A non-contact three-dimensional (3 D) scanning system was first employed to capture the corrosion features of three naturally corroded steel beams and to reconstruct the 3 D geometries. A more reasonable corrosion grade parameter φ, which could comprehensively consider the effects of uniform and non-uniform corrosion, was proposed for evaluating the corrosion degree of these beams. Then, four-point bending tests were carried out on the corroded beams to investigate the effects of corrosion damage on the flexural performance. A theoretical formula and a detailed calculation process, which considered the thickness reduction and strength degradation based on the corrosion morphology, were established to evaluate the bending capacity of the corroded beams. Then, a series of parameter analyses was conducted to determine the influence of different corrosion characteristics. Finally, morphology-based numerical methodologies were also proposed to predict the flexural performance including the load and deformation of corroded beams. The research results indicated that the theoretical formula and numerical methods could both provide accurate predictions, which could supply further technical support for the non-destructive detection and assessment of existing steel structures.

To investigate the degradation law of the mechanical properties of corroded steel, the standard specimens from machining steel members in service for 9 years in an industrial environment were subjected to tensile tests. The influences of different degrees and types of corrosion on the fracture path, stress-strain curve, and mechanical properties of specimens were discussed. Finally, the damage constitutive model of corroded steel was established based on the damage mechanics theory. The results showed that the failure modes of corroded specimens were related to the degrees and types of corrosion. The fracture morphology of specimens with general corrosion were step-like and the fractures of steel were uneven. However, those with local corrosion were mainly flat-like, and the fracture path was along the cross section where the larger corrosion pits were located. The fracture path of the specimen was related to the interaction of the corroded surface and internal material defects (holes). Meanwhile, with the increase of corrosion degree, the yield platform of stress-strain curve gradually became shorter, or even disappeared, and the ultimate strain and elongation at break decreased, implying that the ductility of steel became worse. Ultimately, the good agreement between the curves of the model and test indicated that the damage model could reflect the damage development process of corroded steel in the tensile process better. Corrosion damage resulted in the decrease in the damage threshold, and the damage variable D decreased by the time fracture occurred and the maximum reduction rate was up to 62.5%.