Cause analysis of abnormal setting (dry material) of concrete

Concrete mixer

The abnormal setting time of concrete is generally manifested as excessive retarding, rapid setting and false setting. There are many factors affecting the abnormal setting of cement concrete. Concrete admixtures, cement, admixtures, ambient temperature and wind speed may eventually lead to the abnormal setting of cement concrete. In terms of the quality of concrete engineering at the present stage, the main causes of abnormal setting of cement concrete and even concrete engineering accidents are the variety and amount of retarder, the incompatibility between admixture and cement, and ambient temperature.

Causes of excessive retarding of concrete

1. The retarder components in concrete admixtures are excessive, especially when the content of sucrose is high;

2. Over mixing of concrete admixtures caused by human or mechanical failure;

3. Powdered ash or mineral powder is misused as cement;

4. Improper design of concrete mix proportion and excessive admixtures, especially when the temperature of concrete pouring environment is low;

5. The temperature is too low due to the influence of air temperature;

6. The maintenance is not in place, especially when the temperature is too low;

7. The air content of concrete is too large, resulting in the air content of concrete reaching 8-10%;

8. The concrete slump is too large, even segregation and bleeding, resulting in high fly ash content and high water cement ratio on the concrete surface;

Causes of rapid setting of concrete

1. No retarder is added to the concrete admixture or the retarding dose is not enough;

2. For cement with anhydrite and fluorinated gypsum as setting agent, when lignin sulfonate water reducer is used, it is easy to produce rapid setting of concrete;

3. Low water binder ratio, inadequate maintenance, high ambient temperature, strong wind, premature water loss of concrete and accelerated cement hardening;

4. Sulphoaluminate cement or high alumina cement is used;

5. The content of C3a and alkali in cement is high;

Causes of false setting of concrete

1. The ambient temperature is too high, the wind is strong, the concrete loses water too early, the maintenance is not in place, and the concrete is not covered in time. Especially when the carboxylic acid admixture is used to prepare high-grade concrete, the concrete surface loses water and hardens quickly, and the concrete at the lower layer is still relatively soft and does not set;

2. The adsorption of cement, admixtures, sand and gravel aggregates on concrete admixtures suddenly increases with time, pump pressure and ambient temperature, the slump is quickly lost, and the concrete is very scattered like bean curd residue. It is mainly related to the change of adsorption capacity of cement and other additives and their types;

3. When the cement temperature is too high, the dihydrate gypsum in the cement is dehydrated into hemihydrate gypsum or anhydrous gypsum;

terms of settlement

1. In summer, it is recommended not to use cement with a temperature of more than 80 ℃ to mix concrete in the concrete mixing plant;

2. In summer, use underground cold water (add ice if necessary);

3. The concrete shall be cured in time to avoid premature water loss or freezing of the concrete;

4. Before the construction of the project, the adaptability test of cement and concrete admixtures shall be conducted, the fluidity loss of mixed concrete shall be observed, the setting time of concrete under the current temperature shall be investigated, and the suitability of concrete admixtures shall be judged;

5. Maintenance and reliable operation of instruments and equipment in the mixing plant;

6. Concrete admixture products shall be inspected, reliable and stable in quality;

7. Proper concrete mix proportion;

8. In case of abnormal setting concrete, the specific problems shall be analyzed in detail. If it endangers the project quality, it shall be chiseled sooner or later, the sooner the better. Generally speaking, if the excessive retarding exceeds 4 days, the loss of concrete strength is more serious; In case of false setting of concrete, cement mortar with the same strength grade of concrete can be used for re mixing before construction and pouring, or the mortar can be used to finish the South plastering of concrete surface. After rapid setting, if the concrete is not poured into the formwork, it shall be resolutely abandoned. If it is poured into the formwork, it shall be chiseled or retained according to the specific situation.