Column Strengthening

Project Overview

An office building in Xingtai City, Hebei Province, with a length of 68m, a width of 16m, a building area of about 8900m2, one basement level, seven floors above ground, a frame structure, and a raft foundation. It was invested and constructed in 1980 and invested the following year use. When it was refurbished at the end of 2006, it was discovered that in the middle of the second and third floors of the building, there were three middle pillars with cracks at the roots of the pillars. The concrete peeling off at the roots of the pillars was severe, and the corroded longitudinal bars were clearly visible. Later, the construction engineering quality inspection center carried out inspection and appraisal. It was found that the above-mentioned three central pillars of the building had vertical cracks about 1m away from the root of the pillars. The maximum crack width was 2.5mm. The main reinforcement and stirrups were corroded heavily, resulting The effective area of the original reinforcement bars has been reduced, and the concrete at the root has fallen off seriously, which has reduced the column section to varying degrees. In addition, several frame beams also showed serious cracks, all of which need to be reinforced. This article only introduces the reinforcement column.

Reinforcement method 

At present, the common methods that can be used to reinforce steel reinforced concrete columns are the method of enlarging the column section, the method of wrapping steel, and the method of FRP jacketing. The encased steel method is more effective than the reinforced concrete enlarging section method, because the enlarging section method increases the structural weight and section size. Both of these methods are more labor intensive and on-site construction is sometimes difficult. In addition to the heavier steel, the outer steel enclosure is less capable of resisting harsh environmental erosion.

In recent years, the use of FRP materials to reinforce reinforced concrete columns has become a new type of reinforcement method that replaces outer-clad steel, and it has been used more and more widely. This method has the advantages of high strength, high efficiency, convenient construction, good durability and corrosion resistance, thin thickness, and light weight. Among them, the most common form of FRP jacketing column is externally bonded fiber cloth or strip. This project chooses to reinforce column with FRP jacketing method.

There are several different processes for using FRP materials to reinforce reinforced concrete columns. According to the molding process, the reinforcement methods can be divided into three types: wet bonding, continuous fiber winding, and prefabricated shell method. Among them, the most commonly used method is the wet bonding method, which has outstanding advantages such as flexibility and ease of field operation. This example chooses this method. A unidirectional fiber cloth is used to wrap the surface of the column with a resin-impregnated adhesive paste so that the direction of the main fiber is along the circumferential direction of the column. The column can be completely wrapped by a single layer or multiple layers of fiber cloth, or it can be partially wrapped with fiber cloth formed into continuous spiral hoops or spaced hoops. This project chooses carbon fiber cloth to completely wrap and strengthen.

Reinforcement design

1 Basic assumption

1) Assumption of flat section;

2) There is no slippage between the carbon fiber cloth and the concrete, and no peeling damage occurs;

3) Carbon fiber cloth adopts linear elastic stress-strain relationship, and can only bear tensile force.

2 The number of layers of unidirectional carbon fiber cloth

In this case, 3 layers of unidirectional carbon fiber cloth are required to be wrapped and pasted along the full height of the column. When pasting, each layer of carbon fiber cloth needs to be stitched and pasted in a staggered manner.

Key points of FRP construction process

1) Pick up loose concrete, remove rust from steel bars, and pour high-strength concrete. First, manually pick up the loose concrete of the reinforcement column. Do not use too much force to destroy the dense concrete. The corroded parts of the main reinforcement and stirrup should be derusted to expose the metallic luster, repair the steel bar, and paint the epoxy Then pour high-strength concrete with grout, vibrate and compact.

2) Crack grouting uses chemical pressure grouting for the cracks on the surface of the column. First, the cracks are closed with epoxy mortar to form a closed loop, the grouting nozzle is embedded, and pressure grouting is performed through the grouting nozzle to seal the cracks.

3) Change treatment of the cross-sectional shape of the rectangular column The corners of the original column are chamfered, and the expanded cement mortar is used to level and repair.

4) Surface treatment Grind the surface of the column concrete with a grinding wheel to remove the loose layer and oil stains on the surface until the new concrete interface is completely exposed, and the surface floating dust is removed with compressed air. After wetting, apply expanded cement mortar to form an oval shape.

5) Brush the base resin and mix it thoroughly according to the product instructions, then use a roller brush to evenly smear the concrete surface of the column.

6) Carbon fiber cloth pasting After evenly spreading the structural glue, quickly paste the cut carbon fiber cloth in place, and squeeze out air bubbles to make the carbon fiber cloth tightly bond with the concrete surface. Care should be taken to ensure that the joint is firm, otherwise the strength of the carbon fiber cloth cannot be fully utilized. When pasting, each layer of carbon fiber cloth should be pasted in staggered stitches.

7) Surface treatment and pasting for at least 8 hours, apply a layer of base resin on the surface of the carbon fiber cloth, and dry it with cement mortar as the surface layer.

Conclusion

The reinforcement construction of the project was completed in September 2007. After more than a year of use, there is no abnormality in the reinforcement part, indicating that it is very effective to use FRP jacketing to concrete column to strengthen the reinforcement column.