# Black Loaded Columns--State of the Art Review
## 2 Experimental Testing
### Building Columns
[20] tested two types of CFDST columns, circular and square, with inner and outer tubes, as shown in Figure 9. They concluded that columns filled with normal strength concrete experienced greater crushing of concrete and higher steel buckling than columns filled with UHPCC. Moreover, UHPCC proved to be very resistant to spalling or crushing. The tested CFDST and CFST samples have similar oscillation periods and displacements, so it is concluded that they behave similarly. [PERSON] et al. [21] tested six ultra-high-performance concrete-filled double skin tube columns with square hollow sections. At a standoff distance of 1.5 m, the specimen exposed to 35 kg of TNT did not experience any localized damage or steel buckling. Moreover, the axial load (25% of the maximum load) contributed to the reduction in maximum deflection in the middle of the column. They concluded that the ratio of the cavity and the section influence the overall column deflection and period of oscillation; therefore, it is recommended not to go above 0.5. [PERSON] et al. [19] exposed four circular Reactive Powder Concrete Filled Steel Tubular columns to explosion and fire durations of 0, 60, and 105 min. Fixed supports were simulated, and on one side, an axial load was introduced. Steel tube protects columns against cracks and spalling of concrete. After the detonation of the 17.5 kg explosive, the column experienced bending and after 35 kg bending-shearing deformations. With an increase in the number of explosives and with longer exposure to fire, maximum and residual displacements increased. [PERSON] et al. [24] studied four square and four circular CFST columns under close-range blasts. The column length was 2.5 m, and the thickness of the steel tube was 2.8 mm or 3.8 mm. The standoff distance of the emulsion explosive was 1.5 m, and the charge weight expressed through the TNT equivalence was from 25 to 50 kg. Only a 10% increase in the amount of explosive in square columns increases mid-span deflection by 200%, which is assumed to be caused by a large surface exposed to the blast load. Moreover, increasing the thickness of the steel tube by 1 mm (from 2.8 to 3.8 mm) significantly reduced the displacement (by over 50%). Global failure mode was a flexural failure, and after removing the steel, the square columns sustained spalling and crushing of concrete. Circular columns were broken into several parts. Figure 10 shows the damage to the concrete after removing the steel tube after the blast load. [PERSON] et al. [14] investigated the effects of close-in blast loading on the full-scale reinforced concrete column. They conducted experimental tests and numerical simulations to calibrate the numerical model. The observed column had a square cross-section of 230 mm \(\times\) 230 mm and a free height of 2.44 m. The column was tested in a horizontal position, and the standoff distance from the center of the charge (8 kg of equivalent TNT--the used explosive is Gelamon VF65, which is equivalent in a mass to 65% TNT) to column 1 was 100 cm and to column 2 was 60 cm. Both columns experienced flexural damage, spallation of the concrete on the bottom side, and crushing of concrete on the exposed side, shown in Figure 11.  
Figure 8: Test setup [24] and test pit [21]. Copyright permission obtained from authors. Figure 10: Columns after blast test [24]. Copyright permission obtained from authors. Figure 9: Cross-sections of two CFDST specimens [20]. Copyright permission obtained from authors. [PERSON] et al. [15; 16] investigated the behavior of RC columns and RC columns with two types of protection (steel jacket and reinforced polyurethane bricks) exposed to a near field explosion. At a standoff distance of 60 cm, 8 kg of TNT equivalent shaped into a cylinder was placed, resulting in a scaled distance \(Z\) = 0.30 m/kg\({}^{1/3}\). Comparing the test results of three types of columns, the steel-jacked column had the best results in residual capacity and in reducing final deflection.