There are some common diseases in the use of concrete bridges, such as steel corrosion, bridge deck cracks, expansion joint damage, etc., which seriously affect the performance of the bridge. If it is not maintained in time, the main structure will be destroyed. So far, there is no good solution to how to carry out the correct examination, master the exact condition and formulate a reasonable and economical protection and maintenance plan in time. The main reason is the lack of systematic and clear understanding of the disease.
Damage and diseases will inevitably occur in bridge structures, so it is necessary to strengthen the prevention of diseases. Specifically, the first is to strengthen regular maintenance and establish a daily inspection mechanism to ensure the safety of bridge operation and prevent the spread of diseases. The second is to carry out regular "physical examination" of the bridge to evaluate the operational potential of the bridge structure. The third is to implement the principle of "prevention first". In order to prevent and control bridge diseases, it is necessary to carry out bridge inspection first, and if necessary, carry out bridge load test, master the technical and safety status of the structure, analyze the causes and severity of the diseases, and comprehensively consider whether to carry out maintenance and reinforcement from the aspects of safety, economy and applicability. For bridges that can be repaired and reinforced, it is also necessary to distinguish the situation, analyze the causes of the disease and take corresponding treatment measures. Therefore, it is of great economic value and social benefit to correctly analyze the causes of bridge diseases, master the characteristics of different types of bridges and diseases, and quickly and simply formulate corresponding countermeasures for specific problems.
Through the operation of Quanzhou Bridge, Cabinet Bridge of Provincial Highway 306, Dongshan Bridge and Dongguan Bridge after reinforcement, the reinforcement effect is very successful, and some diseases and prevention measures of concrete bridges are summarized.
2. Cause analysis of bridge diseases
Some common diseases of medium and small span concrete bridges are: steel corrosion, expansion joint damage, bridge deck crack and bearing damage.
(1) rust.
Corrosion of steel bars is an important factor affecting the life and safety of bridge structures. The corrosion of steel bars is mainly caused by electrochemical reaction. The dense oxide film on the surface of steel bar can protect steel bar from corrosion. The destruction of oxide film directly leads to corrosion. Carbonization of concrete, air pollution and excessive cl- content in concrete all reduce the pH value in concrete, and the oxide film on the surface of steel bar is gradually corroded and destroyed in acidic environment. In many areas of our country, "acid rain" caused by industrial pollution is widespread, which provides a suitable external environment for the corrosion of bridge reinforcement. At the same time, salt spray is widely used in winter to prevent bridge deck from freezing, and cl- rich salt water penetrates into concrete structures, which greatly accelerates the corrosion of steel bars.
When the steel bars are corroded, the volume of the corroded part can expand to more than 10 times of the original volume, thus squeezing the surrounding concrete, cracking and peeling the concrete, reducing the effective cross-sectional size and reducing the bearing capacity of the structure. The direct consequence of corrosion is that the cross-sectional area of steel bars is reduced. For concrete bridges with steel bars as tensile materials, the reduction of cross-sectional area will directly affect the flexural capacity of the structure. Corrosion of steel bars will also reduce the bond strength of concrete to steel bars. The outflow of rust forms rust on the structure surface, which affects the appearance of the structure. It can be seen that the corrosion of steel bars is very serious and sometimes even fatal to bridge structures.
(2) Freeze-thaw disease.
Freeze-thaw phenomenon is also one of the main reasons that cause bridge diseases and affect the bearing capacity and service life of bridge structures. Due to the influence of weather, the western region is prone to freeze-thaw phenomenon, and long-term repeated action is easy to cause structural cracks. Some bridges have been reinforced, and the newly poured concrete is not well combined with the post-poured concrete, which is easy to produce cracks, and the freeze-thaw action intensifies this influence. Freeze-thaw disease is mainly caused by the following aspects: first, the influence of natural factors; Second, the concrete construction quality of the bridge itself is not high, and the drainage facilities are defective; Third, the post-maintenance is weak.
(3) expansion joint disease.
According to the current investigation and study, the causes of expansion joint diseases are as follows: ① Poor design causes expansion joint damage. The bridge deck adopts a simple expansion joint device system of "overlapping angle steel and rubber strip", the joint top is the same height as the cast-in-place bridge deck concrete, and asphalt concrete is paved. Because the design emphasizes solving the smoothness problem of the bridge deck at the expansion joint, it ignores the synchronous expansion and contraction of the bridge deck concrete and the bridge deck. Therefore, after opening to traffic, the surface of asphalt concrete cracks irregularly along the joints, widens in winter and arches in summer. Under the long-term action of vehicle load, the bridge deck pavement (or pavement surface layer) gradually shears from the joint to the inside, resulting in edge gnawing or large-area potholes, which brings great difficulties to future repair. ② The expansion joint is damaged due to improper selection. (3) Due to the pier construction and the precast size of the beam (slab), the gap between the actual slab end and the design is quite different, which leads to the damage of the expansion joint. (4) The expansion joint is damaged due to the inconsistency between the design and the actual expansion. This causes the asphalt bridge deck to arch at the expansion joint in summer, and the asphalt concrete bridge deck cracks seriously along the joint in winter. ⑤ Due to construction error or rubber plate damage, the plate rubber expansion joint severely bumps at the expansion joint.
⑥ For slab rubber expansion joints or steel expansion joints, the concrete construction of expansion devices precedes the asphalt concrete pavements at both ends, and the ends of expansion joints trip. ⑦ Inadvertent construction operation of "reverse grooving method" leads to expansion joint uplift. At present, the expansion device of "reverse channel method" construction has wool joint or concealed expansion joint. Although the "reverse groove method" construction theoretically solves the problem of vehicle jumping at the end of expansion joint, if the operation is improper during construction, there is still obvious reflection when the vehicle passes. ⑧ Improper material selection causes damage to expansion joints. There are many reasons for the damage of expansion joints. Due to the complex stress of expansion joints, there are problems of the products themselves, as well as problems of design and construction. From the design point of view, the design engineer only pays attention to calculating the expansion of the bridge in the process of expansion joint design, and selects the type accordingly. However, the performance of the product is often not fully understood, and the corresponding technical requirements of the product are ignored, such as the length, thickness, elevation, position and depth of embedded parts of the concrete in the transition section, which are not clearly marked on the design drawings. Many of these problems will not be discovered until the products are shipped to the site for installation, or even after installation, but it is too late. From the construction point of view, the installation of expansion joints is one of the last working procedures of bridge construction. In order to catch up with the progress, the construction was sloppy and did not follow the installation procedure. In addition, opening the traffic ahead of time before the concrete reaches the strength after installation is also the cause of the expansion joint damage, because the anchorage concrete in the transition section has been damaged early.
(4) Bridge deck pavement diseases.
Bridge deck pavement diseases include irregular network cracks, regular longitudinal and transverse cracks and serious cracks. This not only increases the maintenance cost, but also leads to a large-scale renovation. At the same time, the bridge deck pavement directly bears the impact, shear and wear of high-speed traffic, and directly bears the influence of climate, which is different from the main beam (slab) in terms of warming up in the sun and cooling down at sunset. Therefore, the stress of bridge deck pavement is not only difficult to analyze qualitatively, but also difficult to calculate quantitatively. The stress of bridge deck pavement is complex and diseases occur from time to time. Therefore, the design and construction of bridge deck pavement should attract enough attention to prevent the occurrence of diseases.
Diseases such as bridge deck subsidence, railing and sidewalk system damage are mainly caused by lack of maintenance. The maintenance department does not pay enough attention to bridge maintenance, which leads to long-term malnutrition, serious appearance damage and poor drainage. In the past, bridge deck pavement was mostly mud-bound macadam structure, with low strength and poor waterproof performance, and bridge deck water easily penetrated into the structure. At the same time, the expansion and deformation of the structure will also affect the bridge deck pavement. The slab width of the integral slab bridge is large, and under the load, the slab will not only bend longitudinally, but also bend transversely. The reason why longitudinal cracks appear on the tensile side of the bridge deck is that the transverse distribution of steel bars has not reached the specified number. Transverse reinforcement can not meet the requirements of normal service load, which leads to the inability to limit the width of longitudinal cracks.
(5) Other diseases.
Cracks are common diseases of bridges. The investigation shows that there are other diseases in the bridge structure, such as bearing diseases, insufficient concrete protective layer, failure, looseness, bulging, cracks and even falling off; The carbonation speed of concrete is fast, and the carbonation depth has reached the surface of steel bar; The bridge deck has poor drainage, the waterproof layer has no protective layer, which is seriously damaged, the quality of drainage pipe is not good, the construction quality is poor, and the thickness of concrete protective layer changes greatly; Many factors, such as concrete shrinkage cracks, natural wear of bridge deck, aging of materials, serious overloading of large vehicles, chemical infiltration during snow melting in winter, and construction quality during bridge construction, are also important reasons for aggravating bridge diseases.
3. Countermeasure analysis
In the reinforcement of Quanzhou Bridge, no new cracks were found in the control section of arch rib in the static load test, and the original cracks did not develop further. The cracks in the cracked parts such as arch ribs and pile caps have not developed and widened obviously after one year of test, which also shows that the cracks are basically stable under normal use conditions and will not endanger the normal use of the bridge. Therefore, cracks in arch ribs and caps should be blocked with epoxy mortar and reinforced with carbon fiber sheets. Generally, two layers of carbon fiber cloth should be wrapped, and the width of carbon fiber cloth on both sides of the crack should not be less than 150mm, and the structure should meet the requirements of the corresponding Technical Specification for Strengthening Carbon Fiber Cloth. Due to the serious peeling and damage of the bridge deck pavement, the steel mesh and concrete were all chiseled away, and the original 39 expansion joints were reduced to 15. In the reinforcement project of Dongshan Bridge on Provincial Highway 306, cracks appeared in most beams of the bridge superstructure, which affected the lateral connection of the structure. In the design and construction, not only the cross-section size of the beam is increased, but also five beams are added, and steel plates are pasted on both sides of the main rib and the bottom of the main arch, considering that the steel plates and the main rib bear the same force.
For the problem of steel bar corrosion, when repairing the corroded area, the concrete in all corroded areas should be removed first, and then the steel bar should be derusted by sandblasting and ultra-high pressure water. If the steel bars are seriously corroded, the repair parts should be determined according to the inspection results and the number of steel bars should be increased to ensure the steel bar content, and then high-strength concrete should be poured. When the concrete layer is too thin to support the formwork, shotcrete can be used for pouring. At the same time, physical methods can also be used to prevent rust: for example, epoxy resin coating can be added to the surface of steel bars. A good coating can effectively prevent the corrosion of steel bars and greatly prolong the service life of the structure. According to the evaluation of relevant investigation data, the service life of the bridge can be prolonged by using epoxy resin coated steel bars 10 years or more. In addition, epoxy resin or other polymer materials are added to the surface that is often in contact with water as a waterproof layer, so as to improve the water resistance of the surface and achieve the effect of corrosion protection.
In medium and small span bridges, the use of seamless bridges can avoid the occurrence of expansion joint diseases.
For bridge deck pavement diseases, cold-rolled ribbed steel bars and steel fiber concrete materials can be used for protection. Compared with ordinary steel bars, cold-rolled ribbed steel bars have greatly improved design strength and strong bond strength with concrete. At the same time, the cold-rolled ribbed steel mesh has good overall stiffness and is not easy to deform, and the local collapse of the steel mesh will not occur during the construction of bridge deck concrete. Using steel fiber reinforced concrete, a new high-strength composite material, to repair and strengthen the bridge deck can not only improve the crack resistance, wear resistance and durability of the bridge deck, but also prolong the service life of the bridge and reduce the maintenance. In addition, it has high early strength and can be opened to traffic in advance, which has obvious technical, economic and social benefits and good development prospects.
4. Conclusions and recommendations
According to the actual situation of concrete bridge, through the investigation and analysis of diseases such as steel bar corrosion, bearings, expansion joints and deck pavement of bridge, combined with related projects, the design methods for preventing diseases are put forward, so as to completely solve the diseases of concrete bridge. The specific implementation process is as follows:
(1) Based on the extensive investigation of various diseases of concrete bridges in typical areas, various diseases are classified and analyzed, and their influences on structures are evaluated.
(2) Select some bridges with typical and universal diseases, compare them with those with the same or similar scale but without problems, and combine the information of design, construction and maintenance to investigate the causes of diseases, and analyze various influencing factors, their mechanism and development law.
(3) With reference to the current engineering practice of bridge maintenance, study the disease prevention countermeasures, put forward the treatment methods to reduce and eliminate existing diseases, structural measures and construction requirements to prevent diseases, and discuss and study the disease prevention design.
(4) Applying the research results to supporting projects, testing its effect and constantly improving it.
(5) Summarize the causes and transformation methods of various diseases, as well as the design points and construction methods of disease prevention, form a disease prevention information system, and summarize the advantages and disadvantages and application scope of various treatment methods.
Different types of bridges will have different degrees of diseases in different parts under different conditions. At present, the domestic research level only stays on the specific analysis of specific diseases, and there is no systematic research. There are many bridges in China and the terrain environment is changeable, which not only provides rich materials for this research, but also provides broad prospects for future practical application. The train of thought provided in this paper systematically analyzes the causes of bridge diseases, and studies the new materials, new construction technologies and new design methods in the field of bridge disease prevention and control, which will play a positive role in improving the service performance of bridges and perfecting the bridge design methods.
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