RC columns of square section – Passive and active confinement with composite ropes

This paper addresses the use of high extension capacity fiber ropes as external confining reinforcement in cases of plain or reinforced concrete columns with square section and very low corner radius. The composite ropes were made of polypropylene and were applied by hand in four or three layers. Special mechanical devices were constructed in order to apply pretension on the external wrapping and compare their response with the columns without pretension. The columns were subjected to multiple cycles of increasing compressive deformation to simulate seismic actions. No column wrapped by polypropylene fiber ropes reached fiber fracture for concrete axial strains up to 6.7%. The novel pretension technique showed a satisfactory performance throughout loading while high pretension levels could be achieved without friction between the external wrapping and the concrete column. Full unwrapping after test termination, revealed extensive cracking of the concrete core and buckling of multiple steel bars. Yet, all strengthened reinforced concrete columns presented enhanced stress–strain behavior with plastic-like or hardening inelastic branch that denoted a restriction of premature buckling of the slender bars. The pretension of the external wrapping achieved in this study, resulted in the prolongation of the elastic response of the column by 40% in terms of load. Also, the inelastic stress–strain modulus was remarkably enhanced.