Non-destructive diagnostics of fiber orientation in large planar fiber-reinforced concrete specimens

TL;DR

Non-destructive diagnostics address fiber orientation and content in large planar fiber-reinforced concrete using coil quality factor $Q$ as the key indicator. The approach relies on perturbation theory where eddy currents in steel fibers damp the coil, with skin depth $\,\delta$ governing losses and $Q$-dependence on fiber geometry and angle via $\alpha$. A surface-oriented half-toroid coil and an industrial quality-factor meter are developed and validated against conventional LCR meters, demonstrating strong correlation between $Q$ and fiber parameters. The method offers a practical pathway for in-situ quality control in prefabrication, while recognizing depth-related shielding limits for thicker elements.

Abstract

The mechanical performance of fiber-reinforced concrete is critically dependent on the orientation and concentration of its reinforcing fibers. This paper presents a non-destructive method for diagnosing fiber parameters in large planar specimens. The proposed technique utilizes an electromagnetic measurement system using electrical coils with commercial LCR meters, but also a novel custom-made meter suitable for factory environments, offering a practical solution for quality control in prefabrication of high-performance structural elements. This study details the theoretical background, experimental setup and methodology, and provides an evaluation of the diagnostic system's effectiveness and accuracy. The results demonstrate a strong correlation between the electromagnetic measurements and the actual fiber parameters, confirming the method's reliability.

Figures (16)

• Figure 1: Cross-section of a fiber with diameter $R$ with indicated current path with diameter $r$ and thickness $dr$.
• Figure 2: Intensity of the magnetic field in the fiber according to linearly increasing magnetic potential, where $x$ is a coordinate parallel to the magnetic field, $\varphi$ is the magnetic potential, $\alpha$ is the fiber's angle relative to the magnetic field and $H_f$ is the intensity of the magnetic field inside the fiber.
• Figure 3: Example of influence of frequency and fiber angle on the quality factor. Applicability of the theoretical equation.
• Figure 4: Series resonance circuit -- G is the signal generator, $U_G$ is the generator's voltage, $L_S$ is the coil's inductance, $R_S$ is the coil's loss resistance, C is the variable capacitor and $U_C$ is the voltage across the capacitor.
• Figure 5: Measuring coil for measuring specimens inside the coil. The cardboard specimen is inserted.