Unidirectional carbon fiber wraps have excellent physical and mechanical properties such as high strength, light weight, corrosion resistance, and fatigue resistance, as well as convenient on-site construction. Therefore, they are ideal wraps for reinforcement of old bridges. In recent years, this technology has been used in such projects as the Shenze Chunlun River Bridge on the Anxin Line, the reinforcement of the 307 National Highway old bridge, and the 107 National Highway old bridge reinforcement.

Requirements for unidirectional carbon fiber wrap

The unidirectional carbon fiber wrap used to reinforce concrete members is a soft sheet made of unidirectional carbon fiber filament warp knitting. When weaving unidirectional carbon fiber wrap, a large number of unidirectional carbon fiber filaments are evenly laid out in one direction, and the main direction unidirectional carbon fiber filaments are woven and connected together with very few non-main direction unidirectional carbon fiber filaments, form a very thin unidirectional carbon fiber wrap that is stressed in the direction of the main fiber.

When reinforcing concrete members, according to the different stress characteristics of the members, the unidirectional carbon fiber wrap is wound and pasted on the surface of the members in an orderly manner with the bonding wrap to restrain the deformation of the members, thereby improving the ultimate strength and bearing capacity of the members.

The tensile strength of unidirectional carbon fiber wrap is generally 3 550 MPa, and the elastic modulus is 2.35 x 105 MPa. Depending on the quality of the unidirectional carbon fiber wrap, its thickness is 0.11 -0.43 mm, the width is 20-100 cm, and the coil length is 50-100 m.

Unidirectional carbon fiber wrap construction technology

1 base surface treatment

(1) First, the degraded layer on the concrete surface (such as laitance, weathered layer, etc.) should be removed and polished with a grinder;

(2) The dislocation and protruding parts of the base surface should be smoothed, and the corners should be chamfered;

(3) Epoxy resin slurry should be injected into the cracks for repair.

2 Cleaning of the base surface

(1) First use a wire brush to remove loose scum on the surface, and then use compressed air to remove dust;

(2) Wipe the surface with acetone or absolute alcohol, or rinse with clean water to ensure that it is fully dry before proceeding to the next process.

3 Apply primer

(1) Preparing the primer accurately in proportion and mixing it evenly. Note that the amount of one-time blending is used up within the available time, and it must not be used beyond the time to ensure the bonding quality;

(2) Spread evenly on the base surface with a roller or brush, pay attention to spread evenly across the board, and air dry. If it is applied in winter, the viscosity of the glue is high, so it should not be applied too thickly;

(3) After the primer is hardened, if there are convex parts on the surface, polish it with a sander or sandpaper;

(4) After the primer is dry to the touch, enter the next process.

4 Paste repair

(1) If there is a concave part on the sticking surface, use leveling glue to repair it to ensure the smoothness of the sticking surface to ensure the reinforcement effect.

(2) After the leveling glue is dry to the touch, enter the next process.

5 Paste unidirectional carbon fiber wrap

(1) Mark the position of each layer on the top of the paste to be pasted;

(2) When cutting unidirectional carbon fiber wrap according to the design size, the cutting length should be determined according to the site construction experience and operation. If it is necessary to extend the length, the length of the joint should be determined according to the actual situation, generally not less than 15 cm;

(3) The quantity of cutting wraps should be used up on the same day;

(4) When pasting the unidirectional carbon fiber wrap, it should be pasted from top to bottom, from left to right according to the design position, and the patch is pressed with a roller to fully combine the unidirectional carbon fiber wrap with the vivid resin, and the pressure plate is now used to remove air bubbles;

(5) Observe immediately whether the patch is pasted tightly, and if there is a gap, it should be dealt with in time.

6 Cover protection treatment

(1) After pasting the unidirectional carbon fiber wrap, immediately apply a layer of impregnated resin on the surface of the unidirectional carbon fiber wrap, and let it dry naturally;

(2) After ensuring that the surface of the patch has been fully air-dried and combined, coat the surface with masking glue or take other measures to ensure the durability of each layer of glue.

Paste construction requirements

1 Requirements for the base surface of the reinforced component

Because unidirectional carbon fiber wrap is used to reinforce the bonding efficiency of the surface of the concrete member, the concrete strength level of the base surface is required to be no less than C15. At the same time, it is required that the reinforced component should have a good protective layer, that is, the base surface is flat and has a certain strength. For components that have surface defects such as peeling, peeling, corrosion, cracks and severe unidirectional carbonization, they must be repaired first, and the base surface of the paste should be polished and cleaned, and there should be no sharp corners and floating dust. Prevent the partial peeling of the unidirectional carbon fiber wrap and the failure of the paste.

2 Paste cut of unidirectional carbon fiber wrap

unidirectional carbon fiber wrap is used to add concrete components to the concrete members, which should adopt a thin wrap multilayer pasting method to fully infiltrate the bonding wrap to ensure bonding efficiency.

For flexural members, it is advisable to stick unidirectional carbon fiber wrap shoes straight along the axial direction in the tension zone for reinforcement, and perform additional tracing and solidification at the cross-section end in the main fiber direction.

3 Lap and cut of unidirectional carbon fiber wrap

Reinforced unidirectional carbon fiber wrap is generally not suitable for arching along the main fiber direction, especially for the reinforcement of tension members and bending members in the tension area. According to the test results of bonding and tracing between unidirectional carbon fiber wrap and concrete at home and abroad, the bonding stress is mainly concentrated within 100 mm of the end, the bonding failure is brittle, and the bonding stress generally does not expand. Therefore, if the unidirectional carbon fiber wrap needs to be lapped in reverse, the lap joint should avoid the section with the largest component stress, the lap length should not be less than 100 mm, and the lap end should be flat and free of warping. The multi-layered joint positions should not be in the same section, and the clear distance of the joint positions of each layer should be greater than 200 mm.