In this paper, a method to obtain crack initiation, location and width in concrete structures subjected to bending and instrumented with an OBR system (Optical Backscattered Reflectometer) is proposed. Continuous strain data with high spatial resolution and accuracy are the main advantages of the OBR system. These characteristics make this Structural Health Monitoring (SHM) technique a useful tool in early damage detection in important structural problems. In the specific case of reinforced concrete structures, which exhibit cracks even in-service loading, the possibility to obtain strain data with high spatial resolution is a main issue. In this way, this information is of paramount importance concerning the durability and long performance and management of concrete structures.The proposed method is based on the results of a test up to failure carried out on a reinforced concrete slab. Using test data and different crack modelling criteria in concrete structures, simple non-linear finite element models were elaborated to validate its use in the localization and appraisal of the crack width in the testing slab.