# Ultimate Compressive Strains and Reserves of Bearing Capacity of Short RC Columns with Basalt Fiber
## 4 Results of Testing Short Columns
### Ultimate Strain of Concrete and Reinforcement in Test Columns  
When assigning the design resistances of reinforcement \(R_{SC}\), the current standards take into account not only the properties of steel, but also the possible ultimate compressibility of concrete. When testing short columns with high-strength reinforcement class A1000 for axial compression, the strains of concrete in specimens reinforced with fiber exceeded similar strains of specimens without fiber, not only in the ultimate supercritical stage of operation, but also over the entire range of increasing load (Table 9). An increase in the value of the peak load during testing led to an increase in the ultimate compressibility of concrete, which in turn led to an increase in the actual values of the ultimate compressive stresses in the reinforcement. It was experimentally established that the ultimate compressive stresses in the reinforcement of reinforced concrete elements with fiber reinforcement are higher than in similar ones without fiber. In particular, under axial compression, the discrepancy \(\sigma_{\text{sc}}\) in the samples of expanded clay concrete with fiber was 18.1% and 12.3%, respectively. The given curves' analysis shows that in the specimens-racks with combined reinforcement, the ultimate compressive strains in reinforcement and concrete were higher than in the same specimens but without fiber. Thus, the actual limiting values of strains in samples of heavy concrete with \(\mu_{f}=0\) (code T-1) were 2.85\(\cdot 10^{-3}\), and with fiber 3.08\(\cdot 10^{-3}\) (Table 9). Even higher values of ultimate compressive strains were obtained in expanded clay concrete specimens. In particular, in racks made of lightweight concrete with the code K-3 (without fiber), the ultimate values were \(\varepsilon_{s}=2.65\cdot 10^{-3}\) and expanded clay BFRC \(\varepsilon_{s}=3.05\cdot 10^{-3}\). In expanded clay BFRC specimens reinforced with standard hot-rolled steel grade A400, the actual ultimate compressive strains in the reinforcement were significantly higher than the ultimate compressive strains recorded in the columns without fibers. With an eccentric load application, the limiting compressive stresses in the reinforcement of the compressed zone increased even more (samples with the codes KK-1 and KKF-1). The absolute values \(\sigma_{\text{sc}}\) in the reinforcement class A1000 with forces of 0.98\(N_{u}\) reached 520 MPa, exceeding the standardized SP 63.13330.2018 [30], up to 30%. At the same time, at the same load level (the forces perceived by the structures during their operation were taken \(N=0.7N_{u}\) as the baseline), the actual stresses in the reinforcement of the FRC of the samples were lower than in the racks without fibers. It should be noted that, in quantitative terms, the ultimate compressive strains of concrete in short columns with combined reinforcement with steel of the A400 class and fiber were lower than those of similar FRC specimens reinforced with high-strength steel of the A1000 class. Nevertheless, the general tendency to increase the ultimate compressive strains in samples with fiber is traced during data processing in all tested racks. Ultimate compressive strain of concrete in specimens with combined reinforcement exceeded similar strains in specimens without fiber. In the case of axial compression, the absolute values \(\varepsilon_{\text{c}u}\) in the racks made of expanded clay BFRC with a section of 120 mm \(\times\) 200 mm were from 2.41 \(\cdot 10^{-3}\) to 2.75 \(\cdot 10^{-3}\), and in the samples without fibers, respectively, from 1.94 \(\cdot 10^{-3}\) to 2.21 \(\cdot 10^{-3}\). With an increase in the longitudinal force's relative eccentricity, fiber reinforcement efficiency in this group of samples also increased. Therefore, the ultimate values \(\varepsilon_{\text{sc}}\) and \(\varepsilon_{\text{c}u}\) in the samples with the code KKF-0.15 and KKF-1 exceeded the normalized ones by 12-18 and 33-46%, respectively.