With the development of technology, concrete admixtures have become the fifth necessary material in addition to concrete, large and small stones, admixtures and water. Additives are the key to reliable design of concrete mix proportions and improvement of concrete performance. this key strategy. Therefore, the newly revised "Hydraulic Concrete Pouring Standard" (DL/T5144-2001) emphasizes that appropriate admixtures must be added to hydraulic concrete.
1. Classification of concrete admixtures
Concrete admixtures can be divided into the following four categories according to their key functions:
(1) Improve concrete mixing freight logistics Admixtures with variable properties. Including general water-reducing agents and high-efficiency water-reducing agents, air-entraining agents and underwater concrete agents, etc.
(2) Admixtures that adjust the setting time and hardening characteristics of concrete. Contains retarder, retardant and accelerating agent, etc.
(3) Admixtures that improve the performance of concrete. Contains air-entraining agents, waterproof coatings, rust inhibitors, etc.
(4) Admixtures that improve other properties of concrete. Contains expansion agents, concrete antifreeze, additives, etc.
The common types of admixtures for hydraulic concrete mainly include water-reducing agents, retarders, air-entraining agents, and various composite admixtures, such as retarding water-reducing agents or retarding high-efficiency admixtures. Water reducing agent, early strength water reducing agent, air entraining water reducing agent, and other types of admixtures are also mixed according to special needs, such as underwater concrete agent, waterproof coating, concrete antifreeze, etc.
2. Water-reducing agent
Water-reducing agent is also known as plasticizer or thickener. Adding magnesium water-reducing agent when mixing concrete can disperse the concrete particles evenly and release the moisture encapsulated in the concrete particles, thereby significantly reducing the water demand of the concrete.
The function of the water-reducing agent is to improve the workability of the concrete while maintaining the mix ratio; or to reduce the water demand and improve the rebound of the concrete while maintaining the workability; Or reduce the amount of concrete used while maintaining the compressive strength without changing, saving concrete and controlling costs. At the same time, adding water-reducing agent to concrete will make it more uniform and dense, improve a series of physical properties, such as impermeability, cold resistance, adhesion resistance, etc., and improve the performance of concrete.
3. Retarder
Retarder can slow down the solidification and hardening time of concrete, which is beneficial to engineering construction; it can slow down the hydration rate of concrete slurry and increase the water content. The whole process of chemical exothermic reaction is beneficial to the temperature control of large-volume concrete. The retarder will slightly reduce the initial strength of concrete in 1 to 3 days, but it will not affect the normal development of strength in the later stages.
Usually retarders can increase the initial setting time of concrete by 1 to 4 hours, but this is insufficient for concrete pouring in large warehouses under high temperature conditions. In order to meet the construction needs of large-volume concrete projects in high-temperature areas and high-temperature seasons, the national "Eighth Five-Year Plan" scientific and technological research project has developed high-temperature retarder. This type of retarder can be used when the average temperature is (45+2)℃ and the air humidity is The initial setting time of concrete under the standard of (65+5)% is 6 to 8 hours. Currently, many retarding and high-efficiency water-reducing admixtures used in Xilingxia and other engineering projects can make the initial setting time of concrete exceed about 6 to 8 hours by appropriately increasing the dosage. Taking into account the large warehouse surface during high temperatures around 45°C Concrete pouring is required.
4. Air-entraining agent
Air-entraining agent is a surface-active chemical substance and one of the four common admixtures for concrete. It can remove air from the concrete during the entire mixing process. A large number of uniformly closed small bubbles are introduced into the concrete to make the concrete contain enough gas. A good air-entraining agent can introduce as many as 1.2 billion bubbles into concrete, with diameters ranging from 0.05 to 0.3 mm. They are usually discontinuous closed spheres with even distribution and good reliability, which can significantly improve the quality of concrete. Cold resistance and performance (the freeze-thaw cycle resistance of the concrete inside the Three Gorges Project is about 130 times, and that of the external concrete is about 400 times).
5. Composite admixtures
Composite admixtures are admixtures with two main functions. For example, retarding water-reducing admixture has both retarding and water-reducing functions. Air-entraining and water-reducing agents have both air-entraining and water-reducing effects.
Many hydropower installation projects, especially the Three Gorges Project, use a combination of two types of admixtures, such as a combination of retarding high-efficiency water-reducing admixture and air-entraining agent, which simultaneously have high-efficiency water reduction, air entrainment, and The retardation effect has achieved very good results, which not only meets the requirements for concrete retardation in large warehouse surface concrete pouring, but also achieves the purpose of reducing water and improving performance.
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