Traditional Culture Encyclopedia - Weather forecast - Don’t worry about high-temperature construction of concrete in summer. Six major precautions must be kept in mind?

Don’t worry about high-temperature construction of concrete in summer. Six major precautions must be kept in mind?

As the weather gets hotter and hotter, there is a problem before us, and that is concrete construction in summer. Concrete is a material that is easily affected by temperature. How to control the spread of the impact? Look down.

The main characteristics of concrete construction in summer are hot climate and high temperature. The components of concrete have high temperature.

Due to high temperature and sunlight, the sand and stone materials used to make up the concrete mixture have high temperature. ; New cement from the factory often has a high temperature. Due to the hot climate, the heat in the cement is not easy to dissipate, and the influence of factors such as sunlight makes the temperature of the cement higher. The temperature of some cement can be as high as 70°C. Supports such as concrete foundations or formwork have high temperatures and large exposed surfaces, which are greatly affected by the environment and climate change

The main reason for cracks

Evaporation of surface water The rate is large, causing water loss on the surface of the newly poured concrete, forming plastic shrinkage cracks. Water loss and plastic shrinkage on the surface of the concrete are the main reasons for cracks. When the evaporation rate of the concrete surface is greater than 1kg/m·h, plastic shrinkage cracks are prone to occur on the surface. Air temperature, temperature of the concrete mixture, relative humidity of the air and wind speed are the main factors affecting the evaporation of water on the concrete surface. The higher the temperature of the concrete itself, the smaller the relative humidity of the air, the greater the wind speed, and the greater the evaporation rate of water on the surface of the concrete. In summer, due to the higher temperature, the heat of cement hydration in the concrete is generated in a shorter period of time, which promotes the early development of concrete. Increase in temperature. Relevant data show that when the temperature is 14°C, 43% of the total heat of hydration is generated in the first 24 hours after the concrete is mixed; when the temperature is 30°C, 62.5% of the total heat of hydration is generated in the first 24 hours after the concrete is mixed. %. On the one hand, the high temperature of the coarse and fine aggregates and the cement itself increases the temperature of the early concrete. On the other hand, it makes the hydration heat of the cement more concentrated. The high temperature of the environment makes it difficult for the heat in the concrete to dissipate, so the overall temperature of the concrete is relatively high. The temperature of concrete constructed in other seasons is much higher. Under the influence of the dry wind in summer, high-temperature concrete increases the evaporation rate of water on the surface, causing rapid water loss on the surface and severe plastic shrinkage. The high temperature inside promotes the rapid hydration of cement and hardening of concrete. Under the combined action of severe plastic shrinkage on the surface and internal constraints, plastic shrinkage cracks on the concrete surface are caused. Evaporation of water causes more than just surface cracks. Moreover, due to water loss, the cement hydration water on the surface of the concrete is insufficient, which affects the hardening and strength growth of the surface concrete.

The difference in temperature between concrete sections causes thermal cracks on the surface. During summer construction of concrete, the temperature is higher than the ambient temperature due to the effects of sun exposure on the foundation or formwork. Its height makes the cement hydration speed and hydration heat generation speed in the concrete adjacent to it larger than the surface, causing the temperature difference between the surface and the internal concrete to increase. When the difference between the surface temperature and the internal temperature exceeds 15°C, it is easy to occur. Surface thermal cracks. The foundation or formwork is affected by high-temperature concrete and is prone to expansion deformation. However, the thickness of large-surface concrete is generally small. The expansion deformation of the lower part exerts reverse constraints on the shrinkage of the surface concrete, increasing the tensile stress and causing surface cracks. Adverse effects.

High temperature has an adverse effect on the microstructure of concrete. It is generally believed that when the concrete temperature is below 50℃, the change in the base microstructure is negligible. When the concrete temperature is below 70℃ When the temperature is above ℃, the changes in its microstructure are considered unfavorable. The impact of high temperature on large-volume concrete is often ignored. General concrete structures constructed in summer are concentrated due to the air temperature, material temperature, formwork or foundation temperature, and the concentrated release of cement hydration heat. The temperature rise caused by the impact will often make the temperature exceed the unfavorable maximum temperature. Due to the small resistance to deformation of early-age concrete, changes in the microstructure of concrete not only affect the overall strength of the concrete, but also easily form thermal cracks on the surface.

The changing climate makes the concrete surface vulnerable to cold shock. Due to the changeable climate in summer construction, such as sudden rainfall, the temperature will suddenly drop, and the sudden drop in the temperature of the concrete surface will Temperature shrinkage occurs on the surface to produce surface temperature shrinkage cracks.

The increase in cement dosage and water-cement ratio during construction increases the plastic shrinkage deformation of concrete. During summer construction, due to the influence of evaporation and other factors during mixing, transportation, etc., it is easy to cause concrete The mixture loses water and its workability is reduced. Therefore, it is easy to increase the cement dosage and water-cement ratio during construction to improve the workability of concrete. However, the increase in cement dosage and water-cement ratio is more likely to cause shrinkage. When shrinkage is restricted, cracks will easily form.

When hardened concrete is constructed at high temperatures, the temperature at which it forms a solid determines the base length of the concrete. When the whole body cools, it begins to shrink from this length and temperature, which is very easy to Produce overall temperature shrinkage cracks, such as pavement breaks, etc.

Prevention measures

Choose appropriate maintenance methods and start maintenance as early as possible. Starting maintenance as early as possible and keeping the concrete surface moist can prevent evaporation, reduce shrinkage, and ensure Concrete surface hydration proceeds smoothly.

For newly poured concrete that adopts various types of curing, try to start curing it after the surface shaping of the concrete is completed and before the surface water film disappears. However, during curing, the formed surface of the concrete must not be polluted or damaged. Therefore, appropriate curing methods and the best quality must be selected. maintenance start time.

Increase the relative humidity of the air. Increasing the relative humidity of the air can effectively reduce the evaporation rate of water on the concrete surface. Spray water mist in the wind direction or around the newly poured concrete to increase the evaporation rate. The relative humidity of the air is a simple, easy, low-cost and effective measure. A simple spray method can be to use a plastic hose with pinholes to set up around the newly poured concrete site or in the upwind direction, and inject water with a certain pressure to form a pinhole spray.

Reducing the temperature of the concrete can effectively reduce the plastic shrinkage caused by the evaporation of water on the surface of the concrete, prevent the occurrence of thermal cracks, and at the same time reduce the temperature of the hardened concrete and reduce the Possibility of shrinkage cracks due to temperature contraction. The following methods can be used to cool down: (1) Use measures such as building a shed to protect the newly poured concrete from direct sunlight and reduce the ambient temperature of the concrete surface; (2) Cool down the various constituent materials of the concrete and control the fresh mixing of the concrete. The temperature of the object is below 32°C.

(1) Cooling of coarse aggregate. The heat dissipation effect is better by spraying water in advance. This method is the most economical and effective. It is necessary to strictly control the moisture content and adjust the construction mix ratio. (2) Cooling of fine aggregate. It is not advisable to spray water, and pay attention to the temperature difference between layers. Before loading, push the outer fine aggregate away, use the inner fine aggregate that has not been exposed to the sun, and strictly control the moisture content to ensure that the test sample is consistent with the material used. (3) Cool the mixing water with water. Use a reservoir to cover it to avoid direct sunlight, and use well water as mixing water. (4) Cement cooling. Cement cooling directly affects the exit temperature of concrete. Each mixing station adopts the method of first entering, first using, and then storing as much as possible before using to cool down. And strengthen the detection of cement temperature and stability.

When transporting concrete, the concrete mixer truck is equipped with sun protection facilities to shorten the transportation time as much as possible. Mix slowly during the transportation of concrete. After testing, a slump of 1 to 2 is added to the concrete mixing station to ensure the workability of the concrete when it arrives at the site. It is strictly prohibited to add water for mixing during transportation.

After the concrete is poured, the surface is covered with a clean plastic film. After the initial setting, the plastic film is removed, covered with soaked burlap, and sprinkled with water intermittently. After the formwork is removed, the pier columns are wrapped and covered with plastic film, and water buckets are placed on the upper part for self-flow maintenance; the cap platform is backfilled in time, and the table is sprinkled with water for maintenance; the culvert is covered with straw curtains and sprinkled with water for maintenance. Concrete structures must be kept in a cured and moist state for no less than 14 days. Wet curing must be continuous and no dry-wet cycle must occur. During moisturizing and curing, sunshade and wind-shielding measures are taken to control the temperature and the influence of hot and dry wind. When curing is stopped, the structure should be allowed to dry gradually, so that the creep properties of concrete can be used to "unload" the temperature and drying shrinkage stress to avoid the occurrence of cracks.

For more information about engineering/service/purchasing tender document writing and production to improve the bid winning rate, you can click on the official website customer service at the bottom for free consultation: /#/?source=bdzd