During the preparation process of concrete, mortar and paste, a small amount (not exceeding 5% of the cement dosage) is added into a product that can change the performance of concrete, mortar or paste, which is called concrete admixture.
Adding appropriate admixtures to concrete can improve concrete quality, improve concrete performance, reduce concrete water consumption, save cement, reduce costs, and speed up construction progress. With the advancement of technology, admixtures have become the fifth essential material in addition to cement, coarse and fine aggregates, admixtures and water. Adding admixtures is an important measure to optimize the design of concrete ratio and improve the durability of concrete. Therefore, the newly revised "Hydraulic Concrete Construction Specifications" (DL/T5144-2001) emphasizes that appropriate admixtures must be added to hydraulic concrete.
1 Classification of admixtures
Concrete admixtures can be divided into the following four categories according to their main functions:
(1) Improve the flow properties of concrete mixtures Admixtures. Including ordinary water reducing agent and high-efficiency water reducing agent, air entraining agent and pumping agent, etc.
(2) Admixtures that adjust the coagulation time and hardening properties of concrete. Contains retarder, early strength agent and accelerating agent, etc.
(3) Admixtures that improve the durability of concrete. Contains air-entraining agents, waterproofing agents, rust inhibitors, etc.
(4) Admixtures that improve other properties of concrete. Contains expansion agents, antifreeze, colorants, etc.
The types of admixtures commonly used in 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 water-reducing agents. , early-strength water-reducing agent, air-entraining water-reducing agent, and other types of admixtures are also mixed according to special needs, such as pumping agents, waterproofing agents, antifreeze, etc. National and industry standards such as GB8076-1997 "Concrete Admixtures" and DL/T5100-1999 "Technical Specifications for Hydraulic Concrete Admixtures" have strict requirements on the performance indicators and technical specifications of these admixtures, which can be based on the different needs of concrete. Make selection.
2 Water-reducing agent
Water-reducing agent is also called plasticizer or dispersant. Adding appropriate water-reducing agents when mixing concrete can disperse the cement particles evenly and release the moisture wrapped in the cement particles, thereby significantly reducing the water consumption of concrete.
The function of the water-reducing agent is to improve the workability of the concrete while maintaining the same mix ratio; or to reduce the water consumption and improve the strength of the concrete while maintaining the same workability; or to maintain the concrete strength. When the strength remains unchanged, the cement consumption can be reduced to save cement and reduce costs. At the same time, adding water-reducing agent to the concrete will be more uniform and dense, improving a series of physical and chemical properties, such as impermeability, frost resistance, corrosion resistance, etc., and improving the durability of the concrete.
In the past, the water-reducing agents used in hydraulic concrete were usually ordinary water-reducing agents such as pulp waste liquid, calcium lignin, and molasses. The water-reducing rate was not high, usually 5% to 10%. As the quality requirements for hydraulic concrete improve, the quality requirements for water-reducing agents are also getting higher and higher. Naphthalene-based high-efficiency water-reducing agents are widely used in large hydropower projects such as Ertan and Three Gorges. The water-reducing rate is as high as 20% to 30%. They are mainly used to make high-strength and high-flow concrete or concrete that requires a significant reduction in water consumption. High-efficiency water reducing agents play a significant role in improving cement utilization efficiency and can save about 20% of cement consumption. When the granite artificial aggregate concrete of the Three Gorges Project uses ordinary water-reducing agents, the water consumption is as high as about 110 kg/m3. The optimized ZB-1A high-efficiency water-reducing agent is used, and is jointly mixed with DH9 air-entraining agent and Class I fly ash. , the water consumption is reduced to about 85 kg/m3, reaching the advanced level at home and abroad. The Three Gorges Project also used acrylic high-efficiency water-reducing agent X404 with a higher water-reducing rate of more than 30%. However, this type of water-reducing agent is expensive and is mainly used in high-strength concrete parts.
3 Retarder
Retarder can delay the coagulation and hardening time of concrete, which facilitates construction; it can slow down the hydration rate of concrete slurry and prolong the hydration heat release process, which has the effect of Conducive to temperature control of large-volume concrete. Retarder will reduce the initial strength of concrete for 1 to 3 days, but has no effect on the normal development of later strength.
Usually retarders can extend the initial setting time of concrete by 1 to 4 hours, but this is not suitable for concrete construction in large warehouses at high temperatures. In order to meet the needs of large-volume 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. This retarder can be used when the temperature is (35+2) ℃ and the relative humidity is (60 Under the condition of +5)%, the initial setting time of concrete is 6 to 8 hours. At present, many retarding and high-efficiency water-reducing admixtures are used in projects such as the Three Gorges. By adding appropriate dosage, the initial setting time of concrete can reach more than 6 to 8 hours, which meets the needs of concrete construction in large warehouses at high temperatures around 35°C.
4 Air-entraining agent
Air-entraining agent is a surface-active substance and one of the commonly used concrete admixtures. It can cause concrete to be introduced from the atmosphere during the mixing process. Many small air bubbles that close evenly make the concrete contain a certain amount of air. A good air-entraining agent can introduce as many as 1 billion bubbles into concrete, with pore diameters ranging from 0.05 to 0.2 mm. They are usually discontinuous closed spheres with even distribution and good stability. This can significantly improve the frost resistance of concrete. , Durability (the internal concrete of the Three Gorges Project can resist freeze-thaw cycles of more than 150 times, and the external concrete can resist more than 300 times); at the same time, it can also improve the workability of concrete,Especially when the artificial aggregate or natural sand has coarse particles, poor gradation, and poor cement concrete, it has a very good effect; it can improve the bleeding and segregation of concrete; reduce the permeability of concrete, and improve the corrosion resistance of concrete.
The dosage of air-entraining agent is usually in the range of 0.3 to 20,000 of the cement content. Since the dosage is small, it must be accurately weighed and mixed evenly. In addition, there are many factors that affect the amount of air entrainment, such as water-cement ratio, cement dosage, sand rate, aggregates, vibration methods, mixing time, collapse degree, molding temperature, etc., all of which need to be strictly standardized, otherwise the desired effect will not be achieved. .
5 Composite admixtures
Composite admixtures are admixtures with more than two main functions. For example, retarding water-reducing admixture has both retarding and water-reducing functions. The water agent has both air-entraining and water-reducing functions.
Many hydropower projects, especially the Three Gorges Project, use two kinds of admixtures in combination, such as retarding high-efficiency water-reducing admixture and air-entraining agent, which together have the functions of efficient water reduction, air entrainment, and retardation. Excellent results have been achieved, which not only meets the requirements for retarding the concrete poured on the large warehouse surface, but also achieves the purpose of reducing water and improving durability.
6 Criteria for Selection of Concrete Admixtures
What kind of admixtures should be used in a project should be carefully selected according to the engineering design and construction technology requirements before the project starts, and strictly based on the raw materials. The adaptability experiment proves the conclusion. In the early stage of the construction of the Three Gorges Project, more than 30 types of products produced by nearly 30 admixture standard manufacturers across the country were initially tested in accordance with national standards. On this basis, several products with excellent quality were selected by 3 Qualified experimental units conducted all concrete adaptability experiments based on the raw materials of the Three Gorges Project. After sufficient verification and rigorous evaluation, they finally selected 2 to 3 admixture types with excellent quality and suitable for the actual conditions of the Three Gorges Project. They were satisfactory. It has met the construction needs of large warehouse area and high-strength concrete, improved various functions of concrete, and achieved outstanding economic benefits.
In order to facilitate management, it is appropriate to select 1 to 2 types of similar admixtures (including backup) for a large or medium-sized project. Under normal circumstances, do not replace the types of admixtures casually during project construction.
Compared with other raw materials, although the dosage of admixtures is small, it is crucial to the quality of concrete. Therefore, after its dosage is proved and determined through experiments, it should be strictly controlled, and the quality of admixtures is extremely important. Stability should be strictly inspected during delivery and use in accordance with relevant standards, and the transportation and storage of concrete admixtures should also be strictly implemented in accordance with relevant standards.
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