Corrosion and protection design of reinforced concrete bridges in coastal areas

Corrosion and protection design of reinforced concrete bridges in coastal areas

Wen Baolian 1 Wang Xingang 2 Yang Liang 3 Wang Wenxin 3

(1. Tianjin Municipal Engineering Research Institute, Tianjin, 300074; 2. School of Civil Engineering, Lanzhou Jiaotong University, Lanzhou, 730070; 3. Tianjin Municipal Engineering Design Research Institute, Tianjin, 300070)

Abstract: In view of the durability of reinforced concrete bridges in China's coastal environment, it is proposed that the newly-built reinforced concrete bridges should be specially designed for anti-corrosion from a structural perspective and a material perspective. If there is a bridge, the corresponding secondary waterproof and anti-corrosion design should be made according to the form and location of the disease. Finally, it is proposed that the repaired bridge should be treated with overall waterproof and anti-corrosion treatment.

Keywords: concrete, bridge, corrosion, durability, design

introduction

Once the reinforced concrete bridge is put into use, it is surrounded by a variety of complex natural and man-made environments, especially in the northern coastal areas. Under the combined effects of salt corrosion, frost damage, and heavy traffic entering and leaving the port, the bridge will deteriorate prematurely. , Various diseases occurred.

From the point of design, the scientific anti-corrosion design of the bridge; for the existing bridges with durability defects, a reasonable secondary anti-corrosion design based on the diagnosis of health status is an essential measure to ensure the durability of the bridge.

1. Waterproof and anti-corrosion design of new bridge

In many professional literatures, the deterioration modes of steel bars and concrete in coastal environments are described in detail, such as freeze-thaw, salt crystallization, carbonization, salt corrosion, alkali aggregate reaction and the resulting steel bar corrosion, which will not be repeated here. But after understanding the reaction mechanism of various damages, it can be seen that water is the culprit causing all damages. Without the presence of water, all damages will not occur. If you do waterproof work, you will have good anti-corrosion.

1.1 Waterproof design of structure

1) The shape of the component facilitates the removal of water

The structure of the structure should try to avoid the accumulation of rain, moisture and harmful substances on the concrete surface. In the past, the bridge design was not well considered. Rainwater flows to the side beams through the outside of the railing, so the diseases on the outside of the side beams are the most serious. From the perspective of waterproofing, the outside structure such as railings is required to be smooth and have dripping eaves. The top surface of the pier and abutment is designed as a 5% outwardly inclined surface, or an inclined surface inwardly so that the water can be concentrated and excluded.

2) Waterproof design of pier columns

The bridge pier column is in the worst part of the surface adsorption area, suffering from salt corrosion, crystal corrosion, freeze-thaw cycle, and carbonization. Therefore, the focus of protection is to prevent capillary from absorbing water. It is recommended to consider the silane impregnation protection, the special small molecular structure of silane (or siloxane), so that it penetrates the surface of the concrete along the capillary pores, chemically reacts with the moisture in the air and the substrate, and generates hydroxyl groups on the inner wall of the capillary pores. Thus, a firm water-repellent barrier is formed on the wall of the capillary pores, making it difficult for water and the salt carried by the water to rise along the capillary pores, greatly improving the waterproofness and comprehensive performance of the concrete.

3) The upper structure should try to use cast-in-place

In the past, there were many prefabricated installations of bridges, especially between prefabricated beams welded and connected, and stress concentration was likely to occur at the joints during use, causing premature disease. The cast-in-place structure, the simple-supported-continuous structure, and the simply-supported beam structure are relatively light-weight and should be preferred. For example, the cast-in-place anti-collision wall, leveling layer and wet joint concrete are used together. Pouring makes the joint of prefabricated beam and leveling layer of bridge deck into one, and participates in the work together, which can greatly improve the tensile strength of the joint.

4) Large-span prestressed beams should be used in severely corrosive environments

In saline soil or coastal areas, the corrosion of concrete is mainly reflected in the lower structure of the bridge. The effect of salt in the atmosphere on the upper structure is relatively weaker. The prestressed technology is used to increase the span of the beam body, and the number of lower pile foundations and abutments is reduced as much as possible. It is possible to focus on increasing the protection of the lower structure under the premise that the investment scale is unchanged or even smaller.

5) Increase the thickness of the concrete protective layer

The main function of the concrete protective layer is to protect the steel bars in the beams from corrosion. In the design, the protective layer should be properly thickened to delay the corrosion of the steel bar caused by carbonization, thereby delaying the occurrence of the steel bar corrosion. The minimum thickness of the concrete protective layer of the longitudinally stressed steel bar should be determined according to the environmental category and the strength of the concrete.

6) Detailed waterproof design of bridge structure

The well-designed and smooth drainage system of the bridge deck can keep the bridge structure free or less eroded by water. The longitudinal and transverse slopes of the bridge deck must meet the design requirements, the layout of the drain pipe should be reasonable, and the drainage of the bridge deck should be unobstructed. In addition, the drainage of the asphalt concrete interlayer should be considered in the design of the bridge deck pavement and the construction of the drain pipe.

Consider the design of the waterproof layer in the entire bridge. In particular, the continuity of the waterproof layer is achieved at special locations such as expansion joints, drain pipes, anti-collision walls and dividers, and the installation of the waterproof layer tends to be more reasonable.

7) Cathodic protection measures are reserved for important parts

Cathodic protection is an effective measure that is widely adopted and is widely used in offshore structures. In the design of bridges, protection nodes should be reserved in some important parts. Once the corrosion is found, the protection system can be started quickly.

8) Real-time monitoring system for preset steel bar working status

Combined with the surrounding environment, the reinforced concrete monitoring system is embedded in the most prone to safety and durability hazards for bridge health detection and monitoring, which can timely discover certain defects of the bridge, track the development of defects in real time, and take necessary when appropriate The technical measures can take economic solutions at the most appropriate time to ensure the safety of the bridge, and at the same time provide a reference basis for subsequent bridge design.

1.2 Anti-corrosion design of materials

1) High performance of ordinary concrete

In the previous design codes, the strength of concrete was mainly considered. As the awareness of durability increases, ordinary concrete gradually develops towards high performance. By adding finer admixtures than cement particles such as micro-silica fume, high-quality fly ash, slag, and the use of high-efficiency water-reducing agents, the concrete can adopt a lower water-cement ratio and a smaller amount of water. The performance has been greatly improved, and the air-entraining agent should be added in the environment with anti-freezing requirements, the density is increased, the concrete's own impermeability is improved, and the durability of the concrete is greatly improved.

2) Application of steel rust inhibitor

For reinforcement protection, concrete quality is the most important in any case. If the quality of concrete materials or construction is poor, or the design is defective, etc., the occurrence and development of diseases will be accelerated. Adding steel rust inhibitor on the basis of high-quality concrete is considered to be the simplest, most economical and effective technical measure for long-term protection of steel bars to delay corrosion damage and achieve design life [[1]]. Adding steel rust inhibitor can play two roles: on the one hand, it delays the time when the steel bar starts to rust; on the other hand, it slows down the development of steel bar corrosion.

3) Bridge deck waterproof layer

Because the bridge is subjected to vibration load, flexible coatings and coils should be used for the bridge deck waterproof layer. The waterproof coatings and coils should be considered based on the comprehensive factors such as the structure and construction environment. The designer should pay close attention to the development of new waterproof materials and construction processes at home and abroad, and on the basis of fully understanding the performance of waterproof materials, determine the waterproof setting scheme according to the structural stress characteristics and the performance of the bridge deck pavement materials and construction characteristics. The detailed waterproof design and material selection instructions are reflected in the above, and the construction personnel or the owner cannot determine the selection of materials and construction technology [[2]].

2 Secondary waterproof and anti-corrosion design of existing bridge

Over time, the bridge will have corrosion and performance degradation sooner or later. The timely secondary waterproof and anti-corrosion design can ensure that the bridge continues to serve in health. The secondary design includes material, process, and maintenance effect evaluation. For the maintenance material, it must be bonded to the base The strength is high, the elastic modulus is basically the same as the base layer, the shrinkage rate is small, the temperature change coefficient is basically the same as the base layer, and the calorific value during construction and maintenance is low. The effect after maintenance must meet the requirements from the perspectives of safety, durability, applicability and aesthetics, and the part after maintenance must be consistent with the surrounding color,

Combined with our work, we summed up some remedial measures after the occurrence of bridge diseases.

1) Waterproof treatment of T-beam joints

Due to the water seepage at the longitudinal joint of the two beams of the bridge deck, or the rainwater flowing along the outside of the T-beam, it penetrates into the concrete structure and causes the corrosion of the steel bar, which causes the concrete structure to crack and break, and the steel bar is exposed. After cracking, rainwater and deicing salt are more likely to enter the structure, which in turn accelerates the corrosion process of the steel bars, making the disease more serious and causing extensive damage to the lower edge of the wing.

Cracks at the indirect joints of T-beams should be treated with the principle of prevention and control, and the repair method should be selected according to the damage of the bridge deck pavement:

a. Full repair

Thoroughly clean the pavement layer at the joint of the T-beam, clean it and repair it as follows.

i. The gap between the T-beams is sealed with quick-setting cement;

ii. Apply two layers of 20cm flexible flexible water stop (a polymer-modified cement-based infiltration crystalline waterproof material) to the gap, and add a reinforcing fiber mesh, the total thickness of the coating should not be less than 1.6mm

iii. Rigidly brush the surface of other structural layers with 1.5Kg / m2 in two passes to stop the water and maintain it for 24 hours;

iv. Apply a water shield waterproof rubber primer (a modified asphalt waterproof material) to the surface of the rigid excellent water-stop coating at an amount of 4 m2 / L;

According to the amount of 1m2 / L, apply the water shield waterproof rubber topcoat in two layers, and then use fine sand as the protective layer for bridge deck pavement.

For the repair of gaps under the bridge deck, see the following local repairs.

b. Local repair

The damaged concrete at the expansion gap of the T beam is completely chiseled to expose the fresh surface of the concrete. After the steel bar is rusted, use a steel brush to remove the rust and then apply two FP inhibitors. Use fresh UP2000 (a polymer modified repair cement) to repair the fresh concrete surface.

Repair method for the damage of the outer side of the side beam and the beam body: the drainage is mainly dredged to make the drainage of the bridge deck smooth, and corresponding structural repair measures are taken.