Externally bonded carbon fiber(CFRP) or steel plate reinforcement is to use epoxy resin or building structural glue to paste carbon fiber or steel plate on the surface of the tensioned or weakened part of the reinforced component, so that it and the reinforced component form an integral joint force. To increase structural strength and improve structural stress performance. Pasting carbon fiber and steel plate is a commonly used passive reinforcement method in bridge reinforcement, and both can improve the bearing capacity of the beam. It is suitable for the reinforcement of beams and slabs where the original component's main or diagonal reinforcement is insufficient and the original component's tension steel is seriously corroded or damaged.

Externally pasted carbon fiber(CFRP) reinforcement method

Externally pasted carbon fiber reinforcement is to paste carbon fiber materials with a bonding agent along the tensile direction or perpendicular to the crack direction on the structure to be reinforced. A reinforcement method that uses the high tensile strength of carbon fiber to increase the tensile or shear resistance of the structure. Suitable for bending and shear reinforcement of beams and plates.

The weight of the carbon fiber cloth is very light, the thickness is 0.111-0.333mm, and the pasting does not exceed 4 layers, which has little effect on the weight of the entire reinforcement structure and the clearance under the bridge. The carbon fiber cloth is pasted on the surface of the concrete, which not only closes the cracks of the original concrete, but also restricts the expansion of cracks in the concrete structure due to its high-strength and high modulus characteristics, thereby improving the overall rigidity of the concrete members.

Externally bonded steel plate reinforcement method

Externally glued steel plate reinforcement is to use epoxy resin or building structural glue to stick the steel plate along the tensile zone or the shear weakened part of the member on the edge of the concrete member. A reinforcement method that uses steel plates to supplement the lack of reinforcement of the original members to improve the bearing capacity and stiffness of the structure. If the bearing capacity of the normal section of the T beam is insufficient, a steel plate can be pasted on the bottom of the beam when the beam bottom cracks occur. When the main tensile stress area of the bridge structure is insufficient, the steel plate can be pasted diagonally on the side of the concrete member and perpendicular to the direction of the shear crack. It is also possible to stick strip steel plates vertically or use U-shaped and L-shaped hoop plates.

The thickness of the pasted steel plate is generally 4-10 mm, which basically does not affect the shape of the reinforced member and the bridge clearance, and the increase in the weight of the bridge is also small. It can be constructed without or less affecting the traffic. The construction process is simple and the construction quality is easy to control. Sticking steel plate at the bottom of the beam can improve the flexural rigidity of the structure, and it is very effective for reinforcement whose main purpose is to control the deformation of the structure.

Comparison of the effect of reinforcing flexural members with carbon fiber(CFRP) or steel plate

1) Comparison of bearing capacity after reinforcement

In the commonly used thickness, the effect of sticking carbon fiber cloth to strengthen the bending capacity is not ideal. The bearing capacity after sticking 3 layers of carbon fiber cloth reinforcement is not as good as the bearing capacity after 4 mm steel plate reinforcement. Pasting two layers of carbon fiber cloth has a small increase in the bearing capacity. Pasting three layers of carbon fiber cloth is more reasonable, and the bearing capacity is increased by 6.5%, and adding a layer of carbon fiber cloth increases the bearing capacity by 5.1%. With 4mm steel plate attached, the bearing capacity is increased by 7.9%, and each additional layer of steel plate can increase the bearing capacity by 3.7%. Compared with the increase in the bearing capacity of adding a layer of adhesive, the bearing capacity of carbon fiber cloth is higher than that of steel plate, and the increase of the area of carbon fiber cloth has a greater effect on the improvement of bearing capacity.

2) Comparison of total stress of materials

Considering that the load before reinforcement is borne by the original member, and the load after reinforcement is borne by the combined section composed of the original member and the paste, the material has the characteristic of being stressed in stages.

With the increase in the number of layers of bonded carbon fiber cloth and the thickness of steel plate, the compressive stress of concrete, the stress of tensile steel bar and the stress of bonded materials gradually decrease, and none of them exceed the strength design value. After reinforcement, all materials can work safely. The area where the carbon fiber cloth or steel plate is pasted increases. The carbon fiber cloth or steel plate and concrete work together to increase the moment of inertia of the cracked section and the bending moment of inertia of the section. Under the same load, the stress of each material is reduced. Due to the small width of the ribs of the T beam and the limited area of the carbon fiber cloth, the carbon fiber cloth has a small effect on the moment of inertia of the beam section. After reinforcement, the compressive stress of concrete and the tensile stress of steel bar are both greater than the reinforcement of the steel plate, and the stress of the carbon fiber cloth is much smaller than the design value of tensile strength, and the high-strength effect of carbon fiber cloth is not exerted. The increase in the area of carbon fiber cloth reduces the stress of each material to a small extent, and the reinforcement effect of carbon fiber is not as good as that of steel plate. Increase or decrease the area of the steel plate, the difference between the concrete compressive stress and the tensile stress of the steel bar will decrease. However, due to the strength of the original component's tensile steel bars, the hysteresis strain of the new and old materials, and the limitation of the bond surface strength between the adhesive and concrete, the reinforcement effect will not change much.

3) Comparison of stiffness

Generally, the area of carbon fiber cloth is small, and the area of 2 mm steel plate is much larger than that of carbon fiber cloth. The area equivalent to one-layer steel plate as steel reinforcement is also much larger than that equivalent to one-layer carbon fiber cloth as steel reinforcement. Therefore, the rigidity of the beam with one layer of steel plate is greater than that of one layer of carbon fiber cloth. The fiber-impregnated primer has a certain degree of penetration into concrete, and has good adhesion with concrete. Carbon fiber controls the crack development of beams better than steel plates. Carbon fiber reinforcement is suitable for improving the crack resistance of beams.

Sum up

1) Reinforced concrete beams are strengthened by pasting carbon fiber(CFRP). After the steel bar yields, the carbon fiber in the tensile zone may not reach the ultimate tensile strain and fail. When the reinforced concrete beam strengthened by the steel plate is damaged, the tensile steel (steel plate) yields and the concrete in the compression zone reaches the ultimate compressive strain. There are two forms of beam failure, and the corresponding bearing capacity calculation formulas should be adopted for different failure modes.

2) In the usual pasting range, the effect of pasting 2-3 layers (4-6 mm) of steel plate on the bearing capacity and stiffness of the beam is better than pasting 2-3 layers of carbon fiber cloth. The reinforcement of pasting carbon fiber cloth does not exert carbon fiber The high-strength effect of the cloth, the reinforcement of carbon fiber cloth does not significantly improve the tensile stress of the concrete in the compression zone and the steel bar in the tension zone.

3) Carbon fiber cloth(CFRP) reinforcement is not suitable for reinforcement aimed at improving the flexural capacity of beams. In order to improve the crack resistance of components and restrict the development of cracks, carbon fiber cloth can be used for reinforcement. To improve the bearing capacity and overall rigidity of the beam, it is more reliable to use glued steel plate reinforcement.