Reinforced concrete is a versatile modern construction material. Despite its advantages as a composite material, corrosion of the embedded reinforcing steel leads to infrastructure deterioration and loss of service. Non-Destructive Testing (NDT) methods are required to quantify the reinforcement condition, and to help manage human and financial risks arising from unexpected outright failure or service restrictions. Reinforcement condition can be assessed using a novel, time- and cost-efficient NDT method based on the self-magnetic behaviour of ferromagnetic materials. In this study, the magnetic properties of three similar rebars, each having three similar sized longitudinal defects, are recorded and assessed through experiments and a numerical simulation model. Strong correspondence is demonstrated between the magnetic properties from numerical simulation and from the experimental objects. For instance, applying the experimentally obtained defect detection threshold to the mathematically simulated results allows accurate defect detection in the simulations, showing that self-magnetic behavior is a powerful tool for condition assessment of ferromagnetic reinforcing materials.