Piezoelectric-based monitoring of the curing of structural adhesives: a novel experimental study

Structural adhesives are employed as bonding agents in the construction industry, to externally bond fibre-reinforced polymer (FRP) composite materials onto existing structures for strengthening and repair purposes. The strength development of such structural adhesives throughout the curing process is critical to the success of the system. Monitoring such strength development is thus important, to ensure that the adhesive has been suitably installed. This paper reports a proof-of-concept experimental study that assesses the feasibility of adopting lead zirconate titanate (PZT)-based monitoring techniques for real-time monitoring of the curing process of structural adhesives. More specifically, the electromechanical impedance (EMI) technique is used. As verification of the EMI technique, the well-established wave propagation (WP) technique is employed. Tensile tests, which are conducted in conjunction with the PZT-based monitoring tests, serve as control specimens. For the EMI technique, both the structural and PZT resonance peaks increased rapidly in the first 24 h of curing. The rate of movement of the resonance peaks, however, decreased for curing durations ranging from 24 h to seven days. The peaks then showed negligible movement for curing durations of seven to 21 days. A similar observation is found in the velocity of the elastic wave obtained from the WP technique. Overall, both monitoring techniques correlate well with the physical changes of the structural adhesive throughout the curing process and the tensile tests. The piezoelectric-based technique is therefore confirmed as a viable technique for monitoring the curing process of structural adhesives.