With the development of technology, concrete additives have become the fifth necessary material except cement, coarse and fine aggregate, admixture and water. Admixture is an important measure to optimize the mix design and improve the durability of concrete. Therefore, the newly revised code for construction of hydraulic concrete (DL / t5144-2001) emphasizes that appropriate additives must be added to hydraulic concrete.
1. Classification of concrete additives
Concrete additives can be divided into the following four categories according to their main functions:
(1) Admixture to improve the rheological properties of concrete mixture. Including ordinary water reducing agent and high-efficiency water reducing agent, air entraining agent and pumping agent.
(2) Additives for adjusting setting time and hardening performance of concrete. Including retarder, early strength agent and accelerator, etc.
(3) Additives to improve the durability of concrete. Including air entraining agent, waterproof agent and rust inhibitor.
(4) Additives to improve other properties of concrete. Including expansion agent, antifreeze, colorant, etc.
The main types of additives commonly used in hydraulic concrete are water reducing agent, retarder, air entraining agent and various composite admixtures, such as retarding water reducing agent or retarding high-efficiency water reducing agent, early strength water reducing agent and air entraining water reducing agent. According to special needs, other kinds of admixtures are also used, such as pumping agent, waterproof agent, antifreeze agent, etc.
2. Water reducing agent
Water reducing agent is also called plasticizer or dispersant. When mixing concrete, adding a proper amount of water reducing agent can make the cement particles disperse evenly and release the water wrapped by the cement particles at the same time, which can significantly reduce the water consumption of concrete.
The function of water reducing agent is to improve its workability under the condition of keeping the concrete mix proportion unchanged; or to reduce the water consumption and improve the concrete strength under the condition of keeping the workability unchanged; or to reduce the cement consumption and save the cement and reduce the cost under the condition of keeping the strength unchanged. At the same time, after adding water reducing agent, the concrete is more uniform and dense, improving a series of physical and chemical properties, such as impermeability, frost resistance, erosion resistance, etc., improving the durability of concrete.
3. Retarder
The retarder can delay the setting and hardening time of concrete, facilitate the construction, slow down the hydration speed of concrete paste, extend the hydration heat release process, and facilitate the temperature control of mass concrete. The retarder can reduce the early strength of concrete in 1-3 days, but has no effect on the normal development of the later strength.
Generally, retarder can prolong the initial setting time of concrete by 1-4 h, but it is not enough for the construction of large warehouse concrete under high temperature. In order to meet the needs of mass concrete construction in high temperature areas and high temperature seasons, the national "Eighth Five Year Plan" scientific and technological research project has developed a high-temperature retarder, which can set concrete for 6-8 hours at (35 + 2) ℃ temperature and (60 + 5)% relative humidity. At present, the retarded superplasticizer widely used in the Three Gorges project can make the initial setting time of concrete more than 6-8 hours by increasing the dosage properly, which can meet the needs of large warehouse concrete construction in the high temperature season of about 35 ℃.
4. Air entraining agent
Air entraining agent is a kind of surface active substance, which is one of the common additives for concrete. It can introduce a large number of uniformly closed small bubbles from the atmosphere during the mixing process of concrete, and make the concrete contain a certain amount of air. Good air entraining agent can introduce as many as 1 billion bubbles into the concrete, and the pore diameter is mostly 0.05-0.2 mm. Generally, it is a discontinuous closed sphere with uniform distribution and good stability, which can significantly improve the frost resistance and durability of the concrete (the number of cycles of frost and thaw resistance of the internal concrete of the Three Gorges project is more than 150, and that of the external concrete is more than 300).
5. Compound additive
Composite additives are additives with more than two main functions, such as retarding and water reducing functions of retarding water reducer, air entraining and water reducing functions of air entraining water reducer.
Many hydropower projects, especially the Three Gorges project, combine two kinds of additives, such as retarding superplasticizer and air entraining agent, which have the functions of high-efficiency water reduction, air entrainment and retarding, and have achieved good results. They not only meet the requirements of retarding concrete pouring on large warehouse surface, but also achieve the purpose of reducing water and improving durability.
