Even under normal service loads, reinforced concrete structures in the field of civil engineering are full of micro cracks. More than twenty years of development of distributed fibre optic sensing techniques in terms of accuracy and spatial resolution made them capable of not only monitoring strain and temperature changes, but also detecting and localizing cracks. In order to detect safety-related issues and better maintenance and to offer better maintenance and management strategies, continuous monitoring of the crack width is also paramount. The paper focuses on the application of a theoretical mechanical strain transfer model in concrete substrate. The experimental setup involved employment of the Optical Frequency Domain Reflectometry (OFDR) technique with high spatial resolution. Three-point bending tests were performed on 1m length reinforced concrete beams instrumented with embedded and surface attached fibre optics cables. Results showed that the Crack Opening Displacement (COD) in single and multiple neighboring micro cracks cases can be well estimated.