Carbon fiber strengthening can be successfully used for the seismic, shear and bending structural strengthening reinforcement of concrete structures.

However, it should be pointed out that because of the inherent stress characteristics of the carbon fiber sheet reinforcement structure, many concrete structures are not suitable for strengthening with carbon fiber, so it is necessary to analyze its applicability.

So under what circumstances is it not suitable to use carbon fiber strengthening method for construction reinforcement? 

01 Stiffness question

When the concrete structure and roof structure are lack of rigidity, which causes excessive deformation and severe cracking, which affects the application, it is not suitable to be strengthening by the method of posting carbon fiber sheet.

Studies have shown that, for beams, slabs, and other transversely stressed members, when strengthening with carbon fiber, under normal operating limit conditions, the effect of carbon fiber on structural stiffness is less than 10%, or no progress at all.

02 Scope of application of carbon fiber flexural strengthening

When the concrete structure is not strong due to the flexural bearing capacity, when the carbon fiber cloth is used forstrengthening, the damaged shape of the strengthening structure depends on the strengthening of the original structure on the one hand, and the amount of carbon fiber on the other hand. Now assume that the original structure is a suitable strengthening member, the damaged shape of the strengthening structure can be divided into the following three conditions.

⑴ Less carbon fiber consumption. When damaged, the marginal concrete in the compression zone is crushed, the tensile steel bar is yielded, and the carbon fiber can reach a higher tensile strain.

⑵The dosage of carbon fiber is moderate. When damaged, the marginal concrete in the compression zone is crushed, the tensile steel bar is yielded, and the carbon fiber can reach a certain intermediate tensile strain.

⑶ The amount of carbon fiber is more. When damaged, the marginal concrete in the compression zone crushes, the tensile steel bar yields, and the carbon fiber strain is very low.

The above three conditions, the third condition is not suitable because of the large amount of carbon fiber and low strength utilization rate. The first strengthening method is suitable for beams with large cross-section height, and the second strengthening method is suitable for slab or flat beam with small cross-section height. This is mainly because for the slab or flat beam, when the carbon fiber does not reach a high tensile strain, the structure can be damaged due to excessive deformation. The detailed limit depends on the ultimate tensile strain of the carbon fiber and the loading method.

03 Torsion doubt

For the torsional strengthening of concrete structures with carbon fiber materials, there is little research at home and abroad. From the perspective of the torsional damage mechanism of the concrete structure, regardless of whether the fiber direction of the carbon fiber is perpendicular to the axis or parallel to the axis, it can restrain and change the development of oblique cracks and play a certain role in strengthening.

However, when carbon fiber is used for strengthening, on the one hand, the cracks of the component are now formed, and the distribution of the cracks has been determined. On the other hand, because the ultimate strain of carbon fiber can be as high as 10000με or more, while the strain of ordinary steel bar is less than 2000με, it has already yielded. If the strain is increased to more than 4000με, the crack width of the concrete has already expanded greatly, and it must start to develop on other surfaces, and the structure is close to damage. At this moment, the stress that the carbon fiber can reach is still very low, so the role of the carbon fiber in strengthening the torsion member needs to be determined after further study.