## 3 Results and Discussion
### Axial Load-Displacement Behavior  
Figures 9-11 illustrate the axial load-axial displacement behaviors of the columns in CC, GC, and GCD groups, respectively, tested under concentric and eccentric (15, 35, and 50 mm) axial loads. The columns CC-0F-0E in CC group; GC-0F-0E and GC-0.75F-0E in GC group; and GCD-0F-0E and GCD-0.75F-0E in GCD group were subjected to concentric axial loading. It can be noted from Figures 9-11 that the initial ascending phase of the load-displacement responses of the columns subjected to concentrical loading test was nearly linear till the point of peak axial load. The compressive strength of the core concrete mainly played a dominant role in this phase of the load-displacement curve. The concrete cover of the tested columns was not spalled off till the maximum axial load was attained. However, hairline cracks were observed on the face of columns at 90% of the ultimate load. When the ultimate load-carrying capacity was attained, plain columns of CC, GC, and GCD groups exhibited a drop in the axial load, owing to the spalling-off of concrete cover, whereas a mere disintegration of the cover concrete was observed in the fiber-reinforced GC and GCD columns. The columns GC-0F-0E and GC-0F-15E experienced a sudden drop in the descending phase of the load-displacement curve after attaining the ultimate load-carrying capacity. This rapid decline can be attributed to the sudden spalling of the concrete cover at the top and bottom ends of the GC-0F-0E and GC-0F-15E specimens. All the plain specimens of GCD group, i.e., GCD-0F-0E, GCD-0F-15E, GCD-0F-35E, and GCD-0F-50E, experienced an immediate decline in the descending phase of the load-displacement curve, owing to the cover concrete spalling detected in these columns after reaching the peak load capacity. However, the fiber-reinforced columns GC-0.75F-0E, GC-0.75F-15E, GC-0.75F-35E, and GC-0.75F-50E exhibited a steady drop in the load-carrying capacity till the point of failure. Figure 6: The failure mode of conventional concrete (CC) columns after the test. Due to the presence of steel fibers, the brittle behaviors of the GC specimens were changed to gradual flexure behavior owing to the confinement effect provided by the fibers. The addition of fibers in these specimens, i.e., GC-0.75F-0E, GC-0.75F-15E, GC-0.75F-35E, and GC-0.75F-50E, resulted in improving the ductility by 55%, 40%, 35%, and 25%, respectively, which is higher than the corresponding specimens with no fibers in the same group. The variation of mid-height lateral deformation of the tested specimens with applied axial load is presented in Figures 12-14. It is obvious that the general behavior of concentrically tested columns of CC, GC, and GCD groups is different to that of the columns tested under eccentric loading. The concentrically tested columns (i.e., CC-0F-0E, GC-0F-0E, and GCD-0F-0E) were observed to fail in a brittle manner as the peak load dropped abruptly after the ultimate load was reached. All the plain columns in the GC and GCD groups exhibited an abrupt drop in the descending phase of the axial load-lateral displacement curve after reaching the ultimate load. On the contrary, the fiber-reinforced specimens of both groups (GC and GCD) experienced a steady failure mode, as the load decline was steady with a prolonged plateau in the descending phase of the axial load-lateral displacement curve. It is evident from the axial load-lateral displacement behavior of all columns that the mid-height lateral displacements of the tested columns are increasing for all the specimens with the addition of fibers. Figure 7: The failure mode of GC columns after the test. Figure 8: The failure mode of GCD columns after the test. Figure 9: The axial load–axial displacement behavior of CC columns. It can be noted from Figures 13 and 14 that fiber-reinforced specimens of GC and GCD groups exhibited more displacement than the corresponding plain columns in the same group. The addition of steel fibers considerably affected the post-peak load-lateral displacement behavior of the tested columns.