Why does concrete need to be maintained? What is the purpose of curing? I have been in the construction site for many years and found that many concrete engineers do not have a clear understanding of the purpose of concrete maintenance. Without a clear understanding, it is difficult to find the correct method of maintenance, and it is also difficult to judge the degree of effectiveness of the maintenance measures taken, and thus it is difficult to ensure the quality of hardened concrete.
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Concrete maintenance work is very important, it is not too much to say that it plays a decisive role in the anti-seepage and anti-cracking properties of concrete and the quality of hardened concrete. We have made it clear from the theory that the mixing water of the concrete mix ratio cannot be lost after the concrete is compacted and formed, which is the purpose of curing. In actual engineering, the quality of concrete construction and maintenance can be evaluated according to the amount of water loss in concrete after dense forming and the thoroughness of water loss defects elimination, and the quality of hardened concrete and its impact on durability.
For a long time, from the theoretical community to the engineering community, the purpose of concrete curing is debatable. A.m. Neville in the book “The performance of concrete”, the purpose of curing is described as follows: “The purpose of curing is to keep the concrete saturated or as saturated as possible until the space in the fresh cement paste initially filled by water is filled with cement hydration products to the desired desired level”. Next it says: “In order to ensure that the hydration of concrete continues, the relative humidity within the concrete should be no less than 80%, and when the relative humidity of the surrounding air is not lower than this value, there is basically no water migration between the concrete and the surrounding air, and no effective curing is required to ensure subsequent hydration.” From this we can understand that the so-called “Concrete saturation”, is to maintain the relative humidity of the environment is not less than 80%.
This book is highly authoritative in the academic field of concrete and represents the previous theoretical formulation of the purpose of concrete curing. Neville is an internationally renowned expert on concrete, and his work has had a profound impact on the international scene and will always be held in high esteem and respect. Neville also pointed out realistically that “The technical level of the concrete industry is still not as mature as other industries”, and he attached great importance to solving practical problems in engineering practice, arguing that the quality of the concrete elements constructed on site “Depends almost entirely on the concrete production process to ensure “. This statement of his is very relevant. In his work, more attention is paid to the production practice of concrete and the engineering behavior of pouring into the components, rather than the study of small specimens in the laboratory.
Engineering practice has shown that previous theoretical statements about the purpose of concrete curing failed to accurately reveal the hardening laws of concrete. Theory suggests that effective curing can be dispensed with if the ambient relative humidity is not less than 80%. In fact, as long as the ambient relative humidity is lower than 100%, concrete will lose water. Once the concrete loses water, regardless of the amount of water loss, it will produce defects and internal stresses. Under the effect of internal stress, the concrete system deviates from its original equilibrium and becomes unstable, and its volume becomes unstable. The more water loss, the more serious the defects, the more unstable the volume. When the relative humidity of the environment is not less than 80%, even if the water loss is not a lot, the concrete is not cracked, but the water loss exists water loss channels, the existence of connected capillary pore defects, resulting in a reduction in the seepage resistance of concrete. These connected pore defects intensify the continued water loss of concrete after hardening in unfavorable environments, so that the internal stresses continue to increase and accumulate, and the chances of concrete cracking in the middle and late stages are high. Environmental harmful media also tend to invade the concrete, resulting in reduced durability of concrete. Therefore, during the hardening process of concrete, water loss channels must not occur. To avoid the appearance of water loss channels, it is necessary to prevent the concrete from losing water. Therefore, it is not the case that concrete does not need effective conservation if the ambient relative humidity is not less than 80%, but concrete should be cured whenever the ambient relative humidity is less than 100%. The purpose of curing is to prevent concrete from losing water and prevent the formation of water loss defects.
“The purpose of conservation is to keep the concrete saturated or as saturated as possible”, literally, this narrative is correct, the problem is how to define the concept of “Concrete saturation”. “Concrete saturation” Is not the state of the concrete when the relative humidity of the environment is not less than 80%, but “All the water-filled space of the concrete are filled with mixing water”. For non-sewage, no segregation of the concrete mix, the system is a good equilibrium system after compacting, when all its water-filled spaces are filled with mixing water. It is from this point that we want to keep the concrete saturated and prevent loss of mix water.
The phrase “Until the space initially filled by water in the fresh cement paste is filled to the desired degree by the cement hydration products” Suggests how long is the proper time to maintain the concrete. But “The desired degree” Is a vague concept, there must be indicators to measure, only to become clear. In fact, as long as the concrete does not lose water after compacting, the mixing water in all its water-filled spaces during hardening can be maintained in a full state. Because there is no water loss, there is no water migration channel, and the hydration products can fill the water-filled space completely dense. At this point, the concrete achieves high impermeability. Achieved high impermeability of the concrete, greatly slowed down the loss of mixing water in the late concrete, the environment is also difficult to enter the concrete harmful media. Therefore, the author believes that this “Desired degree”, is the high impermeability of concrete. Normal hardening conditions, concrete to achieve high impermeability does not require 28d, as long as 3 to 7d. From this perspective, the specification of the general concrete maintenance period of 7d is reasonable, but the 7d concrete should not lose water.
In the construction site, i heard some construction workers say that the maintenance is to ensure that the cement hydration has enough water. This statement should be consistent with the expression “To ensure that the hydration of concrete continues” In the book. However, according to the hydration theory of cement, silicate cement fully hydrated theoretical water-cement ratio of 0.227, modern concrete cementitious materials are basically composite cementitious materials, the activity of the admixture is lower than the cement, can generate less hydration products than cement, therefore, composite cementitious materials fully hydrated theoretical water-cement ratio should be below 0.227, while the actual water-cement ratio in the production of concrete are basically in 0.3 or more, the current c30 and below low strength concrete, many in the 0.5 or more. These mixing water filled in the water-filled space is much higher than the theoretical water-cement ratio water consumption when the cementitious material is fully hydrated, thus ensuring the continuation of hydration not only during hardening but also in the middle and later stages of concrete, provided that these mixing waters are not lost. Why the concrete buried in the ground, its late strength is always higher than the concrete exposed to the atmosphere, is this reason. Therefore, the real purpose of concrete curing is still to prevent the loss of mixing water.
Whether a theory is correct depends on whether it can stand the test of practice. In the past, under the guidance of the maintenance theory of some specifications of the maintenance method, as well as the construction site to take some maintenance methods, have failed to meet the needs of concrete hardening. The outstanding problem with these maintenance methods is that after the concrete is densely formed, there is a widespread phenomenon of indulgence in water loss, the concrete does not get the normal growth and development, the formation of many water loss defects during hardening, resulting in long-term concrete cracking and seepage of quality problems, durability problems.
I recently engaged in the technical work of dry mortar mixing plant preparation and laboratory preparation work. Mortar is actually concrete without coarse aggregates. Due to the use of different engineering objects, mortar and concrete are very different, but there are also commonalities. During this period, i contacted an entrepreneur who has been engaged in the production and construction of dry mortar since 2005, and his insight on how to improve the crack resistance of mortar is unique. He said that to improve the crack resistance of mortar we must improve its water retention, and the higher the water retention rate, the better the crack resistance. Perhaps the author has read too little technical information about mortar, which says that adding cellulose ether and redispersible emulsion powder can substantially improve the crack resistance of mortar, but i have not yet seen anyone linking the crack resistance of mortar with water retention. This entrepreneur has broken the essence of the problem in one word, which makes me quite surprised. It is not strange to say that it is a summary of his practical experience. The problem is also well explained. It is because of the addition of cellulose ether and re-dispersible latex powder, improve the water retention capacity of mortar, reduce the evaporation loss of mixing water and grass-roots water absorption loss after mortar construction, and strive to make the mortar water-filled space of mixing water to maintain a full state during the mortar hardening, hydration products to fill the water-filled space completely dense, reduce the formation of water loss channels and its resulting internal stress due to water loss, so that the hardened mortar the quality of hardened mortar is improved and the crack resistance is increased.
How to maintain concrete in summer? The key is to find out the purpose of maintenance, the essence of maintenance. When the purpose is clear, the methods are also available. These methods are the methods that can effectively prevent the loss of mixing water, the methods that can completely eliminate the defects of water loss.
The hot summer sun, exposure to the conditions of the fastest water loss of concrete, hardening is also fast. For a large area of concrete, after vibrating and smoothing, pay close attention to master the second smearing time before the initial set. If you miss the appropriate smear time, the quality of hardened concrete will cause irreversible effects. When the second smear, be sure to cover while smearing, and pour enough water to moisturize. The smear is to completely eliminate the water loss defects formed before the initial set, and the cover is to prevent the concrete from continuing to lose water after the smear. Covering the best use of absorbent sacks or geotextiles, to be connected to each other, and keep the cover has been in a full state of water. Only in this way can effectively prevent the concrete from losing water. Covering plastic film is a faster and less labor-intensive method, but pay attention to its water loss prevention effect. Recently, a construction worker told the author such a case: A project in the summer of 2006 construction, mixing station technical instructions may not be clear enough, the construction unit according to the technical instructions to cover plastic film, a more serious early cracking, the final result is to knock off the floor panel, re-pouring, concrete companies suffered losses. The author’s analysis speculates that the cover may be worn out, very thin film, but also not close to the concrete surface, some places and the concrete surface has a large gap, in the hot sun baking concrete water loss very quickly, water evaporated out or evaporated through the film, or into small water droplets attached to the film, or into steam hovering between the concrete surface and the film. These evaporated water is not as some information that can maintain the concrete, because it can not stop the continued water loss of concrete under the hot sun. More water loss, concrete cracking is undoubtedly. Cracking is serious, indicating serious water loss. So cover film should be selected impermeable, relatively thick film, a smoothing or second smoothing immediately after covering the concrete surface, and the film pressure, so that it is tightly attached to the concrete surface, so that the mix water is difficult to evaporate out. If the sun is very fierce, for important projects, or large volume of concrete, after the completion of the smear cover, it is best to store water maintenance, so that the quality is more assured.
China’s northwest water-scarce areas and african desert water-scarce areas of summer concrete construction, there is no sufficient water conservation of large-area concrete, the second wiping pressure after the best to the concrete surface drift once a water splash, so that the concrete surface fully wet, and then covered with impermeable impermeable film and pressed, and then covered with dry straw mat or dry sacks on the surface, to be interconnected, do not leave the exposed surface, to avoid the film is directly baked by the sun. After this, although no longer drenching, should also have a better anti-cracking effect. This is a maintenance method for water-scarce areas. Never let the dense formed concrete exposed to the sun for a long time and let it lose water.
Another feature of summer construction is that there is a lot of rain, and sometimes the rain comes very quickly and fiercely. The slump of pumped concrete is usually larger in summer because of the high temperature, but the slump should be as small as possible during the rainy season construction. Low-slump concrete has a better ability to retain slurry in the rain, reducing the loss of rain-washed slurry. Before the arrival of heavy rain should make early judgments, plastic film will have been vibrancy smoothing and unvibrancy of the concrete are covered. There are construction units because it is too late to cover only the vibro-smoothed concrete, but not the vibro-concrete is not covered. I think this is the opposite of the method. The unvibrated concrete has a lot of footprints, and after heavy rain, these footprints are filled with rainwater, which is generally not excluded before vibration, and it is difficult to remove it. The process of vibration, rainwater into the concrete, greatly increasing the water-cement ratio, coupled with the loss of some of the slurry, the impact on the quality of concrete is great. I suggest to the construction unit, if the rain comes quickly, it is too late to cover all, the first to cover the unvibrated concrete, the concrete has been vibrated and smoothed, but also be sure to cover the perimeter well, so that the film is close to the surface of the surrounding concrete, to prevent rain washed away cement paste. Concrete vibration smoothing, it is best to slightly press the perimeter a little higher, so that rainwater accumulation in the concrete surface. Doing so, played a “Instant water conservation” Role. The construction unit to do so, the effect is still good.
In short, the maintenance method is flexible. Different construction units may have different maintenance methods, to judge whether a maintenance method is reasonable, can meet the needs of concrete growth and development, to ensure the quality of hardened concrete, depending on whether it can effectively prevent the loss of mixing water, whether the complete elimination of water loss defects. If such an effect is achieved, the purpose of maintenance will be achieved. It should also be noted that different components may have different ways of water loss, and the corresponding water loss prevention measures should be taken for different ways of water loss. The purpose of maintenance discussed in this paper is to provide the necessary conditions for the normal growth and development of concrete. Regardless of any season, what kind of components, as long as the ratio is reasonable, as long as these conditions are met, the normal hardening temperature we will be able to ensure the quality of hardened concrete, its durability quality will also be improved.