Compressive behaviour and design of rectangular concrete columns strengthened with high strength thin-walled octagonal steel tube and sandwiched ECC layer

Traditional strengthening techniques like concrete jacketing and steel jacketing may no longer suffice to meet the demands of high-performance, efficiency, and low carbon footprint. This paper presents an experimental investigation on a novel strengthening method for reinforced concrete (RC) columns, using external thin-walled octagonal high strength steel tube and sandwiched ECC layers. The impacts of steel grade, sandwiched layer material, sandwiched layer thickness, tube aspect ratio, and ECC-concrete interface roughness on the failure mode, load-bearing capacity, and strain responses of the strengthened concrete columns were explored. Uniaxial compression tests results illustrated the effectiveness of the proposed strengthening technique in utilizing the compressive strength of the steel tube and the advantages of confinement. This method resulted in a notable enhancement in load-bearing capacity, ranging from 304 % to 814 %, with a modest increase in section size of 51 % to 140 %. Furthermore, a significant improvement of up to 24 % in strength was observed when comparing the strengthened column to its nominal strength. Design recommendations were provided based on the design approaches from GB 50936–2014, EN 1994-1-1:2004 and ANSI/AISC 360-22 for CFSTs. The results showed that the design approaches for circular CFSTs in EN 1994-1-1:2004 combined with the confinement coefficient for octagonal section could reasonably capture the compressive capacity of the strengthened columns.