1. Vibration mode
In the construction process, the concrete should be fully vibrated to ensure that there is no missing vibration but no vibration. The excessive vibration of concrete results in layered segregation and floating slurry on the surface, resulting in uneven settlement and shrinkage of concrete and cracks at the junction of thickness and thickness of the structure. Leakage vibration is not conducive to concrete compaction, resulting in concrete honeycomb, pitted surface and other defects. The concrete vibration time shall be determined according to the workability of concrete. The concrete should be fully vibrated, and its signs are: the slurry appears on the surface of the concrete, there is no bubble overflow, the concrete is full of the test mold, and there is no significant subsidence.
2. Secondary vibration and multiple plastering
According to the characteristics of commercial concrete, the slump of commercial concrete is relatively large, and the setting time is relatively long (generally 8-12h). The secondary vibration and plastering pressure should be done well to make the plastic shrinkage cracks fully heal in time. The secondary vibration can eliminate the internal delamination caused by plastic settlement, block the continuous channel left by bleeding, improve the interface structure of aggregate, and improve the strength and permeability of concrete.
In the dry spring, the surface of concrete is dried by air after adding high efficiency retarding and water reducing agent, and the water evaporates quickly, forming a layer of "hard shell" on the surface. When the air temperature is high and the evaporation is large, the method of spraying water at the same time should be adopted to solve the problem of concrete "crusting".
3. On site maintenance
After the concrete pouring, if the surface is not covered and watered in time, the surface moisture will evaporate rapidly, which is easy to produce shrinkage cracks. Especially in the case of low relative humidity and high wind speed, drying shrinkage is more likely to occur. Some data show that when the wind speed is 16m / s, the evaporation rate of water in concrete is four times of that in the absence of wind.
At present, some construction sites are unable to cover and maintain insulation in time when pouring concrete. Generally, it is necessary to wait until the last finishing time before covering. There are many construction sites that are not covered at all, or only send people to water every few hours. After the first troweling, the concrete shall be covered with plastic film immediately to prevent water from evaporating, and the concrete shall be kept moist by its own moisture. When the second troweling is needed, the film shall be uncovered and covered after finishing. The moisturizing curing time shall not be less than 7d, and the curing time of impermeability and other special requirements shall not be less than 14d.
4. Time of formwork removal
When removing concrete form, attention should be paid to the time and sequence of formwork removal. For beams, wallboards and other structures, the formwork removal time should be appropriately extended, and measures should be taken to maintain after the formwork removal. And it is not easy to carry out construction work or stack heavy objects on the floor, so as to reduce or avoid the shrinkage deformation cracks of reinforced concrete structure.
5. On the time of condensation
In spring, the temperature is changeable, the temperature difference between day and night is large, and the temperature at night is low, which is easy to prolong the setting time. Commercial concrete companies also take measures to strictly control the slump and the amount of additives, especially when using retarded superplasticizer, more attention should be paid to the influence of temperature on the setting time of concrete. If the concrete slump loss is large and the workability can not meet the construction requirements, please timely inform the commercial concrete company for adjustment. Do not add water or additives at will to adjust the slump of concrete, so as to avoid concrete quality problems.