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.