Knowledge of the stress state at the interface between carbon fibre-reinforced polymer (CFRP) and concrete is critical when assessing the performance of externally bonded concrete repairs under different loading conditions. High levels of interfacial shear and normal stresses (mixed mode loading) at the CFRP-concrete interface can result in premature bond failure (debonding) in FRP retrofitted concrete members under bending, which is detrimental in bond-critical applications. Of the various potential bond failures (loss of composite action), particular focus has been directed toward plate-end interfacial debonding and concrete cover separations. This paper aims to gather essential field data on the performance of CFRP composites on deteriorated 57-year-old concrete. The samples were extracted from the diaphragms of the recently deconstructed Champlain Bridge in Canada. This study compares shear stress at the interface that arises during the debonding process, initiating at the tip of flexural cracks in deteriorated concrete beams, with the shear stress obtained in single and double shear lap tests. These shear stress values are subsequently compared with the bond strength values obtained from the standardized field test method of direct tension pull-off test usually used for quality assurance purposes. Beam test samples were instrumented with distributed fibre optic sensors along their length and discrete electrical resistance strain gauges to monitor the strain and stress distribution of defective and non-defective regions during the debonding process. The debonding progression occurring during the shear-lap test was captured using the full-field 3-dimensional digital image correlation method. The preliminary experimental results from beam tests and the examination of 490 direct tension pull-off samples indicate a substantial reduction in load transfer within areas with field defects identified from the various NDT methods used for condition assessment.