Creep of epoxy–clay nanocomposite adhesive at the FRP interface: A multi-scale investigation

Externally bonded FRP laminates are widely used for strengthening structures due to their high strength-to-weight ratio and extended durability. However, creep of epoxy adhesives at the FRP interface has been reported, creating a concern about the efficiency of the FRP strengthening with time. Nanoclay platelets have been suggested to control creep of epoxy adhesives at the FRP interface. A multi-scale experimental investigation of creep of the epoxy–clay nanocomposite was performed. This investigation included examining macrocreep of epoxy at the FRP–steel interface using double lap shear test set-up, followed by investigating nanocreep of the epoxy–clay nanocomposite using nanoindentation. Microstructural characterizations including X-ray diffraction (XRD), transmission electron microscope (TEM), differential scanning calorimetry (DSC), 29Si nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy of the epoxy–clay nanocomposite were performed. Nanoclay platelets dispersion, intercalation or exfoliation was found to significantly affect creep of epoxy. Exfoliated nanoclay was found to reduce epoxy cross-linking and thus increase creep. However, agglomeration of intercalated nanoclay platelets allowed them to work as micro-particulate reinforcement in the epoxy matrix and therefore reduce epoxy-chain mobility and limit creep. Such complexity in behavior does not allow upscaling creep behavior of epoxy–clay nanocomposites from nano to macroscale.