2. The influence of curing environment on the formation of concrete strength
Maintenance environment mainly includes two factors: maintenance temperature and humidity. Curing temperature plays an important role in the formation of concrete strength. The hydration of cement plays an important role in the hardening of concrete. Higher curing temperature can accelerate the initial hydration speed of concrete, and the initial strength of concrete is also higher. However, the higher the initial curing temperature, the greater the later concrete strength decline. Because the rapid initial hydration reaction will lead to uneven distribution of hydration products, the place with low distribution of hydration products will become the weak link in cement paste, thus reducing the overall strength; The area with high distribution of hydration products is wrapped around cement particles, which hinders the hydration reaction, thus reducing the output of hydration products. Under the condition of low curing temperature, the hydration speed is slow and the diffusion time is sufficient, and the hydrate is evenly distributed in the cement paste, which is beneficial to the development of later strength. In actual construction, the curing temperature is generally not lower than 5 degrees Celsius. When the temperature drops below zero, because most of the water in concrete freezes, cement particles cannot react with ice, and the strength of concrete stops developing.
In addition, the curing humidity of concrete must be ensured during curing. The humidity of the surrounding environment has a significant influence on whether the hydration of cement can be carried out normally: moderate humidity can make the hydration of cement go smoothly, thus giving full play to the strength of concrete. If the humidity is not enough, concrete will lose water and dry, which will affect the normal hydration of cement, or even stop hydration and reduce the strength of concrete. With the progress of cement hydration reaction, the volume of cement gel in concrete increases, while the volume of capillary cavity decreases, and the density and strength of concrete increase. If the curing of concrete in air is compared with that in water, it can be found that the strength of concrete cured in air will be reduced, the strength will be attenuated in the later period, and the strength of concrete cured in water will be obviously improved. Under normal curing conditions, the strength of concrete will increase with the increase of age. Because cement hydration can only happen in capillary filled with water. Moreover, a large amount of free water in concrete soil is gradually absorbed by the generated gel in the process of cement hydration, and the water for internal hydration reaction is less and less. This not only seriously reduces the strength of concrete, but also increases the porosity and water permeability of the internal structure of concrete due to the failure of hydration, or forms shrinkage cracks, thus affecting durability. Therefore, in order to make the concrete hydrate normally, it is necessary to maintain a certain humidity in the surrounding environment for a certain period of time after molding. Even when water is not completely saturated, concrete will hydrate. Cement can absorb water from these reservoirs and further hydrate, but the hydration speed slows down with the decrease of relative humidity in cement paste. Therefore, the hydration and strength growth rate of concrete closed to prevent water loss will be higher than that of concrete that has been in wet curing. If the humidity drops below 80% of the relative humidity, all hydration will stop, which may happen in concrete with low water-cement ratio. The longer the wet curing time of concrete, the higher the strength, but it is difficult to maintain it for a long time in practical engineering. In engineering, the concrete surface is usually covered with straw bags and watered constantly to prevent the abnormal shrinkage of concrete. The experiment shows that the strength of wet curing for 28 days can be guaranteed by wet curing for about 7 days. Therefore, the water curing time of concrete is usually 7 days, which can be extended or shortened according to air humidity, temperature, cement varieties and additives.
Therefore, it is very important to control the temperature and humidity of curing after concrete pouring. Pay special attention to watering in summer and maintain the necessary humidity, and pay special attention to maintaining the necessary temperature in winter. The first few days after concrete pouring is the key period of maintenance, so more attention should be paid in construction. It is necessary to maintain the surrounding environment at a certain temperature and humidity within a certain period of time after molding, so as to better enhance the strength of concrete and better ensure the strength and durability of the building during construction.
3. The influence of moist heat treatment on the strength formation of concrete.
In actual construction, especially in winter, wet heat treatment method is generally used for concrete. Wet heat treatment can improve the development speed of concrete strength and achieve good production efficiency and economic benefits. In wet heat treatment, the principle of delayed heating curing can be applied to minimize the influence of early high temperature on concrete strength. Delayed temperature rise is an important problem in wet-heat treatment. From the beginning of concrete mixing to the curing temperature, if the delay time is enough, the rapid heating has no adverse effect, but if the delay time is small, it will have an adverse effect on the strength, and the higher the curing temperature, the greater the impact. The temperature of concrete during setting has a great influence on the later strength. Excessive temperature rise will lead to the loss of strength in the later stage, which can be as high as 1/3 of standard curing. The greater the water-cement ratio, the greater the influence, and the more obvious the influence on the quick-hardening portland cement.
4, concrete curing method
At present, the common curing methods in highway concrete engineering are water curing, closed curing and steam curing. Water curing Water curing is the most common concrete curing method. It is a good curing method to provide sufficient water for concrete hydration, replenish water loss by sprinkling water, and avoid the phenomenon of concrete self-drying. However, this maintenance method is time-consuming and laborious, and it must be monitored 24 hours a day, and it is not good enough to maintain the vertical plane. Spraying water on the covering layer can reduce the above defects, which can not only save a lot of water, but also prevent water evaporation. At the same time, these covering layers can also play the role of insulation layer and improve the curing temperature of concrete. Absorbent materials, such as sacks and straw curtains, are usually used as covering materials. This type of mulch needs to be watered regularly, because it is easy to dry and saves time and effort compared with simple sprinkler maintenance. At present, sprinkler maintenance with cover is mainly used for concrete pavement maintenance. Sealing curing is to wrap concrete with sealing materials such as plastic film to prevent moisture in concrete from evaporating. Sealing maintenance saves time and effort, especially suitable for sprinkler maintenance. Steam curing is a method to accelerate the hydration reaction of concrete, which can achieve the required compressive strength in a short time and greatly shorten the construction period. Steam curing is generally used in winter construction, and its maintenance cost is generally high.
5, problems that should be paid attention to in the process of concrete curing.
The purpose of curing is to ensure the normal hydration process of cement, and how to control the temperature and humidity of curing reasonably during construction is the key to concrete curing. The common cracks in concrete are mostly surface cracks with different depths, which are mainly caused by temperature gradient. The main purpose of early curing of concrete is to maintain suitable temperature and humidity conditions, so as to achieve two effects. On the one hand, it can protect concrete from unfavorable temperature and humidity deformation and prevent harmful cold shrinkage and dry shrinkage; On the one hand, the hydration of cement goes smoothly to achieve the designed strength and crack resistance.
Cement hydration can only occur in capillary filled with water. Therefore, it is necessary to create conditions to prevent water from evaporating from the capillary. Moreover, the surface area of cement gel produced in the process of cement hydration is large, and a lot of free water becomes surface adsorbed water. During the curing period, the concrete must remain saturated or close to this state. Only in the saturated state, the hydration speed of cement is the largest. The temperature difference between concrete and surrounding air will also affect water loss. Under the condition of low curing temperature, the hydration is slow and there is enough diffusion time to make the hydrate evenly distributed in the cement stone. In the early curing stage of concrete, there is an optimal curing temperature, at which the strength of concrete is the greatest at a certain age. When the outside air temperature is higher than 5℃, concrete is generally covered with water to ensure the curing humidity, especially when the temperature is high in summer, it should be covered immediately after pouring to prevent water evaporation. And the mulch film should be wet frequently, and the days of watering mainly depend on the types of cement: generally, for ordinary cement, silicic acid cement and slag cement concrete, it is not less than 7 days under normal circumstances; Concrete mixed with retarder or with impermeability requirements shall not be less than 14 days. When the outside air temperature is lower than 5℃, concrete can be mixed with antifreeze or treated with damp heat. Concrete mixed with antifreeze shall not be cured by spraying water, and the exposed surface, weak structure or vulnerable parts must be covered, and the curing temperature shall not be lower than the critical temperature specified by antifreeze.