One of the misunderstandings is that the purpose of concrete watering and curing is only for the needs of cement hydration.
After concrete pouring and molding, it must be covered with water to meet the requirement of keeping the concrete surface moist for a certain period of time. At the same time, in order to prevent the rapid evaporation of curing water, it should be covered with plastic film, sacks or straw bags. However, the maintenance of concrete is not only watering, but also contains extensive and profound contents, which can be summarized as follows: first, keep concrete moist enough in a certain period of time to meet the needs of cement hydration; Second, it is necessary to ensure that the concrete can maintain an appropriate maximum temperature, an appropriate internal and external temperature difference and an appropriate temperature difference between the surface and the surrounding atmosphere at different environmental temperatures, and at the same time have an appropriate cooling rate and heating rate.
Myth 2: The latest start time of concrete watering and curing is 12h after pouring and molding.
The Code for Quality Acceptance of Concrete Structure Engineering (hereinafter referred to as "Quality Code") stipulates that moisture curing shall be covered within 12h after concrete pouring. However, many construction workers misunderstand that the latest time to start watering and curing after concrete pouring is 12h, that is to say, as long as watering and curing are carried out after concrete pouring and before 12h, it will meet the specification requirements. So the construction site often urges technicians to maintain and water, but some people will say that it is only a few hours before concrete pouring, and it is still far from 12h! Don't worry.
Due to the continuous progress and development of cement and concrete technology, especially the wide application of high performance concrete, early strength concrete, high strength concrete and ready-mixed concrete in recent years, the concrete strength grade and cement strength grade are high, the cement dosage is large, the early strength is high, and the water-cement ratio is small, which makes the temperature deformation, shrinkage deformation and self-shrinkage deformation of concrete large, and concrete cracks occur from time to time, among which the late watering and curing time of concrete is one of the important reasons for early cracking.
Myth 3: The longer the water curing time of concrete, the better.
According to the quality standard, the water curing time of concrete mixed with Portland cement, ordinary Portland cement or slag Portland cement shall not be less than 7d, and the water curing time of concrete mixed with retarding admixture or with impermeability requirements shall not be less than 14d. It should be pointed out here that the specification only stipulates the minimum time of water conservation, but does not give the optimal duration and maximum time of water conservation. However, the longer the water curing time, the higher the hydration degree of cement and the greater the irreversible shrinkage of cement. If all cement particles are hydrated, the cement gel generated by them will not only improve the strength of concrete, but also cause great shrinkage, which will cause concrete cracking in severe cases. Like aggregate in concrete, a certain amount of unhydrated cement particles or other inert substances are needed to stabilize the volume. Therefore, the longer the water curing time, the better. It is obviously wrong to blindly extend the watering maintenance time as "strengthening maintenance".
The test shows that the drying shrinkage of concrete with standard curing for 7d and standard curing 14d is basically the same at all ages, as shown in Table 1, but long-term curing can not further reduce the shrinkage. At this time, if the concrete is watered for a long time, the shrinkage of the concrete will increase to a certain extent due to the increase of hydrate generated in the concrete. Long-term wet curing can not effectively reduce the dry shrinkage of concrete, although it can delay the start time of shrinkage, but it has little effect.
Table 1: Influence of initial curing and time after curing on drying shrinkage of concrete
The "just-in-time" curing time is related to many factors, such as the choice of composition materials, concrete mixture ratio, environmental temperature and humidity, wind speed and curing mode. The lower the water-binder ratio of concrete, the more it is necessary to strengthen the supply of external supplementary water in time, but the time for watering and curing can be shortened appropriately; When the water-binder ratio is large, there is more free water in concrete. If the concrete is located in an area with high relative humidity, the influence of wet curing is not great, but its curing time is relatively long to make its permeability stable. If the water-binder ratio is large, but it is in an area with low relative humidity, wet curing cannot be underestimated and the curing time cannot be shortened.
The water-binder ratio of concrete mixed with mineral admixtures such as fly ash is small. If the external supplementary water supply is insufficient, the adsorbed water on the surface will easily evaporate. The anti-cracking effect of admixtures with slow reaction, such as fly ash, is the same as the strength increase. In the case of low water-binder ratio, only water conservation can play an effective role, and water conservation is not only sufficient but also takes a long time. For concrete with retarding admixture and impermeability requirements, the water curing time should be appropriately extended according to the requirements of quality specifications. At present, there are few scientific research materials in this field, which need further research and summary to guide the specific construction. Now, from the perspective of permeability, the required wet curing time is listed in Table 2.
Table 2: Curing Time of Concrete with Permeability Stability
Myth 4: The concrete has finally solidified, and the surface is still wet, so don't worry about water conservation.
As we all know, the early cracking of concrete is a new problem brought by the technical progress and development of cement concrete, and self-shrinkage and temperature shrinkage are the main reasons for the early cracking of high performance concrete, high strength concrete and high early strength concrete.
The size of self-shrinkage of concrete depends on the degree of self-drying in cement paste, elastic modulus and creep coefficient of cement paste. In the early stage after concrete pouring, especially in the first 24 hours after initial setting, its elastic modulus is low and its creep coefficient is large. Therefore, the degree of self-drying becomes the main factor to determine its self-shrinkage. Wet curing of the surface of concrete during its initial setting can combine the curing water with the moisture in the concrete capillary, thus supplying the cementing material inside the concrete to hydrate it. The further hydration of cementitious materials promotes the refinement of pores. When the resistance of capillary wall exceeds the surface tension of water and cannot continue to migrate into concrete, the water supply will stop. It can be seen that the water supply function of early water conservation can well inhibit the early shrinkage of concrete.
If the early strength of concrete is regarded as the internal cause of its early cracking, then its water retention lags behind the external supply and supply interruption after the rapid evaporation of surface water, which is the external cause of its early cracking. Therefore, it is necessary to start watering and curing the concrete in advance, so that the evaporated water on the concrete surface can be replenished in time and watered and cured as soon as possible. Specifically, after concrete pouring is completed and initial setting begins, water conservation should be carried out "as soon as possible and in time" without artificially damaging the concrete surface. Special emphasis should be placed on the word "as soon as possible" here, so as to ensure adequate and timely water supply conditions for concrete in the early stage and avoid the joint action of plastic shrinkage, autogenous shrinkage and drying shrinkage of concrete.
Myth # 5: It is best to water the concrete, so that the water can be fully and thoroughly supplemented.
Cover the concrete after pouring and molding. First, prevent curing water from evaporating quickly to save water. The second is to prevent the rapid loss of hydration heat of cement during the cooling stage, so as to ensure that there is a suitable temperature gradient on the concrete section. Some people use high-pressure water to save covering materials and not cover concrete, which not only wastes water, but also easily damages the concrete surface. More importantly, the pressure water flows over the concrete surface and quickly takes away its heat, resulting in a sudden drop in the temperature of the concrete surface. If the temperature difference between curing water and concrete surface is large at the peak of concrete hydration heat, "thermal shock" may occur due to the sudden drop of concrete temperature, which will lead to excessive temperature difference between the inside and outside of concrete and the surface and environment, resulting in concrete. At the same time, it should be remembered that maintenance and watering can not be interrupted, and repeated "thermal shock" may aggravate concrete cracking. The appropriate water conservation method should be a small amount of water spraying.
Myth # 6: In order to accelerate the hardening of concrete, only heat preservation is needed in the curing stage without cooling.
The initial pouring temperature of concrete is an important part of the highest temperature of concrete. If the concrete in plastic state is cooled, the maximum temperature will be reduced, and the cracking temperature of concrete will be reduced accordingly. Therefore, cooling concrete in plastic state is one of the effective methods to prevent concrete from cracking.
From the beginning of concrete hardening, tensile stress will be generated until the highest temperature is reached. Although the cooling treatment is continued at this stage, the tensile state of the whole concrete section generally does not change. However, pouring excessive cooling water on the surface of concrete with a temperature lower than the ambient temperature will cause the temperature of concrete to drop sharply and increase the temperature gradient on the concrete section, which may cause "thermal shock" of concrete. Although the cooling treatment will also reduce the maximum temperature and cracking temperature at this stage, in order to prevent the surface cracking caused by the sudden increase of the temperature difference between the inside and outside of the concrete, the concrete should be cooled in time before the tensile stress is generated.
Myth # 7: Thermal insulation coverage starts from the time of watering coverage, and I don't know when it starts.
Based on the above problems, it can be seen that concrete should be in the heat dissipation stage before reaching the highest temperature of cement hydration, so as to obtain a lower maximum temperature and cracking temperature. If the thermal insulation is advanced from watering, curing and covering, it will be counterproductive, but it will increase the maximum temperature and cracking temperature of concrete. The correct heat preservation time should start from concrete cooling, and should not be earlier.
One of the purposes of thermal insulation in concrete cooling stage is to reduce the heat loss in concrete and the temperature gradient on the cross section. The second purpose is to delay the heat dissipation time of concrete, so that it can effectively exert its strength growth potential, and the relaxation and creep of concrete can be fully manifested, and its internal tensile stress can also be reduced accordingly. At the same time, due to the increase of concrete age, the tensile performance of concrete is improved faster than its compressive performance, which can also prevent and reduce concrete cracking.
The above are collected and sorted by Zhong Da Consulting Company.
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