**FRP wrap the column**

Wrap the concrete column

When the outer surface of the column is coated with fiber reinforced polymer wrap, the transverse expansion of the concrete causes tensile stress of the outer FRP wrap, and the carbon wrap in turn imposes restraints on the concrete to prevent its expansion, so that the concrete is in a three-dimensional state of stress, thus improving the ultimate compressive strength and deformation performance of the concrete.

**The method of strengthening reinforced concrete columns is to wrap CFRP around the columns. The concrete steps are as follows:**

1. The length and width of each piece of cloth is designed according to the height of the column.

2. Clean up the surface of the reinforced concrete column and remove the plaster layer on the surface of the cylinder to clean it up.

3. Leveling the surface of the column, putting the putty on the surface of the column and leveling the surface of the column.

4. Brush base glue;

5. Apply adhesive to the adhesive and dry with adhesive.

6. Stick the fiber cloth and use the special plastic brush to scrape the surface of the fabric to make viscose permeable fiber cloth.

7. Adhesive plaster.

The reinforced concrete columns in the structure are subjected to axial pressure and bending moment at the end of the column. Under the action of axial pressure and bending moment at the end of column, the concrete in the compression zone will not only produce axial deformation, but also produce transverse expansion due to Poisson effect. If the concrete's lateral expansion is not restrained, the concrete will crack. When the fiber cloth is wrapped on the outer surface of the column, the transverse expansion of the concrete causes the tensile stress of the fiber cloth in the outer layer, and the fiber cloth in turn imposes constraints on the concrete to prevent its expansion, so that the concrete is in a three-dimensional stress state. Thus the ultimate compressive strength and deformation performance of concrete are improved, and the plastic deformation performance of longitudinal reinforcement in tension zone is brought into full play. Therefore, the fiber cloth wrapped on the surface of reinforced concrete columns can not only improve the bearing capacity of the columns, but also improve the ductility and seismic resistance of the columns.

**Factors affecting compressive bearing capacity of post strengthened columns**

**1.**Section shape of column

The reinforced concrete columns in engineering are generally circular or rectangular or square sections. When a circular cross-section column is confined by fiber cloth, it is subjected to circumferential tension, and a continuous confined lateral pressure is provided to the concrete along the periphery of the section. When FRP confines rectangular columns, the concrete lateral pressure will bend the FRP outward, which makes the FRP provide the maximum lateral pressure at the corner and the minimum lateral pressure at the middle of the edge. Therefore, only the corner and the central section of the concrete are better constrained by the arch action.

It is pointed out that the section shape has a great influence on the ultimate bearing capacity and deformation capacity of short columns, and the improvement degree is cylinder, square or rectangular column with elliptic, chamfered corner, square or rectangular column without chamfered corner from high to low. Therefore, in practical engineering, the circular or elliptical section should be used to wrap the fiber. If the square or rectangular section is to be used, the right angle of the section should be chiseled into rounded corners and then reinforced by fiber cloth wrapped.

**2. **Dimension parameters of rectangular section

The aspect ratio of rectangular section is an important factor affecting the axial bearing capacity of columns. It is pointed out that when the ratio of length to width of concrete changes from 1:1 to 6:1, the increase of bearing capacity decreases from 1.17 to 1.08, almost 10%. When the ratio of length to width increases to a certain extent, the increase of the ratio of length to width has less and less influence on the improvement of bearing capacity. This is because the constraints provided by FRP become smaller and smaller with the increase of aspect ratio of rectangular section, and the influence of aspect ratio is smaller and smaller.

In addition to the aspect ratio of the section, the fillet radius of the section will also affect the bearing capacity of the column. The larger the fillet radius, the greater the constraint of the concrete, the more the bearing capacity will be improved. However, the radius of fillet has little effect on the increase of bearing capacity.

**3.** the number of stickers for transverse fiber cloth.

The reason why the reinforced concrete columns can improve the bearing capacity and ductility is that the concrete is restrained by transverse fiber cloth, from one-way force to complex three-way force, therefore, the number of fiber layers increases, the restraint on concrete increases, and the compressive capacity of concrete increases. It can be seen that the smaller the aspect ratio is, the greater the influence of the number of bonding layers is. The greater the aspect ratio is, the smaller the influence of the number of bonding layers is.

In practical engineering, the number of fiber cloth layers should be determined by design calculation. For reinforced concrete columns with large difference in length and width, it is difficult to improve the reinforcement effect by adding fiber cloth, and other methods should be considered.

**4. **Effect of load before reinforcement

Reinforcement is usually carried out under a certain load. Unless other temporary supports are used to transfer the external load transmission path, the influence of pre-reinforcement load should be considered, that is, the influence of secondary force on columns. Before reinforced concrete columns are strengthened, the concrete is in a strain state of initial stress under external load. After reinforcement, no change will take place in the reinforced section before loading, and no change will take place in the section strain. Therefore, CFRP is in the state of zero strain and has no restraining effect on concrete.

**5.** creep of concrete

The creep of concrete will also affect the bearing capacity and deformation capacity of reinforced concrete columns under long-term loading. When considering the creep of concrete, the bearing capacity of reinforced concrete columns is lower than that of the unconsidered cases, and the deformation capacity is higher than that of the unconsidered cases.

In addition, the slenderness ratio of reinforced concrete columns, compressive strength of concrete, corrosion of steel bars, mechanical properties of FRP and other factors will affect the compressive strength of reinforced concrete columns.