A technique called circularisation where segmental circular concrete covers made of different concrete strengths (40 MPa, 80 MPa and 100 MPa) was used to change a square column to a circular column. The applicability of the circularisation method was experimentally studied for a wide range of concrete strengths (from 40 MPa to 100 MPa). The behaviour of the strengthened specimens under different loading conditions including concentric loading, eccentric loading (25 mm and 50 mm) and flexural bending is investigated. The experimental results demonstrate that using high strength concrete (HSC) for the additional covers to strengthen existing square reinforced concrete (RC) columns provides higher load-carrying capacity than covers made of normal strength concrete. The HSC covers and the concrete cores worked as a composite material to failure. The FRP strain at peak load was observed for the purpose of estimating the specimens’ capacity. The distribution of FRP strain around the circumference of the column section is also reported.