Microwave Waveguide Nondestructive Testing Sensor for Assessment of Steel Fiber Reinforced Concrete

Concrete structures exhibit improved damage resistance and performance with the inclusion of steel fibers. The performance of fiber-reinforced concrete (FRC) is significantly influenced by fiber content, distribution, and orientation. However, there is currently no widely accepted technique for efficiently evaluating these factors, especially on-site. This paper explores the viability of employing a surface electromagnetic sensor (rectangular waveguide) as a non-destructive microwave testing apparatus to assess the concentration, dispersion, and orientation of steel fibers in concrete. The testing requires access to only one side of the FRC material and necessitates direct contact with the surface to measure the microwave electromagnetic characteristics at various points. Additionally, the sensor's ability to rotate allows for the assessment of fiber orientation by polarizing the electric field at different angles. The study reveals that the measured reflection properties exhibit a rise in mean value with increasing fiber content and improved fiber distribution. Furthermore, the findings indicate that the microwave reflection characteristics are minimized when the fibers are oriented perpendicular to the electric field and maximized when aligned parallel to it. Regression analysis was conducted to establish the relation between the measured microwave electromagnetic characteristics and fiber content and flexural strength. The proposed models can estimate the fiber content and the flexural strength based on the measured wave reflection with determination coefficients of 0.98 and 0.945, respectively at the 10 GHz frequency range.