**4. Results**

Results from DMA tests were shown as storage and loss moduli changes (Figure 3). The glass transition temperature determined from loss modulus changes along the temperature growth was equal to 107.5 ◦C (Figure 3).

**Figure 3.** Results from DMA test.

The results from tensile strength tests were summarized (Table 1). The medium values of elasticity modulus were in the range from 43.3 to 44.6 MPa, while tensile strength medium values were 930.5 to 1073.1 MPa.


**Table 1.** Summary of the results—BFRP bars in tension.

<sup>1</sup> determined only for one specimen. <sup>2</sup> mean value calculated on specimens tested from two directions.

Two out of the three Ø12 specimens with caps mounted with epoxy resin did not fail during the test as a result as achieving stresses equal to tensile strength, but the FRP bars with hardened epoxy resin started to slide out of the steel caps. Therefore, maximum strength values should not be considered as tensile strength in that case, and were excluded from the analysis. Nevertheless, elasticity moduli were calculated for these specimens. Figure 4 shows the typical mode of failure for the analysed specimens.

**Figure 4.** Specimen after failure in tension.

The results of the compressive tests at room temperature are summarized in Table 2, while Figure 5 shows the typical form of failure in compression. The medium values of compressive strength were in the range of 441.2 to 456.0 MPa, and medium values of elasticity modulus were in the range of 31.0 to 38.4 MPa.



<sup>1</sup> mean value calculated on specimens tested from two directions.

**Figure 5.** Specimen after failure in compression.

Compressive strength along with temperature at the surface of the specimen at failure time, at compressive strength tests at elevated temperatures (100 ◦C and 200 ◦C), are summarized in Table 3. Additionally, the results for four reference specimens tested without heating on the same day are included in this table. The strength retention ratio calculated for the medium temperature at failure equal to 97.3 ◦C was 24%, and for 191.0 ◦C, it was 8%.

**Table 3.** Compressive strength tests during heating results.


### **5. Discussion**

Tensile and compressive strength for BFRP bars may strongly vary depending on the type of used matrix, fibres, and volumetric proportions between matrix and fibres.

Basing on a comparison of the results from available experimental studies on mechanical properties of the BFRP bars ([2,3,12–16]—Table 4) the differences between tensile strength may vary from under 600 to even over 1500 MPa, which is a very wide range. In most cases, the tensile strength of the BFRP bars was higher than the typical value of tensile strength for steel reinforcement (about 500–600 MPa). However, no yielding occurs for non-metallic bars. As a result, rupture failure modes were noted in most cases in tension, which may result in a low safe reserve for design purposes.

In terms of tensile strength, the results from this study are similar to the works of Protchenko et al. [13], Urbanski et al. [14] and Włodarczyk and Trofimczuk [16].

Regarding compressive strength of the BFRP bars, there are few data available in the literature concerning this parameter. The reason for that may be the fact that reinforcement bars are typically submitted to tension during their lifecycle in most concrete structures. However, it is worth considering during designing that the compressive strength of the bars can be significantly lower than their tensile strength, and also lower than compressive strength of most of steel reinforcement bars (typically around 500–600 MPa). Moreover, similar results for compressive strength were noted within this study and by Thiyagarajan et al. [3] (about 450–500 MPa).

Elasticity modulus in tension measured within this study was equal to about 45 GPa, which is similar to other studies (38.34–52.0), apart from the Elgabbas et al. studies [2,15], in which the bars had a significantly higher stiffness (59.5–90.4 GPa). Even the highest value of elasticity modulus for BFRP bars amongst the available analysed literature (90.4 GPa) is much lower than typical values for reinforcement steel (about 210 GPa). This may lead to excessive deflections and crack propagation in bent concrete elements with non-metallic reinforcement.

Additionally, this study aimed to experimentally determine elasticity moduli at compression, which were 15–30% lower than the values measured in tension.

Further research will concern the examination of tensile strength at elevated and high temperatures in comparison to the available literature data [17–20]. The results from the current study will also be used for numerical modelling purposes regarding axially compressed concrete columns with BFRP reinforcement bars at room and high temperatures.


**Table 4.** Mechanical properties of the BFRP bars—comparison.
