Commercial concrete is an emerging industry and a process product. Its quality control must be checked through various links. Due to the difference in seasons and climates, the focus of attention in quality control and construction is different. Based on years of experience in the concrete industry, the author briefly lists some key points and construction measures for the benefit of colleagues.
1 Quality control and precautions of concrete construction in spring and autumn
1.1 Seasonal Characteristics
The spring and autumn seasons in Huai'an have some things in common: large temperature difference between day and night, less rain, and high wind speed, generally at level 4-5; dry air with low humidity (generally < 30%). These will cause frequent shrinkage and air-drying cracks in the concrete, or a series of other problems.
1.2 Key points
1.2.1 Material Control
Strengthen the quality control of raw materials, strictly control the mud content of sand and stone materials, increase the detection and use of admixtures, because the temperature changes greatly in this season, and the strength of concrete increases abnormally, so the use of admixtures should be reduced as much as possible . Put an end to the use of unqualified raw materials. The use of admixtures should be adjusted more. The admixtures have a great impact on the performance of concrete. High or low temperature makes the setting time of concrete different day and night.
1.2.2 Mix ratio control
Strictly control the water-cement ratio and reduce the amount of admixtures and admixtures. Increase the amount of cement and water to ensure that the water-cement ratio remains unchanged, and reduce the amount of admixture to prevent abnormalities during coagulation. The slump can ensure the construction performance on site, and the slump cannot be increased blindly.
1.2.3 Construction site control
Ensure that the concrete is used up immediately within 1 to 1.5 hours to prevent the slump loss and change due to long waiting time and affect the quality of the concrete. For any concrete that enters the construction site, it is strictly forbidden to add water to the mixer truck pump without permission, and it is even more necessary to prevent the behavior of adding water while constructing on the pouring surface.
Immediately after the concrete is poured, it should be leveled immediately, and before the initial setting, the finishing should be carried out no less than twice. After the finishing, the surface should be covered with film, straw bags, etc., and curing agent should be sprinkled if necessary. When using a polishing machine to finish the floor, it should be done before and after the initial setting, and the machine should not be used after the final setting, otherwise, due to the low strength of the concrete, microcracks caused by vibration are prone to occur, and the cracks are not easy to heal.
After the beam slab (floor) concrete is poured, it is forbidden to apply force too early. The hoisting materials should be placed lightly and dispersedly. vibration, otherwise it will affect the floor to produce destructive cracks.
2 Summer concrete construction quality control and precautions
2.1 Seasonal characteristics
Summer is a season with continuous high temperature and complicated changes of cloudy and sunny in a year. In the hot summer, the evaporation of water is large, and the newly poured concrete will dry quickly (surface), set fast, false set, set for a long time, reduce strength, etc., and will cause cracks, which will affect the quality of concrete.
2.2 Key points
2.2.1 Temperature control of concrete
In the high temperature season, the higher the ambient temperature, the higher the exit temperature of the concrete, the faster the hydration rate of the cement in the concrete, and its various construction performances are reduced. If the slump loss is too large, the construction cannot be guaranteed; the setting time is shorter In addition, it can cause a series of quality problems such as temperature cracks and dry shrinkage cracks, so the temperature control of concrete in summer is particularly important. In addition, it is to delay the hydration speed of cement and reduce the early hydration heat of concrete.
2.2.2 The main points of controlling the temperature of concrete leaving the machine
(1) Cement temperature. The cement that has just been ground cannot be used, and the cement that is put into the warehouse cannot be used immediately (the temperature of the cement in the warehouse can reach up to 90-100°C). The temperature of the cement and the temperature during use need to be checked, and the amount of admixture can be adjusted in a targeted manner.
(2) Stirring water temperature. The main method is to reduce the temperature of the agitated mixture. Generally, more groundwater is used. If necessary, ice cubes can be added to control the temperature of the admixture.
(3) Other aggregate temperature. Water spraying and other cooling methods can be used to improve the temperature of the aggregate entering the machine, thereby changing the temperature of the concrete mixture leaving the machine.
2.2.3 Reduce the hydration rate of concrete (using admixtures)
In summer, due to the higher temperature, the hydration rate of cement is faster, and the setting time of concrete is shorter, which is not conducive to the later finishing and plastering of concrete. Heat, reduce slump loss, in order to meet the site construction requirements.
2.2.4 Construction control in rainy weather
There is a lot of rain in summer, and sometimes thunderstorms, rainstorms and other weather will be encountered during construction. If the control is not good, the quality and appearance of the concrete will be affected. Therefore, pay close attention to the weather forecast, try to avoid construction in the rain, and take measures such as covering and leaving construction joints in case of unexpected encounters during the construction process, so as to avoid rainwater directly washing the freshly poured concrete.
2.2.5 Other measures
To avoid construction at high temperature, you can choose to pour at night. At this time, the concrete is less affected by wind and temperature, and can be finalized close to sunrise. The possibility of high temperature drying and cracking during the day will be greatly reduced.
3 Winter concrete construction quality control and precautions
3.1 Seasonal characteristics
The temperature in winter is around -5～10℃, there are many cold winds, the temperature is low, and the temperature difference between day and night is large. At this temperature, the cement hydration of concrete is in a dormant state. If you do not pay attention and take effective measures, quality accidents will easily occur.
3.2 Key points
3.2.1 Material Control
(1) Cement: In strict accordance with the requirements of the "Code for Winter Construction of Construction Engineering", Portland cement and ordinary Portland cement are preferred. The cement grade is not lower than 32.5 grades, and the amount of cementitious materials in various mix ratios is quite large. At 300kg/m3.
(2) Crushed stone and sand: In order to prevent the difficulty of purchasing crushed stone and sand in winter, or the inconvenience of production due to the freezing of temporary sand due to excessive moisture content, good quality sand and gravel materials should be stored at the wharf and freight yard in advance.
(3) Admixture: adjust the admixture components in time according to changes in temperature to meet the needs of winter construction. Generally, the admixtures used can be used for anti-freezing in the temperature range of -5 ~ 0 ℃. If the weather changes (the temperature is too low), a professional anti-freezing agent should be selected to meet the construction requirements.
(4) Mixing water: When the temperature is too low, the water should be heated or ground water should be used.
3.2.2 Personnel training
In order to ensure the smooth progress of concrete production and construction in winter, each team should be organized to conduct special training on technical precautions in winter construction to improve the construction personnel's understanding of winter construction. In addition, fire prevention, antifreeze, safety and other knowledge training should be carried out.
3.2.3 Mix ratio design
According to changes in temperature, various materials should be actively prepared and tested; the production mix ratio should be adjusted in time and the monitoring during the production process should be increased.
3.2.4 Mixing and transportation of concrete
The production is carried out in strict accordance with the specified mix ratio, and the mixing is carried out according to the mixing time stipulated in the "Code for Winter Construction of Construction Engineering" to ensure the temperature and workability of the concrete. The drum speed is guaranteed during transportation to ensure that the concrete mixer truck reduces heat loss during transportation. When necessary, the concrete mixer truck should be insulated.
3.2.5 Construction preparation and pouring
(1) The construction unit should actively prepare materials for cold and frost protection. Pay close attention to the changing weather conditions and try to avoid pouring concrete when the temperature is too cold.
(2) Before concrete pouring in winter, the ice, snow and dirt on the formwork should be removed, and protective measures against wind and frost should be taken. If necessary, preheat the steel bars and formwork.
(3) It is not advisable to leave construction joints in winter construction. If continuous pouring cannot be performed due to technical or organizational reasons, and the pause time may exceed the initial setting time of concrete, construction joints should be left correctly.
(4) When pouring the construction joints, the surface cement slurry film and loose stones should be removed first, the surface should be moistened, and the temperature of the original concrete at the joints should be higher than 2°C, and then the cement slurry or mortar with the same composition as the concrete mortar should be spread. For the first floor, the pouring can only be continued when the strength of the poured concrete is higher than 1.2MPa.
4 Curing of concrete
No matter what season, the maintenance of concrete is particularly important, it is directly related to the increase of concrete strength and the stability of performance. Early key maintenance can reduce the carbonation rate of concrete and enhance the durability of concrete such as impermeability and freeze-thaw resistance. Therefore, from the perspective of paying attention to project quality and service project quality, industry practitioners and construction coordinators should pay attention to the maintenance system. A century-old plan, quality first!
