# Behavior of Two-Chord Steel-Concrete Composite Columns under Axial Compression
## 2 Experimental Program
### Material Properties  
In the case of normal-strength concrete and normal-strength steel, the peak strain of concrete is very close to the steel yield strain, which implies that the materials are strain compatible. Because of that, both materials in the composite section can simultaneously reach their full strengths. However, high-strength concrete and steel are not strain compatible since concrete reaches its peak strain before the yielding of high-strength steel is attained. Steel is unable to develop its full strength since concrete will fail before steel, causing a partial utilization of the materials' capacities. The highest strength of structural steel and concrete that can be fully exploited in a composite section can be determined using the strain compatibility method. In this study, steel with a yield strength of 600 N/mm\({}^{2}\) was used in combination with concrete that had a cylinder compressive strength of 95 N/mm\({}^{2}\), ensuring full strain compatibility. The composition of the concrete mix was formulated for the required compressive strength class. The proportions of the fresh concrete mixture were as follows: cement 670 kg/m\({}^{3}\), coarse aggregate 600 kg/m\({}^{3}\), fine aggregate 984 kg/m\({}^{3}\), fly ash 85 kg/m\({}^{3}\), water 135 kg/m\({}^{3}\), and superplasticizer 12 kg/m\({}^{3}\). Crushed diabase rock was used as the coarse aggregate, while quartz sand was the fine aggregate, and the cement was CEM 152.5. A high-range water-reducing admixture was utilized to ensure sufficient viscosity for the casting of the fresh concrete into the chord tubes. The standard test specimens used to \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No.** & **Label** & \(A\) **(mm\({}^{2}\))** & \(I\) **(mm\({}^{4}\))** & \(\lambda_{n}\) **(–)** & \(\xi\) & \(N_{u}\) **(kN)** \\ \hline
1 & S-21 & 1415 & 563,458 & 25 & - & 732 \\
2 & S-22 & 1415 & 563,458 & 25 & - & 720 \\
3 & SC-21 & 2451 & 740,538 & 29 & 2.76 & 1252 \\
4 & SC-22 & 2451 & 740,538 & 29 & 2.76 & 1276 \\
5 & SC-23 & 2451 & 740,538 & 29 & 2.76 & 1260 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of test results. Figure 1: Dimensions of column models (**a**); filling of the chords with fresh concrete (**b**). define the properties of the cured concrete included 150 mm cubes, 150 mm \(\times\) 300 mm cylinders, and 100 mm \(\times\) 100 mm \(\times\) 400 mm prisms according to SRPS EN 12390-1 [25]. All test specimens were cast and cured under conditions similar to those of the TCCFST column specimens according to SRPS EN 12390-2 [26]. The mean values for the cylinder, cube, and characteristic compressive strength are listed in Table 2. Additionally, the flexural tensile strength, hardened concrete density, Poisson's ratio, and elastic modulus are listed. The mechanical properties of the concrete were determined on the same day when the testing of the TCCFST specimens was performed. The tests were conducted according to SRPS EN 12390-3 [27] (compressive strength), SRPS EN 12390-5 [28] (flexural tensile strength), and SRPS EN 12390-7 [29] (density). Thus, the characteristics of the used concrete were precisely identified at the moment of the column model testing. The stress-strain curves of the concrete prism recorded during the compressive test are presented in Figure 2a. The axial (\(\varepsilon_{c,A}\)) and the lateral (\(\varepsilon_{c,L}\)) strains were measured with strain gauges placed in the middle of the test prism (Figure 2b). Under axial compression, the concrete exhibited an almost linear response up to around 0.8\(f_{c}\); the deformations then increased, and sudden brittle failure occurred after reaching \(f_{c}\), which is typical behavior of high-strength concrete. All steel tubes used for built-up column models were factory-manufactured. Three standard test samples were cut from the original steel tubes and were tested in tension according to SRPS EN ISO 6892-1 [30].