Overview

The aging and destruction of bridges is a worldwide problem. The main factors of aging are: age, transportation and environmental impact (rainwater, melting water salt, etc.), among which the cyclic load of heavy trucks is the key factor. The repeated loading of vehicles will cause fatigue failure of the bridge deck. The use of carbon fiber reinforced polymer (CFRR) to reinforce aging bridges has become popular. CFRP has many advantages: it has a high "strength-to-weight" ratio, strong corrosion resistance and fatigue resistance, simple construction and low cost.

In the past ten years, due to the emergence of large vehicles and the increase in traffic, the fatigue problem of highway bridges has begun to become prominent. There are few studies on the fatigue problem of highway bridges in our country, and even less on the fatigue after CFRP reinforcement. In the past, CFRP reinforcement was mostly in the positive bending moment area, and there was very little reinforcement in the negative bending moment area. Slotting in the concrete protective layer of the negative bending moment area, then embedding carbon fiber slats, and bonding with epoxy resin, the reinforcement of the negative bending moment area is completed. Here, static and fatigue loads are applied to the reinforced bridge members, and the bearing capacity and fatigue performance before and after reinforcement are compared and analyzed.

The fatigue resistance of concrete members reinforced with CFRP has been greatly improved. The fatigue resistance of CFRP itself is related to its composition, which is determined by its matrix material and reinforcing fibers. Carbon fiber has good fatigue resistance. Therefore, when it is used to reinforce concrete members, it will not cause fatigue damage by itself. However, as the fatigue load increases, the structure will accumulate damage and eventually cause fatigue failure. The aging of the contact interface between CFRP and concrete will also affect the fatigue performance of the reinforcement system. The magnitude of the stress change greatly affects the fatigue life of the component. Unreinforced components will reduce the stress of the steel bars, and the failure mode is CFRP peeling off. .

Test

The test method to observe the reinforcement effect is divided into three cases:

(1) The cantilever bridge deck is destroyed under static load to test the bearing capacity of the structure;

(2) The cantilever bridge deck bears 360,000 fatigue loads and observes the fatigue resistance of the structure;

(3) Apply static load and cyclic load respectively in the two consecutive negative bending moment areas until failure, and observe the effect of reinforcement with the embedded method.

Material

Concrete 28d has a strength of 26MPa, a tensile strength of 1.9MPa, and a modulus of elasticity of 33GPa. The yield strength of the steel bar is 350MPa and the elastic modulus is 206GPa. CFRP composite material is carbon fiber reinforced epoxy resin, which is a finished slat prefabricated by the factory. The tensile strength of CFRP is 2664MPa, the elastic modulus is 188GPa, and the ultimate elongation is 1.42%. The compressive strength of epoxy resin adhesive is 79MPa, the tensile strength is 37MPa, the elastic mold is 5.8GPa, and the linear expansion coefficient is 38X10^-6.

Specimen

Embedded CFRP is used for reinforcement in the negative bending moment area. One is a cantilever bridge deck, and the other is a continuous beam. The concrete protective layer is 30-40mm, and the negative bending moment area is embedded with a single-layer CFRP strip for reinforcement. First slot in the concrete protective layer with a width of 10cm and a depth of 0.5cm. Fill the groove with a part of epoxy resin, then insert the CFRP strip, and then fill it with epoxy resin to smooth it. The cross-sectional size of the CFRP strip is 5cm×0.12cm.

Experimental procedure

For unreinforced members, the cracking moment is 20.6 kN·m; the ultimate bending moment of unreinforced members is 66.8 kN·m, and the reinforced member is 112.5 kN·m. In addition, because the negative bending moment area is embedded with CFRP, the peak value of the bending moment is reduced, the stress is redistributed, and the cracks are more dispersed and more uniform. When the component is not reinforced, when the fatigue load reaches only 6000 cycles, the damage accumulation increases rapidly and the deflection increases. After the use of CFRP reinforcement, the damage accumulation becomes slow, knowing 240,000 cycles. This means that negative bending moment reinforcement increases the service life of the structure.

Summary and conclusion

At present, most of the reinforcement of old bridges is aimed at the positive bending moment and is pasted with carbon fiber cloth. The method of beam bottom is very popular, but it is often helpless to strengthen the negative bending moment. It is a very simple and very effective method to use carbon plate embedding method to strengthen in the negative bending moment area. The conclusions are as follows through experimental observation:

(1) After the negative bending moment area of the cantilever plate is reinforced with embedded carbon fiber board, the ultimate bending resistance is increased by 68.4%. The negative bending moment cracks above the fulcrum are small and uniform. These are the credits of the carbon fiber board.

(2) When the negative bending moment area of the cantilever plate is not reinforced, under the action of fatigue load, the damage of the component accumulates rapidly, and it quickly reaches the fatigue limit and fails. After reinforcement by the embedding method, the rate of damage accumulation slowed down greatly. The embedded carbon fiber board itself has no obvious signs of damage, it has been effectively controlling the distribution and development of cracks. The fatigue life of the reinforced structure can be increased by 25%.

(3) After the continuous beam is strengthened by the negative bending moment, the damage accumulation becomes very slow under the action of fatigue load, and its fatigue life is more than 10 times that of the unreinforced beam.

(4) The fatigue failure of the unreinforced amount is that the steel bars are first broken, and the reinforced beams are first opened by CFRP, which effectively reduces the stress of the steel bars.