Engineers from Novosibirsk State Technical University (NETI) together with scientists from Novosibirsk State University of Architecture and Civil Engineering (Sibstrin) have developed a unique prototype device that allows monitoring the distribution of steel fibers in steel-fiber concrete structures.
Steel fiber concrete is a composite material made of a concrete matrix reinforced with short steel fibers — fibers with a diameter of 0.25—1.2 mm, with a volume content of 0.5—3%. It is an effective replacement for classic concrete with core steel reinforcement. Steel fiber concrete is made by adding steel fiber to a specially selected concrete mix. The desired material characteristics are achieved provided that the steel fibers are evenly distributed throughout the entire volume of concrete.
Steel fiber reinforced concrete is used in structures of load-bearing elements, such as non-removable wall formwork, including for enclosing structures of nuclear power plants; in monolithic structures and structures (industrial floors, thin partitions, platforms, walls); hydraulic structures (dams, berths); precast reinforced concrete products (slabs, piles, beams, sleepers); particularly durable concrete structures and structures (bunkers, defense structures); in structures, structures and products for transport infrastructure (tunnels, bridges, road and airfield slabs).
According to Arseniy Morozov, head of the scientific research laboratory of the technical experiment of NSTU-NETI, one of the still unresolved tasks in the production of building structures made of steel-fiber concrete is to control the uniformity of fiber distribution in the product. Such control can be achieved using the electromagnetic properties of steel fiber concrete.
To solve this problem, mathematical modeling was carried out at the Department of Physics and Chemistry of NGASU, a mock-up of a fiber distribution control device was made and appropriate tests were carried out. Based on the test results of the mock-up, a compact portable quality control device for thin-walled steel-fiber-concrete formwork has been developed at the NSTU-NETI research laboratory. It allows you to control the uniformity of fiber distribution in steel fiber products up to 40 mm thick.
"The device contains a high-frequency oscillator, a magnetic field sensor and an analyzer with an indicator. The magnetic field sensor consists of two identical magnetically coupled coils (transformers). The first coil is an exemplary one, and the second is a measuring one. The sample coil is located away from the measured sample. The measuring coil is placed on the surface of the test sample. The device allows you to measure the effective value of the differential (difference) signal between the signals from the sample and measuring coils, as well as its phase relative to the master generator," said Arseniy Morozov.
The developed device measures the content of steel fiber in a specific location of the building structure in the sensor area. In this case, the readings will be most reliable with an even distribution of fibers. It is also possible to determine the depth of lowering of fibers in thin-walled products, which is important when using cast concrete mixtures. The calibration dependence is determined by specially made samples or by selected sections of the structure. To diagnose the distribution of steel fibers in structures with a thickness of more than 4 cm, it is necessary to change the shape of measuring electromagnetic circuits, for example, according to the "figure of eight" type, which has a greater directivity of field lines deep into the measured samples.
The device operates in two modes. The first is the comparison mode, in which the measurement results are compared with the measurement results of the test samples stored in memory. In the second mode, the absolute values of the measured values are displayed on the display screen. The first mode is most convenient for technological quality control of thin-walled steel-fiber-concrete formwork.
The main advantage of the developed prototype measuring device is its compactness, notes Arseniy Morozov. At its core, it is a metal detector configured to search for metal fibers. At the same time, the device is able to distinguish fiber from large metal objects (fittings, corners, channels). Thus, in addition to monitoring the uniformity of fiber distribution, the device allows searching for reinforcing rods and embedded metal structural elements in the presence of fiber.
The development of the measuring device was carried out with the involvement of students of the design bureau, which allowed them to acquire practical skills in the field of design, configuration and operation of measuring equipment. Based on the results of the work, one graduation project was completed.
Other developments of NIL TE NSTU-NETI can be found on the website.