Old concrete bridge strengthening and repair

Brief introduction of design of overpass reconstruction project

The Jian Xiang overpass is located at the intersection of the north four ring road and the Badaling Expressway in Beijing. It was completed in 1998. The superstructure of the bridge is 6 span, L = 18m prestressed concrete simply supported beam. The isolation belt is located in the center, and the two sides are two-way motorized lanes. The two outer side is a non motorized lane with a full width of 55m. With the growth of urban volume, the existing width of the overpass can not meet the needs of traffic volume. In order to ensure that the 4th Ring Road becomes an urban expressway, it is necessary to widen the existing bridge to 72m on both sides of the South and north, to reconstruct the existing non-motor vehicle bridges on both sides into auxiliary road bridges, and to build new non-motor vehicle bridges. At present, the main girder and the middle pier cap girder of the superstructure of the bridge are designed according to the automobile - Super 20 grade and Trailer - 120 grade. The load of the main girder and the middle pier cap girder of the superstructure corresponding to the non-motorized lane are designed according to the automobile-15 grade and the trailer-80 grade, and the load is required to be Automobile-20 grade and trailer-120 grade when the auxiliary road bridge is rebuilt. At present, the main girder of the non-motor vehicle bridge is removed and replaced, but the middle pier cap beams of motor vehicles and non-motor vehicles have not been disconnected, which is a three-span and one-joint variable cross-section ordinary reinforced concrete continuous structure.

After repeated calculation and comparison, in order to minimize the reinforcement work in the positive and negative bending moment area of the cap beam, on the one hand, the measures to reduce the dead weight of the bridge are adopted, and the concrete deck is changed into lightweight (unit weight is 19KN/m3) concrete deck. On the other hand, the width of the pier column under the cover of the middle pier will be widened, and the positive and negative bending moment and shear force of the cover beam will be reduced correspondingly. In this way, the current reinforcement configuration of the partial cover beams of the motor vehicles can meet the design requirements, and the stress of the partial cover beams of the corresponding auxiliary roads (now non-motor vehicle bridges) is 10-15% different from the design requirements. In order to save the project, and according to the situation that the traffic of the 4th Ring Road is not allowed to be interrupted, 20 positive and negative moment zones of the five rows of middle pier cap beams of the whole bridge are reinforced and strengthened with the approval of the relevant departments. The area of the whole bridge cap beams is 395m2.

Reinforcement scheme and material selection

Structural reinforcement scheme

The original design of cover beams for middle piers was invisible cover beams, which brought great difficulties to the reinforcement work. The reinforcement scheme of sprayed steel fiber reinforced concrete (SFRC) and outer steel plate was first proposed. Because the end of the main beam is only 5 cm away from the outer side of the cap beam at the span, the feasibility of the two schemes is less than that of the construction space limitation. Finally, unidirectional carbon fiber fabric is selected to reinforce the cap beam. The ultimate tensile strength of CFRP is 10 times higher than that of ordinary steel bars, and its elastic modulus is similar to that of steel bars. It is suitable for reinforcement of reinforced concrete structures. Besides, the construction is simple and suitable for reinforcement works requiring short construction period.

Selection of materials

In the past two or three years, China has introduced carbon fiber composites made in the United States and Japan, and carried out some reinforcement projects, and achieved good results, but the high cost affects the promotion and application. Recently, the appearance of domestic series of carbon fiber composites fabric has laid a foundation for the selection of this engineering material. Carbon fiber fabric and matching resin of Horse Construction are compared. Carbon fiber fabric is made of imported high quality wire, which is reliable and stable in quality and low in cost. The product has passed the stringency of the national quality inspection department, and all the performance indexes have reached the level of similar products abroad.

Construction technology of unidirectional carbon fiber fabric

In order to ensure that the original stressed steel bar and the reinforced carbon fiber can work synchronously, the difference of stress and strain between the original steel bar and the unidirectional carbon fiber fabric should be shortened as far as possible. The unloading measures should be taken into account when strengthening the cap beams, so that the time of transferring the reinforcing bars of the original cap beams to unidirectional carbon fiber fabric can be postponed, the cap beams can be improved actively, and the original reinforcing bars and unidirectional carbon fiber fabric can enter the limit state respectively when they are destroyed, and the overall strength of the cap beams can be significantly improved. The unloading measures for strengthening the cap beam are: dismantling the main beam with low original load standard, pasting unidirectional carbon fiber fabric to reinforce it, and installing the new standard load main beam.

The application of unidirectional carbon fiber fabric reinforcement of middle pier cap beam in Jianxiang flyover reinforcement project saves about 25 million yuan, reduces the construction period, and solves the problem of uninterrupted construction. Great benefits have been obtained, which has been praised by the relevant units.