The three major problems and measures for summer concrete engineering are high temperatures and heat, which have a significant impact on the use of various types of engineering. How to operate to ensure that the quality of concrete meets the construction requirements? The editor has summarized three common problems and preventive measures. Let's take a look together.
1、 Cooling down
■ Coarse aggregate cooling
Cooling down coarse aggregates is the most economical and effective method. The aggregate temperature drops by 2 ℃, and the outlet temperature drops by about 0.5-1 ℃. Setting up a shed for shade and pre watering for heat dissipation have better effects. But the moisture content should be strictly controlled during use.
■ Cooling of fine aggregate
Not suitable for watering, pay attention to the temperature difference between layers. Strictly control and test the moisture content to ensure consistency between the tested samples and the materials used.
■ Cooling of mixing water
This is the most important cooling method. For every 4 ℃ decrease in water temperature, the outlet temperature decreases by approximately 0.8~1 ℃. Cooling water can be achieved using an ice maker or a refrigerator, the latter being more economical.
Cement cooling
The cooling of cement directly affects the outlet temperature of concrete. If cement is cooled by 8 ℃, the outlet temperature of concrete will decrease by about 1 ℃. Currently, due to the tight supply of materials in cement plants, there is no time for cooling down after production, and the temperature of storage tanks is generally as high as 60-80 ℃. The construction unit should proactively contact the factory and adopt the method of adjusting and adding intermediate tank compartments to cool down. Generally, staying for an extra day can lower the temperature by about 1-1.5 ℃, and strengthen the detection of cement temperature and stability.
■ Time period cooling
When the conventional cooling cannot meet the standard, construction should be carried out at night. The temperature measurement records of a certain mixing station at different time periods show that the temperature difference of aggregates can reach 8-10 ℃, and the temperature difference of water can reach about 4 ℃.
2、 Maintenance
Immediately carry out curing after the initial setting of concrete.
Adopting the method of watering for continuous maintenance. After 1-2 days of pouring concrete, it should be ensured that the concrete is fully moist, but it is not advisable to pour cold water directly onto the surface of the concrete to prevent temperature difference cracking.
When removing the formwork after completing the prescribed curing time, cover the concrete surface with a damp covering layer for moisture curing.
During maintenance construction, the following requirements must be ensured:
(1) Determination of concrete curing period. The moisturizing and maintenance time is 14 days; The curing time for large volume concrete is 28 days.
(2) Determination of concrete curing time. During the maintenance period, spraying maintenance should be carried out at least once per hour. When the average temperature is above 30 ℃, maintenance should be carried out every half hour.
(3) After the concrete pouring is completed, when the surface of the concrete has collected water and initially set, it should be covered with burlap bags or grass curtains and watered for curing. (4) The requirement for maintenance water is to use edible tap water.
(5) Horizontal structural beams and slabs are directly maintained by watering.
(6) For vertical structures of columns and walls that are difficult to water and maintain on the surface, after demoulding, damp burlap bags and straw bales should be hung, and a dedicated person should use a water sprayer for moisturizing and maintenance.
3、 Cracks
During summer concrete construction, large volume concrete is prone to cracking problems. To alleviate the occurrence of cracks in large volume concrete, precautions should be taken from the following aspects.
■ Material configuration
(1) Cement should prioritize stable quality, improve concrete crack resistance, and control low C3A content.
(2) Fine aggregate should use well graded medium sand, control the fineness modulus, and increase the particle size of coarse aggregate appropriately during non pumping construction. Coarse aggregate should be continuously graded.
(3) To control the slump and mixing water consumption of concrete mixtures on the pouring face, the cement consumption can be reduced by adding fly ash and slag powder, but the total amount should not exceed 50% of the cement weight in the concrete.
(4) When filling and placing rubble, it should be noted that the number of stones buried should not exceed 20% of the volume of the concrete structure, and the stones should be free of cracks, interlayers, and stains on the surface.
Construction control
(1) During pouring construction, integral layered continuous pouring or push type continuous pouring can be used; Large volume concrete is often poured in layers, with secondary vibration before initial setting, and the pouring of the next layer must be carried out after the initial setting of the previous layer of concrete.
(2) Large volume concrete should be cured using insulation and moisture retention methods. Ordinary cement and slag cement should be cured for at least 14 days, while volcanic ash and fly ash cement should be cured for at least 21 days. After pouring construction, reasonable moisture retention coverage and insulation curing should be carried out, and cold water pipes can be embedded for cooling.