| Corrosion of concrete by marine environment |
The composition of seawater is almost the same in different regions. Table 2-1 shows the salt composition of 77 seawater samples worldwide, with chloride being the most abundant, accounting for almost 90% of the total salt content. But the salinity of the surface layer of seawater varies from place to place, and even within the same region, it can also differ with seasonal changes. Table 2-2 shows the distribution of seawater salinity values in various sea areas of China.
Chloride ions in the marine environment can migrate from the surface of concrete to the interior of concrete. When chloride ions accumulate at a certain concentration (critical concentration) on the surface of steel bars, they may cause steel bar corrosion. The degree of steel corrosion caused by chloride ions is more severe than that caused solely by carbonization in general environments, and it is a key issue in durability research.
The marine environment is generally divided into two categories: coastal environment and sea breeze environment.
1. Coastal environment
Coastal environment refers to the vertical environment of the ocean, including the underwater zone, tidal zone, splash zone (water level fluctuation zone), and offshore atmospheric zone. The chloride ion source in the underwater area mainly comes from seawater; The chloride ion sources in tidal and splash zones come from waves or sprays, and vary periodically with waves; The main source of chloride ions in the oceanic atmosphere is salt spray in the air above the ocean. The higher the salt concentration in seawater, the higher the salt content in the salt spray. In the long run, for concrete structures in the marine environment, whether it is the water level fluctuation zone, splashing zone or atmospheric zone, the concentration of chloride ion sources can be considered to depend on the concentration of chloride ions in seawater
Mainly the concentration of chloride ions in the surface layer of seawater.
The Technical Specification for Corrosion Prevention of Concrete Structures in Port Engineering (JTJ275-2000) and the Design Specification for Hydraulic Concrete Structures (SL191-2008) both specify the division of concrete parts in seawater environments, as shown in Tables 2-3 and 2-4, respectively. However, it should be noted that only environmental conditions and chloride ion erosion mechanisms can be described for each area, and it is not possible to simply distinguish each area based on altitude
2. Sea breeze environment
Sea breeze environment refers to the atmospheric environment near the coast. There are many factors that affect the salt spray content in the atmosphere near the sea. In addition to the salinity of seawater, there are mainly two factors that affect it: climate conditions (wind direction, wind speed, humidity, etc.) and natural environment (coastline topography, distance from the sea, etc.). Among these two factors, distance from the sea is the most important and is closely related to the magnitude of wind speed. The seasonal winds in China are mostly southeast winds in spring and summer, and northwest winds in autumn and winter. If the wind direction blows from the ocean to the land, it is conducive to an increase in salt content over the mainland. The higher the wind speed on the sea surface, the more salt content in the atmosphere. In places farther away from the sea, the salt spray content in the air is usually lower. During a storm, its value may increase by 10 times; when the wind speed remains constant, the salt content in the air will decrease with increasing humidity. In inland environments, the salt spray content in the atmosphere is generally low, but the concentration of salt spray in the air above saline alkali land and inland salt lakes is high, and in some places it is even higher than that of coastal land
Much higher.
The Guangzhou Institute of Electrical Science in China conducted multiple measurements of salt spray content in the air of some coastal areas in China in the 1960s and 1980s, with the maximum value ranging from 0.024 to 1.375mg/m, which is related to the distance from the sea. In 1994, Xu Guobao analyzed that the relationship between salt spray content in the air and distance from the sea decreased exponentially, and the salt spray content outside 30000m was close to normal environment, as shown in Figure 2-2
According to the law that the salt spray content in the air near the sea decreases with increasing distance from the sea, many countries have also formulated corresponding zoning standards and concrete structure construction indicators and regulations: the European standard EN 206-1/2000 and the German industrial standard DIN 1045-2/2000 stipulate that the minimum strength grade of concrete components along the coast is C40 and the water cement ratio is not greater than 0.5, but the distance range from the shore is not clear; The manual of the Japan Highway Association stipulates that the area within 100m offshore is a first level anti-corrosion zone, and the area beyond 100m is a second level anti-corrosion zone; The design and construction recommendations for high durability concrete issued by the Japan Institute of Architects stipulate that the area immediately adjacent to the coast (0m) is a medium salt hazard zone, the area within 50m is a salt hazard zone, 200m is a quasi salt hazard zone, and beyond 200m is not considered a salt hazard zone. It is believed that there will be no significant accumulation of chloride ions on the surface of concrete beyond 200m. In the Chinese "Design Code for Hydraulic Concrete Structures" (SL191-2008), the salt spray zone is divided by 50m from the rising tide coastline, and the area of severe salt spray is the onshore environment 50m from the rising tide coastline; The area affected by mild salt spray is the onshore environment 50500m away from the rising tide shoreline. According to the "Code for Durability Design of Concrete Structures" (GB/T 50476-2008) in China, the mild salt spray zone refers to the outdoor environment on land that is 100-300m away from the rising tide shoreline; The severe salt spray area refers to the outdoor environment on land within 100m from the rising tide shoreline. I, King Ice and others, have studied salt spray zoning using Fick's second law and believe that areas within 100 meters of the sea are classified as severe salt spray
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Overall, it can be seen that from the perspective of external environmental factors, the concentration of chloride ions on the surface of structures in the marine environment increases with seawater salinity, and 100-300m belongs to the mild salt spray zone. The concentration of chloride ions on the surface of structures near the coast increases with the increase of surface chloride ion concentration and wind speed, while it decreases with the increase of distance from the coast.
There are three environmental factors that affect chloride ion erosion in marine environments.
The diffusion of chloride ions is caused by the concentration difference of chloride ions. The higher the surface concentration, the greater the difference in chloride ion concentration between the inside and outside, and the more chloride ions will diffuse into the interior of the concrete. The chloride ion concentration on the surface of the structure is mainly related to the environmental chloride ion concentration, and also to the adsorption performance of the concrete material on chloride ions.
1) Environmental chloride ion concentration
Temperature has a dual impact on the durability of concrete: on the one hand, an increase in temperature accelerates water evaporation, resulting in an increase in surface porosity and permeability; On the other hand, an increase in temperature can accelerate the hydration rate of the internal concrete, increase the compactness of the concrete, and reduce its permeability.
2) Environmental temperature
3) The relative humidity of the environment and precipitation also play an important role in chloride ion erosion. Because the humidity of concrete is an important factor affecting the diffusion coefficient, pore water is required as a carrier for the transfer of chloride ions from the surface of components to the interior of concrete through absorption, diffusion, infiltration, and other pathways. In fact, in the exposed environment, the moisture saturation of concrete is also greatly affected by precipitation, and it is still difficult to specifically express the relative humidity in the life prediction model. However, in nearshore and marine environments, the relative humidity of the atmosphere is not significantly different due to the influence of marine climate.
2.1.3 Freeze thaw damage of concrete in freeze-thaw environment refers to a phenomenon in which concrete undergoes peeling, structural looseness, and strength reduction from the surface layer under alternating cycles of negative and positive temperatures, until it is damaged. The freeze-thaw cycle directly acts on concrete, but due to the failure of the bond between the concrete and the steel bars, as well as the peeling of the concrete protective layer, the steel bars are indirectly affected. Cold regions such as Northern Europe and North America, as well as the Soviet Union, had already attached importance to frost resistance as early as the 1940s and adopted air entrainment technology. However, the damage to concrete roads and bridges was still severe, mainly due to the effects of de icing salts and freeze-thaw cycles. The main reasons for premature failure of concrete structures in northern China are freeze-thaw and salt freezing.
The damage caused by freeze-thaw cycles to concrete structures can be divided into two categories [13,14]. The first category is internal damage, which is caused by the volume expansion of about 9% due to the freezing of water inside the concrete. The concrete cracks or even peels off, usually occurring when the water content inside the concrete exceeds a critical value. The damage is mainly manifested as a decrease in the compressive and tensile strength of concrete, the bond strength with steel bars, and the elastic modulus, resulting in a comprehensive decrease in the compressive, tensile, bending, shear, and torsion resistance of the components. Especially the loss of elastic modulus can significantly reduce the bearing capacity of prestressed concrete structures. When concrete is frozen, the water in the coarse pores freezes first, and under the push of the expansion of water freezing, the unfrozen water in the pores migrates to the surrounding area, forming static water pressure. When the hydrostatic pressure exceeds the strength that concrete can withstand, it will damage the concrete. The higher the saturation degree of concrete, the faster the freezing rate. The static water pressure and destructive power of concrete are specified in the "Code for Durability Design of Concrete Structures" (GB/T 50476-2008), which defines the mild salt spray zone as the outdoor environment on land 100-300m away from the rising tide shoreline; The severe salt spray area refers to the outdoor environment on land within 100m from the rising tide shoreline. I, King Ice and others, have studied salt spray zoning using Fick's second law and believe that areas within 100 meters of the sea are classified as severe salt spray
. 18·
Overall, it can be seen that from the perspective of external environmental factors, the concentration of chloride ions on the surface of structures in the marine environment increases with seawater salinity, and 100-300m belongs to the mild salt spray zone. The concentration of chloride ions on the surface of structures near the coast increases with the increase of surface chloride ion concentration and wind speed, while it decreases with the increase of distance from the coast.
There are three environmental factors that affect chloride ion erosion in marine environments.
The diffusion of chloride ions is caused by the concentration difference of chloride ions. The higher the surface concentration, the greater the difference in chloride ion concentration between the inside and outside, and the more chloride ions will diffuse into the interior of the concrete. The chloride ion concentration on the surface of the structure is mainly related to the environmental chloride ion concentration, and also to the adsorption performance of the concrete material on chloride ions.
1) Environmental chloride ion concentration
Temperature has a dual impact on the durability of concrete: on the one hand, an increase in temperature accelerates water evaporation, resulting in an increase in surface porosity and permeability; On the other hand, an increase in temperature can accelerate the hydration rate of the internal concrete, increase the compactness of the concrete, and reduce its permeability.
2) Environmental temperature
3) The relative humidity of the environment and precipitation also play an important role in chloride ion erosion. Because the humidity of concrete is an important factor affecting the diffusion coefficient, pore water is required as a carrier for the transfer of chloride ions from the surface of components to the interior of concrete through absorption, diffusion, infiltration, and other pathways. In fact, in the exposed environment, the moisture saturation of concrete is also greatly affected by precipitation, and it is still difficult to specifically express the relative humidity in the life prediction model. However, in nearshore and marine environments, the relative humidity of the atmosphere is not significantly different due to the influence of marine climate.
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