Example 1, cement dosage is partial more, concrete produces self-shrinking crack:
A project to connect the original building from the beginning of the 8th floor. The concrete construction site is shear wall, and the strength grade is C40. After removing the mold, it was found that the wall appeared vertical cracks, cracks are very regular, every 1.5m ~ 1.8m a vertical crack, crack tip at both ends, the middle width, the widest about 0.2mm ~ 0.3mm, individual cracks are through cracks. Concrete strength has reached the design requirements and even meet the requirements of C45.
Cause analysis:
1. The concrete mix ratio cement dosage is large, the concrete shrinkage produces the pull crack.
2. Design less stirrup, large spacing.
3. Concrete curing is not in place.
Prevention Measures:
1. The concrete mix ratio should reduce the amount of cement as far as possible under the premise of meeting the strength.
2. It is recommended to increase the amount of stirrup and reduce the spacing between stirrups.
3. Strengthen the maintenance, when the strength reaches 1N/mm2, can make the template from the concrete, and then close the template to continue curing to the final mold removal, which can strengthen the wet curing of concrete, prevent dry shrinkage caused by crack increase.


Example 2: The production of concrete specimens is not qualified:
The strength of concrete specimen in a certain project appears the problem of high and low, the deviation of concrete specimen is too big, the concrete strength is evaluated as unqualified, and at the same time, the same part, the same mix ratio of other sites of concrete is all evaluated as qualified, and the deviation of concrete is small.
Treatment: Contact the technical personnel of the mixing station and the construction unit for joint analysis.
Cause analysis:
1. The construction unit uses concrete test mold unqualified, test mold itself size error is large, some test mold diagonal error ≥3mm, so the specimen error is large.
2. The concrete specimen is made rough, not according to the relevant standards and specifications, and the specimen has the problem of missing edge and Angle.
3. Because the specimen itself is not qualified, so the concrete specimen in the press area does not meet the requirements of local compression, resulting in low strength.
Prevention Measures:
1. It is recommended that test personnel of construction units receive technical training to learn relevant test standards and specifications.
2. It is recommended that the construction unit replace the unqualified test mold. To adopt the test model should strengthen the detection, do not meet the requirements of the firm.


Example 3: Bubbles appear on the surface of concrete:
A project in the construction of the first floor beam, plate, column, concrete C30. When the concrete mold is removed, it is found that there are many bubbles of different sizes in the concrete parts of the wall panels and columns. Although the concrete strength can meet the design requirements, the bubbles affect the appearance.
Treatment: Repair the parts with large bubbles.
Cause analysis:
1. The admixture contains many entraining components.
2. Construction personnel in pouring concrete vibration time is short, did not grasp the vibrator should be "quick insertion and slow drawdown" to vibrate concrete.
3. Concrete pouring is too much, some bubbles float to the surface of concrete.
Prevention Measures:
1. Readjust the admixture formula, reduce the entraining component, and add antifoaming agent appropriately.
2. Train the concrete construction personnel.


It is strictly prohibited to use different grades of cement and fly ash in the same part of pouring:
C30 foundation concrete is produced in a mixing plant, with a total quantity of 2100m3 and a thickness of 1.1m. Belongs to large volume foundation concrete, when the concrete production to 1/2, because the ash of the fly ash bin has been used up, mixing station to solve the "pressing danger" temporarily from other power plants to transfer a car of Ⅲ grade fly ash and the original bin is used in Ⅱ grade fly ash. The second day after construction, it was found that the strength of the concrete produced with grade Ⅲ fly ash was obviously low, and the color of the concrete produced with grade Ⅲ fly ash was obviously different from that of other concrete.
Treatment: It is suggested that the construction unit strengthen concrete maintenance, and carry out CTBJ146 for acceptance of underground mass concrete, and the acceptance period is 90 days.
Cause analysis: In addition to cement, different grades of the concrete poured at the same project site can not be mixed, and fly ash can not be mixed with different grades by different manufacturers. Because fly ash from different manufacturers has different raw material sources; At the same time, different grades of fly ash fineness, water demand, carbon content are different. If it is mixed in the same engineering part, it is bound to cause the difference of setting time and strength.
Prevention Measures:
1. In the production of large volume and generous amount of concrete, the mixing station should be prepared in advance to prevent the occurrence of material "out of file" problem in production.
2. It is strictly prohibited to use two kinds of cement or fly ash in the production of concrete at the same construction site in stirring station.


Example 5, in the winter spring or autumn winter season exchange period construction, mixing station should choose different varieties of admixture, to prevent due to the sudden change in temperature affect the setting time of concrete:
In a project, concrete 12-storey wall panel C30 was constructed in early March. Due to the unstable temperature in early March in northern spring, strong wind and temperature drop occurred during the construction. Usually, the minimum temperature was above 0℃, but the minimum temperature suddenly dropped to -10℃ during the construction.
Treatment: delay the removal of template time, strengthen heat preservation and maintenance.
Cause analysis: pumping admixture is not suitable. Usually the lowest temperature is above 0℃ can be selected without antifreeze pumping agent. When the temperature drops suddenly during construction, pumping agent with antifreeze component should be selected.
Prevention Measures:
1. Mixing station should be equipped with two or more admixture reserve tanks, respectively equipped with different types of admixture, in order to prevent the temperature in winter and spring or autumn and winter exchange period is not stable, according to the specific situation to be selected, to prevent the production of concrete mixing station setting time deviation is too large.
2. When the temperature is not stable during the seasonal exchange period, ordinary Portland cement or Portland cement should be selected.


Example 6, pumping retarded admixture can not exceed the amount of admixture, to prevent concrete abnormal phenomenon:
During the summer construction of a certain mixing station, the loss of concrete slump was too large due to the power failure of the site for nearly 4 hours. In order to increase the slump without increasing the water-cement ratio, the method of adding pumping agent was used to adjust the slump in the mixing station. However, due to the excessive mixing of pumping agent, the concrete was not solidified for 12 hours after construction.
Treatment: Notify the construction unit to delay the removal of the mold and prohibit the people to step on it, and strengthen the maintenance. Although the setting time of concrete is delayed, it will not affect the later strength.
Reason analysis: the pumping agent used in summer concrete production is a kind of compound admixture with retarded setting components, if blindly use mixed pumping agent to adjust concrete slump, will inevitably retarded setting components into concrete, causing retarded setting components over amount, concrete setting time greatly elongated.


Prevention Measures:
1 slump loss is too large, mixing station technicians should analyze the reason, even if the admixture should be appropriate, the adjustment of slump can be appropriate admixture or add a small amount of water according to the specific situation, should not be added by the construction personnel at will, to prevent the water cement ratio increases and reduce the strength of concrete.
2. Concrete that is difficult to adjust to meet slump requirements can be demoted to other sites for use.