(2) Case 2

The slump of the concrete mixture suddenly became larger and bleeding, and it was returned to the construction site. A mixing station used P · o42.5r cement from a cement plant to prepare C30 concrete. The contract required the slump to be 150 mm ± 30 mm at the construction site. When the concrete was delivered from the factory, the measured slump was 180 mm. After it was delivered to the construction site, the measured slump was 210 mm. Two consecutive trucks of concrete were returned, Return to the factory to verify that the slump is still 210mm, and there is bleeding and stratification.

reason:

1. The cement has good adaptability with this water reducing agent, and the dosage of water reducing agent is slightly larger.

2. The mixing time is not enough, and the slump of concrete is not the real slump due to the short mixing time.

prevention:

1. For the cement which is sensitive to the dosage of admixture, the dosage of admixture should be appropriate and the measurement accuracy should be high.

2. Properly extend the mixing time, even if the double horizontal shaft forced mixer, the mixing time should not be less than 40 seconds, preferably more than 60 seconds.

(3) Case 3

The dosage of water reducing agent is too large, and there are too many bubbles on the surface of concrete structure. A mixing station has been using polycarboxylate water reducing agent to prepare concrete for a period of time. Suddenly one day, a construction site reflected that there are too many bubbles on the surface of shear wall after removing the formwork, and the perception is too poor.

reason:

1. On the day of concrete pouring, the site repeatedly reflected that the slump was small and the fluidity was poor, and the personnel on duty in the laboratory of the concrete mixing station increased the amount of admixture.

2. The large shaped steel formwork is used in the construction site. Too much material is poured at one time and the vibration is uneven.

prevention:

1. Polycarboxylate superplasticizer and naphthalene superplasticizer also have a saturation point. For different kinds of cement and different cement dosage, the saturation point of the admixture in concrete is different. If the amount of admixture is close to its saturation point, the fluidity of concrete mixture can only be improved by adjusting the amount of slurry in concrete or other methods.

2. Strengthen the communication with the construction site. It is suggested that the height of one-time feeding and the vibration method should be operated in strict accordance with the specification requirements.