Nondestructive evaluation on dispersion of steel fibers in UHPC using THz electromagnetic waves

•Dispersibility of the steel fiber in UHPC is destructively evaluated like coring.•THz waves can be used as a new test method for detecting steel fiber in UHPC.•Dispersibility of steel fiber is quantitatively evaluated using THz 3D images.•This technology can be used as automated NDT method for enhanced quality control. Ultra-high performance concrete (UHPC), which has better mechanical properties and durability than reinforced concrete, can greatly reduce the weight of structures due to its high strength and reliability. So, it is very effective for the construction of high-rise buildings and long-span bridges. The mix design for UHPC is not only composed of traditional materials (i.e. cement, sand, and aggregate) but also includes binder and admixture. Moreover, steel fiber is necessary to improve ductility. Since the tensile strength of UHPC depends greatly on the direction of the steel fiber, it is important to confirm the dispersibility and orientation of the steel fiber. However, verification of the dispersibility of the steel fiber after curing is indirectly performed through destructive methods like strength tests through coring. Thus, for a better method of dispersibility confirmation, a more effective dispersion analysis technique is required. In this study, the dispersibility of steel fiber in UHPC was evaluated through tomography of UHPC using THz electromagnetic waves. These waves have both infrared straightness and microwave penetrability. Three UHPC specimens were fabricated with steel fiber and were tested with THz electromagnetic wave spectroscopy by experimentally scanning the specimen. From the experimental results, it was possible to obtain a 3D image of the steel fiber arrangement inside the specimen through scanning of the specimen. Also, through image analysis, the dispersibility of the steel fiber in each specimen was numerically derived. For the future, it is expected that the dispersion analysis technology proposed in this study will be applied as an automated nondestructive testing method for enhanced quality control.