Design principles of carbon fiber structural strengthening technology

The use of any kind of material has its limitations. Carbon fiber reinforced bridge technology also has a certain scope of application and applicable conditions. Blind use beyond this scope can only be a waste of investment. Even buried safety hazards. Therefore, after passing the inspection, testing, and static and dynamic load tests on the old bridge, it is a very important task to choose a reasonable strengthening plan and understand the scope of application of various strengthening plans. 

The strengthening principles of carbon fiber materials are:

1) The concrete strength of the components to be reinforced shall not be lower than C15. Otherwise it cannot be used

Carbon fiber material is reinforced.

2) The cross-sectional dimensions of reinforced concrete members must meet the requirements of not greater than the design value of shear force. otherwise. Carbon fiber materials cannot be used for processing.

3) When the number of layers n≤3, the rigidity and bearing capacity of the component can be enhanced. The effect of the externally attached carbon fiber on the stiffness of the component depends on the thickness of the carbon fiber used for strengthening (single layer thickness and the thickness of the adhesive layer). Therefore, when using carbon fiber materials for strengthening, if the fiber configuration rate is relatively low. The increase in stiffness and bearing capacity increases faster with the increase in the number of fibers. If the fiber configuration rate is relatively high. The increase in stiffness and bearing capacity increases slowly with the increase in the number of fibers. Considering the increase in the thickness of the carbon fiber adhesive layer. The shear stress and normal stress generated in the anchoring zone also increase. Paste damage may occur. In addition, it is affected and restricted by the elastic modulus and ultimate elongation of the adhesive and the construction climate. The number of adhesive layers of stressed carbon fiber cloth on the same section should not exceed 3 layers.

4) When the beam is strengthened by shearing, the U-shaped strip net distance is not more than 200mm.

Control of bending strengthening

When calculating the flexural capacity of the normal section, in addition to the flexural capacity of the normal section, the shear capacity of the member should be checked. Avoid that the shear failure precedes the bending failure due to the excessive increase in the bending bearing capacity. Therefore, the increase in flexural capacity after strengthening should not exceed 40%. After strengthening, the stress of the tensile steel bar under short-term load combination should not exceed the design value of the tensile strength of the steel bar, and the converted strengthening after strengthening should not be greater than 2.5%.

Shear strengthening

It is required that when carbon fiber is used to strengthen the beam, the carbon fiber cloth sometimes needs to be arranged in strips. At this time, the net spacing should be greater than 0.7 times the maximum stirrup spacing specified by the current state, and the size of the shear section of the beam should conform to the size specified in the current national code.

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