degrees. [ABSTRACT FROM AUTHOR]

Adhesive bonding is nowadays an integral part of timber engineering and sets the basis for efficient joint typologies, as well as for most Engineered Wood Products (EWP). Regarding the manufacturing of joints, in particular under on-site conditions, two-component (2K) adhesives, in particular epoxies (EPX) and polyurethanes (PUR), are preferred over one component (1K) adhesives, because of their ability to cure under ambient conditions. If compared to mechanical fastening, however, the relatively slow curing of most 2K-adhesives used in practice remains a serious disadvantage. This paper summarises investigations on induction heating as a way to speed up significantly the curing process of Glued-in Rods (GiR), a joint type used in timber engineering. Firstly, four commercially available adhesives were investigated with regard to their potential for accelerated curing, resulting in the determination of maximum curing temperatures, and heating rates. Based thereupon, a series of large-scale GiR, involving threaded rods and beech Laminated Veneer Lumber (LVL), were manufactured, and inductively cured. The curing time was dramatically shortened from days to 5 min. Resulting joint capacities depended upon the time elapsed since induction. The resulting findings served as a basis for subsequent investigations to be presented in the second part of this series Figure 16. [ABSTRACT FROM AUTHOR]

The curing behaviour of inductively heated glued-in rods (GiR) experimentally tested in Part I of this work was further investigated with regard to the kinetics of adhesive curing, and evolution of strength with time. For that, dynamic DSC measurements were performed, and appropriate kinetic models were derived. These investigations were complemented with rheological experiments in which the adhesive were subjected to the same thermal regime as in the inductive heating of the GiR. Curing kinetics of the GiR were then compared to the increase of strength with time. The comparison showed that curing progress, defined by conversion degree, does not result in proportional strength increase. It was then attempted to estimate the influence of potential residual stresses from the induction curing on joint capacity. A relatively simple FE model was thus derived, which considered two different loading cases: a tensile force acting on the rod, and a shrinkage resulting from the induction process. Their superposition allowed for a general description of the fact that joint capacities increased with time elapsed since induction heating. Depending upon the adhesive considered, fair to very good quality of prediction were achieved. [ABSTRACT FROM AUTHOR]

• Resistive heating is a simple technique to speed-up the curing of glued-in rods. • The technique enables bonding of glued-in rods under low temperatures. • Glued in rods with epoxies exhibited no loss of strength under low temperatures. Adhesive bonding is a joining technique that enjoys increasing popularity in industry, including timber engineering. However, two major shortcomings still delay their widespread use, in particular on-site: long process times, and temperature requirements for curing. Accelerated curing is seen as one way to overcome both. In this paper, resistive heating is investigated as a method to both substantially accelerate curing, and to make it possible even under low temperature. The technique is illustrated on glued-in rods (GiR), a typical adhesively bonded structural joint widely used in timber engineering. Resistive heating was achieved by wrapping a thin Constantan wire within the threads of a rod, which was then subjected to an electrical current. In a series of tests, the performance of three two-component epoxies (2 K-EPX) and one two-component polyurethane (2 K-PUR), all commercially available, were investigated in terms of load capacity after resistive heating under room temperature (RT), +5 °C, and –10 °C. The results indicate that GiR bonded with two of the 2 K-EPX exhibited no significant differences in terms of joint capacity, regardless of the temperature under which they were cured, but that the 2 K-PUR proved very sensitive from departures from the recommendations of the technical data sheet. The results give, in particular practitioners, valuable indications on how to implement resistive heating in real-life applications. [ABSTRACT FROM AUTHOR]