MECHANICS OF CONFINEMENT
1. Preliminary Remarks
Confinement of columns by means of FRP jackets is done by retraining the dilation of concrete by wrapping the fibers in the hoop direction of concrete columns. Confinement of columns by FRP wraps is termed as a passive strengthening technique since the fibers do not play any role in the elastic response of the axially loaded column. The tendency of concrete to dilate after cracking and the radial stiffness of the confining jacket to restrain the concrete dilation, are considered to be two important factors affecting the concrete confinement. By wrapping the concrete with an external continuous FRP jacket, the fibers in the hoop direction resist the transverse expansion of the concrete providing a confining pressure. At low levels of longitudinal stress; however, the transverse strains are so low that the FRP jacket induces little confinement, if any. At higher longitudinal stress levels, the dramatic increase in transverse tensile strains activates the FRP jacket and the confining pressure becomes more significant. The general confining pressure induces a tri-axial state of stress in the concrete. It is well understood that concrete under tri-axial compressive stress exhibits superior behavior, in both strength and ductility, as compared to concrete in uniaxial compression.
2. Parameters affecting confined compressive strength of concrete: The confined compressive strength of concrete columns is influenced by several factors apart from the material properties of FRP composites. Parameters such as size and shape of the cross section of the columns, grade of concrete, material properties of FRPC, nature of loading collectively affect the confined compressive strength. These parameters are discussed briefly in the following paragraphs. 1) Tri axial state of stress:
Experimental evidence shows that the circumferential failure strain mostly occurs at strains lower than the ultimate strain εfu obtained by standard tensile testing of the FRP sheet. This is because in case of full composite action, the jacket undergoes both transverse and longitudinal stresses. The ultimate stresses and strains are then reduced, with potential microbuckling and delamination to develop. Thus, failure of the specimen occurs at even lower circumferential strains than in case of no composite action. 2) Effect of cross section of shape
The shape of cross section of the column is the most important factor that influences the confined compressive strength. The effectiveness of FRP reinforcement is less in the case of a square or rectangular cross section compared to a circular cross section because of the concentration of stresses at the corner of the non circular section and also because of the smaller effectively confined concrete core in a non circular cross section with respect to a circular cross-section. Rectangular columns may have different confinement effect in two orthogonal directions and have different levels of confinement pressure between the long and short sides of the cross-section. Confinement pressure along the long side plays a dominant role on concrete strength. As a consequence of this, in order to obtain in columns having a square or rectangular cross-section, analogous performance in terms of strength as in columns having a circular cross-section and reinforced with FRP, an increase in the volumetric ratio of FRP is required and/or a transformation of the square section into one with rounded corners, utilizing adequate corners radii. 3) Effect of corner radius
The sharpness of the section corner is a significant influence parameter to the stress–strain curve of FRP-confined prisms. The sharpness of the section corner can be expressed by the ratio of the radius corner (r) to the longer side length of the section (h). Round corners are necessary to reduce...
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