Ultimate strength and axial strain of FRP strengthened circular concrete columns

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Strengthening of concrete columns is one of the most important tasks in civil engineering, as well as strengthening other elements and structures such as beams, bridges, municipal buildings, transportation systems, and parking. This might be required for a change in the structural system of the building such as removing of some load-bearing members or the damage caused by external factors or when the function of the building is changed from previously planned which could lead to an increase in the applied load and consequently the need to increase the column capacity. Conventional methods like concrete or steel jacketing were used for repair, strengthening, or providing a lateral confinement for the RC columns (Chai, Priestly, & Seible, 1991; Priestly, Seible, Xiao, & Verma, 1994a, 1994b). Although both methods are effective in increasing the structural capacity, they are labor consuming, mostly results in a substantial increase in the cross-section of the strengthened column, high density, long time of installation, difficulty in application, and the need for continuous maintenance. For these reasons and to replace outdated techniques, the innovative rehabilitation and strengthening methods for reinforced concrete structures, especially with composite materials, have taken a large portion of the research and application work in the field of repair and restoration of structural elements. lately emerged fiber reinforced polymer FRP as a new material which has been used in aerospace, marine and automobile industries and to be used in different structural elements for its mechanical properties such as high tensile strength, low density, high resistance for corrosion, low thermal coefficient, short time of installation, easy application, and low cost for maintenance (Ferguson, Breen, & Jirsa, 1981). Moreover it is considered as an alternative to steel in reinforced concrete structures because of the continuing decline in the cost of FRP. FRP sheets for strengthening and rehabilitation of structural concrete elements has attracted great attention in the recent years (Hollaway, 2003; Mufti, 2003; Mufti, Labossiere, & Neale, 2002; Nanni, 1993). First applications for FRP were in the form of rebar. Then, FRP laminates were used for strengthening of concrete bridge girders by binding them to the tension face (Meier, 1992) as well as for rehabilitation of concrete columns (Saadatmanesh, 1994). FRP are available in the form of rods, grids, sheets, and winding strands. Review of literature up to 1996 can be found in ACI Committee 440 (ACI Committee 440, 1996). Another general review on class of materials including FRP used in civil construction was presented by Bakis et al. (Bakis et al., 2002). Some research by Parvin and Wang on FRP confined columns, the effect of FRP jacket thickness were investigated (Parvin & Wang, 2001a, 2001b). Shear strength of RC columns strengthened with FRP was studied by Ye et al. (Ye, Yue, Zhao, & Li, 2002) who finally concluded that shear strength of RC column can be effectively increased through external strengthening by using FRP sheets. In this paper, several methods for predicting strength and behavior of CFRP strengthened concrete columns were reviewed and a new equation is proposed.

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