Concrete admixtures refer to substances added to improve and adjust the performance of concrete. The application of concrete admixtures in engineering has received more and more attention. The addition of admixtures can play a certain role in improving the performance of concrete, but the selection, addition method and adaptability of admixtures will seriously affect its development.
Concrete admixtures are divided into four categories according to their main functions:
Concrete admixture is a material added before or during the mixing process to improve the performance of freshly mixed and hardened concrete. The characteristics of concrete admixtures are low dosage (<5%) and great effect (essential components of concrete)
2.1 An admixture to improve the rheological properties of concrete mixtures. Including various water reducing agents, air-entraining agents and pumping agents.
2.2 An admixture that adjusts the setting time and hardening performance of concrete. Including retarder, early strength agent and accelerator, etc.
2.3 Additives to improve the durability of concrete. Including air-entraining agents, waterproofing agents and rust inhibitors, etc.
2.4 Additives to improve other properties of concrete. Including air-entraining agents, expansion agents, coloring agents, antifreeze agents, waterproofing agents and pumping agents.
The initial use of admixtures was just to save cement. With the development of construction technology, admixtures have become the main measure to improve the performance of concrete.
Thanks to the superplasticizer, high-fluidity concrete, self-compacting concrete, and high-strength concrete are used; thanks to the thickener, the performance of underwater concrete is improved: thanks to the retarder, the setting time of cement is prolonged , It is possible to reduce the slump loss and extend the construction operation time: due to the antifreeze, the freezing point of the solution can be lowered, or the ice crystal structure will not cause frost damage. It is possible to carry out construction at negative temperature.
In general, admixtures have the following effects in improving the performance of concrete:
(1) The water consumption of concrete can be reduced. Or the fluidity of concrete can be increased without increasing the water consumption.
(2) The setting time of concrete can be adjusted.
(3) Reduce bleeding and segregation. Improve workability and resistance to washing with water.
(4) It can reduce the slump loss and increase the pumpability of pumping concrete.
(5) Can reduce shrinkage. Adding expansion agent can also compensate for shrinkage.
(6) Delay the initial hydration heat of concrete. Reduce the temperature rise rate of mass concrete and reduce the occurrence of cracks.
(7) Improve the early strength of concrete. Prevent freezing under negative temperature.
(8) Improve strength, increase frost resistance, impermeability, abrasion resistance and corrosion resistance.
(9) Control the alkali-aggregate reaction. Prevent the corrosion of steel bars and reduce the diffusion of chloride ions.
(10) Make concrete with other special properties.
(11) Reduce the viscosity coefficient of concrete, etc.
After adding an admixture to concrete, due to different types, the effects are different, most of which are physical effects, such as adsorption on the surface of cement particles to form an adsorption film, which changes the potential and produces different suction or repulsion; some will destroy Flocculation structure improves the stability of cement diffusion system and improves the conditions of cement hydration: some can form macromolecular structure and change the adsorption state of cement particles; some will reduce the surface tension and surface energy of water, etc.: there are a few It directly participates in chemical reactions and generates new compounds with cement.
Because admixtures can effectively improve the performance of concrete and have good economic benefits, they have been widely used in many countries and have become indispensable materials in concrete. Especially the use of high-performance water-reducing agent. Cement particles can be fully dispersed, water consumption is greatly reduced, and the potential of cement is fully utilized. As a result, the cement stone is denser, and the pore structure and microstructure of the interface area are well improved. The physical and mechanical properties of concrete have been greatly improved, whether it is water impermeability, chloride ion diffusion, carbonization, sulfate erosion resistance, impact resistance, abrasion resistance and other aspects are better than concrete without admixtures. It not only improves the strength and workability. It can also improve the durability of concrete. Only by mixing high-efficiency water-reducing agents to formulate high-performance concrete with high workability, high strength and high durability can it be realized.
The commonly used concrete admixtures are mainly naphthalene series superplasticizers, polycarboxylic acid superplasticizers and aliphatic superplasticizers.
Naphthalene series superplasticizer: Naphthalene series superplasticizer is a non-air-entraining superplasticizer synthesized by chemical industry. The chemical name is naphthalenesulfonate formaldehyde condensate, which has a strong dispersing effect on cement particles. For the preparation of large-flow concrete, cast-in-place concrete and prefabricated components with early strength and high strength requirements, have a good use effect, can comprehensively improve and improve various properties of concrete, and are widely used in highways, bridges, dams, and ports Wharf, tunnel, power, water conservancy and civil construction projects, steam curing and natural curing prefabricated components, etc.
1. Find the best dosage through experiments, the recommended dosage is 1.5-2%;
2. The aliphatic superplasticizer is added to the concrete together with the mixing water, or a post-addition method can also be adopted, and the aliphatic superplasticizer concrete should be stirred for 30 seconds;
3. As the water reduction rate of aliphatic superplasticizer is relatively large, a layer of yellow paste will be secreted on the surface before the initial setting of the concrete, which is a normal phenomenon. After the concrete is polished, the color will be eliminated, or after the concrete is strengthened, the color will be eliminated naturally, and the color will be eliminated faster by watering and curing, without affecting the internal and surface properties of the concrete.
Admixtures can improve the anti-carbonation performance of concrete. There is a certain linear relationship between the carbonation depth and porosity of concrete; admixtures have a great influence on the frost resistance of concrete. Concrete with air-entraining water-reducing agent is anti-freezing The performance is better, but the early strength agent alone has no effect on improving the frost resistance of concrete.
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