On the Effect of External Active Confinement on Spirally Reinforced Concrete Columns

Abstract eng:
External confinement has been adopted as an efficient technique of seismic upgrading of existing RC columns. This paper presents the experimental and analytical results of a research on strengthening small scale spirally reinforced columns. Experiments included axial compressive test of cylindrical concrete specimens with 12 cm diameter and 30 cm height. The specimens were internally confined with spiral reinforcements at two pitches of 4 and 6 cm. Post-tensioned high strength metal strips were wrapped around specimens. These strips had two different sizes of 16 and 32 mm with different strengths and ductilities. In addition to the area and material properties of confining strips, the effect of some other parameters on strength, peak strain and ultimate strain of columns were studied including spacing, number of layers and pretensioning force in the strips. The effect of mechanical confinement ratio on strength and toughness of columns was studied. Test specimens included “only internally reinforced specimens with spirals” and “doubly confined specimens with internal spirals and external strips”. An equivalent volumetric ratio for internal and external confinements is defined. The observed stress-strain relations of columns with different levels of internal and external confinement were compared to those calculated based on Mander 1988 and Madas 1992 theoretical confinement models. It was found that although these models were capable to predict the behavior of specimens with either internal or external confinements, failed to forecast the response of doubly confined specimens. Based on the observed behavior of tested concrete specimens, a confinement model was proposed for doubly confined concrete which accounted for both internal and external confinements.

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