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8 Misunderstandings About Concrete Maintenance!

Thomas Huang posted this article in August 03, 2020

Concrete curing is a common thing on construction sites. It is very common to cover and water the poured concrete. The mechanism and effect of its cover watering and maintenance are now analyzed in order to get out of several misunderstandings about it.
Misunderstanding 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 of keeping the concrete surface wet for a certain period of time. At the same time, in order to prevent the rapid evaporation of the curing water, plastic film, sack pieces or straw bags are also used to cover. However, the curing of concrete is not only watering, but also contains extensive and profound content. In summary, there are mainly the following two points: One is to keep the concrete sufficiently wet 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.
Misunderstanding 2
The latest start time for concrete watering and curing is 12h after pouring.
The concrete should be covered and moisturized within 12h after pouring. However, many construction workers misunderstand that the latest start time for watering and curing after the concrete is poured is 12 hours thereafter, that is to say, as long as the watering and curing is performed before 12h after the concrete is poured, the specification requirements are met. . Therefore, technicians often urge maintenance and watering on the construction site, but some people will say that it is only a few hours after the concrete has been poured, and it is still far from 12h! 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, etc., the concrete strength grade and cement strength grade used are higher, and the amount of cement is relatively high. Large, high early strength, low water-cement ratio, etc., make the temperature deformation, dry shrinkage deformation and auto-shrinkage deformation of concrete large, and concrete cracking occurs from time to time. Among them, the late watering and curing time of concrete becomes early cracking One of the important reasons for this is that the construction staff must pay attention.
Many years ago, plastic concrete with high fluidity was often encountered on construction sites. Its pouring volume was not large, the concrete strength grade and cement strength grade were low, the amount of cement was small, the early hydration degree was not high, and the dry shrinkage was not high. There is no auto-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, too late watering and curing will cause cracking and potential quality Bring adverse effects.
Misunderstanding No. 3
The longer the concrete watering and curing time, the better
For the concrete mixed with Portland cement, ordinary Portland cement or slag Portland cement, the watering and curing time shall not be less than 7 days. For the concrete mixed with retarding admixture or impermeability requirements, it shall not be less On 14d. It should be pointed out here that what the specification stipulates is only the minimum time for watering and curing, but does not give the best duration and maximum time for watering and curing. However, the longer the watering and curing time, the higher the degree of cement hydration and the greater the irreversible shrinkage of the cement. If the cement particles are fully hydrated, the cement gel generated will not only increase the strength of the concrete, but also produce Large shrinkage can cause concrete cracking in severe cases. Like the volume stabilization effect played by aggregates in concrete, a certain amount of unhydrated cement particles or other inert substances are required to stabilize the volume in cement stone. Therefore, the longer the watering and curing time, the better. It is obviously wrong to blindly extend the watering and curing time as a "strengthening maintenance" approach. The advancement and development of modern cement and concrete technology require watering and maintenance "just in time".
Tests have proved that the dry shrinkage of the concrete of standard curing for 7 days and standard curing for 14 days is basically the same, as shown in Table 1. However, the long-term curing cannot further reduce the shrinkage. At this time, if the concrete is poured for a long time Water curing, due to the increase of hydrates generated inside the concrete, increases the shrinkage of the concrete to a certain extent. Long-term wetting and curing cannot effectively reduce the dry shrinkage of concrete, and although it can delay the onset of shrinkage, it has little effect.
The length of "just in time" curing time is related to many factors such as the selection of constituent materials, concrete mix ratio, ambient temperature and humidity, wind speed and curing methods. The lower the concrete water-binder ratio, the more it is necessary to strengthen the external supplemental water supply in time, but the watering and curing time can be appropriately shorter; when the water-binder ratio is large, there is more free water in the concrete. If the concrete is in an area with high relative humidity, The effect of wet curing is not great, but the curing time is relatively longer to make the permeability stable; if the water gel is relatively large, but in an area with low relative humidity, wet curing should not be underestimated, and the curing time cannot be shortened; Concrete with mineral admixtures such as fly ash, because its water glue is relatively small, if the external supplementary water supply is insufficient, the adsorbed water on the surface will easily evaporate, and the admixtures such as fly ash that react slowly will have an anti-cracking effect. The strength growth is the same. Under the condition of low water-to-binder ratio, only by strengthening watering and curing can it be effectively brought into play. Watering and curing should not only be sufficient but also take longer. For concrete mixed with retarding admixtures and impermeability requirements, as required, the watering and curing time should be appropriately extended. At present, there are very few scientific research materials on this aspect, and it is necessary to strengthen research and summary in the future to guide concrete construction.
Misunderstanding 4
The concrete has finally set, and the surface is still wet, so do not worry about watering and curing
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 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 concrete self-shrinkage depends on the degree of self-drying inside the cement stone, the elastic modulus and 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 creep coefficient is large. Therefore, the degree of self-drying becomes the main factor that determines self-shrinkage. The wet curing of the surface of the concrete during the initial setting can make the curing water and the moisture in the capillary holes in the concrete become 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 pores. When the resistance of the 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 supplement effect of early watering and curing can well inhibit the early shrinkage of concrete.
The autogenous shrinkage of concrete has started from the initial setting. The early development is very fast. Most of it can be completed within 24 hours, and then it will quickly decay, its value can reach (0.025~0.050)×10-3, and it is also glued with water. The ratio decreases and increases, and increases with the increase in temperature. At the same time, with the gradual increase of concrete strength, its ultimate tensile strain also drops sharply from 4.0×10-3 in 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 regulations, according to the requirements of traditional plastic concrete, the watering and curing can start at the latest within 12 hours after the completion of pouring. The time is obviously far behind the dangerous period of concrete cracking. Water curing time is no longer suitable for modern concrete curing requirements. Many people mistakenly believe that the watering and curing of concrete can be started at any time within 12h after the concrete is poured. That is to say, the watering and curing can be done sooner or later within the 12h time frame. This kind of understanding and practice is obviously wrong.
If the early strength of concrete is regarded as the internal cause of its early cracking, then its watering and curing lags behind the external water replenishment and interruption of water replenishment after the rapid evaporation of surface water, which is the external cause of the early cracking of concrete. Therefore, it is necessary to greatly advance the time when the concrete starts to be watered and cured, so that the evaporated water on the surface of the concrete can be recharged in time, so as to achieve "early and timely" watering and curing. To be more specific, after the concrete has been poured, at the beginning of its initial setting, "as soon as possible" is the limit of watering and curing to prevent the surface of the concrete from being artificially damaged. The word "as soon as possible" should be emphasized here to ensure that the concrete is ready in time. Sufficient water replenishment conditions to avoid the combined effects of plastic shrinkage, auto-shrinkage and dry shrinkage of concrete.
Misunderstanding No. 5
The best way to water and maintain the concrete is heavy water pouring, so that water replenishment can be fully and thoroughly
Covering after concrete pouring is to prevent the rapid evaporation of curing water to save water; the other is to prevent the rapid loss of cement hydration heat during the cooling stage, so as to ensure that the concrete section has a suitable temperature gradient. In order to save covering material, some people do not cover the concrete and use high-pressure water to pour it. This not only wastes water, but also easily damages the concrete surface. More importantly, the pressure water flows over the concrete surface and quickly removes its heat. , Resulting in 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, the temperature of the concrete may drop suddenly, causing the temperature difference between inside and outside and the temperature difference between the concrete surface and the environment to be too large. Produce "thermal shock", causing the concrete surface to crack; at the same time, keep in mind that the maintenance and watering cannot be intermittent. Interruption of repeated "thermal shock" may increase the possibility of concrete cracking. The proper watering and maintenance method should be small water flooding.
Misunderstanding 6
In order to accelerate the hardening of concrete, only heat preservation during the curing stage without cooling down
The initial pouring temperature of concrete is an important part of the maximum temperature of concrete. Cooling the concrete in a plastic state will reduce the maximum temperature while reducing the cracking temperature of the concrete. Therefore, cooling the concrete in a plastic state is one of the effective methods to prevent concrete from cracking.
Tensile stress is generated from the concrete hardening to the maximum temperature. Although the concrete is continued to be cooled at this stage, it will generally not change the tension state of the entire concrete section, but the concrete surface is poured with a temperature that is lower than the ambient temperature. 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 to prevent concrete The sharp rise in temperature between the inside and outside causes surface cracks, and the cooling treatment and watering and maintenance at this stage must be careful. Before tensile stress is generated in the concrete, it should be cooled in time.
Misunderstanding No. 7
Insulation coverage starts when watering and covering, I don’t know when to start
Summarizing the above issues, it can be seen that the concrete should be in the heat dissipation stage before reaching the maximum temperature of cement hydration in order to obtain a lower maximum temperature and cracking temperature. If the heat preservation is advanced to the beginning of watering, curing and covering, it will be counterproductive, but increase In order to determine the maximum temperature and cracking temperature of the concrete, the correct heat preservation time should start from the cooling of the concrete and should not be advanced.
One of the purposes of implementing thermal insulation during the cooling stage of concrete is to reduce the heat loss inside the concrete 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 give full play to its potential for strength growth, 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 growth 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 occurrence of 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 steep temperature change will inevitably affect the steep temperature change between the surface of the concrete and the ambient temperature. The effective coverage of thermal insulation materials can reduce The temperature gradient on the concrete section.
Engineering practice has proved that temperature change is an important and very complex load of concrete structures. The steepness of the temperature gradient can be regarded as the speed of "loading" the concrete and has an important influence 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, which reduces the ultimate tensile of concrete and weakens the crack resistance. The slow loading of concrete can cause the tensile stress and elastic modulus of concrete to be reduced compared with the rapid loading, while the ultimate tensile of concrete is increased. 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, the external insulation and internal slow-down can prevent and reduce concrete cracking.
In summary, it can be seen that no matter the ambient temperature is high or low, that is to say, whether the outside temperature in spring, summer, autumn and winter is high or low, the thermal insulation of concrete not only increases the surface temperature of the concrete, but also makes the temperature inside the concrete slow down. It also reduces the temperature difference between inside and outside and the temperature difference between the surface of the concrete and the atmosphere. For this reason, this "external insulation and internal slow-down" curing method can prevent and reduce concrete cracking.
Misunderstanding No. 8
Not based on the specific actual situation of the concrete, mechanically copy the specifications
In order to prevent the occurrence of early concrete cracks, people usually control the maximum temperature of the concrete, the temperature difference between the inside and outside, the temperature difference between the surface and the environment, the heating rate and the cooling rate and other technical indicators to achieve it. Among them, the temperature difference between the inside and outside of the concrete is generally considered not to be greater than 25℃; The temperature difference with the environment should not exceed 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; and some emphasize that the surface The instantaneous temperature difference caused by water spraying and mold removal should not exceed 15°C. Engineering practice has proved that the temperature difference between inside and outside of some engineering concrete is greater than 25℃, but the structure is not cracked; while the temperature difference between inside and outside of some engineering concrete is less than 20℃, but the concrete is cracked.
At the same time, the control indicators of the daily cooling rate are also different. Some believe that the daily cooling should not exceed 3°C, some believe that it should not exceed 2°C, and some even believe that it should not exceed 1.5°C.
The emergence of the above-mentioned differences in the technical data is actually very normal. Although some data are regulated by the regulations, no doubt about the regulations can be raised. Due to the randomness, diversity, and heterogeneity of concrete material composition, as well as the heterogeneity of concrete and differences in construction quality, it is not surprising that there are some differences in the technical data shown. This requires on-site technicians to follow the actual situation of the project. Considering the temperature control comprehensively, certain specifications should not be used mechanically.

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