For existing bridges, one is that with the continuation of the operating time, they gradually approach their design service life, and the other is that they are overloaded compared to the load rating standards when the bridge was originally designed. Therefore, the main load-bearing component beams of beam bridges with relatively limited load-bearing potential have an increasing probability of fatigue damage. Once the beam body is damaged, the bearing capacity is reduced and the operation needs cannot be met, it can be demolished and rebuilt. It can be done once and for all, but the resource consumption is large. Therefore, it is particularly important to reinforce existing bridges to adapt them to the needs of modern transportation development.

1 Features of externally prestressed carbon fiber strip strengthening bridges

1.1. The advantages of externally prestressed carbon fiber strip to strengthen the bridge

This article describes the use of special fixture anchors for externally prestressed carbon fiber strip to strengthen the bridge technology, its advantages are mainly reflected in:

①The loss of prestress is small. The maintenance of its effective prestress completely relies on the anchoring ability of the anchoring carbon fiber strip anchors, does not rely on the bonding effect of any adhesive, does not need to consider the aging of auxiliary materials, and has good durability;

② It has good economic and technical characteristics. Not only the initial cost of reinforcement is low, but later maintenance is basically not required, and the environmental applicability is strong;

③The flexural and tensile resistance of the beam is significantly enhanced, and the load level of the bridge can be greatly improved;

④ In the process of maintenance and reinforcement, the secondary damage to the bridge is small and the traffic is not interrupted;

⑤The new dead load is less, compared with the original dead load and live load of the bridge, the increased dead load is almost negligible;

⑥Carbon fiber strip has simple layout, convenient construction and reliable quality. No need for large-scale equipment, capable personnel, and short construction period.

1.2 The shortcomings of externally prestressed carbon fiber strip to strengthen the bridge

① The working environment temperature of prestressed carbon fiber strip should not be higher than the temperature when it is solidified during manufacture, which is about 90 degrees Celsius;

②Due to the poor shear resistance of carbon fiber strip, in the process of anchoring the carbon fiber strip, the tightening sequence and design torque of the bolts must be strictly observed when tightening the anchor bolts.

2 Reinforcement principle of external prestressed carbon fiber strip

The external prestressed carbon fiber strip is arranged on the outer side of the beam body subjected to bending and tension (positive and negative moments), and the anchor bolt anchors the anchor base to the reinforced concrete beam. The prestressed carbon fiber strip is anchored by the fixture, and the prestress is transferred to the beam body through the anchor bolt and the anchor base. The inverse bending moment generated by the prestress on the beam partly offsets the bending moment generated by the external load, and plays a role in suppressing the development of cracks and flexural deformation, thereby greatly improving the bearing capacity of the bridge and the rigidity of the bridge structure. Reinforced concrete beam bridges with straight bases (such as continuous or simply supported beam bridges such as T-beams, rectangular slab beams, box beams, etc.) at the bending and tensile parts (positive and negative moment zones) can be reinforced with external prestressed carbon fiber panels. The basic composition of the external prestressed carbon fiber strip system mainly includes: external prestressed carbon fiber strip, lubricating and bonding agent, and special anchor for external prestressed carbon fiber strip.

3 External prestressed carbon fiber strip reinforcement construction process

The construction process of external prestressed carbon fiber strip reinforcement is: construction platform-lofting-concrete surface structure treatment-slotting-drilling anchors and bracket bolt anchor holes-bolt planting reinforcement-bolt planting reinforcement curing-installing anchor base-carbon fiber strip blanking-install tensioner-lay carbon fiber strip—fasten the carbon fiber strip at the fixed end—fix the carbon fiber strip at the tensioning machine—tension—anchor the carbon fiber strip at the tension end—tighten the supporting bolts—cylinder unloading—removing the tensioning machine—coating the protective mortar of anchors and prestressed carbon fiber strip—removing the construction platform.

4 Key points of external prestressed carbon fiber reinforcement construction

(1) Stakeout positioning. According to the bridge reinforcement design and construction drawings, the positions of the carbon fiber strip, anchors and bolt holes of the anchors are marked on the surface of the beam.

(2) Concrete surface treatment of beam base surface. Clean the concrete surface of the beam base surface, sand it with sandpaper, and remove sharp protrusions.

(3) Slotting. Slot the anchor (tensioner) base position on the beam. The groove should not damage the steel bars in the beam. The groove depth is about the thickness of the protective layer of the beam steel bars, and the depth is about 3cm.