Civil and Environmental Engineering Vol. 17, Issue 1, 9-22, DOI: 10.2478/cee-2021-0002 EXPERIMENTAL WORK TO STUDY THE STRENGTHENING EFFECT OF SFR AND CFRP ON A PART OR WHOLE OF THE LENGTH OF SLENDER RC COLUMNS Mohammed AL-HELFI 1, *, Ali ALLAMI 1 1 Technical Institute of Amara, Southern Technical University, Iraq. * corresponding author: mohmsce1974@stu.edu.iq 1 Introduction Studies on slender reinforced concrete columns have historically focussed on the behaviour of columns with regard to parameters such as slenderness ratios and cross-sections [1-6]. In last few years, various researchers have investigated the strengthening of slender reinforced concrete columns (SRCCs) using carbon fibre reinforced polymers (CFRP) composites and steel-fibre- reinforced (SFR) columns. Research in this field was started by G. P. Lignola et al. (2007) [7], who tested slender RC hollow columns by jacketing them with CFRP and computing induration diagrams for hollow columns encased with FRP. K. Gajdošová and J. Bilčík (2011) [8] investigated the effects of the manner CFRP strengthened slender reinforced concrete columns, for which they used the ATENA 3D modelling program. Numerical and theoretical analysis proved that strengthening with CFRP strips was more effective than CFRP sheet confinement. Waleed A. et al. (2012) [9] experimentally studied RC columns strengthened with CFRP according to various parameters such as the thickness of the layer CFRP and eccentricity. The study results proved that there was an increase in the carrying capacity of the strengthened columns compared with normal columns. Katarina G. and Juraj B. (2012) [10] showed the effects of slender rectangular reinforced concrete columns strengthened by CFRP via two methods. Additionally, they proved that the effects of CFRP in terms of increasing the strength of the columns was comparatively greater than for short RC columns subjected to compression. Further, the research showed that longitudinal fibres in the form of CFRP strips affixed to the grooves in the cover of concrete were of greater influence in increasing the flexural load capacity of SRCC. Caballero-Morrison et al. (2012) [11] focussed on the behaviour of SFR and normal strength SRCC under cyclic loading. Again, various associated parameters were studied such as axial load level, Abstract This paper is focussed on the investigation of the strengthened column by steel-fibre-reinforced (SFR) and carbon fibre-reinforced polymer (CFRP) on a part or a whole of the length of the column under axial force, and to obtain the effective length, which is responsible for the increasing of the ability of the column to resist buckling. Seven samples were designed as slender reinforced columns with dimensions of 2000 mm length and cross-sections of 120 mm x 60 mm. It was shown that the strengthening of the middle half of the column length by SFR gave an ultimate load similar to the strengthening of the whole column with same material. Also, it was found that the column strengthened by SFR increased its ultimate load by 42.6 %, 42.1 % and 33.3 % for strengthened lengths of dimensions L, L/2 and L/3, respectively, compared to the non- strengthened column. The increase in ultimate load that could be borne by the column strengthened by CFRP were 53.0 % and 33.8 % for strengthened lengths of L and L/2, respectively, only. Interaction diagram had been depended as a theoretical analysis for slender concrete columns. The interaction diagram for the column strengthened by CFRP is greater than for the column strengthened by SFR, which in turn is greater than the non-strengthened column. Keywords: Slender reinforced concrete column (SRCC); Steel-fibre-reinforced (SFR); Carbon fibre reinforced polymers (CFRP); Effective length; High slenderness ratio.