Proceedings of the 8 th U.S. National Conference on Earthquake Engineering April 18-22, 2006, San Francisco, California, USA FRP-JACKETED AND SHAPE MODIFIED COLUMNS USING CHEMICAL POST- TENSIONING C.P. Pantelides 1 , Z. Yan 2 , and L. D. Reaveley 3 ABSTRACT The axial compressive strength and ultimate compressive strain of columns inadequately designed for seismic forces can be improved by confinement with fiber reinforced (FRP) composites. To improve the confinement effectiveness of FRP composite jackets for square and rectangular columns, shape modification is performed by using prefabricated FRP shells combined with expansive cement concrete. Chemical post-tensioning using expansive cement concrete is used to change the FRP confinement from “passive” to “active”. Experimental results of large-scale columns in compression with externally bonded FRP jackets, or post- tensioned FRP shells were performed on plain concrete columns. Square sections modified with FRP shells and expansive cement concrete to circular columns demonstrated a compressive strength increase by a factor of three and a ductility increase by a factor of two. Rectangular sections had a smaller but significant increase in both strength and ductility. Introduction Collapse of reinforced concrete buildings and bridges in large earthquakes is caused largely by failure of the supporting columns. Fiber Reinforced Polymer (FRP) composites have been used in the retrofit of concrete columns to improve their axial strength, displacement ductility or both. It is well-known that FRP composite jackets can provide effective lateral confinement for circular concrete columns, and substantially enhance their axial compressive strength and ultimate axial compressive strain; the latter is an important parameter in seismic resistant design. FRP confinement is much less effective for square and rectangular columns compared to circular columns; this has been verified in tests performed by Rochette and Labossière (2000), Pessiki et al. (2001), and Yan (2005). Stress concentrations at the corners and inefficient confinement at the flat sides cause reduction in the confinement of the section. Because of the presence of steel ties, rounding of the corner radius of the concrete section in existing square/rectangular columns is limited. In addition, lower FRP confinement effectiveness results in softening behavior for square and rectangular columns since the FRP composite ruptures prematurely; therefore, the high strength of FRP composite materials cannot be fully utilized. A possible approach to increasing the effectiveness of FRP-confined rectangular columns is to perform shape-modification that is to modify the column cross-section into an elliptical, oval, or circular section. One method for performing shape-modification is to use prefabricated (non-bonded) FRP composite shells combined with expansive cement concrete. For this method, a prefabricated elliptical/oval/circular FRP shell may be used as stay-in-place formwork for casting additional expansive cement concrete around the square or rectangular 1,3 Professor, Dept. of Civil & Env. Engineering, University of Utah, Salt Lake City, UT 84112 2 Engineer, Nishkian Menninger Consulting Structural Engineers, San Francisco, CA 94103 Paper No. 500