Summer construction refers to concrete construction under the condition that the daily average temperature exceeds 25°C. There are three main unfavorable factors for concrete construction under this climatic condition, namely high temperature, dryness and strong wind. These factors will lead to increased loss of concrete slump, too fast setting time, and rapid evaporation of free water in concrete, resulting in plastic shrinkage cracks and dry shrinkage cracks. If the pouring time is too long, there will be poor connection between the old and new concrete interfaces and transportation And the difficulty of pumping, etc., will eventually lead to the decline of construction quality and the deterioration of concrete performance. In order to avoid these problems as much as possible.
1. Control of Materials
According to the characteristics of concrete construction in summer and the climate factors in summer, measures should be taken in the control of concrete raw materials to ensure the quality of concrete.
(1) For aggregate yards, enterprises should adopt closed yards to reduce environmental pollution caused by aggregate dust; secondly, reduce aggregate surface temperature and reduce aggregate absorption and mixing water consumption. If the enterprise cannot achieve closed conditions under the existing conditions, the easiest way is to pre-cool the aggregates, such as drying and covering.
(2) Appropriately increase the amount of mineral admixture. The increase in temperature will accelerate the hydration reaction of the cementitious material, making the slower reaction rate of the mineral admixture a secondary problem, and conversely the setting time and slump loss become the main problems. The hydration speed of cement is fast, so reducing the amount of cement and appropriately increasing the amount of mineral admixture can delay the setting time of concrete and reduce the slump loss of concrete.
(3) The management personnel of commercial concrete laboratories should communicate with cement enterprises in a timely manner, and require cement manufacturers to adjust material parameters, such as reducing the C3A content in cement that has a greater impact on the heat of hydration. At the same time, the temperature of cement entering the site should not exceed 70°C, otherwise it can be put into the tank after the temperature drops. The temperature of cement entering the mixer should be controlled at 40°C, and the freshly produced cement is strictly prohibited to be used directly.
(4) Communicate well with the chemical admixture manufacturer, take the cement used by the enterprise to the chemical admixture manufacturer for testing, adjust the composition parameters of the chemical admixture, so that the compatibility of the two is the best, and at the same time, the retarding component should be appropriately increased , prolong the setting time of concrete.
2. Production and Transportation Control
(1) The testers should measure and mix in strict accordance with the company's summer mix ratio. When the concrete fails to meet the requirements, the tester should report to the director of the laboratory, who will re-measure and calculate. It is strictly forbidden for other personnel to change the mix ratio without authorization. The author visited many mixing plants and found that many mixing plant operators directly changed the ratio according to their own ideas without communicating with the laboratory director, and the reason for the adjustment was basically a single slump change.
(2) After the first set of concrete is discharged from the machine, the slump, air content, and discharge temperature of the mixture should be measured in time to determine whether the construction requirements are met, and to guide the batching of the lower set of concrete.
(3) The tank of the mixer truck should have sunshade measures, or the tank body should be painted with white paint to enhance the reflection ability of the tank body surface to sunlight.
(4) The concrete should be mixed slowly during transportation, and it is strictly forbidden to add water to stir during transportation.
(5) Laboratory staff should regularly measure the loss of concrete over time, conduct statistical analysis on the data, explore the law of slump loss of freshly mixed concrete in the process of temperature and distance, and do a good job for future adjustment of mix ratio.
3. Control of Pouring Maintenance
(1) When pouring concrete in summer, formwork, steel bars, and old concrete bases should be exposed to water to moisten and cool down. During the pouring process, it is necessary to reasonably divide and layer the concrete so that the interval between new and old concrete pouring is shortened. Avoid pouring in direct sunlight, and try to arrange pouring in the morning, evening and night. The pouring and vibrating process should be as fast and compact as possible, and the interval should not be too long.
(2) In order to facilitate the construction in summer, the general slump is required to be relatively large. During the construction vibration process, the cement slurry, mortar, and coarse aggregate will be distributed from the upper layer to the lower layer, so that the cement slurry on the concrete surface is in the lower layer. Shrinkage cracks are easily produced under the constraints of stones and stones. Therefore, in the process of concrete pouring, the distribution of materials should not be concentrated, but scattered distribution should be adopted, and then the concrete should be basically flattened with tools. , The point where the vibrator is inserted should generally be about 40cm apart. The vibrating rod should be vertically up and down, and when operating the vibrating rod, the vibrating rod should be inserted quickly and slowly pulled out. For concrete with a large slump in summer, the vibrating time should not exceed 15s; it is strictly forbidden to insert the vibrating rod into the concrete and let it vibrate; Uniform and dense, based on the observation that the coarse aggregate can be evenly distributed on all layers of the concrete, so that the concrete will not sink, bubble, or bleed, and the surface will be flat.
(3) In summer construction, the most important thing is the maintenance work after pouring. No matter how good the previous control is, if the maintenance is not in place, there will still be quality problems in the concrete. Because concrete evaporates water very quickly at high temperatures, and the evaporation of a large amount of water will affect the hydration of cementitious materials and affect the development of concrete properties. At the same time, a large amount of water evaporates rapidly, which not only produces a large drying shrinkage deformation, but also has a faster drying shrinkage rate, and occurs when the strength of the concrete is not high, which can easily cause cracking of the concrete. Therefore, after the fresh concrete is poured, the concrete should be well maintained. Usually, it can be covered with plastic film, sprinkled with water and sprayed with concrete curing fluid.
As a laboratory manager of a ready-made concrete enterprise, not only must he have excellent technical ability, but also he must be able to communicate with the construction unit. In the process of concrete construction in summer, as a laboratory personnel of the enterprise, we must make sufficient preparations to minimize possible problems, so that there will be very few problems about the quality of concrete.
4. Measures for Three Major Problems of Concrete Engineering in Summer
The high temperature in summer has a great impact on the use of various projects. In order to ensure that the quality of concrete meets the construction requirements, what should be done? Below are some measures for you.
1. Cool down
(1) Coarse aggregate cooling
Coarse aggregate cooling is the most economical and effective method. When the temperature of the aggregate is lowered by 2°C, the exit temperature will drop by about 0.5-1°C. Setting up a shed for shading and spraying water in advance has a better cooling effect. However, the water content should be strictly controlled during use.
(2) Fine aggregate cooling
It is not suitable to spray water, and pay attention to the temperature difference between the layers. It is necessary to strictly control and test the water content to ensure the consistency between the test samples and the materials used.
(3) Mixing water to cool down
This is the most important means of cooling. Every time the water temperature drops by 4°C, the outlet temperature will drop by about 0.8~1°C. Cooling water can be done with an ice maker or a refrigerator, the latter being more economical.
(4) Cement cooling
The cooling of cement directly affects the exit temperature of concrete. If the cement cools down by 8°C, the exit temperature of concrete will drop by about 1°C. At present, due to the shortage of raw materials in cement plants, the supply exceeds demand, and there is no time to cool down after production. Generally, the temperature of storage tanks is as high as 60~80°C. The construction unit should take the initiative to contact the factory, and adopt the method of adjusting and adding intermediate tanks to cool down. Generally, if you stay for one more day, you can lower the temperature by about 1~1.5°C, and strengthen the detection of cement temperature and stability.
(5) Period cooling
When the conventional cooling fails to meet the standard, the construction should be carried out at night. The following table shows the temperature measurement records of a mixing station in different periods. It can be seen that the temperature difference of the aggregate can reach 8~10°C, and the temperature difference of the water can reach about 4°C.
2. Maintenance
During seasonal concrete maintenance construction, the following maintenance methods should be paid attention to:
(1) Concrete should be cured immediately after initial setting.
(2) Adopt the method of sprinkling water for continuous maintenance. 1~2 days after the concrete is poured, the concrete should be kept in a fully wet state.
(3) When the formwork is removed after the specified curing time is completed, the concrete surface is covered with a damp covering layer for moisture curing.
During maintenance and construction, the following requirements must be ensured:
(1) The concrete curing period is determined. The moisture curing time is 14 days; the mass concrete curing time is 28 days.
(2) The concrete curing time is determined. During the maintenance period, the spraying maintenance method should be no less than once every hour. When the average temperature is higher than 30°C, the maintenance should be done every half an hour.
(3) After the concrete is poured, when the concrete surface receives water and initially sets, cover it with sacks or straw curtains and water it for curing.
(4) Conservation water requirements use edible tap water.
(5) Horizontal structural beams and slabs are directly cured by watering.
(6) For the vertical structure of the column wall whose surface is inconvenient to water and maintain, after the formwork is removed, wet sacks and straw bales are hung and water sprayers are used for moisturizing and maintenance.
3. Cracks
During concrete construction in summer, cracks are prone to occur in large-volume concrete. In order to alleviate the cracks in mass concrete, precautions should be taken from the following aspects.
1. Material configuration
(1) Cement should be given priority to stable quality, improve the crack resistance of concrete, and control the low C3A content.
(2) The fine aggregate should use well-graded medium sand, and the fineness modulus should be controlled. The particle size of the coarse aggregate can be appropriately increased during non-pumping construction, and the coarse aggregate should be continuously graded.
(3) To control the slump of the concrete mixture on the pouring face and the amount of mixing water, the amount of cement can be reduced by mixing fly ash and slag powder, but the total amount should not exceed 50% of the weight of cement in the concrete.
(4) When filling rubble, it should be noted that the number of buried stones should not exceed 20% of the volume of the concrete structure. The stones have no cracks, no interlayers, and no stains on the surface.
2. Construction control
(1) The overall layered continuous pouring or push-type continuous pouring can be used during pouring construction; layered pouring is often used for large-volume concrete, and secondary vibration is performed before the initial setting. When the next layer is poured, the initial setting of the previous layer must be after
(2) Massive concrete adopts heat preservation method and moisture retention method. Ordinary cement and slag cement shall be cured for no less than 14 days, and volcanic ash and fly ash cement shall be cured for no less than 21 days. After pouring construction, reasonable moisturizing coverage and heat preservation maintenance shall be carried out, which may be pre-embedded The cold water pipe cools down.
