The durability design standard of 1. 1 is low.

Although the durability of concrete and the safety of bridge structure will be affected by the environment, the problems such as bridge defects and diseases are mainly caused by unreasonable design. After many bridges are put into use, they do not pay attention to maintenance and bridge management is not in place. In the process of bridge design, the durability of concrete is not fully considered, but the structural strength and bearing capacity are considered, and only the main design is paid attention to, not the details, which leads to the low durability of concrete [1]. Bridge design usually pays more attention to the safety and strength of the structure, but not enough attention to the selection of structural materials and structural design. There is no concrete durability standard according to the construction environment, and technical measures are not in place. Coupled with the influence of the environment, the durability of the bridge is insufficient.

1.2 is affected by environmental factors.

Environment is one of the most important reasons that affect the durability of concrete. The influence of concrete material characteristics and environmental reasons will lead to concrete cracks, steel corrosion, concrete falling off and damage. The long-term circulation in the past will lead to the gradual reduction of the area of steel bars and concrete, which will reduce the structural performance, reduce the bearing capacity of the structure and lack the durability of the structure. These problems are also related to design. If the reinforced protective layer used in bridge design is thin, the thickness can not meet the requirements, or the water-binder ratio of concrete is too large, it will lead to concrete damage.

2 Bridge durability design method

The design requirements of bridge durability are put forward in the Regulations on Bridge Construction, and relevant technical standards and design specifications are formulated, indicating that the design requirements of concrete bridge structures should be formulated according to the construction environment, limit state and design life. Generally speaking, the durability design of bridge structures mainly focuses on the following points.

2. 1 The performance and strength of concrete should meet the design requirements of bridge structure.

The design of bridge structure should not only pay attention to the performance and strength of concrete, but also pay attention to the durability of concrete, so as to ensure that the bridge still has good corrosion resistance, carbonation resistance, freeze-thaw resistance and impermeability under environmental influence. Therefore, in the design of bridge structure, concrete requirements should be formulated according to the environment and performance of the structure, concrete with suitable durability should be selected, and admixtures should be used reasonably to ensure the damage resistance of concrete and improve its compactness [2]. If the bridge is built in some coastal areas, concrete with high durability can be used for the bridge bottom construction. The mix proportion of concrete and relevant technical standards should be strictly followed to ensure that the alkali content and chloride ion content in concrete meet the design requirements, the strength grade can meet the design requirements, the water-binder ratio meets the limit standard, the dosage of cement is strictly controlled, and qualified raw materials are used. Only in this way can the durability standard of concrete be guaranteed.

If the bridge is affected by harsh environment, it is the durability of concrete that determines the structural strength of the bridge, not the bearing capacity of the bridge. For example, in some northern areas, near the sea, environmental factors are more complicated and have a greater impact on bridge structures. If the durability of concrete is poor, the structural strength will be reduced. When the environment of the bridge structure changes greatly, it should meet the requirements of concrete durability and structural strength of the bridge in different environments. If the components are often in wet environment and contact with water sources, expansion joints should be designed according to the requirements of freeze-thaw environment to ensure the durability of the top surface of pier cap; If the bridge is a cross-sea bridge and is corroded by chloride for a long time, prestressed and reinforced concrete cement should be selected as far as possible, and air entraining agent should be used for pier construction. In addition, relevant anti-corrosion and anti-scour measures should be formulated to improve local strength.

2.2 Pay attention to detail design to prevent cracks.

If the environment of the bridge is complex, we should try our best to design simple shapes of concrete members to avoid exposing too many concrete edges and corners, and try our best to replace concrete edges and corners with rounded corners. In the structural layout and design, attention should be paid to concrete curing and vibrating ventilation. According to the drainage requirements, design the surface shape of concrete, rationally design structural joints, especially structural design, to avoid deformation caused by increased load and stress concentration. Some parts where the water level changes greatly, dry and wet alternate, and are corroded by chloride salt, protective measures should be taken in time to avoid corrosion of components. Hydrophobic treatment and waterproof treatment can be carried out, and corrosion can also be prevented by indicating coating measures.

2.3 Ensure that the design of concrete protective layer meets the requirements and increase its thickness.

Using a protective layer with appropriate thickness can not only reduce the carbonation speed of concrete, but also avoid the diffusion of chloride ions and effectively improve the durability of concrete. Therefore, in order to prevent the corrosion of steel bars, a protective layer with a larger thickness should be used as much as possible.

2.4 Do a good job in bridge deck waterproof design and drainage design.

When designing the longitudinal slope of viaduct and main bridge deck, we should consider the inclination of transverse slope and longitudinal slope, and design drainage facilities, such as drainage holes, to ensure the traffic safety of the bridge, solve the water accumulation on the bridge deck and ensure the effectiveness of the drainage system. The bridge deck should be designed with waterproof layers, such as flexible waterproof layers and rigid waterproof layers. If conditions permit, two kinds of waterproof layers can also be designed at the same time to improve the waterproof effect of the bridge [3].

2.5 attention should be paid to details in the design of ancillary structures and structures.

In the process of superstructure design, many bridges are connected with precast simply supported beams by vertical and horizontal wet joints, which does not require high durability of concrete in the later period, resulting in low structural strength. This design is easy to cause shrinkage, cracks, even water leakage and corrosion of concrete, resulting in concrete damage. Therefore, this design method should be avoided as far as possible in the design of ancillary structures or structures. If this design structure must be adopted, protective measures should be taken to add expansion agent to some concrete in post-pouring zone to avoid concrete shrinkage and improve the durability of concrete [4]. The joint between main girder and expansion joint is easy to be damaged, which requires high durability of concrete. Therefore, concrete with good durability should be used for construction, and an appropriate amount of fiber toughening materials or expansion agents should be added to the concrete. At the same time, steel bars and expansion devices should be designed and constructed reasonably to make them connect reasonably. The concrete with the same label should be selected for the component part and the post-pouring belt part, so as to achieve the sealing effect of the metal anchorage and avoid the change of prestress.

2.6 Strengthen the steel structure to avoid concrete cracks.

Once cracks appear in concrete, it will lead to the infiltration of water and gas, corrode reinforced concrete and reduce the durability of concrete. Therefore, attention should be paid to crack control and the durability of concrete should be improved as much as possible. In the design of bridge concrete structure, the prestressed concrete structure is the main structure, the anti-crack design is increased, and the steel bar structure is arranged reasonably to avoid cracks caused by excessive structural spacing or diameter.

3 Conclusion

To sum up, the durability of concrete is related to the overall bearing capacity, practicability and safety of the bridge. We should try our best to optimize the bridge structure through scientific and reasonable structural design, improve the durability of concrete, update the design concept, pay attention to detail design, avoid cracks, falling off and damage of concrete, improve the strength of the bridge and prolong its service life.

For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd