Post-tensioned containment structures are widely used in nuclear power plants. In case of any accident, these concrete structures are the last passive barrier against contaminating radioactive materials. During the construction-phase of these cylindrical concrete structures, hidden delamination defects may develop and remain undetected. This study introduces a new method based on acoustic emission (AE) to monitor the onset of delamination in a large-scale experimental model of a containment structure as it is being post-tensioned. In particular, advanced data processing algorithms are used to interpret AE data in terms of the delamination mechanism.