The most effective measures to prevent cracks are to protect the concrete pouring surface from the wind and the sun, cover the surface immediately after concrete pouring, and sprinkle water in time for health preservation. In addition, adding an appropriate amount of air entraining agent to concrete is also helpful to reduce shrinkage cracks.
2. Secondary vibration before initial setting
When the surface of the formwork is uneven, or the release agent is uneven, the friction of the formwork prevents this settlement, which will produce cracks on the vertical plane of concrete. This situation is generally easy to happen at the corner of concrete columns or other long and narrow structures. The second vibration before the initial setting of concrete is the best way to avoid this defect.
3. Add fly ash and cool the water pipe.
When the shrinkage of surface concrete is blocked, the concrete will be in tension, and once it exceeds the strain capacity of concrete, cracks will occur. In order to reduce the shrinkage constraint as much as possible and make concrete have enough strength to resist the resulting stress, it is necessary to effectively control the internal heating rate of concrete. Adding proper amount of mineral fly ash into concrete can slow down the release rate of hydration heat; Controlling the temperature of raw materials and circulating water in concrete structures by using cooling pipes can also release hydration heat in time.
4, it is forbidden to mix cement.
The mixed use of different brands of cement will also cause concrete cracking. Different brands of cement have different fineness, strength, initial setting and final setting time, stability and chemical composition, and there are still compatibility problems. In concrete construction, it is forbidden to mix different brands and labels of cement.
Extended data:
Preventive measures of concrete cracks
1, increase the structural reinforcement to improve the crack resistance, and the reinforcement should adopt small diameter and small spacing. The reinforcement ratio of the whole section should be between 0.3 and 0.5%.
2. When designing concrete mixture ratio, the unit water consumption of concrete should be reduced as much as possible under the condition of ensuring good workability of concrete.
3. Avoid stress concentration caused by structural mutation, and take strengthening measures in weak links that are prone to stress concentration.
4. Set hidden beams at the crack edge to improve the reinforcement ratio of this part. Improve the ultimate tensile strength of concrete.
5. In the structural design, the climate characteristics during the construction period should be fully considered, and the post-pouring belt should be set reasonably. Under normal construction conditions, the spacing of post-cast strip is 20-30m. The retention time is generally not less than 60 days. If it is impossible to predict the specific situation during construction, design changes can also be made temporarily according to the specific situation.
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