*1.1. Fibers in Concrete*

While ordinary concrete is the world's most frequently used building material, its damaging structural qualities, such as brittle strain failure, must be improved. It has been established that adding fibers to the concrete mix improves the aforementioned properties by increasing the material's strength and decreasing brittleness.

The notion of reinforcing fibers is not a recent concept. Fibers have been used since ancient times as reinforcement; for example, horsehair has been used traditionally in mortar, and straw in mudbricks.

Asbestos fibers were used in buildings in the 1900s. Some of the topics of concern were the development of composite materials starting in the 1950s and fiber-reinforced concrete. When the health hazards related to asbestos were discovered, concrete and other products were required to remove the substance. In the 1960s, steel, glass (GFRC) and synthetic fibers (e.g., polypropylene) were used in concrete. Research on new concrete structures, made of fiber-reinforced concrete, continues today [1–3].

**Citation:** Olafsson, B.L.; Einarsson, G.J.; Thorhallsson, E.R. Flexural Performance of Chopped Basalt Fiber Reinforced Concrete Beams. *Mater. Proc.* **2023**, *13*, 29. https://doi.org/ 10.3390/materproc2023013029

Academic Editors: Katarzyna Mróz, Tomasz Tracz, Tomasz Zdeb and Izabela Hager

Published: 16 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Paul Dhé made the first attempts at basalt fiber production in the United States in 1923 and was later granted the US patent of 1.462.446. Scientists in the United States, Europe and the Soviet Union continued to develop these attempts in aerospace and military applications during the Second World War. Basalt fibers have been used since their declassification in 1995 in a broader range of civil applications.

Basalt fiber (BF) manufacturing technology is a single-stage process: melting, homogenization of basalt and extraction of fibers. Basalt is heated just once. BF is further refined into "cold processing" materials, with low energy costs [4].

BFs consist of one component, crushed basalt, from a carefully chosen source of a quarry. Basalt of high acidity (over 46% silica) and low iron element is considered suitable for fiber production. Basalt is an inert and naturally occurring material found around the world. Basalt products are environmentally friendly and not dangerous. Unlike other composites, such as glass fiber, no materials are substantially applied during manufacturing; the basalt is simply washed and then melted. The production of basalt fibers includes melting of the crushed and washed basalt rock at about 1.500 ◦C. The molten rock is extruded by small nozzles when the desired temperature is reached to create continuous basalt fiber filaments [5,6].

High strength and high modulus with excellent shock resistance are the main features of basalt fibers; they are a low-cost alternative to carbon fibers and can replace them in some applications. Basalt fiber reinforced concrete (BFRC) research has largely focused on fundamental mechanical properties: compressive, split tensile and flexural strength. In the case of BF, the research shows, in general, that the addition of fibers is beneficial up to approximately 0.3–0.5 percent by volume and detrimental thereafter [6]. A new basalt concrete reinforcement product, called minibars (MB), has recently been developed. The minibars are basically a scaled-down version of basalt fiber reinforced polymer rebar. In contrast to steel (SF) and fiberglass fibers, basalt is not affected by corrosion or acids. Basalt fibers even possess high corrosion and chemical durability to corrosive media such as salts, acid and alkali solutions. Basalt fibers have better mechanical properties than glass fibers in most cases, but in a few cases are somewhat worse than silica fibers. Due to high costs compared to other fibers, silica fibers are rarely used for concrete reinforcement. Basalt fibers are characterized by a high tensile strength from approximately 1700 MPa to 4800 MPa. The tensile strength of the basalt fibers (4800 MPa) is higher than that of the steel fibers [7].
