Accelerated curing of G-FRP rods glued into timber by means of inductive heating – Influences of curing kinetics

Abstract

ABSTRACT Glued-in Rods (GiR) are nowadays widely established as an adhesively bonded connection type in timber engineering. The assemblies consist of rods, e.g. threaded rods or rebars, made of steel, or Fibre-Reinforced Polymers (FRP), which are bonded into wooden construction elements. In contrast to mechanical fastening, adhesive bonding often results in a better load transfer between the adherends, and consequently a reduction of local stress peaks in comparison to mechanical fasteners. Bonding of GiR is typically performed with 2 K polyurethanes (2 K-PUR) or 2 K epoxies (2 K-EPX), for which full curing can last up to several days. Consequently, before the connection can be stressed, or moved for further processing, components have to be maintained in fixed position. This process can be very time, and cost, consuming for contractors. In order to create a speedier, thus more efficient, bonding process, the present study investigated accelerated curing of large-scale GiR specimens with the help of inductively heated Curie particles (CP). The CP are added to the adhesives, and exposed to a high frequency (HF) electromagnetic field (EMF). The EMF heats the mix up until the Curie temperature (T C) is reached, at which point the heating automatically stops. Due to the generated heat, polymerisation is significantly accelerated in a process independent of external monitoring techniques that prevents adhesive overheating. In order to design this practitioner-friendly and controllable manufacturing process, kinetic models were developed, allowing the prediction of the degree of cure, α, in dependency of CP content. The kinetic models were applied to experimentally determined temperature profiles during inductive heating of large-scale GiR specimens, considering three commercially available 2 K-EPX, and one 2 K-PUR, all of which are widely used. The kinetic modelling allowed for a much better interpretation of the heating behaviour, highlighting, in particular, the contribution of enthalpy for adhesive curing, and by extension of the whole induction process.