Influence of anchor design parameters on flexural performance of hybrid bonded-fiber reinforced polymer strengthened reinforced concrete beams

Strengthening concrete structures using adhesively bonded fiber-reinforced polymer (FRP) has been demonstrated as an effective method to improve the structural capacity. However, premature debonding of FRP material has limited the application of this technique. A number of anchorage methods have been proposed to enhance the bond performance, among which the hybrid bonded FRP (HB-FRP) has appeared to be more attractive as it can provide adhesive bond, frictional bond and dowel action. Previous studies have reported that the HB-FRP technique can considerably improve the bond strength and the flexural capacity of the strengthened beam. Yet, the influence of some key parameters on the flexural performance of the strengthened member is still not completely clear, and an accurate design method that can account for these parameters is not available for estimating the ultimate load capacity. In this paper, an experimental study was conducted on the HB-FRP strengthened beams to investigate the effects of anchoring pressure, anchor spacing, anchor distribution pattern, and FRP bonding mode. The test results indicate that the ultimate load-carrying capacity of the HB-FRP retrofitted member can be enhanced by increasing the anchorage pressure or decreasing the anchor spacing provided that the retrofitted beam fails by FRP debonding. The anchor distribution pattern has negligible influence on the flexural strength of the strengthened member, but uniform distribution of anchors along the bond length can yield higher flexural stiffness than the end anchoring approach. In addition, different FRP bonding mode can cause various strengthening mechanism in the HB-FRP system, leading to distinct flexural behavior of the retrofitted member. The dowel action of the anchor is suggested to be released by means of unbonding the FRP with the steel cap plate of the anchor since it may induce premature brittle anchorage failure. Based on the test results, a modified model is proposed for predicting the adhesive bond strength in HB-FRP system, in which a new parameter, referred to as spacing coefficient, is introduced to quantify the influence of the anchor spacing. Lastly, a theoretical model is developed in this study to design the ultimate moment capacity of the HB-FRP strengthened beam. A good correlation is achieved between the model predictions and the experimental results.