**1. Introduction**

In previous practice, concrete additives in the form of mineral ingredients or chemical admixtures were used for two reasons: (1) to obtain concretes with increased strength parameters, and (2) to use waste raw materials and to increase the recycling rate. This article describes the use of concrete additives in the form of various fibers in order to obtain a gentle change of the strength parameters of fragments of the railway surface on the section of the transition zone in order to eliminate, or at least reduce, the impact of the threshold effect on the dynamic interactions at the interface of the rail vehicle with the railway track.

### *1.1. A Brief Overview of Previous Studies*

With the development of construction, there is an increase in demand for concretes with high strength parameters. Due to the occurrence of concrete shrinkage, it is not possible to increase the cement content in it indefinitely. For this reason, a number of mineral additives and chemical admixtures have been used for concrete for decades [1–4]. One of them is diffuse reinforcement in the form of fibers. Currently, such solutions are used in ceilings, tunnels, foundations, road construction and even prefabricated elements [5–8]. The main task of diffuse reinforcement is to prevent the formation of shrinkage microcracks in concrete. Moreover, fibers for concrete have a beneficial effect on mechanical and operational parameters after bonding the concrete mix. They may improve the compressive strength, as well as improve the frost resistance of concrete. Some modern techniques for

**Citation:** Idczak, W.; Lewandrowski, T.; Pokropski, D.; Rogojsz, G.; Rudnicki, T. The Influence of the Type of Fibers on the Reduction of the Threshold Effect in the Transition Zone of a Railway Track. *Materials* **2022**, *15*, 5730. https://doi.org/ 10.3390/ma15165730

Academic Editor: Andreas Lampropoulos

Received: 1 July 2022 Accepted: 18 August 2022 Published: 19 August 2022

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concrete reinforcement with polymers were widely described in [9]. In addition, the use of additives in the form of fibers for the production of concrete has an ecological aspect, because in this case, waste materials, such as used glass or plastics, become additives. The use of used glass or plastics for the production of additives for concrete can improve the recycling rate of these materials. For example, glass recycling in Poland is about 62% and is several percent lower than the average for the European Union. In many countries, this indicator reaches up to 95%. Increasing the use of concrete additives makes it possible to increase this indicator so that as little waste as possible goes to landfills [10]. Another very interesting study, concerning the influence of certain recycled aggregates added in place of sand on the mechanical properties of concrete, is presented in [11]. Extending the life cycle of concrete used in road construction can be achieved, for example, by 100% recycling of the concrete surface even after 80 years or by using cements with a low carbon footprint. A very important element in improving the durability of the road surface is the use of fibers and chemical admixtures that increase the plasticity of the mixture and shape the structure of aeration of hardened concrete [12].

### *1.2. New Applications*

The use of various concrete additives to obtain materials with different, slightly changing strength parameters has not been the subject of research described in the available literature. This type of research was initiated in the work presented in [13]. This article is a continuation of the research presented there, where a simple computational theoretical model of the interaction of a rail vehicle with a railway track, which made it possible to study dynamic phenomena arising in the zones of change of railway surface technology, was described. In the literature on the subject for calculations, a model of the railway surface is adopted, in which the rail is based on the elastic Winkler substrate characterized by the modulus of elasticity of the substrate. The validity of the adopted model was confirmed by previous research and analytical work, among others, presented in [14]. The validity of this approach has also been proven, inter alia, in [15], where a number of variants of rail, including four ways of mapping a vehicle in the form of a stream of concentrated forces, a stream of concentrated masses and streams of one- and two-mass oscillators, were analyzed. The same representations of a moving-rail vehicle were also used in [16–18], each time giving correct, real results. The theoretical model described in [13] was experimentally verified using laser scanning technology on active sections of the railway line. The advantages of measurements performed in this technology include high accuracy and automation, as well as the speed of measurements and the lack of the need to destroy the tested object or exclude it from operation. The main element of the measuring set included the laser scanner of the scanCONTROL LLT2610-50 series [19]. Thanks to positive experimental verification of the computational theoretical model, further theoretical analyses were possible for any/variable material technologies of surfaces in transition zones where the threshold phenomenon occurs. The threshold effect has a negative impact not only on the railway surface, but on the object that is exposed to excessive loads and vibrations as well [20–22]. The negative effects of the threshold effect were reduced in [13] by modifications of the railway ballast surface. In this article, concerning experimental research, an attempt is made to modify the ballastless section of the railway surface. Thanks to this, after applying the obtained set of materials in the elements of the railway surface in the transition zones: in front of and behind the railway engineering infrastructure facility, a significant reduction in the adverse effects of the threshold effect that occurs in these zones will be achieved. Thus, the next points of the article describe research on the mechanical properties of the material samples themselves, as well as tests of laboratory models of specific elements of the railway track, used in the transition zones of the railway line, made of previously tested materials.

### **2. Materials and Methods**
