Effects of nano montmorillonite clay modified epoxy adhesive on confinement efficiency of adhesively bonded CFRP stay-in-place formwork column

The efficiency of load transfer between the concrete surface and carbon fiber reinforced polymer (CFRP) is greatly influenced by the adhesive agent used in any composite system. This study investigates the effects of using nano montmorillonite clay (nC) as a toughening agent with the epoxy adhesive, to improve the confinement efficiency of CFRP-wrapped stay-in-place (SIP) composite column. Twenty scaled columns were tested under axial compressive load including, unwrapped (reference columns) and wrapped with one layer (1 L) or two layers (2 L) of CFRP. Two cross-section column shapes, square and circular, were evaluated based on their nC-modified adhesive efficiency. Results revealed that SIP columns made with nC-modified epoxy adhesive showed superior compressive behavior, stress, and strain capacity, compared to unmodified composite columns, even when wrapped with 2 L of CFRP. Using 1 L and 2 L of CFRP bonded with unmodified adhesive of circular specimens, increased the confinement efficiency (CE) by about 134 % and 183 % respectively. However, the improvements for the circular specimens with nC-modified epoxy adhesive were higher by up to 15 % than their counterparts’ ones made with unmodified adhesive. For square specimens, the use of nC-modified adhesive improved the CE by up to 16 % as compared with their counterparts’ specimens made with unmodified adhesive.Scanning electronic images revealed that using adhesive modified with nC induces several shorter microcracks, instead of a few wide cracks appearing in the unmodified adhesive. This toughening mechanism as well as the enhancement in the IZ between the adhesive matrix and the fabric yarns, explain the improvements in the ultimate strain capacity of the column specimens made with modified adhesive.