Karel Künzel

This study investigated the feasibility and effects of two-dimensional magnetic orientation of steel fibers in high-performance fiber-reinforced concrete (HPFRC) slab elements subjected to punching loading. Although one-dimensional fiber orientation has shown significant improvements in concrete’s mechanical properties, this research explored fiber application in two-dimensional configurations. This could offer advantages in prefabricated construction, such as ceiling panels, traffic barriers, protective structures, and so forth. The scope of the work involved manufacturing HPFRC specimens with various two-dimensional fiber orientation patterns using a custom-built magnetic orientating device. A nondestructive method, utilizing a coil’s quality factor Q measurements, was employed to evaluate the success of fiber alignment. The mechanical performance was assessed using quasi-static punching tests with clamped circumference. Cracking patterns were analyzed to understand failure mechanisms, which clearly revealed the oriented fiber paths. Two-dimensional fiber orientation was achieved successfully, proving that the manufacturing process works well. However, the results indicate that despite this, preferential alignment does not universally enhance punching resistance in all loading scenarios. In fact, semirandom fiber orientation performed comparably to some orientation patterns and significantly better than others, suggesting that specific loading cases, such as punching, benefit from the more-homogeneous properties of randomly distributed fibers. This study proposes possible future research headings to optimize the orientation process and usage.

A key part in optimising the performance of fibre-reinforced concrete (FRC) lies in ensuring the correct orientation of fibres, especially in structural components where directional stress resistance is critical, such as beams and slabs. However, assessing the orientation of fibres in hardened concrete poses a significant challenge. This study explores the use of the non-destructive evaluation of fibre orientation within FRC by measuring a quality factor (Q-factor) of an electrical coil with an inserted FRC specimen. Through numerical simulations, the effectiveness of the Q-factor measurements were evaluated using several scenarios of fibre positions, quantity, and orientations. It was discovered that the Q-factor is sensitive to these variables and, therefore, it is a suitable tool for material diagnostics.

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 ferromagnetic (steel) fiber parameters especially suited for 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 the prefabrication of high-performance structural elements. This study details the theoretical background, experimental setup, methodology and provides an evaluation of the diagnostic system’s effectiveness and accuracy. The experiments were performed using steel fiber specimens with cardboard as a carrier. This allows for the exact placement and orientation of the fibers in a matrix that has negligible influence on the magnetic field, similar to plain concrete. The results demonstrate a strong correlation between the electromagnetic measurements and the actual fiber parameters, confirming the method’s reliability.

Steel fiber-reinforced concrete (SFRC) can exhibit markedly improved mechanical performance when the fibers are preferentially aligned along the principal tensile stress directions. One method of aligning steel fibers is using magnetic methods. However, most existing magnetic alignment techniques rely on solenoids and are restricted to one-dimensional alignment and relatively small specimen sizes. This paper presents a novel planar magnetic orientation device capable of producing arbitrary two-dimensional fiber layouts and demonstrates its applicability from laboratory-scale proof-of-concept tests to large high-performance fiber-reinforced concrete (HPFRC) structural elements. The concept is first verified on transparent ultrasound gel specimens, where image analysis confirms fiber orientation in the prescribed angles. The method is then applied to small prismatic HPFRC specimens (40 mm × 40 mm × 160 mm) with fiber contents of 0.5%, 1.0%, and 1.5%, exposed to different magnetic field intensities (80 mT–140 mT). Flexural tests show increases in average flexural strength compared to non-oriented reference specimens, with 100 mT providing the most efficient alignment for the investigated mixture. A non-destructive electromagnetic method based on the measurement of the quality factor Q of a coil correlates well with flexural strength. Finally, the device is integrated with an industrial robot and used to orient fibers in large HPFRC slabs (1000 mm × 410 mm), achieving an average increase in flexural tensile strength of about 64% relative to non-oriented slabs. The results demonstrate that planar magnetic orientation is a promising approach for tailoring fiber layouts in SFRC structural elements and for enabling automated, programmable manufacturing.

This study investigates fiber content and orientation in prefabricated slab elements made of ultra-high-performance fiber-reinforced concrete (UHPFRC), using novel non-destructive measurement using a coil’s quality factor, where the coil is put to one side of the specimen only. This allows the analysis of slab specimens of arbitrary size. That then allows an accurate quality control of elements made in the prefabrication industry. This study presents an experimental campaign designed to evaluate the non-destructive principle’s accuracy and practical feasibility. Twenty-five large slab specimens were made in an industrial prefabrication plant using various casting methods to introduce different flow-induced fiber parameters. The slabs were subjected to this non-destructive testing, then destructive bending tests and CT scanning to tie the results together and validate the non-destructive results. The results showed that the coil’s quality factor values correspond well to the distribution (concentration) and orientation of fibers, and the method reliably reveals potential defects of the material and can predict the element’s mechanical properties.

This paper explores the practical possibility of using a magnetic field to orient steel fibres in a fresh concrete matrix. This process leads to...

The efficiency of fibre reinforcement in concrete can be drastically increased by orienting the fibres using a magnetic field. This orientation occurs immediately after pouring fresh concrete when the fibres can still move. The technique is most relevant for manufacturing prefabricated elements such as beams or columns. However, the parameters of such a field are not immediately apparent, as they depend on the specific fibre reaction to the magnetic field. In this study, a numerical model was created in ANSYS Maxwell to examine the mechanical torque acting on fibres placed in a magnetic field with varying parameters. The model consists of a single fibre placed between two Helmholtz coils. The simulations were verified with an experimental setup as well as theoretical relationships. Ten different fibre types, both straight and hook-ended, were examined. The developed model can be successfully used to study the behaviour of fibres in a magnetic field. The fibre size plays the most important role together with the magnetic saturation of the fibre material. Multiple fibres show significant interactions.

This paper deals with the description, measurement, and use of electromagnetic properties of ferromagnetic fibres used as dispersed fibre reinforcement in composite mixtures. Firstly, the fibres’ magnetic properties are shown, and a method of measuring the hysteresis loop of fibres is proposed. The results from the measurements are presented and a discussion of the influence of measured parameters on the fibres’ orientation in a magnetic field is performed. Furthermore, methods of non-destructive estimation, of their amount and orientation in the composite specimens, are discussed. The main experimental goal of this paper is to show the relationship between this non-destructive method’s results and the destructive flexural strength measurements. The method is sensitive enough to provide information related to fibre reinforcement.

We show that two-terminal multijunction cells interconnected by tunnel junctions are fairly immune to individual local shunts, thanks to the shunt quenching. Interestingly, they may still suffer from global shunts. We revise the paradigm of a multijunction cell as a simple serial connection of component cells. This paradigm remains valid only for multijunction cells with laterally conductive interlayers. Instead, a new equivalent circuit is proposed and verified by measurement and simulations. As a main approach, selective illumination is applied and the voltage is measured at the end terminals. The global shunt is seen as a shift from logarithmic to linear intensity response. The presence of tunnel junction is important for an optimum configuration of tandem structures such as metal-halide perovskite with crystalline silicon solar cell.