A method to monitor the mechanical behavior and identify the crack growth and location in a concrete structure element using a distributed fiber optic sensor system is demonstrated experimentally by testing concrete specimens in four-point bending. The sensor system consisted of an optical frequency-domain reflectometry (OFDR) interrogator unit paired with an all-grating sensing fiber that was bonded to the surface of the concrete test specimen. High spatial resolution of 10 mm scale strain measurements at various points along a single fiber cable were obtained. Large strain values at the crack locations indicated strain concentrations that could be used to assess the crack growth. The distributed sensing system demonstrated the capability to detect localized, early-stage cracks, with crack width smaller than 0.1 mm, well before they become observable by visual inspection.