Active confinement of weakened concrete columns

This paper investigates the advantages of using of external active confinement to improve the compression load capacity of concrete cylinders using mild steel band clamps. For this, passive and actively pre-stressed steel bands have been attached to concrete cylinders to mimic real structural columns which may otherwise have to be removed and replaced due to poor strength performance. The experimental programme included compression load testing a number of unreinforced concrete cylindrical specimens to establish the effect the confinement had on the compressive load-carrying capacity and the stress–strain behaviour. A number of parameters were investigated in the study including the effect of varying the band spacing, volumetric ratio and passive and active confining forces. It was found that confining the concrete did produce an increase in compressive strength. However, the different levels of pre-stress used had no significant effect on strength but did influence the confined concrete stress–strain behaviour with greater lateral stresses at peak strength observed. Ductility increases were observed for confined specimens and significant axial strains achieved with noticeable peak strength enhancements. However, due to initial misfit between the bands and the concrete in the passive confinement state, confining forces generated only contained the cracked specimens. The actively pre-stressing confinement yielded an increase in load capacity of 53 % as the bands were fully activated.