The driving forces that cause concrete shrinkage can be divided into two categories: temperature and humidity. The early shrinkage caused by temperature action includes the shrinkage of the temperature drop caused by the heat of hydration and the temperature difference between day and night, and the former is particularly significant in large-volume concrete. Early shrinkage caused by humidity includes plastic shrinkage, self-shrinkage, and drying shrinkage. It is worth noting that the shrinkage caused by temperature and humidity occurs at the same time and interacts with each other, which makes the study more difficult. 1. The temperature shrinkage caused by the heat of hydration The temperature shrinkage is mainly generated in the cooling process of concrete after the temperature peak of cement hydration and heat release. Cement will release a lot of heat in the early hydration process, generally 502J heat can be released per gram of cement, and under absolute conditions, 5 8 adiabatic temperature rise will be produced per 45k** slurry hydration. In the absence of retarder, temperature peaks usually occur around the first 12 hours. Subsequently, due to the slowdown of hydration and the decrease of heat release, the temperature begins to drop under the heat exchange with the external environment. Due to the inconsistency of the internal and external heat dissipation conditions of the concrete, the temperature of the surface concrete decreases rapidly, and a temperature gradient occurs along the concrete section, which makes the temperature drop process shrink along the inconsistency of the section, which leads to the tension of the surface concrete, and the temperature crack occurs when the tensile stress exceeds the tensile strength of the concrete. This can be as high as 60 in bulk concrete, which is the main factor causing early cracks in this type of concrete. In addition, it should be explained that expansion cracking usually does not occur in the temperature rise stage of hydration, because the concrete is still in a fluid plastic state during the temperature rise expansion process, and the temperature rise process is rapid, and the temperature rise process is relatively uniform along the cross-section. The subsequent heat dissipation and temperature drop process is relatively slow, the uniformity is poor, and the concrete has gradually hardened, so it is often easy to appear temperature shrinkage cracks at this time. 2. Temperature contraction caused by the temperature difference between day and night, and the temperature difference between day and night will also cause corresponding temperature deformation. For example, for concrete slabs, under the irradiation of the morning sun, the temperature of the surface concrete increases significantly, and its expansion is limited by the bottom concrete and causes the surface layer to arch; During the day, as the temperature of the whole section tends to be the same, the deformation shows free elongation. At night, as the surface temperature begins to decrease, the surface bends again. Therefore, for newly poured concrete, early temperature cracks are very likely to occur when the temperature difference between day and night is large. 3. Plastic shrinkage Plastic shrinkage occurs in the plastic stage before the final setting of concrete, usually about 4 15h after pouring, and most of them occur in the flow plastic stage before the initial setting. At this stage, the cement hydration reaction is intense, the molecular chain is gradually formed, and the phenomena of water secretion, rapid evaporation of water and uneven settlement of aggregate and slurry appear. Therefore, plastic shrinkage can be subdivided into three categories: water loss condensation, chemical reduction, and sedimentation contraction. Water loss and condensation are caused by the migration of water from the inside of the concrete to the outside due to water secretion and other factors during the hydration process of fresh concrete, and the rapid evaporation on the surface, which mostly occurs in dry heat and windy weather. Chemical reduction refers specifically to the shrinkage of the macroscopic volume of the system due to the fact that the average density of the products before and after the hydration reaction is smaller than that of the reactants in the early plastic stage; Settlement shrinkage is the phenomenon that the concrete is unevenly settled after pouring and smashing, and there is delamination and segregation, coarse aggregate sinking, cement slurry floating, and the concrete is separated from each other when blocked by steel bars and other phenomena causing cracking. Relatively speaking, the early cracks caused by plastic shrinkage are relatively easy to deal with, that is, usually this kind of shrinkage cracks can be avoided by sufficient vibration and maintenance in construction. 4. Drying shrinkage Drying shrinkage is usually an irreversible shrinkage that occurs when the concrete stops curing and loses the adsorption water of the internal capillary pores and gel pores in the unsaturated air, and the dry shrinkage of the cement slurry increases with the decrease of relative humidity. The dry shrinkage mechanism is related to the internal pores of the cement slurry, and the result of cement hydration is the formation of hydrated calcium silicate and the formation of a large number of water-filled micropores (> 5nm capillary pores and 05~2.5 nm of gel pores), these micropores store excess water that has not been consumed by hydration. When the concrete is dry, the evaporation rate of the surface water of the concrete may exceed the rate of the concrete to excrete water, therefore, the water surface of the surface layer decreases, and as the evaporation continues, the loss of water gradually develops from the surface layer to the inside of the concrete, and the adsorption water in the capillary pores and gel pores is lost one after another. The loss of moisture in these micropores will create a capillary negative pressure in the pores and promote the formation of a gas-liquid meniscus, which will exert tensile stress on the pore wall (i.e., the hydrated calcium silicate gel skeleton) and cause the cement slurry to shrink. In the past, it was generally believed that the drying shrinkage mainly occurred during the age of 3 90 days after pouring, but in fact, if the early maintenance was not carried out in time, the drying shrinkage within 3 days of adding water and stirring was quite large, which has been studied in the literature. The water-cement ratio is respectively. 32 The deformation curves of the OTC3 and OTC4 specimens measured in the age 3d under the condition of open drying after initial setting, in which OTC3 was not mixed with water reducer, and OTC4 was mixed with 18% aliphatic high-efficiency superplasticizer, from the test results, the drying shrinkage of the two at the age of 3 days is very large, the former is about 5 85 10-6mm, while the latter is about 1110 10-6mm, almost twice the former. However, the ultimate tensile strain of concrete is usually only about 300 10-6mm, which shows that if the sealing or moisturizing maintenance is not done in time after pouring, the drying shrinkage is enough to lead to the generation of early shrinkage cracks. 5. The self-shrinkage of self-shrinking concrete refers to the volume change caused by the self-drying of concrete without humidity exchange with the surrounding environment. The so-called self-drying means that in the process of cement hydration, due to the absence of external water** or the speed of external water migrating to the inside of the system through the capillary pores is less than the speed of water consumption by hydration, the water required for hydration will be absorbed from the pores, so the gas-liquid meniscus is formed in the pores, and the reduction of the absolute volume of the hydration reaction will be compensated in the form of micropores formed inside. The reduction of capillary water reduces the saturated vapor pressure inside the concrete, that is, the relative humidity will decrease, but the reduction of capillary water does not cause the quality of cement stone to be lost, and this phenomenon is called self-drying. It can be seen that the mechanism of self-drying on self-shrinkage is essentially the same as that of drying shrinkage, that is, it is related to the capillary pressure caused by water loss, but the difference is that the water loss mode of the two is different. It is worth noting that self-drying can occur under any water-cement ratio conditions, but the degree of performance varies from concrete to concrete. When the water-cement ratio is low, the general occurrence of this microscopic phenomenon in the capillary pores will be manifested as macroscopic self-contraction. When the water cement is relatively high, the self-drying phenomenon only occurs in the local capillaries, but can be ignored on the macroscopic scale. For w c 042 of the ordinary concrete without water reducing agent, the shrinkage in a dry environment is mainly dry shrinkage, and the self-shrinkage can be ignored. It should be noted that the early self-shrinkage value of concrete has been quite large, if coupled with the temperature drop shrinkage caused by the heat of hydration, the sum of these two shrinkage values alone can easily exceed the ultimate tensile strain of concrete, so it is not difficult to understand that some current high-strength and high-performance concrete, even in the process of constant temperature water cultivation, cracks appear in the early stage. Plastic shrinkage cracksPlastic shrinkage cracking occurs within a few hours of concrete pouring, usually before the surface is finished. As its name suggests, this type of cracking occurs when the concrete is still in a plastic state, and its cause is the shrinkage of the concrete surface. Plastic shrinkage cracking occurs mainly on concrete slab structures, such as floors, floors, pavements, etc., and can also occur on the surfaces of other horizontal structures, such as beams, foundations, and the top of walls. Most of the cracks formed by plastic shrinkage cracking are parallel to each other, of different lengths, and 03 meters to about 1 meter. Of course, you will also see a variety of other shapes of cracks, which are usually not through-cracks. Plastic shrinkage cracks are usually shallow joints in the surface layer, which generally have no impact on the safety of the structure and rarely need to be repaired. But it affects the perception. In addition, it may also have a negative impact on the durability of concrete, especially for reinforced concrete, where the potential for harm is even greater. 
Typical plastic shrinkage cracks
Plastic shrinkage cracking generally does not form penetrating cracksEnvironmental and construction factors1. Construction environment and construction conditions: wind speed is greater than 10km hr, relative humidity is low, ambient temperature and/or concrete temperature is high, etc. 2. Incorrect, untimely and incomplete curing: curing is the simplest and most effective means to avoid plastic shrinkage cracking, and correct, timely and thorough maintenance is the only way to avoid plastic shrinkage cracking. 3. Tools and processes for surface smoothing and polishing: If the tools and processes for smoothing and polishing are correct, plastic cracking can be avoided and repaired. Conversely, it may exacerbate the severity of plastic shrinkage cracking. 4. The cold climate or low base temperature will affect the curing time of concrete, thereby increasing the risk of plastic shrinkage cracking. concrete itself factor1. The amount and type of Portland cement will affect the water leakage and internal temperature, so it will also affect the probability and severity of plastic shrinkage cracking. 2. Mineral admixtures - such as fly ash, silica fume, etc., not only improve the compactness of concrete, but also reduce the water leakage rate, so this kind of concrete is more prone to plastic shrinkage cracking. 3. The type and amount of admixtures, such as retarders, delay the curing time of concrete, so as to increase the risk of plastic shrinkage cracking. 4. A high water-cement ratio will also delay the curing time of concrete, thereby increasing the risk of plastic shrinkage cracking. In fact, plastic shrinkage cracking is often caused by multiple factors, not a single factor. However, controlling the evaporation rate of moisture on the surface of concrete is an important means to avoid plastic shrinkage cracking. For ordinary Portland concrete, the surface evaporation should be controlled to be less than 1kg m2 hr. However, for the concrete mixed with ultra-fine mineral additives, the surface evaporation rate should be reduced as much as possible according to the mix ratio and specific conditions of the concrete. The evaporation capacity available for reference includes 750g m2 hr, 500g m2 hr, 250g m2 hr, etc. Again, correct, timely and thorough maintenance is the simplest and most effective means to avoid plastic shrinkage cracking, even if there are congenital deficiencies in concrete or construction environment, curing can correct these deficiencies, even if it can not completely eliminate plastic shrinkage cracking, at least it can greatly reduce the severity of plastic shrinkage cracking. Therefore, in order to avoid the problem of plastic shrinkage cracking, the construction unit should assume the main responsibility, and must strengthen management to achieve correct, timely and thorough maintenance. The concrete business should timely inform the construction unit of the matters that need special attention in advance, such as whether it is mixed with external admixtures that may affect the water leakage of concrete, etc., and work with the construction unit to formulate a maintenance plan and urge the construction unit to maintain it in accordance with the plan. Precautionary measuresControlling the evaporation rate of water on the concrete surface is an important means to avoid plastic shrinkage cracking. Measures to control the evaporation rate include: 1. Correct, timely and thorough curing of concrete; 2. Set up a temporary windshield wall to avoid the concrete surface being directly blown by the wind; 3. Set up an awning to avoid direct sunlight on the concrete surface; 4. Spray moisturizer on the surface of concrete; 5. Spray water mist on the surface of concrete; 6. Cover the concrete surface with moisturizing film, etc.; Other measures include: 1. Pre-wetting the template; 2. Reduce the temperature of the concrete by using cold water and adding crushed ice cubes; 3. Mix an appropriate amount of fiber into the concrete; Again, no preventive measure can replace "correct, timely and thorough maintenance", which is the simplest, most effective and cheapest means. It should be noted that moisturizers are usually made with one part of the active ingredient and 9 points of water, which contains more water, so it must be used after the surface of the concrete is lightened. Otherwise, the moisture in the humectant enters the concrete surface, which will change the water-cement ratio of the concrete on the surface and negatively affect the durability of the concrete. Caution should also be taken when spraying water mist to prevent water from accumulating on the surface of the concrete, which will also change the water-cement ratio of the surface concrete. In addition, it is necessary to prevent direct spraying of water on the concrete surface, which may damage the concrete surface. 
Spraying water directly onto the surface can cause severe plastic shrinkage cracking: if the curing water temperature is much lower than the surface temperature of the concrete, spraying it on the surface of the concrete will cause the surface to shrink violently, resulting in tortoiseshell cracking. Finally, plastic shrinkage cracks should not be confused with other early cracks, such as plastic settlement cracks, template displacement cracks, early temperature cracks, early temperature gradient cracks, and early uneven settlement cracks.