Abstract: Steel corrosion is the most important and direct factor that leads to the durability damage of reinforced concrete bridges, and it is also one of the main forms of durability damage of concrete bridges. This paper comprehensively discusses the corrosion mechanism, influencing factors and consequences.
Corrosion of steel bars is a common durability problem, which seriously threatens the safety of structures. Among the factors that affect the durability of the structure, it is dominant. The United States, Britain, Germany, Japan and other countries spend huge sums of money on the durability repair of concrete structures every year, in which the corrosion of steel bars accounts for a considerable proportion. A considerable number of reinforced concrete bridges in China have gradually entered the aging period, so it is very important to study and prevent the corrosion of steel bars.
Corrosion of steel bars is the most important and direct factor leading to the durability damage of reinforced concrete bridges, and it is also one of the main forms of durability damage of concrete bridges. The damage of steel bar corrosion to bridge structure can be divided into three periods: in the early stage, due to local corrosion, rust spots and peeling appeared on the surface of steel bar; In the middle period, the whole surface of steel bar is corroded and expanded, separated from the protective layer and peeled off; In the later period, the corrosion of steel bars further expanded, the concrete itself was destroyed, the concrete cracked along the steel bars, and the concrete was detached, until the steel bars were continuously corroded, the effective section was continuously reduced, the bearing capacity of the bridge structure was continuously reduced, and the reinforced concrete members lost their basic bearing capacity.
1. Corrosion mechanism of steel bars in reinforced concrete bridges
In general, due to the high alkalinity of initial concrete, a dense passivation film is formed on the surface of reinforced concrete bridge structure, which makes it in a passive state. However, with the invasion of environmental media, the passive film is gradually destroyed, leading to corrosion.
There are three basic factors for tendon corrosion:
(a) destroying the passivation film on the surface of the rib;
(2) Sufficient oxygen supply;
(3) Appropriate humidity (RH = 60 ~ 80%).
Three elements are indispensable. The first factor is the induced condition, and the corrosion rate depends on the supply of oxygen and water.
The corrosion of steel bars is generally electrochemical corrosion. Electrochemical corrosion must meet three conditions:
1, forming potential difference on the surface of steel bar;
2. There is enough oxygen and water on the surface of the cathode steel bar;
3. In the anode area, the surface of the steel bar at the anode is activated, that is, the passivation film on the surface of the steel bar is destroyed.
Under the joint action of oxygen and water, the surface of steel bar continuously loses electrons, electrochemical reaction occurs, and gradually corrodes, resulting in red rust on the surface of steel bar and concrete cracking.
For reinforced concrete bridges, under general environmental conditions, the corrosion of steel bars is usually caused by two actions: one is the carbonation of concrete; One is chloride ion erosion. Carbon dioxide and chloride ion have no serious damage to the concrete itself, but these two environmental substances are the most important and frequently encountered environmental media for the destruction of the passive film of steel bars in concrete: carbonation of concrete gradually reduces the content of Ca(OH)2 in the pore solution of concrete, and the PH value gradually decreases, which makes the passive film gradually unstable or even completely destroyed, and makes the steel bars in a desensitized state; Chloride ions in the surrounding environment gradually penetrate into the concrete from the concrete surface. When the concentration of free chloride ions in the pore solution of concrete reaches a certain value (critical concentration), even if the alkalinity of concrete is high and the pH value is greater than 1 1.5, Cl- can destroy the passive film, thus causing corrosion of steel bars. The corrosion of steel bars caused by chloride salt develops rapidly, which is far more serious than carbonation corrosion. This situation often occurs in offshore or marine environment and in the environment where deicing salt is often used in winter.
Second, the main factors affecting the corrosion of reinforced concrete bridges
(a) the thickness and integrity of concrete protective layer and the compactness of concrete
These three aspects are all related to the erosion speed of aggressive media, and the influence of protective layer thickness on steel bar corrosion is linear, so the specifications of all countries in the world stipulate the thickness of protective layer. In the newly revised Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts in China, the minimum protective layer thickness of steel bars is stipulated, and the thickness of concrete protective layer increases with the deterioration of service environment. The compactness of concrete affects the permeability of concrete, and concrete with high permeability is more likely to corrode.
(B) the degree of carbonation of concrete
The carbonation of concrete reduces the alkalinity of concrete, which leads to the decrease of PH value and makes it possible to passivate steel bars. The weight loss rate of steel bars is almost linear with the carbonation depth of concrete, so the carbonation degree of concrete has a great influence on the corrosion of steel bars.
(3) Environmental conditions
The influence of environment on steel corrosion mainly includes the following aspects: temperature, humidity, carbon dioxide concentration, oxygen concentration and corrosion medium concentration. For reinforced concrete bridges, humidity is the most important factor. When the bridge is in a high humidity environment, especially the pier and splash area with floating water level, it is most likely to rust.
(D) the impact of chloride ions
Chloride is a dangerous corrosive medium. However, in northern China, in order to ensure smooth traffic in winter, snow melting agent is sprinkled on roads, bridges and urban overpasses, and a large number of sodium chloride and calcium chloride are used, so that chloride ions penetrate into concrete, resulting in corrosion damage of steel bars.
Many engineering experiences and lessons in northern China show that the extensive use of deicing salt is one of the main reasons that affect the durability of reinforced concrete bridge structures. According to foreign related research reports, bridge structures using deicing salts generally begin to corrode and fracture in 5~ 10 years, which leads to steel bar corrosion and concrete cracking. As there is no deicing method that can completely replace deicing salt so far, deicing salt will continue to be used. Therefore, it is very important to take anti-corrosion measures for deicing salt. Thirdly, the influence of steel corrosion on the durability of reinforced concrete bridges.
The direct result of steel bar corrosion is that the cross-sectional area of steel bar decreases. Uneven corrosion leads to uneven surface of steel bars, resulting in stress concentration, which degrades the mechanical properties of steel bars, such as strength reduction, brittleness increase and ductility deterioration, resulting in lower bearing capacity of members.
(1) Mechanical properties of corroded steel bars
The reduction of resistance of corroded steel bars directly affects the bearing capacity of existing structures and components, and may lead to premature failure or even collapse of structures in serious cases. When steel bars are corroded uniformly along the length direction, the weight loss rate of steel bars is approximately equal to the loss rate of cross-sectional area of steel bars, and the reduction of ultimate tensile force that steel bars can resist is basically proportional to the corrosion rate of cross-sectional area of steel bars. At this time, the ultimate tensile capacity of corroded steel bars can be simply obtained by multiplying the actual cross-sectional area of corroded steel bars by the ultimate tensile strength of corroded steel bars.
However, due to the inhomogeneity of concrete materials, the instability of service environment and the different stress levels of various parts of steel bars, the corrosion of steel bars in concrete is rarely uniform. In general, the loss rate of cross-sectional area of steel bars is greater than the weight loss rate, and with the development of corrosion of steel bars, the unevenness and dispersion of corrosion increase, and the difference between weight loss rate and cross-sectional area loss rate is also greater. Therefore, the decline of ultimate tensile capacity of steel bars is due to the corrosion of steel bars and the reduction of effective cross-sectional area. There is another factor: the surface of corroded steel bars is uneven, and stress concentration occurs at the notch after stress, which reduces the yield strength and ultimate strength of corroded steel bars; Moreover, the more serious the corrosion, the more the strength reduction caused by stress concentration.
(2) The influence of rebar corrosion on the cooperative performance of rebar and concrete.
After steel bars are corroded, the bond and anchorage performance between steel bars and concrete is reduced. The test results show that the experimental value of flexural capacity of corroded reinforced concrete beams is less than the calculated value only considering the reduction of cross-sectional area and yield strength of corroded steel bars, which shows that the reduction of bond strength between steel bars and concrete is also one of the main factors affecting the reduction of flexural capacity of corroded reinforced concrete beams. Therefore, the tensile reinforcement must be multiplied by the cooperative working coefficient to consider the influence of bond degradation on the flexural capacity of reinforced concrete beams.
Theoretically, considering the influence of bond strength reduction, the flexural capacity of corroded reinforced concrete beams should be between corroded members and unbonded members, while the flexural capacity of unbonded members is about 70%~80% of normal members under the same conditions, so kb should be between 0.7~ 1.
(C) the impact of steel corrosion on the structural performance of reinforced concrete bridges
Once the steel bars in concrete are corroded, a layer of loose corrosion products will be generated on the surface of the steel bars and spread to the surrounding concrete pores. The volume of corrosion products is much larger than that of corroded steel bars, which can generally reach 2-4 times of the corrosion amount of steel bars. The volume expansion of corrosion products will produce circumferential tensile stress in concrete around steel bars. When the circumferential tensile stress reaches the tensile strength of concrete, internal radial cracks will appear at the interface between steel bar and concrete. With the further aggravation of steel corrosion and the increase of steel corrosion, radial internal cracks will develop to the concrete surface until the concrete protective layer cracks or even peels off, which seriously affects the normal use of reinforced concrete bridges.
The bond between reinforcement and concrete is a complex interaction, through which the stress between reinforcement and concrete can be transferred and the deformation can be coordinated. Therefore, the bond and anchorage performance between steel bar and concrete is the basic premise to ensure that two different materials, steel bar and concrete, work together. Lubrication of corrosion layer between steel bar and concrete, corrosion of transverse ribs on steel bar surface, cracking or peeling of concrete protective layer will all lead to the reduction or even complete loss of bonding and anchoring performance of reinforced concrete, which will ultimately affect the safety, applicability and durability of reinforced concrete bridge structure.