Abstract: Adding admixtures to concrete can improve the rheological properties of the concrete mixture, which has the characteristics of less investment, quick results, and significant technical and economic benefits. However, improper use of concrete admixtures will cause accidents, so what is the method of using concrete admixtures? What are the precautions for using concrete admixtures? Let’s find out
1. How to use concrete admixture
1.1 What should be paid attention to when using concrete admixtures:
- For preparation and measurement, there are two forms of admixtures, powder and liquid. The liquid is measured by volume or mass, and an aqueous solution of a certain concentration is prepared before use. The powder generally has a carrier, such as fly ash, volcanic ash, the measurement must be accurate, and special attention should be paid to its compatibility and the use of air-entraining agent.
- The mix ratio of concrete should meet the technological needs and determine the appropriate mix ratio.
- The construction technology of concrete, such as the workability of concrete.
- For the maintenance of concrete, choose the appropriate maintenance method according to the strength of the concrete in the early and late stages to prevent freezing damage. There must be a reasonable maintenance system. After steaming and stacking, it should be watered and maintained to further improve its performance.
1.2 Adding method of concrete admixture
1.2.1 First mixing method
The powdery admixture is mixed with cement first, and then the aggregate and water are mixed together, which is called the first mixing method. This method is beneficial to the dispersion of the admixture and can reduce the adsorption of the aggregate to the admixture, but it is inconvenient to use in practical engineering and is often used in laboratory experiments.
1.2.2 Blending method
The liquid and powder admixtures are put into the mixer together with the concrete constituent materials for mixing, or the liquid admixtures are mixed with water first, and then mixed with other materials. This method is simple and easy to operate, so that when the concrete is initially hydrated, there is an admixture to intervene, and it is immediately adsorbed to the surface of the cement particles, thereby rapidly reducing the concentration in the liquid phase.
1.2.3 Post-mixing method (stagnation method)
After the concrete is mixed with water for a period of time (sometimes before pouring), the admixture is added for further mixing, that is, after the cement hydration reaction has been carried out for a period of time, the admixture does not participate in the hydration reaction, thus maintaining the admixture in the concrete liquid phase. The concentration of the agent will not decrease anytime soon.
1.2.4 Fractional Participation Method
Add the admixture to the concrete mixture several times in the process of concrete mixing or transportation, so that the concentration of the admixture in the liquid phase of the concrete is always maintained at a certain level. Under the same conditions, the post-blending method and the step-by-step adding method are very effective in reducing the slump loss of the mixture, and can reduce the amount of water reducing agent, especially for the high content of C3A and C4AF in cement deposits. And the effect of fresh cement is the most significant, the main reason is that different cement deposits have different adsorption properties to water reducers, and the order of the adsorption effect of cement deposits on water reducers is: C3A>C4AF>C3S>C2S
2. Matters needing attention in the use of concrete admixtures
- Concrete with high freeze-thaw resistance requirements must be mixed with air-entraining agent or air-entraining superplasticizer, and its dosage should be determined through experiments according to the air content requirements of the concrete.
- Contains toxic antifreeze agents such as hexavalent chromium salt and nitrite, and is strictly prohibited from being used in drinking water projects and parts in contact with food.
- The types and parameters of concrete admixtures must be determined through tests based on the performance requirements of concrete, construction and climate conditions, raw materials used in concrete, mix ratios, and adaptability to cement.
- In steam-cured concrete and prestressed concrete, it is not suitable to mix air-entraining agent or air-entraining superplasticizer.
- When different types of admixtures are used in combination, attention should be paid to their compatibility and their influence on concrete properties. Tests should be carried out before use to meet the requirements before use.
- For concrete in contact with water or in a humid environment, when using alkali-active aggregates, the alkali content (calculated as equivalent sodium oxide) brought by the admixture should not exceed 1kg/m3 of concrete, and the total alkali content of concrete should still be Comply with relevant standards.
- The selected admixture should have the following technical documents provided by the supplier:
(1) Product instructions, and shall indicate the main ingredients of the product.
(2) Factory inspection report and certificate of conformity.
(3) Performance test report of admixture concrete.
- It is strictly forbidden to use concrete admixtures that are harmful to the human body and pollute the environment.
- It is strictly forbidden to use early strength agents and early strength water reducers prepared with chloride salts in the following structures:
(1) Prestressed concrete structure.
(2) Structures used in environments where the relative humidity is greater than 80%, structures located in places where water levels change, open-air structures, and structures that are often showered and washed by water.
(3) Mass concrete structure.
(4) Structures in direct contact with acid, alkali or other corrosive media.
(5) Reinforced concrete prefabricated components that are often exposed to temperatures above 60°C and require steam curing.
(6) Concrete with decorative requirements, especially concrete with consistent color or metal decoration on the surface.
(7) Thin-walled concrete structures, beams, roof trusses and other structures of intermediate and heavy duty cranes.
(8) Structures using cold-drawn steel bars or cold-drawn low-carbon steel wires.
(9) Aggregate has alkali active concrete structure.
- In the following concrete structures, it is strictly forbidden to use early-strength agents and early-strength water reducers containing strong electrolyte inorganic salts:
(1) Structures in contact with galvanized steel or aluminum and iron, structures with exposed steel bars and embedded iron parts without protective measures.
(2) The structure using DC power supply and the structure within 100m from the high-voltage DC power supply.
- Antifreeze agents containing nitrite and carbonate are strictly prohibited in prestressed concrete structures.
- Concrete admixtures of different varieties should be stored separately and marked, and no sundries or contamination should be allowed during transportation and storage.