Strengthening of RC Columns with Partial GFRP Wrapping

Abstract

Fiber-Reinforced Polymer (FRP) materials are used in different configuration for strengthening of Reinforced Concrete (RC) columns to ensure their longer service life. Use of FRP by Externally Bonded Reinforcement (EBR) and Near Surface Mounted Technique is growing now a days. The EBR method is used to increase the concrete confinement and the column shear resistance, as well as to avoid buckling of longitudinal steel bars of RC columns. The near surface mounted technique is one of the innovative strengthening techniques that includes of placing FRP reinforcing bars to increase axial load carrying capacity for deficient RC columns. Limited studies are reported to date on the use of partial GFRP wrapping along with NSM reinforcement for strengthening RC elements in literature.

The present investigation includes evaluation of effectiveness of GFRP wrapping along with different techniques for strengthening RC columns with various cross sectional aspect ratios. The project is covering a wide range of cross-section dimension ratio for RC columns. Four aspect ratio are to be studied e.g. 1.0, 1.5, 2.0, 2.5 for RC columns. Dimensions of RC columns are taken as 150mm x 150mm, 185mm x 125mm, 220mm x 110mm, 240mm x 95mm respectively. All columns are designed by considering the same 0.89% percentage of reinforcement using IS 456:2000 provisions. Total twenty eight RC columns are cast including four control columns. Each set of eight columns are strengthened using partial GFRP wrapping, partial GFRP wrapping with vertical strip of GFRP sheet at middle of the columns, partial GFRP wrapping with HYSD reinforcement at middle using Near Surface Mounted technique respectively. Width for vertical strip of GFRP sheet is taken as 50mm. RC columns are reinforced with HYSD bars by cutting the grooves with specified dimensions in to concrete along longitudinal axis of columns and the size of the groove is taken as 12mm x 12mm. The diameter of HYSD bar is taken as 8mm. All the test columns are failed in axial compression. All columns specimens are 1m in length with having 20mm of corner radius. Design of GFRP wrapping is evaluated using CNR-DT 200/2004 provisions. Different structural parameters i.e., failure load, displacement and strains at various heights are measured. Strain of the columns in the axial and transverse directions has been investigated. GFRP wrapping increases the axial strength by providing addition confinement to the concrete without increasing the original column size. It is observed that use of HYSD reinforcement Near surface mounted technique along with GFRP partial wrapping is more effective in all aspect ratios of RC columns, because it enhances confinement zone of RC columns. The experimental results clearly demonstrate that partial GFRP wrapping, partial GFRP wrapping with vertical strip of GFRP sheet and partial GFRP wrapping with HYSD reinforcement using Near surface mounted technique has enhanced the structural performance of RC columns under axial loading, in terms of structural properties. For all aspect ratios, it is observed that Partial GFRP wrapping along with HYSD reinforcement using NSM technique gives higher axial strain as compared to other strengthening methods of RC columns. Brittle failure and blasting effect is observed in all control RC columns. RC columns strengthened using partial GFRP Wrapping and HYSD reinforcement using Near surface mounted technique gives higher lateral strain at middle and corner portion as compared to other strengthening methods. Brittle failure and blasting effect is observed in all control RC columns. For all aspect ratios, it is observed that failure of partial GFRP wrapping of RC columns occured due to debonding of GFRP sheet at top of the specimen. It is observed that first debonding of vertical strip is occurred, then after GFRP sheet fails in partially GFRP wrapped RC columns along with vertical strip of GFRP sheet. It is observed that first additional reinforcement buckles at the top of the specimen then after GFRP sheet fails in partially GFRP wrapped RC columns along with NSM technique.