On the construction site, it is a very common thing to cover and water the poured concrete. The mechanism and function of its covering and watering conservation are analyzed in order to get rid of several misunderstandings about it.
One of the misunderstandings: The purpose of concrete watering and curing is only for the needs of cement hydration.
After the concrete is poured, it must be covered and watered to meet the requirement of keeping the concrete surface in a wet state within a certain period of time. At the same time, in order to prevent the rapid evaporation of the curing water, it should be covered with materials such as plastic film, sacks or straw bags. However, the maintenance of concrete is not only watering, but also includes extensive and profound content. In summary, there are two main points: one is to keep the concrete in a sufficient wet state within 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 internal and external temperature difference, and a suitable temperature difference between the surface and the ambient atmosphere under different ambient temperature conditions, as well as an appropriate cooling rate and heating rate.
Misunderstanding 2: The latest start time for concrete watering and curing is 12 hours after pouring.
The "Quality Acceptance Specification for Concrete Structure Engineering" (hereinafter referred to as "Quality Specification") stipulates that the concrete should be covered and moisturized within 12 hours after pouring. However, many construction workers misunderstand that the latest start time for watering and curing after concrete pouring is 12 hours later, that is to say, as long as the watering and curing is carried out before 12 hours after concrete pouring, it will meet the specification requirements . Therefore, on the construction site, technicians are often encountered urging maintenance and watering, but some people will say that it is only a few hours after the concrete is poured, and it is still far away from 12 hours! Not in a hurry
Due to the continuous progress and development of cement and concrete technology, especially in recent years, the wide application of high-performance concrete, early-strength concrete, high-strength concrete and ready-mixed concrete, the concrete strength grade and cement strength grade used are relatively high, and the amount of cement is relatively high The temperature deformation, dry shrinkage deformation and self-shrinkage deformation of the concrete are large due to the reasons such as high early strength, small water-cement ratio, etc., and concrete cracking occurs from time to time, and the late watering and curing time of concrete becomes early cracking. One of the important reasons for this must arouse the attention of construction workers.
Many years ago, plastic concrete with high fluidity was often encountered on the construction site. The pouring volume was not large, the strength grade of concrete and cement strength were low, the amount of cement was small, the degree of early hydration was not high, and the dry shrinkage. There is no self-shrinkage. In this case, it may be appropriate to require such plastic concrete to be watered and cured within 12 hours after pouring. However, for modern concrete, late watering and curing will cause cracking and damage the potential quality. bring adverse effects.
The third misunderstanding: The longer the concrete is watered and cured, the better.
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 7 days. The required concrete shall not be less than 14d. It should be pointed out here that what the specification stipulates is only the minimum time for watering and maintenance, but does not give the optimal duration and maximum time for watering and maintenance. However, the longer the watering and curing time, the higher the degree of hydration of the cement, and the greater the irreversible shrinkage of the cement. If the cement particles are fully hydrated, the resulting cement gel will not only increase the strength of the concrete, but will also produce Large shrinkage can cause concrete cracking in severe cases. Like the volume stabilization effect of aggregates in concrete, a certain amount of unhydrated cement particles or other inert substances are required in cement stones to stabilize the volume. Therefore, the watering and curing time is not as long as possible. 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.
Tests have proved that the shrinkage of concrete at different ages is basically the same for standard curing of 7 days and standard curing of 14 days, as shown in Table 1, but too long curing can not further reduce the shrinkage. Water curing, due to the increase of hydrates generated inside the concrete, increases the shrinkage of the concrete to a certain extent. Long-term wet curing cannot effectively reduce the drying shrinkage of concrete, and although it can delay the start time of shrinkage, the effect is minimal.
Misunderstanding four: The concrete has just finally set, and the surface is still wet, so don't worry about watering and curing.
As we all know, early cracking of concrete is a new problem brought about by the progress and development of cement and concrete technology, and autogenous 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 concrete self-shrinkage depends on the self-drying degree of the cement stone, the elastic modulus and the creep coefficient of the cement stone. In the early stage after concrete pouring, especially 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 determining self-shrinkage. When the concrete is initially set, the wet curing of its surface can make the curing water and the moisture in the capillary pores of the concrete connect as a whole, so as to supply the cementitious material inside the concrete for hydration. The further hydration of the cementitious material promotes the refinement of the capillary pores. When the resistance of the capillary wall exceeds the surface tension of the water and cannot continue to migrate to the interior of the concrete, the supply of water 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 has already started from its initial setting, and the early development is very fast, and most of it can be completed within 24 hours, and then rapidly decays, and its value can reach (0.025~0.050) × 10-3, and also with water glue increases with decreasing ratio and increases with increasing temperature. At the same time, with the gradual increase of concrete strength, the ultimate tensile strain also drops sharply from 4.0×10-3 2 hours after forming, and can drop to 0.04×10-3 in 6~12 hours, reaching the risk period of concrete cracking. If according to the provisions of the "Quality Standards" and the requirements of traditional plastic concrete, the latest start time within 12 hours after pouring is mistakenly used to start watering and curing. The time has obviously lagged 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. Many people mistakenly believe that the watering and curing of concrete can be started at any time within 12 hours after the concrete is poured. The plasticity of human beings is very large, and this kind of understanding and practice is obviously wrong.
If the early high strength of concrete is regarded as the internal cause of its early cracking, then the external water replenishment and interruption of water replenishment after the watering curing lags behind the rapid evaporation of surface water is the external cause of early cracking of concrete. Therefore, it is very necessary to greatly advance the time of concrete watering and curing, so that the outwardly evaporated water on the concrete surface can be replenished in time, so as to achieve "early and timely" watering and curing. Specifically speaking, after the concrete has been poured and the initial setting begins, watering and curing should be done "as soon as possible" as long as the surface of the concrete will not be artificially damaged. Sufficient water supply conditions to avoid the joint action of plastic shrinkage, autogenous shrinkage and dry shrinkage of concrete.
Misunderstanding five: The watering and maintenance of concrete is best poured with water, so that the water can be fully replenished.
The covering after concrete pouring is to prevent the rapid evaporation of curing water to save water; the second is to prevent the rapid loss of cement hydration heat during the cooling stage, so as to ensure a suitable temperature gradient on the concrete section. In order to save covering materials, some people do not cover the concrete and pour it with high-pressure water. This not only wastes water, but also easily damages the concrete surface. The most important thing is that the pressure water flows through the concrete surface and quickly takes away its heat. , leading to a sudden drop in the surface temperature of the concrete. 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, it may be caused by the sudden drop in the temperature of the concrete, which will cause the temperature difference between the inside and outside of the concrete and the temperature difference between the concrete surface and the environment to be too large. "Thermal shock" will cause the concrete surface to crack; at the same time, it must be remembered that maintenance and watering should not be intermittent, and repeated "thermal shock" may aggravate the cracking of the concrete. The appropriate watering and maintenance method should be small water flooding.
Misunderstanding six: In order to accelerate the hardening of concrete, the curing stage only keeps warm and does not carry out cooling and cooling treatment.
The initial pouring temperature of concrete is an important part of the maximum temperature of concrete. Cooling the concrete in a plastic state will not only reduce the maximum temperature, but also reduce the cracking temperature of concrete accordingly. Therefore, cooling the concrete in a plastic state is one of the effective methods to prevent concrete cracking.
From the beginning of concrete hardening to produce tensile stress until reaching the highest temperature, although the concrete continues to be cooled at this stage, generally it will not change the tensile state of the entire concrete section, but the surface of the concrete is poured with water that is lower than the ambient temperature. Cooling water will cause the concrete temperature to drop suddenly, which will increase the temperature gradient on the concrete section and may cause concrete "thermal shock". Although at this stage, the cooling treatment of concrete will also reduce the maximum temperature and cracking temperature, but in order to prevent The sudden rise in temperature difference between inside and outside causes cracks on the surface. At this stage, the cooling treatment and watering maintenance must be careful. Before producing the stress of drawing inside the concrete, it should be cooled in time.
Misunderstanding seven: Insulation covering begins when watering and covering, I don’t know when to start.
Summarizing the above problems, it can be seen that 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. Given the maximum temperature and cracking temperature of concrete, the correct heat preservation time should start from the cooling of concrete, and should not be advanced.
One of the purposes of implementing thermal insulation in the concrete cooling stage is to reduce the heat loss inside the concrete, so as to reduce the temperature gradient on the section. The second purpose is to delay the heat dissipation time of concrete, so that it can effectively and fully exert the potential of its strength growth, and make the relaxation and creep of 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 improves 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 restricting the cracks on the concrete surface. The rise and fall of the atmospheric environment temperature affects the temperature gradient on the internal section of the concrete, and the steepness and slowness of the temperature change will inevitably affect the steepness and slowness of the temperature change between the concrete surface and the atmospheric environment temperature. Effective coverage of thermal insulation materials can reduce the Temperature gradient across a concrete section.
Engineering practice has proved that temperature change is an important and very complex load on concrete structures. The steepness and slowness of temperature gradient can be regarded as the speed of "loading" concrete, and has an important impact on the physical and mechanical properties of concrete. A sudden drop in temperature can be seen as a rapid loading of concrete, which can lead to an increase in the tensile stress and elastic modulus of concrete, which reduces the ultimate stretch of concrete and weakens the crack resistance. The slow loading of the concrete can cause the tensile stress and elastic modulus of the concrete to decrease compared with the fast loading, while the ultimate tensile 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, concrete cracking can be avoided and reduced through external thermal insulation and internal slow down.
To sum up, it can be seen that no matter the ambient temperature is high or low, that is to say, no matter whether the outside air 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 concrete, but also slows down the temperature inside concrete. drop, and reduce the temperature difference between the inside and outside and the temperature difference between the concrete surface and the atmospheric environment. Therefore, this "outer insulation and inner slow drop" curing method can prevent and reduce concrete cracking.
Mistake No. 8: Not according to the specific actual situation of the concrete, apply the rules and regulations mechanically.
In order to prevent early cracks in concrete, it is usually achieved by controlling 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°C. However, there are some discrepancies in the previous specifications for the application in actual engineering. Some think that both should not be greater than 25°C; some think that they should not be greater than 30°C; The instantaneous temperature difference caused by water spraying and form removal should not exceed 15°C. Engineering practice has proved that some projects have a temperature difference between the inside and outside of the concrete greater than 25°C, but the structure has not cracked; while some projects have a temperature difference between the inside and outside of the concrete that is less than 20°C, but the concrete has cracked. This can also explain the reason why the revised "Quality Standards" did not make rigid regulations on this.
At the same time, the control indicators of the daily cooling rate are also different. Some believe that the daily cooling rate should not exceed 3°C, some believe that the daily cooling rate should not exceed 2°C, and some even think that it should not exceed 1.5°C.
The emergence of differences between the above-mentioned technical data is actually very normal. Even though some data are stipulated by the norms, it is not possible to raise doubts about the norms. Due to the randomness, diversity, and heterogeneity of concrete material composition, the heterogeneity of concrete, and the difference in construction quality, it is not surprising that there are some differences in the technical data shown. This requires on-site technicians to Taking temperature control into consideration, some normative provisions cannot be mechanically copied.
