Steel Fiber-Reinforcement is a way to strengthen concrete structures which results to be more flexible and less expensive than the conventional hand-tied rebars, while still increasing the tensile strength. In order to ensure the prescribed tensile strength, the conglomerate needs to have a certain volumetric density of fibers uniformily distributed over the structure. In this paper, a novel study on microwave Non-Destructive Testing (NDT) for Steel Fiber Reinforced Concrete (SFRC) is developed, with the objective to produce an image of the fiber density distribution in the concrete. An electromagnetic model is formulated relating the electromagnetic properties of SFRC to physical properties of this conglomerate such as fiber density or humidity content. A measurement system based on two dielectric waveguide antennas working at the appropiate microwave frequency range is designed, according to the environmental constraints and subject to the electromagnetic requirements of the situation. Laboratory non-destructive measurements on real SFRC test structures and images of the fiber density distribution in the SFRC are shown in this paper. Agreement with the theoretical simulated results is achieved.