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Columns are slender vertical load-bearing members, designed with a high degree of utilization of their design resistance. They are efficiently used especially in buildings, where it is a requirement to create free dispositions. Their disadvantage is that they only have a small reserve in resistance, so it is often necessary to proceed to their renovation or strengthening. The requirement for strengthening may be based on the need for repair of a structural failure, to repair mistakes during design or construction, but also on the need for an increase in the resistance of existing structure resulting from a change in its load. There are several methods to choose from, when the strengthening of a column is needed. Traditional methods include enlargement of the original cross-section with the addition of concrete or structural steel members. These solutions increase the compression as well as the bending resistance of the column, but there is a significant increase in the cross-sectional area of the column. Newer methods use progressive materials that allow the required strengthening to be achieved with lower or almost zero cross-sectional enlargement. These methods include the application of FRP composites, but also the use of FRC or ferrocement. Mostly CFRP (Carbon Fiber Reinforced Polymer) strips and sheets are used for strengthening. The strips can be installed by NSMR or EBR method. They serve as additional tensile reinforcement. Sheets can also be bonded in the longitudinal direction as an additional tensile reinforcement, but they are more often applied in the transverse direction in the form of stirrups or helices, thus creating a wrap around the column. When column is then loaded with an axial force, the sheets prevent transverse deformation, a spatial state of stress occurs, which leads to an increase in the compressive strength of concrete. In addition to CFRP sheets, FRC or ferrocement can also be used for wrapping. This article deals with slender columns strengthening. The simple parametric study compares the influence on column resistance by the change of following parameters: concrete compressive strength, amount of reinforcement in the cross-section, and amount of reinforcement added to the cross-section surface, the results are presented by means of interaction diagrams of the column cross-section and the interaction diagrams of the column taking into account the second order effects. The comparison shows that for slender columns a more significant increase in the resistance of the column is achieved by adding longitudinal reinforcement. The increase in the strength of the concrete (achieved by wrapping) has only a minimal effect on the resistance increase of the column. [ABSTRACT FROM AUTHOR]