Effect Of Corner Radius and Confinement Layers On Behaviour Of RC Columns

Abstract

Existing reinforced concrete (RC) columns may be structurally de cient due to vari- ety of reasons such as improper transverse reinforcement, aws in structural design, insu cient load carrying capacity, etc. A poorly con ned concrete column behaves in a brittle manner, leading to sudden and catastrophic failures. Carbon/Glass(G) Fibre reinforced polymer (FRP) con nement can be e ectively used for strengthening the de cient RC columns. The e ectiveness of FRP wrapping for RC columns mainly depends upon corner ra- dius of the specimens as well as number of FRP layers used for the con nement. An attempt has been made hereby to investigate the experimental behaviour of GFRP wrapped small scale square RC columns with varying corner radii. Exper- imentally evaluated behaviour of GFRP wrapped RC columns is compared with the performance observed for non-wrapped RC columns. Experimental behaviour of RC columns is further compared with analytical results. 27 RC columns having cross-sectional dimensions 125mm 125mm and length of 1200mm have been cast and tested under axial compression. Three columns are un- wrapped and have been designated as control specimens. Three columns each with corner radii equivalent to less than cover (15mm), equal to cover (25mm) and greater than cover (35mm), are wrapped with one & two layers of GFRP, respectively. To avoid a premature rupture of the GFRP composite, remaining six columns with cor- ner radius of 5 mm have been wrapped with one and two layers of GFRP, respectively. Analysis and design of control RC column has been done according to IS 456:2000 and ACI 318M - 08 provisions, respectively. IS 456:2000 does not cover provisions pertaining to design of FRP wrapped columns. The FRP wrapped columns are de- signed using provisions of ACI 440.2R - 08. To maintain adequate correlation, the control columns are further designed based on provisions of ACI 318M - 08. Measurements taken during testing included axial compressive strength, displace- ment, axial strain, lateral strain, failure modes and crack patterns. Axial load has been applied from bottom of the column with the help of hydraulic jack. Displacement and axial strain measured with the help of LVDT and lateral strain measured with the help of P3 strain indicator and recorder are observed for the columns. Interval is kept 10 kN constant up to the complete failure of the column specimen. Percentage increment in ultimate failure load is ranging from 8.10 % to 149.45 % for all wrapped columns compared to control columns. GFRP wrapped column goes under higher axial displacement in order to gain higher compressive strength over control column. Lateral strain is more at the mid side of specimen and then reduces at starting of curvature to center of curvature for the columns. From the failure of column, it is clearly shown that the rupture of GFRP sheet transfers from edges (zone 1) to mid of side face (zone 2) of the specimen. The results showed that smoothening of the edges of square cross-section play a sig- ni cant role in delaying the rupture of the FRP composite at the edges. Increasing the number of GFRP layers increases the axial compressive strength of the specimens, but the strength increase is not in linear relation with the number of GFRP layers. From results it is interpreted that the corner radius equal to concrete cover gives better results in terms of ultimate load carrying capacity than the corner radius less than cover and more than cover.