# Ultimate Compressive Strains and Reserves of Bearing Capacity of Short RC Columns with Basalt Fiber
## 3 Results of Testing Cubes and Prisms from Heavy and Light Expanded Clay BFC  
The change in the strength of the concrete, of experimental compositions on light and heavy aggregates, with and without fiber, was determined by the results of the testing cubes at 28 days of age (Table 6). The test results for cubes shown in Table 6 show that fiber reinforcement has a positive effect on increasing the strength of concrete on heavy- and lightweight aggregates. All else being equal, on the 28 th day of hardening, the average strength indicators of heavy concrete was \(R=24.1\) MPa, and for fiber-reinforced concrete was \(R_{f}=25.6\) MPa. Thus, the actual increase in strength was 1.5 MPa or 6%. The effect of fiber on increasing the strength of concrete on a lightweight aggregate was more noticeable. The average strength of concrete without fiber was 21.3 MPa, and fiber-reinforced concrete cubes showed strength of 23.4 MPa. The actual increase in strength was 2.1 MPa or 10%. This effect is due to the fact that, when mixing the concrete mixture, rounded expanded clay grains damage fibers to a lesser extent than gravel grains. The measurement of the deformability of concrete on light and heavy aggregates under compression was determined by the results of testing prisms at 28 days of age (Table 7). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Concrete Type** & **Fiber Percentage,\%** & **Concrete Age, (Days)** &
\begin{tabular}{c} **Actual Strength, MPa** \\ \(R\) (\(R_{f}\))** \\ \end{tabular} & \(R_{f}\)/\(R\) \\ \hline Heavy & \(\mu_{f}=0\) & 28 & 24.1 & 1.06 \\  & \(\mu_{f}=10\) & 28 & 25.6 & \\ Lightweight expanded clay concrete & \(\mu_{f}=0\) & 28 & 21.3 & \\  & \(\mu_{f}=10\) & 28 & 23.4 & 1.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Cubic compressive strength of fine-grained concrete class. The test results of the prisms (Table 7) show that fiber reinforcement has a positive effect on increasing the strength and deformability of concrete on heavy and light aggregates. On the 28 th day of hardening, the average values of the prismatic strength of heavy concrete were 18.3 MPa, and for fiber-reinforced concrete, 21.3 MPa. Thus, the actual increase in prismatic strength was 3.0 MPa, or 16%. Ultimate strains corresponding to the ultimate compressive strength of concrete were 1.87 \(\times\) 10\({}^{-3}\) and 2.24 \(\times\) 10\({}^{-3}\) in FRC (the increase in ultimate deformability was 19.8%). In concrete on lightweight aggregate, the average value of the prismatic strength of heavy concrete was 18.5 MPa, and in fiber-reinforced concrete 21.8 MPa (an increase in prismatic strength by 3.3 MPa or 18%). Ultimate strains corresponding to the ultimate compressive strength of concrete were 2.26 \(\times\) 10\({}^{-3}\) and 2.85 \(\times\) 10\({}^{-3}\) in FRC (the increase in ultimate deformability was 26.1%). The coefficient of transition from cubic strength to prismatic \(R_{b}/R\) (Table 7) for expanded clay FRC was higher than that of specimens not reinforced with fiber. The actual values of Poisson's ratio in the lightweight FRC ranged from 0.22 to 0.27, exceeding the normalized value. The change in the ratio between an elastic and total strain with increasing stresses is presented in Figure 4, which shows the dependence coefficient of elastic-plastic on the stress-strain level \(\sigma/R_{b}\). The coefficient of elastic-plastic strain is the ratio of the elastic strain of concrete to the total strain, measured at different loading levels. In contrast to heavy concrete, the non-linear nature of strain in lightweight concrete appears earlier. Thus, the coefficient of elastic-plastic strain becomes less than one at a ratio \(\sigma/R_{b}\) in the range of 0.26-0.35, and upon further strain, it decreases relatively uniformly with an increase in stresses, i.e., almost linearly until crushing.