1. The purpose of concrete watering and curing is only for the needs of cement hydration
After the concrete is poured and formed, it must be covered and watered to meet the requirement that the concrete surface remains wet for a certain period of time. At the same time, in order to prevent the rapid evaporation of the maintenance water, it is also covered with materials such as plastic film, sacks or straw bags. However, the maintenance of concrete is not only just watering, but also includes extensive and profound contents, which can be summed up in the following two points: one is to keep the concrete in a sufficiently moist state for a certain period of time to meet the needs of cement hydration The second is to ensure that the concrete can maintain a suitable maximum temperature, a suitable temperature difference between inside and outside, and a suitable temperature difference between the surface and the ambient atmosphere under different ambient temperature conditions, as well as a suitable cooling rate and heating rate.
2. The latest start time of concrete watering and curing is 12h after pouring and forming
The Specification for Quality Acceptance of Concrete Structural Engineering (hereinafter referred to as the "Quality Specification") stipulates that the concrete should be covered and maintained with moisture within 12 hours after the completion of pouring. However, many construction workers misunderstand that the latest start time of watering and curing after concrete pouring is 12 hours later, that is to say, as long as the watering and curing are carried out before 12 hours after the completion of concrete pouring, the specification requirements are met. . Therefore, technicians often urge maintenance and watering on the construction site, but some people will say that the concrete pouring is only a few hours away, and it is still far from 12 hours! Not in a hurry.
Due to the continuous progress and development of cement and concrete technology, especially in recent years, high-performance concrete, early-strength concrete, high-strength concrete and ready-mixed concrete are widely used. The temperature deformation, drying shrinkage deformation and self-shrinkage deformation of concrete are all large due to the reasons of high strength, high early strength and small water-cement ratio, and concrete cracking occurs from time to time. One of the important reasons must attract the attention of construction personnel.
Many years ago, plastic concrete with high fluidity was often encountered on construction sites. The pouring volume was not large, the concrete strength grade and cement strength grade were low, the cement dosage was small, the early hydration degree was not high, and the dryness was reduced. There is no self-shrinking. In this case, it may be appropriate to require such plastic concrete to be watered and cured within 12 hours after pouring, but for modern concrete, watering and curing too late will cause cracking and damage the potential quality bring adverse effects.
3. The longer the watering and curing time of concrete
The "Quality Specification" stipulates that for concrete mixed with Portland cement, ordinary Portland cement or slag Portland cement, the watering and curing time shall not be less than 7d. The required concrete shall not be less than 14d. It should be pointed out here that the specification stipulates only the minimum time for watering and curing, and does not give the optimal duration and maximum time for watering and curing. However, the longer the watering and curing time is, the higher the degree of cement hydration and the greater the irreversible shrinkage of the cement. If the cement particles are fully hydrated, the resulting cement gel will not only improve the strength of the concrete, but also produce Large shrinkage can cause concrete cracking in severe cases. Like the volume stabilization effect of aggregate in concrete, a certain amount of unhydrated cement particles or other inert substances are required in the cement stone to stabilize the volume. Therefore, the longer the watering and curing time, the better. It is obviously wrong to blindly extend the watering and maintenance time as "enhanced maintenance". The progress and development of modern cement and concrete technology requires "just-in-time" watering and maintenance.
4. The concrete is finally set, and the surface is still wet, so don't rush to water it for maintenance
As we all know, the early cracking of concrete is a new problem brought about by the progress and development of cement and concrete technology, and self-shrinkage and temperature shrinkage are the main reasons for early cracking of high-performance concrete, high-strength concrete and high-early-strength concrete.
The size of the self-shrinkage of concrete depends on the degree of internal self-drying of the cement stone, the elastic modulus of the cement stone and the creep coefficient. In the early stage after pouring, especially in the first 24 hours after initial setting, the elastic modulus is low and the creep coefficient is large. Therefore, the degree of self-drying becomes the main factor determining the self-shrinkage. Wet curing on the surface of concrete during initial setting can make the curing water connect with the water in the capillary holes in the concrete as a whole, so as to supply the cementing material inside the concrete to hydrate it. The further hydration of the cementitious material promotes the refinement of the capillary pores. When the resistance of the capillary pore walls exceeds the surface tension of the water and cannot continue to migrate into the concrete, the water supply stops. It can be seen that the water replenishment effect of early watering and curing can well inhibit the early shrinkage of concrete.
The self-shrinkage of concrete starts from its initial setting. It develops very fast in the early stage. Most of it can be completed within 24h, and then it decays rapidly. Its value can reach (0.025~0.050) × 10-3. The ratio decreases and increases with the increase of temperature. At the same time, with the gradual increase of concrete strength, its ultimate tensile strain also dropped sharply from 4.0×10-3 2h after forming, and dropped to 0.04×10-3 in 6~12h, reaching the risk period of concrete cracking. According to the provisions of the "Quality Specification", according to the requirements of traditional plastic concrete, the latest start time within 12 hours after pouring is mistakenly started to start watering and curing, and the time obviously lags behind the dangerous period of concrete cracking. The latest time to start watering and curing is no longer suitable for the curing requirements of modern concrete. There are many people who mistakenly believe that the watering and curing of concrete can be started at any time within 12 hours after the completion of concrete pouring, that is to say, the watering and curing within this 12h time range can be early or late. The plasticity is very large, and this understanding and practice is obviously wrong.
If the early high strength of concrete is regarded as the internal cause of its early cracking, then its watering and curing lag behind the external replenishment and interruption of water replenishment after the rapid evaporation of surface water, which is the external cause of early cracking of concrete. Therefore, it is necessary to greatly advance the time of watering and curing of concrete, so that the evaporating water on the concrete surface can be replenished in time, so as to achieve "early and timely" watering and curing. Specifically, after the concrete is poured and the initial setting begins, it is limited to "early and timely" watering and curing so as not to artificially damage the concrete surface. Sufficient water replenishment conditions to avoid the combined effect of concrete plastic shrinkage, autogenous shrinkage and dry shrinkage.
5. The watering and curing of concrete is best to pour water violently, so that the water can be fully and thoroughly replenished.
The covering after the concrete is poured and formed is to prevent the rapid evaporation of the curing water to save water; the second is to prevent the rapid dissipation of the hydration heat of the cement during the cooling stage, so as to ensure that the concrete section has a suitable temperature gradient. In order to save the covering material, some people do not cover the concrete and pour it with high pressure water. This not only wastes water, but also easily washes out the concrete surface. More importantly, the pressure water flows over the concrete surface and quickly takes its heat away. , resulting in a sudden drop in the temperature of the concrete surface. If it is in the peak period of concrete hydration heat, if the temperature difference between the curing water and the concrete surface is large, the temperature difference between the inside and outside of the concrete and the temperature difference between the concrete surface and the environment may be too large due to the sudden drop in the temperature of the concrete. "Thermal shock" occurs, resulting in cracking of the concrete surface; at the same time, it is necessary to remember that the maintenance and watering cannot be intermittent, and repeated repeated "thermal shocks" may aggravate the cracking of the concrete. The appropriate method of watering and maintenance should be a small amount of water.
6. In order to accelerate the hardening of concrete, the curing stage is only kept warm without cooling
The initial pouring temperature of concrete is an important part of the maximum temperature of concrete. For concrete in plastic state, cooling and cooling treatment will reduce the maximum temperature and correspondingly reduce the cracking temperature of concrete. Therefore, cooling the concrete in a plastic state is one of the effective methods to prevent concrete from cracking.
The concrete starts to harden and generates tensile stress until it reaches the highest temperature. Although the concrete continues to be cooled at this stage, it generally does not change the tensile state of the entire concrete section. Cooling water will cause the concrete temperature to drop sharply, which will increase the temperature gradient on the concrete section, which may cause concrete "thermal shock". Although at this stage, the concrete cooling treatment will also reduce the maximum temperature and cracking temperature, but in order to prevent the concrete The surface cracking is caused by the sudden rise in the temperature difference between the inside and outside. The cooling treatment and watering maintenance at this stage must be careful. Before the tensile stress is generated inside the concrete, it should be cooled in time.
7. Insulation mulch starts when the mulch is watered
Before the concrete reaches the maximum temperature of cement hydration, it should be in the heat dissipation stage, in order to obtain a lower maximum temperature and cracking temperature. If the insulation is advanced to start from the watering and curing coverage, it will be counterproductive, but will increase the maximum temperature of the concrete and cause cracking. The correct heat preservation time should start from the cooling of the concrete, not in advance.
One of the purposes of implementing heat preservation in the concrete cooling stage is to reduce the loss of heat inside the concrete, so as to reduce the temperature gradient on the section. The second purpose is to delay the heat dissipation time of the concrete, so that it can effectively fully exert its strength growth potential, and make the relaxation and creep of the concrete fully manifest, and its internal tensile stress can be reduced accordingly. At the same time, due to the increase of concrete age, the tensile performance of concrete is faster than its compressive performance, which can also prevent and reduce concrete cracking.
The temperature gradient of the concrete surface is one of the important reasons that restrict the cracks on the concrete surface. The rise and fall of the atmospheric temperature affects the temperature gradient on the internal section of the concrete, and the steepness of the temperature change will inevitably affect the steepness of the temperature change between the concrete surface and the atmospheric temperature. The effective coverage of thermal insulation materials can reduce the temperature. Temperature gradient across a concrete section.
Engineering practice has proved that temperature change is an important and very complex load of concrete structure, and the steepness of temperature gradient can be regarded as the speed of "loading" of concrete, and has an important impact on the physical and mechanical properties of concrete. The sudden drop in temperature can be regarded as a rapid loading of concrete, which can lead to an increase in the tensile stress and elastic modulus of the concrete, while reducing the ultimate tensile strength of the concrete and weakening the crack resistance. The slow loading of the concrete can lead to a decrease in the tensile stress and elastic modulus of the concrete compared with the rapid loading, while the ultimate tensile strength of the concrete increases. At the same time, the sudden drop in temperature can also lead to an increase in the degree of internal and external constraints. Whether it is a structure dominated by external constraints or a structure dominated by internal constraints, external thermal insulation and internal slow descent can avoid and reduce concrete cracking.
To sum up, it can be seen that regardless of the ambient temperature, that is to say, regardless of whether the outside temperature is high or low in spring, summer, autumn and winter, the thermal insulation and maintenance of concrete not only increases the surface temperature of the concrete, but also slows down the temperature inside the concrete. It reduces the temperature difference between the inside and outside and the temperature difference between the concrete surface and the atmospheric environment. For this reason, the curing method of "external thermal insulation and internal slow drop" can prevent and reduce the cracking of concrete.
8. Not according to the actual situation of the concrete, the regulations are rigidly moved
In order to prevent the occurrence of early cracks in concrete, people usually control the technical indicators such as the maximum temperature of concrete, the temperature difference between inside and outside, the temperature difference between the surface and the environment, the heating rate and the cooling rate. The temperature difference with the ambient atmosphere should not be greater than 20 ℃. However, there are some discrepancies in the application of the previous specifications in actual engineering. Some believe that neither should be greater than 25°C; some believe that it should not be greater than 30°C; some believe that it should not be greater than 15°C; The instantaneous temperature difference caused by water spraying and mold removal should not exceed 15℃. Engineering practice has proved that in some projects, the temperature difference between inside and outside of concrete is greater than 25℃, but the structure does not crack; while in some projects, the temperature difference between inside and outside is less than 20℃, but the concrete cracks. From this, it can also be explained that the revised "Quality Specification" does not make rigid provisions on this.
At the same time, the control indicators of the daily cooling rate are also different. Some people think that the daily cooling rate should not be greater than 3°C, some think that it should not be greater than 2°C, and some even think that it should not be greater than 1.5°C.
The emergence of differences between the above technical data is actually very normal. Although some data are specified by the specification, it is not possible to raise doubts about the specification. Due to the randomness, diversity and heterogeneity of the composition of concrete materials, as well as the heterogeneity of concrete and the difference in construction quality, it is not surprising that the technical data shown are different. Considering the temperature control comprehensively, some normative provisions cannot be copied mechanically.
