As the weather is getting hotter, there is a problem before us, that is, concrete construction in summer. Concrete is a material that is extremely susceptible to temperature. How to control the spread of the impact?
1. The main reason for cracks
01
The surface water evaporation rate is large, causing the surface of the newly poured concrete to lose water, forming plastic and shrinking cracks. When the evaporation rate of concrete surface is greater than 1kg/m·h, plastic shrinkage cracks are easy to occur on the surface. Air temperature, temperature of concrete mixture, relative humidity of air and wind speed are the main factors that affect the evaporation of water on concrete surface. The higher the temperature of the concrete itself, the lower the relative humidity of the air, the higher the wind speed, and the higher the evaporation rate of water on the concrete surface. In summer, due to the high temperature, the cement hydration heat in the concrete is generated in a short time, which promotes the early concrete temperature rise. 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 concrete mixing; when the temperature is 30°C, 62.5% of the total heat of hydration is generated in the first 24 hours after concrete mixing. %. On the one hand, the high temperature of the coarse and fine aggregates and the cement itself increases the temperature of the early concrete; Concrete temperatures constructed in other seasons are much higher. Under the influence of the dry wind in summer, the high-temperature concrete increases the water evaporation rate on the surface, which causes the surface to lose water rapidly and produce serious plastic shrinkage, and the high temperature inside promotes the rapid progress of cement hydration and concrete hardening. 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 does not only cause surface cracks. Moreover, due to water loss, the cement hydration water on the concrete surface is insufficient, which affects the hardening and strength growth of the surface concrete.
02
The temperature difference of concrete section causes thermal cracks on the surface. During the construction of concrete in summer, the temperature is higher than the ambient temperature due to the exposure of the foundation or formwork to the sun. Its height makes the cement hydration speed and hydration heat generation speed in the concrete adjacent to it greater 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 ℃, it is easy to produce Thermal cracks on the surface. The foundation or formwork is affected by high-temperature concrete, and it is also prone to expansion and deformation, while the thickness of the large-surface concrete is generally small, and the expansion and deformation of the lower part exerts a reverse constraint on the shrinkage of the surface concrete, which increases the tensile stress and causes surface cracks. negative effect.
03
High temperature has an adverse effect on the microstructure of concrete. It is generally believed that when the concrete temperature is below 50 °C, the change in the microstructure of the foundation can be ignored. The high temperature effect of bulk concrete is often ignored. The general concrete structure constructed in summer, due to the influence of air temperature, material temperature, formwork or foundation temperature, and the concentrated release of cement hydration heat, the temperature rise often causes its temperature to exceed the unfavorable temperature. At the highest temperature, due to the low deformation resistance of early-age concrete, the change of concrete microstructure not only affects the overall strength of concrete, but also tends to form thermal cracks on the surface.
04
The changeable climate makes the concrete surface vulnerable to cold shock. Due to the changeable construction climate in summer, such as sudden rainfall, the temperature will drop suddenly, and the sudden drop of the concrete surface temperature will cause the surface temperature to shrink and produce surface temperature shrinkage cracks.
05
The increase of cement dosage and water-cement ratio during construction increases the plastic shrinkage and deformation of concrete. In summer construction, due to the influence of evaporation and other factors in the process of mixing and transportation, it is easy to cause the concrete mixture to lose water and reduce its workability. Therefore, it is easy to increase the amount of cement and the water-cement ratio to improve the workability of concrete, while the increase of the amount of cement and the water-cement ratio is more likely to cause shrinkage. When the shrinkage is constrained, it is easy to form cracks.
06
When the hardened concrete is constructed at high temperature, the temperature at which the solid is formed determines the base length of the concrete. When the whole is cooled, it starts to shrink from this length and temperature, and it is very easy to produce overall temperature shrinkage cracks, such as road breaking.
2. Preventive measures
01
Choose the appropriate curing method and start curing as soon as possible Start curing as soon as possible and keep the concrete surface moist, which can prevent evaporation, reduce shrinkage, and ensure the smooth progress of concrete surface hydration. For newly poured concrete that adopts various curing methods, it is best to start curing after the completion of the shaping of the concrete surface and before the surface water film disappears. However, it is generally not possible to contaminate or damage the formed surface of the concrete during curing. Therefore, it is necessary to choose the appropriate curing method and the best maintenance start time.
02
Increasing the relative humidity of the air can effectively reduce the evaporation rate of water on the surface of the concrete. It is a simple, easy and low-cost method to increase the relative humidity of the air by spraying water mist in the wind direction or around the newly poured concrete. effective measures. A simple spray method can be used to set up a plastic hose with a pinhole around the newly poured concrete site or in the upwind direction, and inject water with a certain water pressure to form a pinhole spray.
03
Lowering the concrete temperature Lowering the concrete temperature can effectively reduce the plastic shrinkage caused by the evaporation of water on the concrete surface and prevent the occurrence of thermal cracks. The following methods can be used to cool down: (1) Use measures such as erecting a shed to cover the newly poured concrete from direct sunlight and reduce the ambient temperature of the concrete surface; (2) Cool down various components of the concrete and control the fresh concrete mixing. The temperature of the material is below 32°C.
04
(1) Cool the coarse aggregate. It is the most economical and effective method to use water in advance to dissipate heat. It is necessary to strictly control the water content and adjust the construction mix ratio. (2) Cooling of fine aggregates. It is not advisable to spray water, and pay attention to the temperature difference between the 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 of the test to ensure that the test sample is consistent with the material used. (3) Cool with water for mixing. Cover the reservoir to avoid direct sunlight, and use well water for mixing. (4) Cement cooling. The cooling of the cement directly affects the temperature of the concrete. Each mixing station adopts the method of using the first field first, then entering the field and storing it as much as possible before using it to cool down. And strengthen the detection of cement temperature and stability.
05
When transporting concrete, the concrete mixer truck is equipped with sun protection facilities to shorten the transport time as much as possible. In the process of transporting concrete, mix at a slow speed. Through the test and detection, increase the slump of 1~2 in the concrete mixing station to ensure the workability of the concrete when it arrives at the site. It is strictly forbidden to add water to mix during transportation.
06
After the concrete is poured, the surface is covered with a clean plastic film. After the initial setting, the plastic film is removed and covered with a soaked burlap cloth. After the formwork is removed, the pier column is wrapped and covered with plastic film, and the water bucket placed on the upper part adopts self-flow maintenance; the cap is backfilled in time, and the table surface is sprinkled for health preservation; The concrete structure should be maintained in a moist state for not less than 14 days, and the wet curing should be uninterrupted, and a dry-wet cycle should not be formed. During moisturizing and maintenance, take shading and wind shielding measures to control the influence of temperature and hot and dry wind. When the curing is stopped, the structure should be gradually dried, so that the creep performance of concrete can be used to "unload" the temperature and shrinkage stress to avoid the occurrence of cracks.
