Nanotechnology-enhanced Near-Surface Mounted-Fibre Reinforced Polymers (NSM-FRP) structural retrofitting

Abstract

The retrofitting of concrete structures using fibre-reinforced polymers (FRP) bonded with neat epoxy (NE) adhesives, while proven efficient, has faced challenges related to interfacial debonding at FRP-NE and/or concrete-NE interfaces. These debonding issues pose a threat to both structure performance and safety, since the occurrence of the interfacial debonding weakens the bond between the FRP and concrete, compromising the load transfer mechanism. This compromised bond diminishes the effectiveness of the retrofitting, potentially leading to structural failures, reduced load-bearing capacity and overall instability. From a structural safety perspective, these issues are deemed unacceptable as they can significantly undermine the reliability and safety of the retrofitted structures. Consequently, given the challenges posed by interfacial debonding, exploring advanced bonding technologies, such as nanomaterial-modified adhesives, holds promise for enhancing the long-term stability and performance of retrofitted structures. The addition of nanomaterials to epoxy adhesives has proficiently overcome the drawbacks accompanied with using NE in retrofitting concrete members with FRP materials, by improving their mechanical and physical properties in addition to the interfacial bond strength. However, the use of nano-modified adhesives is currently limited to the application of the externally bonded reinforcement (EBR)-FRP strengthening systems, and there is no recorded application of these adhesives in the near-surface mounted (NSM)-FRP retrofitting techniques. Therefore, this thesis addresses utilising the nanomaterial-modified epoxy adhesives (NMEAs) for the NSM-FRP retrofitting of concrete members. The NMEAs were produced by incorporating various carbon-based (i.e. carbon nanofibres (CNF), cellulose nanocrystals (CNCs) and graphite nano powder) and silicon-based nanomaterials (i.e. silica nano powder and MMT nano clay) into the NE at 0.5, 1.0 and 1.5% by weight. First, inclusive chemical, physical and microstructural characterisations of the NMEAs were conducted to investigate the effect of incorporating different nanomaterials into neat structural epoxy adhesive (Sikadur®-30) on those properties. The NMEAs were also examined for their interfacial bond characteristics through testing the interfacial bond strength and characteristics of cement paste (CP)-adhesive joints bonded by NE or NMEAs. Afterwards, a comprehensive experimental programme was carried out to investigate the overall mechanical (i.e. flexural capacity and ductility response) and bond (i.e. failure modes) behaviours of the NSM-FRP-retrofitted concrete prisms bonded by NE or NMEAs. In regard with the NSM-FRP flexural retrofitting, a total of 68 concrete prisms were retrofitted for the purpose of investigating the effect of several parameters, mainly the types of bonding agents, on the performance of tested specimens. Results showed that using silica, clay and graphite NMEAs rather than NE enhanced the retrofitted concrete capacities by about 17%, 5% and 15%, respectively, while about 37% and 9% strength decreases, respectively were observed with using CNF- and cellulose-modified epoxies. Furthermore, it was found that the specimens bonded with silicon-based NMEAs had, on average, higher capacities than those bonded using carbon-based ones, which, on the other hand, showed more ductile behaviour. Also, using carbon-based NMEAs was able to prevent the interfacial debonding and switched the failure mode from shear to flexural, while slight debonding at the bar-adhesive interface was observed in the specimens retrofitted using silicon-based NMEAs accompanied with cohesive failure in the adhesive layer alongside minor concrete crushing. This research establishes the potential for broader applications of nanotechnology-enhanced retrofitting in concrete structures, particularly in the NSM-FRP retrofitting technique. The enhanced mechanical properties and interfacial bond strength offered by NMEAs hold promise for improving the long-term stability and performance of retrofitted concrete members in various real-world scenarios. Beyond flexural retrofitting, future research could explore the application of NMEAs in shear and torsion retrofitting, as well as their performance under cyclic, fire and earthquake conditions. Additionally, considering the complex nature of concrete retrofitting, further investigations are warranted to comprehensively understand the efficiency of NMEAs-bonded NSM-FRP technique. This could involve exploring new nano-fillers, individually or in combination, to optimize adhesive properties. Furthermore, alternative types and geometries of FRP reinforcement, such as Aramid FRP (AFRP) or different strip geometries, could be studied to expand the applicability of NMEAs across diverse structural configurations. Expanding the application of novel retrofitting techniques to large-scale concrete members, such as full-scale beams, columns or slabs, will provide valuable insights into the behaviour of retrofitted structures under realistic conditions. This includes assessing their performance under cyclic loads and extreme scenarios like fire and earthquake conditions, aligning with the latest NSM-FRP design guidelines provided by ACI. Additionally, the development of analytical formulas based on these guidelines could facilitate the practical implementation of the developed retrofitting system, ensuring its adoption and integration within the construction industry.