Concrete column is a member that mainly bears vertical load and horizontal load. The column is easy to be destroyed under the action of seismic load. Its typical failure modes mainly include the following four types:

• Shear failure,

• Bending failure,

• Longitudinal lap joint failure,

• Bending shear failure.

The use of externally pasted FRP strengthening method can significantly improve the seismic performance of the column, so that the column can have better inelastic deformation before the above-mentioned damage occurs. At present, the commonly used method of seismic strengthening is to stick FRP outward along the column ring. The FRP pasted in the ring direction can be regarded as an additional stirrup to restrain the lateral deformation of the concrete, thereby improving the ductility of the concrete column. Longitudinal FRP can also be used to improve the seismic performance of the column.

Strengthening the column with carbon FRP can significantly improve its seismic performance. There are two main types of strengthening methods:

• Paste the carbon FRP along the column ring

• Paste the carbon FRP longitudinally.

Strengthening method for sticking carbon FRP along the column ring

The ring-directed bonding carbon FRP is used to form a ring-directed enclosing beam to strengthen the concrete column seismically. The strengthening method is the same as that of the normal section of the column. Carbon FRP mainly plays a similar role to stirrups. Carbon FRP forms an effective constraint on the core concrete, which can inhibit concrete spalling and longitudinal bar buckling, so that the column has better inelastic bending deformation ability. In addition, the binding force provided by the carbon FRP can prevent the bonding failure and slippage of the longitudinal rib lap joint, and the carbon FRP can improve the shear resistance of the column. Therefore, sticking the carbon FRP along the circumference of the column can prevent the column from shearing damage, bending plastic hinge damage and longitudinal rib lap damage, and improve its seismic performance.

Strengthening method for sticking carbon FRP along the longitudinal direction of the column

The main purpose is to improve the bending resistance of the column. This pasting method requires the end carbon FRP to be anchored reliably. Normally, the carbon fiber cloth needs to be pasted along the column ring direction. However, it should be noted that this kind of strengthening will also generate a lot of additional force on adjacent members. Therefore, when longitudinally pasting carbon FRP to strengthen the column, attention should be paid to the improvement of the bending capacity of the strengthening column. It is not advisable to increase the bending capacity of the column too much.

Strengthening requirements with FRP 

When the RC column is strengthening with carbon FRP to increase its ductility, the following structural requirements should be met:

1. The number of layers of FRP sheets bundled in the circumferential direction, cylinders shall be no less than 2 layers; square and rectangular columns shall be no less than 3 layers.

2. The overlap length between the upper and lower FRP sheet layers of the hoop bundle should not be less than 50 mm, the extension length of the hoop cutoff point should not be less than 200 mm, and the overlap position of each strip should be staggered.

3. The corners of square and rectangular columns need to be chamfered, and the chamfering radius should not be less than 25 mm when using carbon FRP and glass sheets to reinforce.

4. The height of the dense zone should be the larger of the long side dimension of the column section, 1/6 of the column clear height and 500mm.

5. It should be wrapped along the entire height of the column. For corner columns with seismic resistance grades of one or two, they should be wrapped along the entire height of the column.