Capping of bridge pier with carbon fiber wrap reinforcement

1.Engineering survey  

1.1 Bridge Survey

A bridge is a highway main line bridge. The superstructure of the bridge is 3 x25m prestressed concrete continuous box girder. The substructure is column pier and pile foundation. The whole length of the bridge is 82 M.

1.2 cover beam related parameters

The pier cap beam is 1.5 m high, 1.8m wide and 11.393 m long. A total of 36 28 HRB335 bars were installed at the bottom edge of the cap beam. The concrete strength was C 35 and the thickness of the concrete protective layer was 6 cm.

1.3 cover beam defect condition

During the construction of pier cap beams, the layout and design of the stressed steel bars do not conform to each other, and part of the stressed steel bars move up, which results in the distance between the stressed steel bars and the neutral axis becoming smaller and the bearing capacity of the structure decreasing.

2. Reinforcement ideas

Due to the construction mistake, the main reinforcement of the lower edge of the cap beam moves up, and the bending capacity of the lower edge of the cap beam is insufficient. Because the bridge has not been opened to traffic, it can be considered to paste steel plate and carbon fiber wrap at the lower edge of the cover beam for flexural reinforcement. It is necessary to drill holes and make bolts on the cover beams to stick steel plates. The damage to the concrete and steel bars of the original cover beams is serious. The exposed surface of the steel plates should be treated regularly after the construction of sticking steel plates. Bonding carbon fiber wrap to reinforce the original cover beam structure does not cause any damage, construction is simple and fast. After comparing the two reinforcement methods of sticking steel plate and carbon fiber wrap, we choose the reinforcement method of sticking carbon fiber wrap.

3 Structure analysis

Cross section of cap beam: b*h=1800mm*1500mm.

f_sd=280Mpa,f_cd=161Mpa

In the calculation, it is considered that the bearing capacity of the original design pier cap beam meets the requirements, and the reinforcement is only a partial supplement to reduce the bearing capacity of the cap beam caused by construction mistakes.

According to the basic theory of structural design, the bending moment bearing capacity of the original design cap beam and the actual construction completed cap beam is calculated. The difference between the two is the bending moment value which needs to be strengthened by pasting CFRP wrap. The cross section area of carbon fiber wrap can be obtained by the basic formula of material mechanics.

3.1 calculation of moment bearing capacity of cover beam in original design

3.1.1 calculate pressure zone height:x

 x=(f_sd*As)/(f_cd*b)=(280 ×36 ×615．8)／(16．1 ×1 8001 =2 14．2 m m <sbh_{0(=0．56 ×1 392．6=779．9 m m ).}

No over reinforced beam will occur.

3.1.2 bending moment bearing capacity of cover beam in original design

Bending moment capacity of the cover beam in the original design:

M_u=f_sd*As(h₀*x/2)=280 ×36 ×6 15．8 (1 500一f60+31．6 ×1．5)-214．2／2]=7 979 kN.m．

3.2 calculation of moment bearing capacity of actual cover beam in mid span

3.2.1 calculate pressure zone height: x^'

M_u=f_sd*As(h₀*x/2)

x=291.8mm

3.2.2 Moment bearing capacity of actual cover beam in mid span

Moment bearing capacity of actual cover beam in mid span: M_u^'₌f_sd*As(h₀*x'/2)==280 ×[615．8 X 23 ×(1 500-60-31．6／2-291．8／2)+6 15．8 ×1 l ×fl 500—60—3 1．6／2—3 19-291．8／2)+615．8 ×2 ×(1 500-60-31．6／2—319 ×2-291．8／2)=7 1 10 kN .mm

The actual M_u^' is smaller than the designed M_u. M_u-Mu^'=7 979-7 l10 =869 k N ·m ．

3.3 area calculation of CFRP wrap paste

By placing carbon fiber wap on the lower edge of the cover beam, the bending moment bearing capacity due to construction causes is compensated. The width b of carbon fiber wrap is 20 cm, the elastic modulus Ef is 2.4 *10⁵ M Pa, and the thickness of single layer carbon fiber sheet is 0.167 mm.

M=A_f*F_f*E_f(h-x/2)

3.4 carbon fiber wrap for pier strengthening

A) the CFRP wrap should be banded and should not be more than 3 layers.

B) the lap length of CFRP along the force direction of the fiber should not be less than 100 mm. When multiple or multi-layer CFRP wrap are used, the lapping positions should be staggered.

C) When CFRP is used to circumvent the outer chamfer of the member (section), the section edge of the member should be polished into a circular surface before pasting. The radius of the arc should not be less than 20 m m.

D) When strengthening with multi-layer CFRP, the CFRP wrap should be cut off layer by layer, and the outermost compression strip at the cut-off point of each layer should be used, and the bonding mode should be short inside and long outside.

When the bridge cap beam is strengthened, three layers of carbon fiber wrap with a width of 20 m m are pasted on the bottom edge of the cap beam, and nine layers are arranged transversely, then the cross-sectional area of carbon fiber wrap is 901.8 m > A f: 764.05 m, which meets the requirements.

Summary

Through the introduction above, it can be seen that the bonding of CFRP wrap can make up for the problem of insufficient flexural capacity of pier cap beams caused by insufficient reinforcement in the case of no damage to the original structure. It is also useful for improving the bearing capacity of other flexural members such as reinforced concrete beams. It should be noted that the bonding of CFRP has strict requirements for construction conditions and construction techniques. In order to ensure the construction quality, it is necessary to complete the construction by professional construction units with mature construction experience, and in each construction stage strictly in accordance with the requirements of the specifications for quality control.