Engineering background

The bridge is located at the K294+140 highway from Tianshui to Dingxi on the main line of Lianhuo National Road, crossing the local river ditch. The bridge is divided into left and right. The total length of the bridge is 376m, the total width of the bridge deck is 25m (l2 mx2+1 m), and the bridge deck is combined with drainage. The bridge is located on a flat curve with radius R=760 m.

Bridge disease status

The bridge deck is made of asphalt concrete upper pavement and concrete anti-collision guardrail. The upper structure is a 9-span (4x 40 m+5 x 40 m) prestressed concrete continuous small box girder, which is divided into two links, the first 4 spans are the first link, and the rear 5 spans are the second link. A single horizontal cross-section of 4 small box beams, with a beam height of 2.0 m, adopts the method of simple support and continuous construction. The lower structure is column type pier and platform, and bored pile foundation. The bridge uses plate rubber bearings.

Deck disease

No obvious diseases were seen on the asphalt concrete deck of the bridge. Expansion joints are filled with soil and the concrete protection band is cracked. Anti-collision guardrail honeycomb pitted surface, exposed steel bar corrosion, fixed bolt missing. Affected facilities cone slope and many local collapses of protection works.

Diseases of superstructure

The superstructure of the bridge is a continuous prestressed concrete box girder with multiple repair marks. The reinforcing steel of the bottom plate is exposed. There are vertical cracks in the web. Longitudinal cracks occurred at the root of the flange plate, accompanied by water seepage and alkali. The concrete in the post-flap section of the flange plate is not densely packed, filled with debris, and the cavity is 6.5 cm deep. The steel bar is exposed and corroded. The diaphragm is not densely cast, the aggregate is exposed, and the upper and lower positions are misaligned. The cast-in-situ section of the box girder on the pier top seeped water and alkali, and the aggregate was exposed. The reserved holes for hoisting are not closed and seepage water and alkali.

Substructure disease

The bridge uses plate rubber bearings, and no obvious defects are seen in the appearance. Debris piled on the top of the cover beam of the pier, there are 1 horizontal cracks on the front side of the cover beam, 3.4 m long and 0.26 mm wide, and the crack cracking depth is small when observed from the top of the cover beam.

Disease cause analysis

(1) Most of the prestressed steel beams are arranged with web plates, and the steel beams are arranged densely, with a vertical spacing of 11 cm. If the construction is not vibrated properly, the rented aggregate is difficult to distribute evenly, which reduces the density of the concrete and is one of the reasons for the decrease in strength.

(2) The quality is not strictly controlled during the construction process, the concrete is poured too fast, the water quality is poor, the water-cement ratio is not suitable, and the mold is demolished too early during construction and other factors. It is also an important reason for the low strength of concrete and the appearance of open bars, honeycomb pits and voids.

(3) By comparing the calculation results, it can be concluded that the vertical cracking of the web is an important reason for the cracks in the web due to the small protective layer at some locations, the shrinkage of the concrete and the construction quality.

(4) The poor construction quality of the connection between the wet joint and the diaphragm is the main reason for the weak lateral rigidity. The low strength of the concrete may have a certain impact on the lateral rigidity.

In summary, the main causes of main beam diseases are poor construction quality and insufficient strength of main beam concrete.

Reinforcement maintenance method

The main beam is reinforced with prestressed carbon fiber laminate

The strength of the main girder concrete does not meet the design strength requirements. The bonded prestressed carbon fiber board reinforcement method is used to strengthen the main girder to increase the strength of the main girder and thus the bearing capacity of the bridge structure. Before tensioning, the carbon fiber board should be coated with special carbon fiber board adhesive in advance. After the tensioning is completed, the card board should be used to adhere it to the bottom of the beam. If there is still an empty area between the carbon fiber board and the bottom of the beam, it should be filled up with adhesive in time. Pre-stressed anchors (including carbon plates, anchors, etc.) should be surface-treated products. After the anchors are installed, epoxy-based protective materials should be used to coat and protect the worn areas of the anti-corrosion layer, and whether to install special supporting protective covers according to the site conditions. Otherwise, special anti-corrosion and protective measures should be taken. 

Conclusion

Compared with the traditional non-prestressed carbon fiber laminate reinforcement, the prestressed carbon fiber laminate reinforcement technology effectively solves the defect of poor working performance of carbon fiber and reinforcing steel. The material strength of carbon fiber is fully utilized, and the problem of strain lag in reinforcement is overcome. The application of prestressed carbon fiber board reinforcement technology can significantly improve the crack resistance and load-bearing capacity of the reinforced structure, reduce structural deformation, increase the structural bending stiffness, and improve the working performance of the structure in the use phase.

The successful reinforcement of the diseased bridge shows that: The prestressed carbon fiber laminate reinforcement bridge structure technology has the traditional carbon fiber laminate reinforcement technology and external prestress unmatched advantages. Can effectively improve the mechanical performance of bridge components.