The use of carbon fibre reinforced polymer (CFRP) wrapping to strengthen plain concrete prisms of square cross section was investigated experimentally. The study was aimed at quantifying the increase in axial compressive strength and ductility that can be achieved and assessing the effect of cross-sectional size on the increases. Thirty prisms of three different square cross-sectional sizes (100 mm × 100 mm × 300 mm, 125 mm × 125 mm × 375 mm, 150 mm × 150 mm × 450 mm) were tested. Ten prisms were constructed in each size. Five prisms in each size were left unwrapped as control specimens, and five were wrapped with two layers of unidirectional CFRP laminate. All prisms were loaded in axial compression until failure. Significant increases in strength and ductility were achieved by wrapping. The effectiveness of the wrap, as measured by the percentage increases in strength and peak axial strain, reduced with increasing cross-sectional size. These tests indicate that the use of CFRP wrapping is an effective technique for strengthening and (or) rehabilitating concrete columns. Test results available in the literature by other authors are also summarized. Although these results are highly scattered, they are consistent with the findings of the current tests.Key words: concrete, prism, column, square, rehabilitation, strengthening, FRP, experimental.