Why the use of admixtures is the most effective measure to improve concrete

Admixtures are mainly divided into water reducing agents, early strength agents, air-entraining agents, retarders, pumping agents, etc.

Water reducer: add less water consumption under the condition of ensuring the same working properties (slump) of concrete.

Air entraining agent: increase the air content of concrete, which is conducive to impermeability.

Retarder: prolongs the setting time of concrete, which is conducive to the construction of large-volume concrete, and the concrete with long transportation distance and long construction time.

Pumping agent: As the name suggests, I won't talk about it.

The working mechanism of admixtures I won't say much if I'm not professional, so as not to smash my feet with an axe The above can be seen that by adding admixtures, we can make a certain aspect of the concrete work performance enhance or weaken according to our own wishes, which is why admixtures are the most effective measures to improve concrete.

The addition of admixtures can improve the fluidity, workability, water retention and compressive strength of concrete on the basis of basic concrete. Special admixtures can also serve special purposes.

To put it simply, the strength index of cement concrete meets the design requirements at the same time, the working performance of concrete is very important, and workability can not or is not conducive to on-site operation of concrete, the site can not be used is also waste, the purpose of adding admixtures is mainly to reduce the water-cement ratio to improve the workability of concrete, cement hydration itself needs 20% of the water is not needed so much, there is no external concrete in most of the water is used to adjust the workability (and workability), In the concrete construction of large slump, it is obvious that there is obvious water leakage phenomenon without admixtures, water is the lightest floating in all concrete substances, and it is very common to add admixtures to achieve workability at the same time, greatly reduce water consumption, reduce the cohesion of water-cement than concrete, and can also be significantly improved, but the adaptability of cement and admixtures should be determined by the test.

Answer]: B2021 2020 2019 Textbook Chain Rough P37

Admixtures to improve the workability of coagulation jujube soil, including water reducer, air-entraining agent for shandong town, pumping agent, etc.

Answer]: B retarder is an admixture that can maintain the workability of concrete for a long time and delay the setting and hardening time of concrete.

Concrete admixtures:

Abbreviated as admixtures. Substances used to improve the properties of concrete are incorporated in the process of mixing concrete. The content is generally not greater than 5 of the cement mass.

It is divided into four categories according to the main functions:

1) Admixtures to improve the fluid deformation properties of concrete mixture, including various water reducing agents, air-entraining agents and pumping agents;

2) Admixtures that adjust the setting time and hardening properties of concrete, including retarders, early strength agents and quick-setting agents;

3) Admixtures to improve the durability of concrete, including air-entraining agents, waterproofing agents and rust inhibitors;

4) Admixtures to improve other properties of concrete, including aerating agents, expansion agents, antifreeze agents, colorants, waterproofing agents and pumping agents, etc.

When the concrete is too dry, it needs to be adjusted with admixtures and a little water. The adjusted concrete should be poured immediately, and if the time is too long, the concrete will dry out again, and it will be drier (because at this time, the role of the original admixture has been completed, and the new admixture does not contribute much to the slump loss, but only releases some excess water, and the slump loss will be aggravated). Therefore, it is mostly adjusted before concrete pouring with admixtures, and played as soon as possible after adjustment.

Concrete admixture: a substance added to improve and regulate the properties of concrete.

Admixtures of concrete refer to substances that can significantly improve the properties of concrete incorporated in the concrete mixing process. Its content is generally not more than 5% of the cement mass. Due to the improvement of the performance of admixtures on concrete, it is used in engineering to account for more and more proportions, and the use of admixtures in many countries has accounted for 60% 90% of the total amount of concrete, therefore, admixtures have gradually become the fifth component of concrete.

1. Classification of admixtures.

Concrete admixtures:

There are many types, according to the "Classification, Nomenclature and Definition of Concrete Admixtures", concrete admixtures are divided into four categories according to their main functions:

1.Admixtures to improve workability: water reducer, pumping agent, air-entraining agent.

2.Admixtures that regulate the time of coagulation and hardening: retarders, early strength agents, and quick-setting agents.

3.Admixtures to improve durability: rust inhibitors, water repellent agents, air-entraining agents.

4.Admixtures to improve other properties: aerated agents, colorants, bulking agents, antifreeze.

2. Water reducer.

The superplasticizer refers to the addition of an admixture that can significantly reduce the amount of water used in the mixing under the condition that the concrete slump is basically the same.

1) Mechanism of action of superplasticizer.

The superplasticizer is a surfactant.

Its molecule is composed of two parts: a hydrophilic group and a hydrophobic group. The cement is mixed with water, and the cement slurry forms a flocculation structure, which wraps a part of the mixed water and reduces the fluidity. The mechanism of action of the superplasticizer is manifested in the following three aspects:

1) The hydrophobic group is oriented to adsorb on the surface of the cement particles, and the hydrophilic group points to the aqueous solution, so that the surface of the cement particles has the same charge, and the repulsion effect separates the cement particles, releases the free water of the flocculation structure, and increases the fluidity;

2) The hydrophilic group adsorbs a large number of polar water molecules.

Increase the thickness of the solvated water film on the surface of cement particles, play a lubricating role and improve workability;

3) Superplasticizer reduces surface tension.

Cement particles are easier to wet, so that the hydration is more sufficient, thereby improving the strength of the concrete.

Figure: Schematic diagram of the flocculation structure of cement slurry and the action of superplasticizer.

2) The technical and economic effects of superplasticizers.

1.Increase liquidity. in water consumption and water-cement ratio.

When unchanged, the concrete slump.

It can be increased by 100 200mm without affecting the strength of the concrete.

2.Increase the strength of concrete. Under the condition of keeping the fluidity and cement dosage unchanged, the mixing water consumption can be reduced 10 15, thereby reducing the water-cement ratio and making the concrete strong.

Increase by 15 to 20.

3.Cement saving. Under the condition of maintaining the fluidity and water-cement ratio, the amount of cement can be reduced while reducing the amount of mixing water.

4.Improve the durability of concrete.

Relatively speaking, the lower the water-cement ratio of concrete, the higher the strength of its concrete. But the low water-cement ratio is easy to lead to insufficient water consumption for concrete mixing, in order to solve the problem that the water consumption of concrete can reach the amount of water used for mixing under the condition of low water-cement ratio, it is necessary to add water reducer admixture to the concrete to increase the strength of concrete.

I have not heard of any admixtures to improve the strength of concrete, only heard that early strength agents can improve the early strength of concrete.

The use of admixtures in concrete is mainly to improve the properties of a certain aspect of concrete. For example, adding retarder to improve the setting time of concrete, adding expansion agent to reduce concrete cracks, and adding antifreeze agent to make concrete hydrate normally at negative temperature. Wait a minute.

Nowadays, water reducing agents are basically added to concrete, mainly to save costs and improve workability and strength.

Concrete admixtures need to be used when preparing concrete with special requirements, and concrete admixtures with different properties can be added according to the actual preparation of the use requirements! Such as: anti-freezing, waterproof, early strength, anti-cracking and so on.

In order to effectively improve the workability, strength and durability of concrete; To reduce the deformability of concrete, sometimes in order to make the concrete quickly coagulate, or delay the coagulation, an appropriate amount of admixtures can be added, so as to improve the working performance of the concrete and meet the construction requirements.

1. For example, when high-rise construction concrete, admixtures should be added to the concrete, the purpose is to slow down the setting and improve the workability of the concrete, and there is no effect on the strength of the concrete.

2. When pouring concrete for basement slab walls, anti-cracking fiber materials and impermeable agents are generally added to the concrete.

3. During the construction of concrete in water conservancy, anti-chloride ion admixtures should be added to the concrete, which is what we often call marine concrete.