Transmission Approach for Near-Field Non Destructive Characterization of Steel Fiber Reincorced Concrete at Microwave Frequencies # G. Roqueta 1 , J. Romeu and L. Jofre # Antennalab, Universitat Polit` ecnica de Catalunya. Barcelona, Spain 1 gemma.roqueta@tsc.upc.edu Abstract 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 de- veloped, 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. 1 Introduction Thanks to its improved mechanical properties, Steel Fiber Reinforced Concrete (SFRC) is being increas- ingly used in civil constructions that need to withstand heavy forces such as pavements, airport runways and subway tunnels [1]. In SFRC composites, uniform density and random orientation of fibers are cru- cial to ensure isotropy and uniformity of the strength properties of the material. Nowadays, destructive methods based on magnetic approaches are the most common technique used to assess the dosage and uniformity of fiber distribution, however, they provide partial information but not a pixeled image of the whole volume of the concrete structure, as it is desirable for civil engineering purposes. To address these problems, a new non-destructive imaging technique using a microwave system for the fiber density diagnostics is proposed in this paper. In this context, microwaves, due to their capability to sense light-opaque materials with reasonable spatial resolution, penetrability and discriminability, offer an interesting approach. The presence of fibers inside the material modifies the complex dielectric permittivity of the medium and, therefore, produces variations in the propagation of microwaves through it. Consequently, the measurement of electromagnetic properties carries information about the fiber density, leading to the obtention of images of the fiber density distribution in the concrete structure. In this paper, the performance of the proposed electromagnetic imaging technology is investigated through an experiment on multiple SFRC blocks with different fiber densities arranged forming a test wall, named Lego wall.