## 2 Experimental Program
### Test Setup  
The experimental part of the presented research included registering two types of deformation, namely measuring strains (local deformation) and measuring column shortening (global deformation). Strain gauges were used to measure the variation of strains at a section located in the middle of model height and were placed symmetrically, due to the longitudinal column axis. Figure 3a shows the plain RC specimen, while Figure 3b shows a strengthened specimen. Strain gauges used to register axial strains were marked as V (vertical), while strain gauges used to register lateral strains were marked as H (horizontal). In strengthened specimens, four vertical strain gauges were placed on the basic RC column before fill concreting. Strain gauges were protected by a special ABM75 elastic cover manufactured by HBM. Conductive cables from the strain gauges to the acquisition equipment were guided to the top of the specimens through fill concrete. Four 0-90\({}^{\circ}\) degree T-rosette strain gauges were placed on the steel tube outer wall, marked by R (rosette), shown in Figure 3b. Two symmetrically placed displacement transducers (DTs) were used to measure the axial deformations (shortening) of specimens, as shown in Figure 3. Readings from all instruments were automatically recorded during the loading process. In all tested columns, registering and analysis included: changes in stress and deformation state for all parts of the composite cross-section, ultimate strength, shape of global deformation in failure, engagement of the individual parts of the composite cross-section. In RC models, besides ultimate force and deformation, registering included: the value of the force at which cracking of concrete started, fracture development, and propagation. In this research, the emphasis was made on short-term loading, and the long-term effects, such as creep and shrinkage of the concrete or temperature changes, were not considered.  
All tests were performed on a 2000 kN capacity hydraulic testing press. The specimen was placed on the rigid steel bed of the machine, and the load was applied from the bottom of the specimen. Figure 3 presents a schematic view of the test arrangements (not to scale). The passive composite column loading method was used, which means that the concentric loads on the specimens were applied over top and bottom steel bearing plates. The bearing plates were 12 by 12 cm so that, on strengthened specimens, only the basic RC column was directly loaded. In that way, simulation of a real structure was possible, since, in a real strengthened column, a beam administers load only to an existing (basic) column.  
Every specimen was loaded continuously until failure. A force control technique was used to operate the hydraulic press with a load step interval of around one-tenth of the estimated load-bearing capacity. All load steps were maintained for about 5 min. The development of deformations, the failure mode, and the capacity load of the specimens were recorded. Loading was terminated when the applied axial force dropped below 20% of the peak value or when the weld failure of the steel tube happened. Figure 4 shows the column specimen of each group just before experimental testing.