Structural health monitoring (SHM) has become an integral part of essential and costly to replace infrastructures such as bridges and buildings which degrade during their lifetime. Understanding the structural behaviour of these infrastructures is critical for assuring their structural integrity and safe operating conditions. In this study, the structural performance of a reinforced concrete beam was examined under flexural loading. Distributed optical fibre sensor (DOFS) was attached to the beam’s bottom surface, and Optical Backscattered Reflectometry (OBR) technology was used to measure the surface strain. A three-point bending test was performed, and the sensor responses were acquired. Unusual high strain peaks were observed at the bottom surface of the beam due to the formation of hairline cracks. Subsequently, the Concrete Damaged Plasticity (CDP) based Finite Element Analysis (FEA) was performed to simulate the plastic behaviour of concrete beam using ABAQUS 2019 commercial software. The FEA results have strong agreement with the strain pattern observed from the DOFS. Therefore, DOFS and CDP technique based FEA can be successfully used to investigate the plastic damage pattern inside the concrete beam and distributed sensing demonstrates a greater capacity for long-term monitoring of the structural health of concrete structures.