1 Project Brief

A reinforced concrete arch bridge was built in the 1980s. Due to many objective reasons such as bridge deck maintenance and excessive load-bearing, the bridge deck suffered serious water seepage. As a result, the longitudinally supporting micro-bending slabs of the bridge body were eroded, the surface of the arch wave was partially cracked, and the reinforced protective layer concrete was cracked and loose. After a comprehensive analysis and attack demonstration, it was decided to use carbon fiber cloth for reinforcement to achieve the original design bearing capacity, prevent excessive deformation, extend the service life, and ensure safe passage.

2 Reinforcement design

First, the rebound method is used to test the concrete, and the actual strength of the undamaged solid concrete meets the design strength requirements. According to the design drawings and the damage of the concrete on site, it is considered to increase the strength level of this reinforcement and increase the thickness of the protective layer after reinforcement. After reinforcement, the tensile and compressive strengths are fully guaranteed.

When reinforcing in the tension zone, first use high-pressure injection to reinforce the cracks, and then reinforce the carbon fiber cloth. The fiber direction of the carbon fiber sheet is consistent with the tensile stress direction in the tension zone. Before reinforcement, peel off the cracked and loose parts of the steel bar protection layer, and thoroughly clean the bonding surface. Afterwards, the arch wave was reinforced with a layer of carbon fiber cloth of 300g per square meter along the bridge. The carbon fiber cloth needs to be pasted with the matching adhesive. The resin bonding material matched with the carbon fiber is prepared by a main agent and a curing agent, and is divided into a winter type and a summer type suitable for use in winter and summer. The main agent and curing agent are packaged separately. When used on site, they should be mixed evenly according to the process requirements and in the specified ratio to form the required bottom resin, leveling resin, and bonding resin. The role of the bottom resin is to enhance the bonding strength of the surface concrete and the leveling material or the bonding resin interface. Leveling resin is to make the surface flatness meet the specified requirements, and have reliable bonding strength with the underlying resin and the adhesive resin. The bonding resin is the main bonding material for sticking carbon fiber cloth, and its function is to make the carbon fiber cloth and concrete fully bond, so that it can bear the role of the structure together.

The main technical indicators of the material-carbon fiber cloth: the weight is 300g per square meter, the design thickness is 0.167mm, the standard value of tensile strength should be greater than 3400MPa, and the elongation at break should be greater than 1.5%. Base rubber, leveling rubber: The positive tension bond strength should be greater than 3.0MPa. Dipping glue: the positive tensile bond strength should be greater than 3.0MPa, the tensile strength of the adhesive butt joint should be greater than 30MPa, the tensile shear strength should be greater than 18MPa, and the elongation rate should be greater than 1.5%.

3 Construction technology

3.1 Treatment of concrete base surface

Use hand hammer steel to remove cracked and loose concrete on the surface, exposing it on a clean, solid surface. After cleaning the dust and loose scum on the concrete surface with a wire angle grinder, clean the base surface with clean water or blow it with an air compressor to ensure that the base surface is clean, free of dust and oil, and dry.

3.2 Apply primer

Use a roller brush or brush to evenly and completely apply the primer on the concrete surface to be reinforced. The boundary of the primer coating surface should not be smaller than the size of the carbon fiber cloth. During construction, the temperature should not be lower than 5℃ or higher than 40~C, and the relative humidity should be less than 70%. In case of rain, the construction should be suspended and the construction can be carried out after the surface treatment is dry. After the primer gel is dry to the touch (about 1h), if there are protruding burrs or glial bumps on the surface, use emery cloth to polish it smoothly. Be careful not to polish the primer through.

3.3 Trimming the leveling layer

After the primer is dry to the touch, if there are defects, anti-sagging, dented corners, height differences at the template joints, etc. on the pasting surface, apply leveling glue for multiple repairs. Ensure that there are no obvious height differences, defects, and potholes at the formwork joints. The corners of the recesses are filled with rounded transitions with R greater than or equal to 20mm. After the leveling material is cured to dryness to the touch, if there are protruding burrs on the surface of the leveling part, use emery cloth to polish it smoothly. If there is a large glioma, it can be removed with a flat shovel, and if it is damaged during the trimming process, it should be repaired immediately. In the inspection, pay special attention to whether there is any virtual stickiness or air bag phenomenon on the surface. This phenomenon is mostly caused by substandard treatment at the basic level, or caused by floating dust, dampness, and looseness. It must be repaired in time.

3.4 Paste carbon fiber cloth

Cut the carbon fiber cloth to the size required by the design. According to the standard dosage of adhesive, calculate the required dosage of the coated area. Use a roller brush or a brush to apply the adhesive on the selected concrete surface evenly and without omission. The surface of the adhesive should not be smaller than the size of the carbon fiber cloth. Lay carbon fiber cloth on the concrete surface that has been coated with adhesive glue. The direction of the carbon fiber cloth is in line with the design requirements. The overlap between the cloths in a layer should be done in the fiber direction, and the overlap width should not be less than 10cm . Use a special rubber roller and a scraper to apply pressure in the direction of the carbon fiber and roll the bandit in one direction or from it or from the middle to two directions, but it is not allowed to roll back and forth repeatedly to prevent fiber relaxation.

3.5 Brushing surface protective glue and surface coating

After the surface protective glue is pasted with carbon fiber and formed into a composite material gel and cured, a brush and a scraper are used to uniformly coat the surface of the composite layer to form a protective glue layer. The surface coating is mixed with cement mortar and polymer glue and applied to the surface of the protective glue layer.

3.6 Inspection

After the paste is finished and cured, it is inspected. The compactness can be used to hit the surface of the component with a small hammer, and the compactness can be judged by different sounds. The density should meet the relevant regulations, otherwise it should be repaired and inspected.

4 Conclusion

The strengthened bridge has undergone the test of the bridge's dynamic load and static load, as well as half a year of real-time tracking and observation, and it has met the requirements of demonstration and design, which proves that the reinforcement scheme is feasible.

Through the construction operation of this project example, I deeply realized the advantages introduced in the introduction of carbon fiber structure reinforcement technology, especially in terms of cost investment and construction period, which are significantly lower than other solutions.