Concrete admixture is an indispensable part of modern concrete, and it is also a method and technology of concrete modification. It can improve the physical and mechanical properties of hardened concrete, improve the quality and durability of building components, save cement, reduce costs and speed up the progress of the project. With the development of modern buildings to high-rise buildings and the development of ready-mixed concrete to concentrated mixing, the application of concrete admixtures is becoming more and more extensive, but if the application of admixtures is improper, it often fails to achieve the expected results, or even quality accidents.

Misunderstanding one  The admixture is directly put into use without inspection.

Before the concrete construction, the construction unit and the supervision unit always send the cement, sand, stone and other building materials products to the inspection agency for inspection according to the specification requirements, but often do not pay enough attention to the performance inspection of the admixture. In fact, there are many kinds of concrete admixtures, and there are corresponding quality standards. Its performance directly affects its role in concrete, such as the size of the water reduction rate, the amount of chloride ion content, etc. Article 7.2.2 of the Code for Acceptance of Construction Quality of Concrete Structures (GB 50204) emphasizes that the quality and application technology of admixtures in concrete shall conform to the provisions of Concrete Admixtures (GB8076), Technical Code for Application of Concrete Admixtures (GB50119) and other provisions on environmental protection, and stipulates the quantity and method of inspection. Explain in the description of the provision that this provision is mandatory and therefore admixtures cannot be used in concrete without testing.

Misunderstanding 2: The admixture is not used at will according to the actual situation of the project

1. The purpose of use shall be considered in the selection of admixture

The admixture shall be selected according to the engineering materials and construction conditions through trial mixing test. All kinds of admixtures have their own characteristics, the use should be based on the characteristics of admixtures, combined with the purpose of use, such as according to save cement, improve concrete performance, speed up the template turnover and other comprehensive indicators to consider, that is, through technical and economic comparison to determine the use of admixture varieties.

2. Several kinds of concrete application of admixture analysis

(1) The purpose of the application of admixtures for waterproof concrete is to improve the impermeability of concrete. To improve the impermeability of concrete, the key is to reduce the pores inside the concrete, block the leakage path, and apply water reducing agent and air entraining agent.

(2) the key of mass concrete construction technology is to ensure the good combination of layered pouring concrete in the initial setting, reduce the temperature rise in the process of concrete hardening, avoid concrete cracking, the use of retarding water reducing agent can meet the above requirements, namely retarding and reducing the temperature rise in the process of concrete hardening, so as to improve the quality of concrete.

(3) in the large span prestressed concrete structure, the structure weight is often the main contradiction, so the purpose of the use of admixture is to prepare concrete, reduce the weight of the structure and the effective use of steel stress, should use water reducing agent, can greatly reduce the unit water consumption, improve the strength of concrete.

Misunderstanding three do not attach importance to the adaptability of admixture and cement

1. What is the adaptability of admixtures and cement

The adaptability of concrete admixtures and cement can be interpreted as an admixture that has been tested to meet the relevant standards is added to the concrete configured with cement that can use the admixture according to the regulations. If it can produce the desired effect, the cement and the admixture are suitable.

2. Examples of applications where admixtures and cement are not suitable

There are adaptability problems between almost all varieties of admixtures and cement. Such as the use of several ordinary Portland cement and mixed with some kind of water reducing agent (after inspection in line with the quality standards of water reducing agent) preparation of concrete, in the preparation conditions are the same, there is a kind of cement preparation of concrete in the water reducing rate of serious deficiencies, it shows that the cement and the water reducing agent is not suitable, while the other several kinds of cement and the water reducing agent to adapt. For example, when a certain cement prepared by the concrete mixed with the test to meet the requirements of the relevant quality standards of the accelerator but can not get the quick setting effect, or mixed with the retarder but can not get the retarding effect, can be considered to be caused by the admixture and cement do not adapt.

In terms of water reducing agent, different kinds of water reducing agent have different dispersion, water reducing and strengthening effects on cement. For the same kind of water reducing agent, due to the difference of cement mineral composition, mixed material type and dosage, alkali content, gypsum type and dosage, etc., the water reducing and strengthening effect is very different. For example, wood calcium and molasses water reducing agent, when it is used in cement concrete with dihydrate gypsum as setting agent, its setting is normal, however, when used in cement with anhydrite (CaSO4) as a moderator, condensation (pseudo-coagulation) occurs. There are also some cement due to insufficient amount of gypsum (but to reach the standard performance of cement) and water reducing agent is not compatible with the occurrence of rapid setting phenomenon.

3. Adaptability inspection of admixture and cement

The adaptability between admixture and cement is a complex and unavoidable problem, which affects its application effect and sometimes leads to serious engineering accidents and incalculable economic losses. Test the adaptability of the admixture to cement before using the admixture, and draw a curve with the admixture as the abscissa and the fluidity as the ordinate. The admixture with low saturation point, large fluidity and small economic loss has good adaptability to cement.