Retrofit of Square Concrete Columns

It is a well recognized fact that external confinement by FRP (or even steel) jackets is not that effective for square column sections. In these cases, the corners of the column are properly confined but the sides remain relatively unconfined. The challenge has been how to build a circular FRP jacket around an existing square column. Of course, one can use temporary forms to cast additional concrete to convert the square column into a circular column, then remove the form and wrap FRP around the circular column. But such an approach is very time consuming and costly.

As depicted in Figure 4, super laminates make this task very easy. For ease of handling, one may use a narrower super laminate, e.g. 24 inch (600 mm) wide, and start construction of the jacket at the bottom of the column. The overlap is typically one half of the super laminate width, 12 inches (300 mm) in this example. By applying a thin layer of epoxy putty, the jacket is constructed as the workers approach the top of the column. To prevent the top end of the super laminate from unraveling (due to its elastic memory), a band of fabric saturated with resin can be wrapped a couple times near the top of the jacket.

This creates a very strong ‘seamless’ structural jacket with desired strength and stiffness in the longitudinal and hoop directions. The longitudinal fibers in the super laminate provide the lateral (hoop) confinement for the column; the transverse fibers in the super laminate will be aligned along the axis of the column and with the aforementioned 12 inch (300 mm) overlap, provide continuous reinforcement along the column. There is a slight reduction in efficiency due to the angle of spiral that can be calculated with simple geometry. The annular space between the jacket and the column can be filled with resin or grout, and if desired, pressurized for further confinement of the column. This can be achieved by pressurizing the resin by air or using expansive grout.

A similar approach can in fact be used on existing circular columns that are deteriorated due to corrosion. At present, once the corroded steel in such columns has been repaired, the column must be coated with concrete to bring it back to its original cylindrical shape before it can be wrapped with FRP fabrics. Super laminate will eliminate this step as the column jacket can be wrapped directly around the column with an uneven surface and the result will be a smooth cylindrical shell; the annular space between the shell and the circular column can be injected with resin or filled with grout.


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