This work shows how the experimental strain data obtained with an OBR (Optical Backscattered Reflectometer) measuring system can be used to locate cracking before being visually observable and also to estimate the crack width for different levels of load. The method is checked in a test up to failure carried out on a reinforced concrete slab, where the deployement of other standard sensors allowed to validate the results from the OBR system. The results are also compared with those coming from a 2D non-linear finite element model, showing also a good agreement.