Uncertainties regarding structural safety of reinforced concrete structures may warrant a need for a detailed assessment. A detailed assessment using nonlinear finite element analysis is one of the alternatives which could help in decision-making about maintaining, upgrading or even demolishing and rebuilding of the structure. A reliable assessment should account for the existing damage in the structure, which may cause re-distribution of stresses within the structure, giving rise to unexpected failure modes. The existing approaches in nonlinear finite element analysis which account for the effect of already undergone damage in concrete, pose a number of limitations which either makes structural analysis ambitious or the uncertainty of concrete damage is not effectively accounted for. An alternative approach is adopted in this thesis, which is phenomenological and probabilistic in nature. Existing damage in concrete is conceived as a statistical field, which can be input into a finite element model, such that the existence of damage is taken as the starting point of the structural analysis. Focusing on indications of damage on the surface of concrete, i.e. crack patterns, an exploratory methodology based on image analysis is developed, to account for information obtained from crack pattern observation into nonlinear finite element analysis of reinforced concrete structures. The methodology is implemented on MATLAB and validated on damaged experimental specimens. Results of the computational analyses indicate good efficiency in predicting residual load carrying capacities and failure modes, alongside insightful numerical crack patterns. Through a critical examination of the obtained results and reflection upon the assumptions and simplification made in the methodology, recommendations for future research are provided.