Main characteristics of concrete construction in summer
The climate is hot and the temperature is high.
The constituent materials of concrete have high temperature.
Due to high temperature and sunshine, the temperature of sand and gravel used to form concrete mixture is high; The temperature of new cement is usually very high. Due to the hot climate, the heat in cement is not easy to dissipate, and the influence of solar radiation and other factors makes the temperature of cement higher, and the temperature of some cement can reach 70℃.
The temperature of supports such as concrete foundation or formwork is very high.
Large exposure, greatly influenced by the environment.
The climate is changeable
The main cause of cracks.
0 1
The high evaporation rate of surface water leads to the loss of water on the surface of fresh concrete, forming plastic and shrinkage cracks. The main reason for cracks is the plastic shrinkage of concrete surface. When the evaporation rate of concrete surface is greater than 1kg/m? H, its surface is prone to plastic shrinkage cracks. Air temperature, concrete mixture temperature, air relative humidity and wind speed are the main factors affecting water evaporation on concrete surface. The higher the temperature of concrete itself, the smaller the relative humidity of air, the greater the wind speed and the greater the evaporation rate of water on the surface of concrete. Due to the high temperature in summer, the hydration heat of cement in concrete is generated in a short time, which promotes the early temperature rise of concrete. Relevant data show that when the temperature is 65438 04℃, 43% of the total hydration heat is generated after the concrete is mixed for 65438±024h. When the air temperature is 30℃, the hydration heat generated at 1th 24h after concrete mixing accounts for 62.5% of the total hydration heat. On the one hand, the high temperature of coarse and fine aggregates and cement itself increases the early temperature of concrete, on the other hand, it makes the hydration heat of cement more concentrated, and the high temperature of the environment makes it difficult to dissipate the heat in concrete, so the overall temperature of concrete is much higher than that of concrete constructed in other seasons. Under the action of dry wind in summer, the evaporation rate of water on the surface of high-temperature concrete increases, which makes the surface lose water quickly and produces serious plastic shrinkage, while the high temperature inside it promotes the rapid hydration of cement and the hardening of concrete. Under the combined action of severe plastic shrinkage on the surface and internal constraints, plastic shrinkage cracks appear on the concrete surface. Evaporation of water will not only lead to surface cracks. Moreover, due to water loss, the cement on the concrete surface is not hydrated enough, which affects the hardening and strength growth of the surface concrete.
02
The temperature difference of concrete section leads to temperature cracks on the surface. In summer construction, the temperature of concrete is higher than the ambient temperature because the foundation or formwork is exposed to the sun. Its height makes the hydration speed and heat of cement in the concrete adjacent to it faster than that on the surface, which increases the temperature difference between the surface and the internal concrete. When the difference between the surface temperature and the internal temperature exceeds 65438 05℃, surface thermal cracks are easy to occur. Under the influence of high-temperature concrete, the foundation or formwork is also prone to expansion deformation, while the thickness of surface concrete in a large area is generally small, and the expansion deformation at the lower part has a reverse constraint on the shrinkage of surface concrete, which increases its tensile stress sentence and is not conducive to the generation of surface cracks.
03
Generally speaking, when the concrete temperature is lower than 50℃, the change of foundation microstructure can be ignored, and when the concrete temperature is higher than 70℃, the change of its microstructure is considered unfavorable, and the high temperature influence of mass concrete is often ignored. Due to the influence of air temperature, material temperature, formwork or foundation temperature and concentrated release of cement hydration heat, the temperature of concrete structures constructed in summer usually exceeds the unfavorable maximum temperature due to the resulting temperature rise. Due to the small deformation resistance of early-age concrete, the change of concrete microstructure not only affects the overall strength of concrete, but also easily forms hot 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 suddenly drop, and the sudden drop of concrete surface temperature will cause the surface temperature to shrink, resulting in surface temperature shrinkage cracks.
05
The increase of cement dosage and water-cement ratio in construction increases the plastic shrinkage and deformation of concrete. In summer construction, due to the influence of evaporation and other factors, it is easy to cause water loss of concrete mixture and reduce its workability. Therefore, it is easy to increase the cement content and water-cement ratio in construction to improve the workability of concrete, and the increase of cement content and water-cement ratio is more likely to cause shrinkage, and when shrinkage is constrained, it is easy to form cracks.
06
When hardened concrete is constructed at high temperature, the solid temperature determines the foundation length of concrete. When the whole concrete is cooled, it starts to shrink from this length and temperature, and it is easy to produce overall temperature shrinkage cracks, such as broken slabs.
Preventive and control measures
0 1
Choosing a suitable curing method, starting curing as soon as possible and keeping the concrete surface moist can prevent evaporation, reduce shrinkage and ensure the smooth hydration of the concrete surface. For new concrete with various curing methods, curing must be started after the concrete surface is formed and before the water film on the surface disappears. However, the formed surface of concrete should not be polluted or damaged during the curing process, so it is necessary to choose the appropriate curing method and the best curing start time.
02
Increasing the relative humidity of air can effectively reduce the evaporation rate of water on the concrete surface. It is a simple, effective and low-cost measure to spray water mist on or around the upwind direction of newly poured concrete to increase the relative humidity of the air. The simple injection method can erect plastic hoses with pinholes around the newly poured concrete site or upwind, and inject water with a certain water pressure to form pinhole injection.
03
Reducing the temperature of concrete can effectively reduce the plastic shrinkage caused by evaporation of water on the surface of concrete, prevent the occurrence of hot cracks, reduce the temperature of hardened concrete and reduce the possibility of shrinkage cracks caused by temperature shrinkage of concrete. The following measures can be taken to reduce the temperature: (1) block the fresh concrete and avoid direct sunlight to reduce the ambient temperature of the concrete surface; (2) Reduce the temperature of various components of concrete, and the temperature of fresh concrete should be controlled below 32℃.
04
(1) coarse aggregate cooling. It is best to spray water in advance to dissipate heat. This method is the most economical and effective. It is necessary to strictly control the water content and adjust the construction mixture ratio. (2) Fine aggregate cooling. It is not advisable to spray water, and pay attention to the temperature difference between layers. Before loading, push away the outer fine aggregate, use the fine aggregate that is not exposed inside, and strictly control the water content of the test to ensure that the sample is consistent with the materials used. (3) stir with water to cool down. The reservoir is covered to avoid direct sunlight, and well water is used as mixed water. (4) Cement cooling. The cooling of cement directly affects the outlet temperature of concrete, so each mixing station adopts the method of first entering the site and then storing it as far as possible, and then using the method of cooling. Strengthen the detection of cement temperature and stability.
05
When transporting concrete, concrete mixer should be equipped with sun protection facilities to shorten the transportation time as much as possible. When transporting concrete, mix it slowly. Through the test, the slump of 1~2 is increased in the concrete mixing station to ensure the workability of concrete when it arrives at the site. Mixing with water is prohibited during transportation.
06
After concrete pouring, the surface shall be covered with clean plastic film, and after initial setting, the plastic film shall be removed, and covered with soaked sacks, without intermittent watering for health preservation. After formwork removal, the pier is covered with plastic film, and a bucket is placed on the upper part for self-flow maintenance; The pile caps should be backfilled in time, and the mesa should be watered and maintained; Cover the culvert with a straw curtain and water it for health. The concrete structure shall be kept in a healthy and moist state not less than 14d, and the wet curing shall be uninterrupted, and no dry-wet cycle shall be formed. During the period of moisture preservation and curing, sun-shading and wind-proof measures should be taken to control the influence of temperature and dry hot wind. When the maintenance is stopped, the structure should be gradually dried, and the temperature and shrinkage stress should be "unloaded" by using the creep characteristics of concrete to avoid cracks.
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