**1. Introduction**

Fibre-Reinforced Polymer (FRP) is a light, high-strength, and durable material. Its electric indifference, high corrosion resistance, high tensile strength, good damage tolerance, good fatigue performance and low energy consumption during the fabrication of raw materials should also be highlighted [1–8]. These advantages make them potentially attractive as an alternative to traditional reinforcement. Nevertheless, there are also important disadvantages, when comparing FRP to steel, which may significantly influence the performance of such a reinforcement in concrete building structures. One of these disadvantages is the fact that FRPs have much lower compressive strength than the tensile strength, another that they have a low elasticity modulus and, finally, poor mechanical performance at even slightly elevated temperatures.

This study aims to analyse thermal and mechanical properties of the basalt type of FRP, which is relatively new and has not as yet been sufficiently examined [9]. The environmental

**Citation:** Wydra, M.; Dolny, P.; Sadowski, G.; Grochowska, N.; Turkowski, P.; Fangrat, J. Analysis of Thermal and Mechanical Parameters of the BFRP Bars. *Mater. Proc.* **2023**, *13*, 24. https://doi.org/10.3390/ materproc2023013024

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

Published: 15 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/).

impact of BFRP composites (especially in terms of costs and amount of energy during production) should be emphasised, as it is lower when compared to CFRP [10].

#### **2. Materials and Specimens**

Three diameters of the same type of BFRP bars were tested: 8, 10, and 12 mm. The fibre content was equal to 77%, and epoxy type of matrix was used. Precise diameters of the bars were measured in five random locations along the bars and were equal to 8.1 ± 0.2, 9.2 ± 0.1 and 11.6 ± 0.3 (mean value ± standard deviation), respectively.

A cuboid specimen was cut out of the inner part of the Ø12 BFRP bar, with the cross section of 9.64 mm and 3.28 mm, on which Dynamic Mechanical Analysis was performed in order to determine the glass transition temperature of the analysed BFRP material.

Ø10 and Ø12 BFRP bars with the length of 1 m were tested in tension. In that case, 330 mm-long steel pipes were mounted at the end specimens in order to prevent crushing of the FRP in the grip of the hydraulic press (see Figure 1). Either epoxy resin (for Ø10 BFRP specimens) or expansive mortar (Ø10 and Ø12) was used to attach the FRP bars into the steel pipes.

**Figure 1.** Specimens for tension tests (Ø12 BFRP, steel caps mounted with the use of expansive mortar).

The specimens in compression at both room and elevated temperatures were tested with the use of steel caps (see Figure 2a), similarly to experiments performed by Khorramian and Sadeghian [11].

**Figure 2.** Specimen for compressive strength test (Ø12 BFRP) (**a**); specimen for compression test wrapped with two layers of ceramic wool (**b**); black–white pattern at the specimen's surface (**c**).

The steel plates (30 × 30 × 2 mm for Ø10 and 50 × 50 × 5 mm for Ø8 and Ø12) were welded with the round pipe pieces (Ø20.0 × 10 × 2 mm for Ø10 and Ø26.9 × 12 × 2 mm for Ø8 and Ø12). After preparation of the steel caps, they were attached at the ends of FRP bars with the use of epoxy resin and positioned with the use of a spirit level. The length of the bars was 4 cm. In the case of Ø12 bars, the method of specimens' preparation was improved in order to enable examination at higher temperatures. Therefore, cementitious expansive mortar was used instead of epoxy resin.

In the case of specimens tested at elevated temperatures, two layers of ceramic wool (see Figure 2b) were used to sustain the temperature after removing the specimens from the thermal chamber and placing them at the test stand.

In the case of specimens tested at room temperature in both compression and tension, a black–white pattern was added at the surface of the specimens (Figure 2c), so that Digital Image Correlation could be used to determine the strains during the tests, and as a result, moduli of elasticity (at compression and tension) could be calculated.
