As the weather gets hotter and hotter, a problem lies in front of us, which is the construction of concrete in summer. Concrete is a material that is highly susceptible to temperature effects. How to control the spread of these effects? Please read on.
Main characteristics of concrete construction in summer
▶ Hot climate and high temperature
▶ The temperature of the constituent materials of concrete is high
Due to high temperatures and exposure to sunlight, the sand and stone materials used to form concrete mixtures have higher temperatures; Newly produced cement often has high temperatures. Due to the hot climate, the heat in the cement is not easily dissipated, and the influence of factors such as solar radiation causes the temperature of the cement to be high, with some cement reaching up to 70 ℃.
▶ High temperature of supporting structures such as concrete foundations or formwork
▶ Large exposed surface, greatly affected by the environment
▶ Climate variability
The main cause of cracks
01
The high evaporation rate of surface moisture causes the surface of newly poured concrete to lose water, forming plastic and shrinkage cracks. The main reason for crack formation is the loss of water on the concrete surface and plastic shrinkage. When the surface evaporation rate of concrete is greater than 1kg/m ² · h, plastic shrinkage cracks are prone to occur on its surface. The temperature, temperature of the concrete mixture, relative humidity of the air, and wind speed are the main factors affecting the evaporation of surface moisture in concrete. The higher the temperature of 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 surface of concrete. In summer, due to the higher temperature, the hydration heat of cement in concrete is generated in a shorter period of time, which promotes the BIM building improvement of early concrete temperature. According to relevant data, when the temperature is 14 ℃, 43% of the total hydration heat is generated in the first 24 hours after concrete mixing; When the temperature is 30 ℃, 62.5% of the total hydration heat is generated in the first 24 hours after concrete mixing. The high temperature of coarse and fine aggregates, as well as cement itself, on the one hand increases the temperature of early concrete, and on the other hand, concentrates the hydration heat of cement. The high temperature of the environment makes it difficult for the heat in concrete to dissipate, resulting in a much higher overall temperature of concrete compared to concrete constructed in other seasons. Under the influence of summer dry winds, high-temperature concrete increases the surface water evaporation rate, causing rapid dehydration 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 are generated on the surface of concrete. The evaporation of water not only causes surface cracks. Moreover, due to water loss, there is insufficient hydration of cement on the surface of concrete, which affects the hardening and strength growth of the surface concrete.
02
The temperature difference of the concrete section causes thermal cracks on the surface. During summer construction, the temperature of the foundation or formwork is higher than the ambient temperature due to exposure to sunlight and other factors. Its height causes the cement hydration rate and hydration heat generation rate in the adjacent concrete to be greater than the surface, resulting in an increased temperature difference between the surface and the internal concrete. When the difference between the surface temperature and the internal temperature exceeds 15 ℃, surface thermal cracks are prone to occur. The foundation or formwork is also susceptible to expansion deformation due to the influence of high-temperature concrete, while large surface concrete generally has a smaller thickness. The expansion deformation of the lower part has a reverse constraint on the shrinkage of the surface concrete, increasing its tensile stress sentence and causing adverse effects on the formation of surface cracks.
03
It is generally believed that high temperatures have adverse effects on the microstructure of concrete. When the temperature of concrete is below 50 ℃, the changes in the microstructure of the base can be ignored. When the temperature of concrete is above 70 ℃, the changes in its microstructure are considered unfavorable. The high-temperature effects of large volume concrete are often ignored. For general concrete structures constructed in summer, the temperature rise caused by temperature, material temperature, formwork or foundation temperature, and the concentrated release of cement hydration heat often exceeds the unfavorable maximum temperature. Due to the small deformation resistance of early aged concrete, the changes in the microstructure of concrete not only affect the overall strength of concrete, but also easily form thermal cracks on the surface.
04
The changing climate makes the surface of concrete susceptible to cold strikes. Due to the unpredictable summer construction climate, such as sudden rainfall, the temperature can suddenly drop. The sudden drop in surface temperature of concrete can cause temperature shrinkage and produce surface temperature shrinkage cracks.
05
The increase in cement dosage and water cement ratio during construction increases the plastic shrinkage deformation of concrete. In summer construction, due to factors such as evaporation during 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 cement dosage and water cement ratio during construction to improve the workability of concrete, and the increase in cement dosage and water cement ratio is more likely to cause shrinkage. When shrinkage is constrained, cracks are easily formed.
06
The temperature at which hardened concrete is formed during construction at high temperatures determines the base length of the concrete. When it cools down as a whole, it begins to shrink from this length and temperature, which can easily lead to overall temperature shrinkage cracks, such as road surface fractures.
prevention and control measures
01
Choosing appropriate maintenance methods, starting maintenance as early as possible and keeping the concrete surface moist can prevent evaporation, reduce shrinkage, and ensure smooth hydration of the concrete surface. For newly poured concrete that adopts various curing methods, it is advisable to start curing after the surface of the concrete is shaped and before the surface water film disappears. However, curing should generally not contaminate or damage the already formed surface of the concrete. Therefore, it is necessary to choose appropriate curing methods and the best curing start time.
02
Increasing the relative humidity of the air can effectively reduce the evaporation rate of moisture on the surface of concrete. Using water spray mist in the upwind direction or surrounding area of newly poured concrete to increase the relative humidity of the air is a simple, feasible, and cost-effective effective measure. A simple spray method can be used to erect a plastic hose with pinholes around the newly poured concrete site or upwind, and form pinhole spray by injecting water with a certain pressure.
03
Lowering the temperature of concrete can effectively reduce plastic shrinkage caused by surface moisture evaporation, prevent the occurrence of thermal cracks, and lower the temperature of hardened concrete, reducing the possibility of shrinkage cracks due to temperature shrinkage. The following methods can be used for cooling: (1) Covering the newly poured concrete with a shed or other measures to protect it from direct sunlight and reduce the ambient temperature on the surface of the concrete; (2) Cool down the various components of concrete and control the temperature of the new concrete mixture below 32 ℃.
04
(1) Coarse aggregate cooling. The use of pre shower integrated decoration water has a better heat dissipation effect, which is the most economical and effective method. Strictly control the moisture content and adjust the construction mix ratio. (2) Cooling of fine aggregates. Not suitable for watering, pay attention to the temperature difference between layers. Before loading, push the outer fine aggregate aside, use the inner fine aggregate that has not been exposed to sunlight, and strictly control the moisture content of the test sample to ensure consistency with the material used. (3) Cool down the mixing water. Cover the reservoir to avoid direct sunlight and use well water as mixing water. (4) Cement cooling. The cooling of cement directly affects the outlet temperature of concrete, and each mixing station adopts the method of using it first and then storing it as much as possible before use to cool it down. And strengthen the detection of cement temperature and stability.
05
When transporting concrete, the concrete mixer truck should be equipped with sun protection facilities to shorten the transportation time as much as possible. During the transportation of concrete, slow mixing is carried out. Through experimental testing, an additional 1-2 slump is added at the concrete mixing plant to ensure the workability of the concrete upon arrival at the site. It is strictly prohibited to add water for mixing during transportation.
06
After the concrete pouring is completed, the surface is covered with a clean plastic film. After the initial setting, the plastic film is removed and covered with soaked burlap cloth. Water is sprayed intermittently for curing. After demoulding, the pier column is wrapped and covered with plastic film, and a water bucket is placed on the upper part for self flowing curing; Timely backfill the abutment and sprinkle water on the tabletop for health preservation; The body of the culvert is covered with grass curtains and watered for health preservation. Concrete structures shall be maintained in a moist state for at least 14 days, with continuous wet curing and no dry wet cycle formed. During the moisturizing and maintenance period, take shading and windproof measures to control the temperature and the influence of dry and hot air. When stopping maintenance, the structure should be gradually dried, which can utilize the creep performance of concrete to “unload” temperature and shrinkage stress, and avoid the occurrence of cracks.
Article source: Architectural Technology Magazine
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