Introduction:

In spring, there is a large temperature difference between day and night, with variable temperatures and dry weather. Strong winds and high temperatures promote increased water loss and plastic shrinkage of fresh concrete. Based on the characteristics of spring construction, corresponding technical measures should be taken to ensure the quality of concrete. The construction unit should do the following:

1. Vibration method

During the construction process, the concrete should be thoroughly vibrated to ensure no leakage or excessive vibration. When using a plug-in vibrator, it should be inserted quickly and pulled out slowly. The insertion points should be evenly arranged, moved point by point, and carried out in sequence, without omission, to achieve uniform compaction. The moving distance should not exceed 1.5 times the radius of action of the vibrating rod (usually 300-400 mm). When vibrating the upper layer, it should be inserted 50mm into the lower concrete surface to eliminate the joint between the two layers. The movement distance of the flat plate vibrator should ensure that the vibrator's flat plate covers the edge of the compacted part.

Excessive vibration of concrete can cause delamination and surface floating, resulting in uneven settlement and shrinkage of the concrete, which can easily lead to longitudinal cracks or cracks at the junction of structural thickness. On the contrary, the leakage of vibration in the construction of concrete mixtures is also detrimental to the quality control of concrete, such as causing defects such as honeycomb and pockmarks in the concrete. Concrete vibration should be determined based on the workability of the concrete, and the vibration time should be determined. The concrete should be fully vibrated, with the sign that there is slurry on the surface of the concrete, no bubbles overflow, the concrete is filled with the test mold, and there is no significant sinking.

2. Secondary vibration and multiple plastering

How to determine the time for the second vibration is a key factor in ensuring that the second vibration achieves the expected effect. If the interval between the second vibration and the first vibration is too short, the effect on eliminating cracks and improving compactness is not significant. On the contrary, if the interval between initial compaction is too long, especially if the poured concrete has already set, it cannot effectively eliminate internal defects and can also damage the hydration structure of the concrete, reducing its quality. Practice has shown that the optimal time for secondary vibration is around 1-4 hours before the initial setting of the concrete, especially 1 hour before the initial setting.

3. On site maintenance

After pouring concrete, if the surface is not covered and watered for curing in a timely manner, the surface moisture will quickly evaporate and it is easy to produce shrinkage cracks. Especially in low relative humidity and high wind speeds, drying shrinkage is more likely to occur. There is data indicating that when the wind speed is 16m/s, the evaporation rate of water in concrete is four times faster than when there is no wind. In dry spring, after adding high-efficiency retarder and water reducing agent to concrete, the surface is air dried, and the water evaporates quickly, forming a layer of "hard shell" on the surface. When the temperature is high and the evaporation is large, the method of spraying water while spraying mist should be used to solve the phenomenon of concrete "crust".

After the first smoothing, immediately cover with plastic film to prevent moisture from evaporating, and rely on the moisture of the concrete itself for moisturizing and curing. When a second round of polishing is required, remove the film and cover it thoroughly. Moisturizing maintenance shall not be less than 7 days, and the maintenance time for projects with special requirements such as impermeability shall not be less than 14 days.

4. Demoulding time

When removing concrete formwork, attention should be paid to the time and sequence of formwork removal. For structures such as beams and wall panels, the demoulding time should be appropriately extended, and measures should be taken for maintenance after demoulding. And try not to carry out construction work or stack heavy objects on the floor slab as much as possible to reduce or avoid shrinkage deformation cracks in reinforced concrete structures.

5. Setting time

The setting time of spring concrete is influenced by temperature. Generally, the initial setting time of spring concrete is 3-4 hours, and the final setting time is 6-8 hours. However, the complete solidification time of concrete in spring is not fixed, and due to seasonal factors, the solidification time also varies. If it is in spring and summer, the complete solidification time of concrete is generally around 8 to 10 hours. If it is in autumn and winter, the setting time of concrete will be extended, usually reaching over 48 hours.