1. Analysis of the causes of freeze-thaw damage of concrete:
Common quality problems such as cracks and spalling in the concrete structure of cooling towers in alpine areas are common during use. Common causes mainly include the following two aspects
1. Damage caused by alternating wet and dry periods and freeze-thaw cycles:
The freeze-thaw damage of concrete is one of the common diseases of concrete projects in alpine areas. It is concrete damage caused by physical effects (changes in dryness and humidity, changes in temperature, changes in temperature thinking, etc.), freeze-thaw changes, etc. The main cause of concrete freeze-thaw erosion is that the water in the concrete voids forms fatigue stress under the combined action of ice expansion pressure and seepage pressure under the action of alternating dry-wet and freeze-thaw cycles, causing concrete to erode from the surface to the interior. Thereby reducing the freeze-thaw erosion of concrete strength.
After repeated freeze-thaw cycles, the damage gradually accumulates and expands, and develops into interconnected cracks, gradually reducing the strength of the concrete, or even losing it completely. It is not difficult to see from practice that there is no freeze-thaw damage problem in concrete in a dry state, so the saturated state is one of the necessary conditions for freeze-thaw damage of concrete. Another necessary condition is positive and negative changes in external temperature, which causes the water in the concrete pores to repeatedly freeze and thaw. These two necessary conditions determine that the freeze-thaw damage of concrete starts from the erosion damage of the first layer of concrete surface.
2: Chemical corrosion and carbonization:
Chemical corrosion is mainly the gas containing SO2 emitted from the chimney, which is further oxidized into SO3 in the air, forming chemical corrosion on the concrete, and is in contact with the concrete. Ca(OH)2 reacts to produce Ca(so)4, causing volume expansion. However, Ca(so)4 reacts with C3a in concrete to form calcium sulfoaluminate, resulting in secondary volume expansion. Expansion stress causes cracks and embrittlement in concrete.
The main cause of carbonization is that the gas emitted by the chimney increases the concentration of carbon dioxide in the surrounding air. Long-term corrosion of high-concentration CO2 causes carbonization of the concrete of the tower body and herringbone columns. Carbonated concrete can crack due to shrinkage in the presence of moisture.
Due to the carbonization and chemical erosion of concrete, the original passivation film on the surface of the steel bars is destroyed, the steel bars corrode, the volume expands, and the expansion stress causes the concrete protective layer to crack, forming cracks along the steel bars, that is, along the steel bars, and the protective layer Collapse.
2. Concrete winter construction technology:
1. Concrete antifreeze admixture: The use of antifreeze admixture is an effective measure to improve the frost resistance of concrete. Negative temperature is very detrimental to concrete. One of the reasons is the long construction period. The second is the impact on project quality. The use of antifreeze is an effective measure in cold areas to ensure the quality of concrete construction in winter, save energy, and reduce project costs. Adding admixtures for winter construction of concrete has the following main functions:
①. Reduces the critical strength of concrete that is exposed to early freezing. In general, adding admixtures can reduce the critical strength (20% to 30%) by r28, which greatly shortens the curing time of concrete, reduces the cost of curing, and shortens the construction period.
②. Promote the crystallization distortion of solid phase water-ice in fresh concrete. The solidification of the liquid phase in concrete mixed with admixtures actually “stores” a part of the water. As the freezing process progresses, the concentration of the admixtures continues to increase due to the reduction of the liquid phase. At the same time, part of the water is used When the concentration of the admixture solution is close to equilibrium in the concrete liquid phase, the amount of water required for the cement is obtained by melting ice. As a result, the ice content in the concrete gradually decreases until it disappears.
③. Change the pore structure of concrete. The frost resistance of both newly poured concrete and hardened concrete is related to the pore structure of the concrete. Air-entraining admixtures are used to make concrete have a certain air content, thereby improving the pore structure of the concrete and improving its durability and ability to resist early freezing.
④. Improve the early strength of concrete. The main effect of early strength is to change the solubility of silicate in cement, thereby accelerating the hardening of cement concrete and generating complex and alkaline hydration products. The generated hydrate crystallization strengthens the structure formation effect of the cement slurry to a certain extent, allowing the newly poured concrete to reach critical strength faster.
⑤. Change the water-cement ratio of concrete and reduce the water consumption of concrete mixture. The water-cement ratio affects the pore structure and structure formation process of concrete. Therefore, winter construction strives to continuously reduce the water-cement ratio of concrete through the water-reducing and plasticizing effect of admixtures. In order to meet the requirements of winter construction, domestic and foreign scientists have conducted unremitting research and pursuit of antifreeze technology and achieved a series of satisfactory results. Antifreeze theory has become increasingly perfect, and antifreeze products and varieties have made great progress. In addition to antifreeze, admixtures that can be used in winter construction include air-entraining agents, water-reducing agents, early-strength agents, etc., which are often used in combination.
2. Air-entraining agent: The incorporation of air-entraining agent can produce an appropriate amount of closed micro-bubbles in the concrete, improve the concrete structure, and help the concrete resist early freezing damage. The additional pores produced when water expands when frozen can act as a buffer and reduce damage, thereby increasing the frost resistance of concrete.
After the air-entraining agent is mixed into the concrete, a large number of uniform, stable and closed micro-bubbles are introduced. These micro-closed bubblesThe bubbles are not connected to each other and are uniformly and stably distributed in the concrete. When the free water in the pores freezes, the bubbles are compressed, which can greatly reduce the expansion pressure brought by freezing to the pores; when dissolved, these bubbles can return to their original shape, so the bubbles in the pores Repeated freezing and thawing of free water will not exert great pressure on the hole wall. These bubbles act like rolling balls in the concrete, which can greatly improve the fluidity of the concrete, improve the workability of the concrete, and reduce bleeding and separation. Due to the improved workability, the unit water consumption of concrete can be reduced. When the cement dosage remains unchanged, part of the strength loss caused by air entrainment can be compensated. As long as the amount of air entrainment is appropriate, ordinary concrete can also obtain very high frost resistance.
Air-entraining agents have been used to improve the frost resistance of concrete for many years. For a long time, its function has been mainly attributed to the physical effect of bubble relief. The physical effects of bubbles and the surface chemical effects of air-entraining agents are both non-negligible reasons for improving the frost resistance of air-entrained concrete.
3. Water-reducing agent:
It can increase the fluidity of concrete clinker without changing the workability, thereby reducing the water consumption of concrete mixing and reducing the water-cement ratio. This reduces the chance of structural defects due to water freezing and enhances the hardening process of concrete. Incorporating water-reducing agents can improve the workability of concrete and make it easier to pour and shape, thereby increasing the density of concrete, reducing the bleeding rate, reducing the internal and external stratification of concrete, and preventing the surface of concrete from freezing and the formation of ice around reinforced stones. Membrane; the water-cement ratio of concrete decreases, and the diameter and spacing of bubbles in the concrete decrease accordingly, which is beneficial to improving frost resistance. The dispersion effect of high-efficiency water reducing agent on cement can improve the effect of early strength components and antifreeze components.
Water-reducing agent has a dispersing effect. It can make cement become small, separated individual particles, evenly dispersed in water, and can also form a stable water film on the surface of cement particles. Increase the workability of concrete mixture and reduce the amount of cement. Therefore, water reducing agents have the ability to improve the void structure of concrete and enhance durability. Because the water-reducing agent reduces the water content in concrete and can make the ice crystals finer and dispersed, it weakens the destructive effect of ice content on the concrete structure.
4. Early strength agent: It can shorten the setting time of cement, accelerate the growth of concrete strength, and early exothermic reaction of cement. Function of early strength agent:
①. Accelerate cement hydration. The c3s and c3a in the cement minerals react quickly with water to form ettringite crystals and ettringite gel that reach critical strength earlier to resist the ice expansion stress when water freezes.
②. Lower the freezing point. On the one hand, the early strength agent is also an electrolyte. On the other hand, a large amount of free water becomes bound water, which increases the concentration of antifreeze, improves the early strength of concrete, and creates conditions for the concrete to enter the critical anti-freeze strength in advance.
3. Summary:
A survey of the ingredients of various antifreezes in my country found that their composition cannot be single, but must be a combination of multiple ingredients. High-efficiency water reducing agent, early strength agent, air-entraining agent and other ingredients are used in combination to make up for each other’s shortcomings and give full play to their respective functions. Only in this way can the antifreeze have anti-freeze, early strength, rust inhibition, catalysis, etc. The best results can be obtained through comprehensive effects. High-efficiency antifreeze is based on the fundamentals of greatly reducing the causes of frost damage, improving the early strength of concrete, increasing the concentration of antifreeze in capillary tubes and refining the diameter of capillary tubes and other methods to reduce the freezing point, and introducing an appropriate amount of gas phase to reduce frost heave stress.
The above descriptions of the frost damage mechanism and antifreeze admixtures are closely related. Only with a more thorough understanding of the frost damage mechanism of concrete can we continue to research and produce more and better antifreeze admixtures on this basis. , in order to continuously improve the frost resistance of concrete, shorten winter construction time, improve winter construction quality, save winter construction costs, and create greater economic benefits.
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