Significance of Midspan Debonding Failure in FRP-Plated Concrete Beams

Abstract

Reinforced concrete beams enhanced in flexure with adhesively-bonded fibre reinforced polymer plates are susceptible to a brittle form of failure, defined by delamination of the cover concrete attached to the adhesive that causes the plates to debond from the beam. This paper demonstrates that, while previous research has focused almost singularly on one debond mode in which concrete delamination progresses from the ends of the plates inwards, there exists another critical debond mode that initiates near flexural cracks in the midspan region of the plated beam and propagates out to the ends of the plates. Data from large-scale experimental work are presented to show that midspan debond action is triggered by high shear stresses transmitted from the plates through the adhesive to the cover concrete. These stresses arise initially from tension stiffening in the cracked concrete and corrosion of the embedded steel. It is shown that strain gauge data are required from both the bonded and exposed surfaces of the plate for accurate quantification of these shear stresses. The shear span of the external load and the stiffness of the plate are cited as parameters that may influence whether in practice end peel or midspan debond occur in plated beams. The paper concludes by describing compulsory features of any numerical modeling of the midspan debond phenomenon.