Bridge retrofitting

At present, many existing bridges in China can not meet the use requirements for various reasons, and need to be strengthened. The maintenance, repair and reinforcement of concrete structures and facilities has become an urgent issue. Traditional materials are often used to reinforce relatively low efficiency, higher degree of mechanization, and usually appear after the poor durability of the problem. The special resin and carbon fiber cloth are applied to the surface of concrete according to the design requirements, so as to achieve the reinforcement effect of the structure. The mechanical properties of beams, culverts and other beams before and after reinforcement have been significantly improved. According to the experimental study, the bending properties of beams, slabs and culverts with a layer of carbon fiber unidirectional cloth can be improved by 5-8%. Compared with other reinforcement techniques such as sticking steel and shotcrete, this technique has the characteristics of small dead weight, simple construction, short construction period and good durability. Because of the advantages of high strength, high efficiency, corrosion resistance and light deadweight, carbon fiber fabric has been widely used to reinforce reinforced concrete structures.

1 Project overview

The expressway section from Nanchang to Jiujiang (section) is the main trunk line in Jiangxi Province, with a total length of 133 km. Its construction has undergone many changes, such as secondary highway reconstruction, automobile special highway widening and so on. After more than ten years of operation and use, the pavement has appeared a more serious and diversified late damage phenomenon with potential safety hazards. In view of this, the owners decided to carry out large-scale comprehensive technical transformation of Chang nine expressway. The main content of this technical transformation is to increase the thickness of the pavement structure layer by 16-25 cm on the basis of reinforcing the existing pavement structure layer. At the same time, the single pavement double-sided slope existing in some sections is changed into single-sided slope, in order to fundamentally solve the hidden danger of transverse drainage of Chang-Jiu Highway. In order to achieve the above-mentioned technical transformation purposes, all bridge and culvert structures along the line should be treated and reformed according to the above requirements, and the existing diseases of various structures should be eliminated simultaneously.

There are altogether 849 bridges and culverts along Chang-Jiu expressway, including 79 main bridges and 26 branch bridges. The superstructure is mainly composed of 13m, 16m and 20m pre-tensioned simply-supported hollow slabs. There are a small number of 8m and 10m ordinary reinforced concrete simply-supported hollow slabs, continuous box girder and I-shaped composite girder bridges. There are 303 round pipe culverts, 441 arch culverts and passages with cover plates, box and mortar masonry. Among them, there are nearly 200 open culverts and covered culverts with a small amount of filling at the top. Through investigation, no obvious structural defects were found in the structure, and the overall bearing capacity of all the structures is still good, only in specific parts such as bridge deck continuous, bridge deck pavement, rubber bearings and other weak positions appeared more damage. Therefore, in addition to eliminating all kinds of existing diseases, the emphasis of this reconstruction is to reinforce and reinforce the main line and overpass bridges, culverts and roof slabs and culverts with a small amount of fill, which are greatly affected by the road surface reconstruction, so as to achieve the goal of ensuring traffic safety.

2 Material properties

At present, the technical status of structural reinforcement, reconstruction and repair can be generally divided into reinforced concrete thickening method, volume pre-addition method, sheet reinforcement method, isolation and Seismic Absorption method. Carbon fiber reinforcement method has the characteristics of low relative density, high fatigue strength, good durability, wear resistance, convenient construction, low cost, and is not restricted by construction conditions basically does not affect the shape of the original structure. The test results of concrete beams strengthened with FRP show that the bending stiffness is increased by 17%-99% and the bending strength is increased by 28%-97% under normal load. In the shear strengthening test, the bending capacity of the strengthened members is greatly improved, about 65%-95%, which meets the seismic requirements of strong bending and weak shear.

3 Basic assumptions of reinforcement design

(1) the role of concrete in the tensile area is negligible.

(2) when the beam is bent, the strain of concrete, steel bar and carbon fiber conform to the assumption of plane section.

(3) the stress-strain relationship of the carbon fiber material is linear elastic:  σcf=Ecf×εcf  andεcf<0.01.

(4) before reaching the ultimate bending capacity state, there is no bond peeling failure between CFRP and concrete.

4 Carbon fiber reinforcement construction

4.1Adhesive configuration

(1) The raw materials are weighed accurately according to different mix ratios, and primer, leveling and bonding binders are respectively allocated. The diluent is added into the main polymer material and stirred evenly, then the filler is added to continue stirring until evenly, and finally the curing agent is added, which can be used after full stirring.

(2) Attention should be paid to the following matters when collocating the binder: 1-2 kg for each collocation of primer, 0.5-1 kg for each collocation of leveling binder and 1-2 kg for each collocation of binder.

(3) all the ingredients are required to be completed in 1H.

4.2 Substrate processing

(1) Deterioration of concrete surface such as spalling, hollowing, honeycomb and corrosion should be removed. Cracks with a width of less than 0.2 mm should be coated with epoxy resin and sealed. Cracks larger than 0.2 mm should be grouted with epoxy resin.

(2) Concrete corner grinder, sandpaper and other equipment to remove the surface of the concrete slurry, grease and other impurities, the base of the concrete component to be polished flat, especially the surface of the convex parts to be polished flat, corner paste to be chamfered and polished into a circular arc (R < 30mm).

(3) the blower will clean the concrete surface and keep it dry.

4.3 Primer

(1) the base glue is allocated according to the ratio of main agent: curing agent = 3:1. The main agent and the curing agent are placed in a container, measured with spring scales, and stirred evenly with agitators. According to the actual temperature of the site, determine the dosage and strictly control the use time. In general, 60mim is used up.

(2) Spread the adhesive evenly on the surface of the concrete component with roller brush or brush, the thickness is not more than 0.4mm, and no leakage brush or flowing air bubbles are allowed. The curing time is 3 ~ 24h.

4.4 Leveling 

(1) Repairing epoxy putty should be used to fill the concave parts of concrete surface, and patching agent should be used to fill the parts with poor height such as template joints, so as to minimize the height difference.

(2) a corner of the corner is also applied to repair a smooth arc with a radius of not less than 30mm. When the surface of the remedial agent is touched and dried, the next step can be carried out.

4.5 Bonding carbon fiber sheet

(1) Make sure that the adhesive surface is dry, the temperature is below 5 C and the relative humidity RH is above 85%. If there is no effective measure, the carbon fiber cloth can not be constructed.

(2) Mix and mix the adhesive material (using the same method as the base glue), then apply it evenly on the part to be pasted, and brush more on the lap joint, concrete corner and other parts. The thickness of the brush is slightly thicker than that of the base glue. It is strictly prohibited to have the phenomenon of leakage and brush, and pay special attention to sticking the edge of carbon fiber.

(3) After the carbon fiber cloth is bonded, the special tools are used to roll along the fiber direction repeatedly to remove the bubbles and make the adhesive permeate the carbon fiber cloth sufficiently, and the scraper is used to scrape the surface of the carbon fiber cloth to make it even. Multilayer pasting should repeat the above steps, and the next layer can be pasted until the surface of carbon fiber cloth touches dry.

(4) apply gluing glue evenly on the surface of the final layer of CFRP. Repeatedly rolling along the fiber direction with the tool, and scraping the surface of carbon fiber cloth with scraper to make it even.

(5) The overlap length of CFRP along the fiber direction should not be less than 10 cm. Resin should be coated on this part, and the defoaming and resin operation should be carried out normally.

5 Quality assurance measures

(1) carbon fiber raw material control.

Homogeneity of materials: carbon fiber bundles only distribute evenly, processed into sheets can play a role of uniform overall force.

Pre-impregnated resin content in carbon fiber sheets: Pre-impregnated resin is the role of fiber mutual restraint, mutual bonding into a common force of the whole, but its content is too much, impregnated too thick, but not conducive to repair and reinforcement. Because the CFRP reinforced concrete structure depends on the epoxy resin coated on the top and bottom of the repeated infiltration of carbon fiber sheets to complete. If the content of preimpregnated resin in the fiber sheet is too much, the resin penetration effect is poor when the carbon fiber sheet is pasted, which directly affects the construction quality.

The percentage of filament yarn of carbon fiber is very easy to appear because of its fine fiber yarn and only 7μm. Therefore, to minimize the breakage of fiber and ensure the continuity and integrity of fiber sheet is an important parameter to ensure the final reinforcement effect.

(2) strictly control the temperature and humidity at the construction site. The construction temperature is within 5~35 C and the relative humidity is no more than 85%.

(3) carbon fiber bonding project: the lap length of carbon fiber cloth along the fiber direction should not be less than 100mm. Carbon fiber cloth should be pasted as far as possible to avoid obstacles, if the obstacles can not be removed and need to be truncated, in the truncated part should be properly treated, specific measures should be determined according to different circumstances.

(4) The site acceptance after construction is mainly to evaluate the bond quality between CFRP and concrete, and the bond effect is judged by the echo of the lightly tapped CFRP surface with a hammer and other tools. If the phenomenon of stickup is not dense, the method of needle injection glue should be used to remedy it. If the bonding area is less than 90%, the bond is invalid and rebuilt.

6 Conclusion

The advantages of CFRP strengthening concrete structures are very obvious, and it has a broad application prospect in highway bridges and culverts reinforcement.