Ji Ronghua, Changzhou Institute of Engineering and Technology, 2003, 9
This paper analyzes the durability of concrete from the engineering point of view, and summarizes some measures to improve the durability of concrete.
Keywords: alkali-aggregate reactive corrosion of durable high performance concrete
1 Durability in concrete engineering
Strength and durability are two important indexes of concrete structure. In the past, only the strength of concrete was concerned, or the durability of concrete was ignored because of the one-sided pursuit of high strength. The durability of concrete is the ability of the structure to ensure normal function during its service life, which is related to the service life of the structure. With the aging of the structure and the aggravation of environmental pollution, the durability of concrete has attracted the attention of various competent departments and the majority of design and construction departments.
A survey shows that most industrial buildings in China need to be overhauled after 25-30 years of use, and the service life of buildings in harsh environments is only 15-20 years, especially for infrastructure projects such as bridges and ports. Many projects were completed within a few years, and the steel bars rusted and the concrete cracked. Some experts pointed out that the climax of infrastructure construction in China needs to be continued. Due to the neglect of durability, there will be a climax of overhaul to meet us.
2 durability analysis of concrete structure
The durability of concrete refers to the long-term evolution of the structure in the use environment due to internal or external reasons, which eventually makes the concrete lose its ability to use. In other words, there are many reasons for durability failure, including frost resistance failure, alkali aggregate reaction failure, chemical corrosion failure and structural damage caused by steel bar corrosion. Let's analyze it in detail.
2. 1 Freeze-thaw damage of concrete
When the structure is in the environment below freezing point, the water in some concrete pores will freeze, resulting in volume expansion, and supercooled water will migrate, forming various pressures. When the pressure reaches a certain level, it will lead to the destruction of concrete. The most obvious feature of freeze-thaw damage of concrete is surface peeling, which can expose stones in severe cases.
The frost resistance of concrete is closely related to the pore structure and bubble content of concrete. The fewer pores, the smaller the damage and the more closed bubbles, the better the frost resistance.
Besides pore structure and air content, the factors that affect the frost resistance of concrete include: concrete saturation, water cement ratio, concrete age, aggregate porosity and its water content.
2.2 Alkali-aggregate reaction of concrete
Alkali-aggregate reaction of concrete refers to the chemical reaction between alkali in concrete and active components in aggregate, which causes concrete to expand, crack and even destroy. Due to the reaction factors in concrete, its harmful effects can not be eradicated, which is a hidden danger in concrete engineering. Many countries have to dismantle dams, bridges, seawalls and schools due to alkali-aggregate reaction, which has caused huge losses. There are also reports of similar alkali-aggregate reaction damage in domestic projects.
There are three conditions for alkali-aggregate reaction of concrete, namely a considerable amount of alkali, corresponding active aggregate and water. There are usually three types of reactions: alkali-silicic acid reaction, alkali-carbonate reaction and slowly expanding alkali-silicate reaction. The methods to avoid alkali-aggregate reaction can be adopted: first, try to avoid using active aggregate; Second, limit the alkali content of concrete; Third, mix with mixed materials.
2.3 chemical erosion
When the concrete structure is in an environment with corrosive medium, it will cause a series of chemical, physical and physical and chemical changes in the cement paste, and it will be gradually eroded, which will seriously reduce the strength of the cement paste and even destroy it. Common chemical corrosion can be divided into five categories: fresh water corrosion, general acid water corrosion, carbonic acid corrosion, sulfate corrosion and magnesium salt corrosion. Flushing with fresh water will dissolve the components in the cement slurry, increase the porosity of the cement slurry and reduce the compactness, thus further damaging the cement slurry. The research shows that the strength of cement paste decreases by 7% when calcium oxide is dissolved by 5%, and by 29% when it is dissolved by 24%. Therefore, fresh water scouring will have a certain impact on hydraulic structures. When some acids are dissolved in water, the cement stone is subjected to both dissolution and chemical dissolution, and the corrosion is obviously accelerated, which often happens in chemical plants. The effects of carbonic acid on concrete include: dissolving cement stone, destroying the alkaline environment in concrete, reducing the stability of cement hydration products, affecting the density of cement stone and causing corrosion to concrete; Sulfate corrosion is characterized in that SO42- ions penetrate into concrete and react with cement components, and the product volume expands and cracks, causing damage; Due to the existence of various ions in seawater, the corrosion forms are complex, but the main reason is that magnesium salts decompose and harden the structural components of cement paste, sulfate action will destroy the cement paste, and magnesium oxide precipitation will block the pores of concrete, thus alleviating seawater erosion.
2.4 Corrosion of steel bars
The corrosion of steel bars is manifested in the electrochemical reaction of steel bars under the action of external media, which gradually generates rust such as iron hydroxide, which is 2-4 times larger than the original metal, leading to cracks in concrete along the steel bars, thus becoming a channel for corrosive media to penetrate into steel bars and accelerating the destruction of structures. Iron hydroxide will form a stable protective layer in strong alkali solution to prevent the corrosion of steel bars, but if the alkaline environment is destroyed or weakened, it will cause the corrosion of steel bars. Such as carbonation or neutralization of concrete. The main reason for carbonation and neutralization of concrete is that the compactness of concrete is insufficient, and acid gases (such as CO2, SO2, H2S, HCL, NO2) penetrate into concrete to react with calcium hydroxide. Secondly, chloride ions have a special destructive effect on the passive film on the surface of steel bars. When the chlorine content in concrete exceeds the standard, steel bars will be corroded, and the existence of water and oxygen is the necessary condition for steel bars to be corroded. Therefore, if cracks appear in concrete, which leads to the passage of water and oxygen, it will accelerate the corrosion of steel bars, which will lead to the further development of concrete cracks and the peeling of concrete protective layer, and finally make the members lose their bearing capacity; Thirdly, the brittle fracture of steel bars under the combined action of tensile stress and corrosive medium can be destroyed under the action of low tensile stress and weak medium; Fourthly, the phenomenon of hydrogen embrittlement of steel bars, that is, prestressed steel bars are brittle in acidic and slightly alkaline media, and a small amount of hydrogen will be produced during the corrosion of steel bars. When there are defects in the steel bar, hydrogen will infiltrate into the steel bar in the form of atoms, generating hydrogen molecules, generating huge pressure and bubbles, making the steel bar brittle.
Measures to improve the durability of concrete
From the above analysis, it can be seen that the external environment, internal pore structure, raw materials, compactness and impermeability of concrete are important factors affecting the durability of concrete. Therefore, corresponding measures should be taken to improve the durability of concrete in engineering according to specific conditions.
3. 1 Selection of raw materials
(1) The strength and engineering properties of cement materials are formed by the setting and hardening of cement mortar. Once the cement stone is destroyed, the durability of concrete will be destroyed. Therefore, we should pay attention to the specific properties of cement varieties when choosing cement, and choose cement with low alkali content, low hydration heat, low drying shrinkage, good heat resistance, water resistance, corrosion resistance and frost resistance. The strength of cement is not the only criterion to determine the strength and performance of concrete, and low-grade cement can also be used to prepare high-grade concrete. Therefore, when choosing cement strength in engineering, we should consider its engineering performance, sometimes its engineering performance is more important than strength.
(2) The selection of aggregate and admixture aggregate should consider its alkali activity to prevent the harm caused by alkali-aggregate reaction, the corrosion resistance and water absorption of aggregate, and at the same time choose reasonable gradation to improve the workability of concrete mixture and improve the compactness of concrete; A large number of studies show that adding additives such as fly ash, slag and silica fume can effectively improve the performance of concrete, improve the internal pore structure of concrete, fill internal gaps and improve the compactness. High-content concrete can also inhibit the alkali-aggregate reaction, so concrete with additives is an effective measure to improve the durability of concrete, that is, high-performance concrete developed in recent years.
3.2 Durability should be considered in concrete design.
While meeting the strength and workability of concrete, the design of concrete mixture ratio should consider reducing the dosage of cement and water as much as possible, reducing the hydration heat, reducing shrinkage cracks, improving the compactness, adopting reasonable water reducing agent and air entraining agent, improving the internal structure of concrete, and adding enough mixture to improve the durability of concrete.
The thickness of concrete protective layer should be designed according to the use environment of structural members, so as to prevent external media from infiltrating into the interior to corrode steel bars.
The joint design of the structure should also consider the overall durability of the components after local damage.
Structural design should still control the crack width of concrete cracks.
3.3 Structural durability should be considered in concrete engineering construction.
Concrete mixing should adopt secondary mixing method, sand wrapping method and sand wrapping method as far as possible to improve the workability and water retention of concrete mixture, improve concrete strength and reduce water consumption; The pouring and vibrating of mass concrete should control the temperature cracks, shrinkage cracks and construction cracks of concrete, establish the pouring and vibrating system of concrete, improve the compactness and impermeability of concrete, pay attention to the surface process of concrete after vibrating, strengthen maintenance and reduce concrete cracks. The concrete construction process is very important to control the appearance cracks and construction cracks of components, so the construction quality management should be strengthened, and special measures should be taken for concrete structures constructed in special seasons.
3.4 Daily maintenance of the structure
In the use stage of the structure, attention should be paid to detection, maintenance and repair, especially for infrastructure projects in the open air and harsh environment. A detection and evaluation system should be established to find and repair the structure in time to ensure the normal use of concrete structures.
refer to
1, Code for Anticorrosion Design of Industrial Buildings (GB 50046-95)—— Translation: China native, China chemical worker.
The first edition of the Ministry of Industry and Information Technology199565438+February edition, China Planning Press, Content 4, Structure 5, Architecture.
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2. Prediction of Durability and Life of Concrete Structures-Niu Beijing Science Press, 2003
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5, "Corrosion of steel bars in concrete under atmospheric conditions"-Xu "Building Technology" magazine 2003.
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