1 Project Overview 

The Longxi Interchange 1 Bridge is located 42 km+295 m from Hening Expressway. The total length of the bridge is 110 m and the net span is 100 m. The upper structure of the bridge is a cast-in-place reinforced concrete box girder with four lanes in both directions.

According to field observation, there are many cracks in the bottom of the box girder, and the cracks have penetrated, and there is water leakage.

According to the design check calculation, there is a large tensile stress at the bottom edge of the box girder mid-span bottom plate under the limit state of normal service, and the cracked concrete must be reinforced to prevent the cracks from expanding further. To this end, the following reinforcement schemes have been formulated for the bridge:

① Take surface sealing treatment for cracks with crack width ≤ 0.02 mm.

② For seam width >0.02 mm, use pressure grouting.

③ After the treatments of ① and ② above, stick carbon fiber cloth to the beam bottom plate for reinforcement.

2 Choice of carbon fiber cloth and viscose

2.1 Carbon fiber cloth (CFRP)

Carbon fiber cloth is a composite material woven with carbon fiber yarns in a certain direction. This project uses HM-300 produced by Horse Construction Co., Ltd., with tensile strength ≥3 000 MPa, elongation ≥1.4%, and the unit mass is 300 g/m. , CFRP cloth is unidirectional fiber, the width is 100 cm

2.2 Resin glue

According to the main purpose, the supporting resin adhesives are divided into bottom adhesives, search adhesives and pasting adhesives. This project uses Horse primer, leveling glue and HM-180C3P carbon fiber paste glue. The bottom layer glue is mainly coated on the surface of the reinforced structure, which can significantly improve the strength of the concrete base surface and its adhesion to the upper carbon fiber material. Leveling glue is mainly used to seal concrete cracks and level the surface. Carbon fiber adhesive is used to bond the fiber cloth to the reinforced concrete.

3 Main construction technology of carbon fiber cloth reinforced bridge

3.1 Construction preparation

(1) Material inspection. For the purchased materials, after reviewing the production inspection report and certificate attached with the goods, they will immediately send samples for inspection. It can be used after passing the inspection.

(2) Set up a facility work platform and prepare the equipment needed for construction.

3.2 Primary treatment

(1) Remove the degraded parts of the concrete surface, such as peeling, honeycomb, looseness, and corrosion. For a large area of inferior layer, it needs to be cleaned repeatedly until a hard layer appears. After removing the inferior layer, apply epoxy JGN-DN structural adhesive to repair, and do the maintenance work in time.

(2) Use concrete angle grinders, grinding wheels, high-pressure air guns and other tools to remove impurities such as scum and oil on the concrete surface, and polish the base surface concrete of the components, especially the protruding parts of the surface, and the internal and external angles should be pasted. Carry out chamfering and form an arc shape with a radius of 2 cm to prevent stress concentration from reducing the repair effect.

(3) The base layer should be flat. In principle, the recess between 1 m should be less than 5 mm, and there should be no partial unevenness between 25 crn. In addition, the level difference is within l Hm, and there is no acute angle part. If there are cracks of <0.02 mm in the reinforcement construction range, the surface shall be sealed with seam sealant, and the cracks of >0.02 mm shall be filled with glue.

(4) After the base level is polished, use a hair dryer to blow off the dust on the concrete surface, then wipe it several times with ethanol and acetone, and dry it.

3.3 Applying primer

(1) According to the requirements of Table 2 and the instruction manual, put the main agent and the curing agent in the container successively, mix them evenly with a stirrer, determine the dosage according to the actual temperature of the site environment, and strictly control the use time.

(2) Use a roller or brush to spread the glue evenly on the surface of the concrete member, and there should be no leakage, flow, or bubbles. After the glue is cured, proceed to the next process.

3.4 Leveling of concrete surface

(1) For the concave parts of the concrete surface, use a spatula to insert and scrape the leveling glue, and partially repair and fill it. The height difference of the formwork joints should also be filled with the leveling glue.

3.5 Paste carbon fiber cloth

(1) According to the size required by the design and the actual size of the site, the carbon fiber cloth is cut, numbered and staked out.

(2) Configure, stir and paste the glue, use a roller to brush evenly on the glued box girder, and apply more on the overlap and corners. At the beginning, the base glue is 0.5 kg/m. , Adhesive veneer is 0.25 ~ 0.3 kg/m, and make a trial match according to the area where the carbon fiber is pasted on site, so that the carbon fiber is fully impregnated.

(3) Spread the carbon fiber cloth on the resin-impregnated concrete surface, and use a special smooth roller to roll the carbon fiber surface in the same direction multiple times on the carbon fiber surface while spreading it until the rubber material seeps out of the outer surface of the carbon fiber cloth. When rolling, from bottom to top, from the inside to the outside, to remove air bubbles, so that the carbon fiber cloth is fully infiltrated with the rubber. When the fiber surface feels dry, the next layer of CFRP cloth can be pasted (the interval should be more than 12 h).

(4) Apply two layers of weather-resistant protective paint on the outer surface of the outermost layer of CFRP cloth to resist the aging of the resin caused by sunlight and ultraviolet radiation. This project uses acrylic water-based high-grade "Fengcai" brand cement paint.

4 The development prospects of carbon fiber composite materials

In order to improve and restore the bearing capacity of the existing bridge, a reinforcement method of sticking carbon fiber composite material outside the tension side of the bridge structure is adopted. Compared with the traditional enlarged section reinforcement method, bonded steel reinforcement method, outer steel reinforcement method, additional component reinforcement method and prestressed reinforcement method, it has simple construction and low reinforcement cost (replacement of steel reinforcement can save 25% of funds). And the advantages of not reducing the clearance under the bridge, and the reinforcement materials do not increase the dead load, and the reinforcement construction can be carried out without affecting the use of the structure. CFRP also has the characteristics of high strength and good durability.