1 Effect of low temperature conditions on concrete properties
1.1 Tests have proven that when concrete is poured, if the temperature is lower, both the initial setting time and the final setting time will be longer. In comparison, the final setting time is more obviously prolonged.
Under low temperature conditions, the slump of concrete should generally not exceed 100mm, and bleeding should be minimized and solidified as early as possible. Bleeding water will stay on the concrete surface for a long time, which will affect the normal progress of the finishing process.
Decoration without treatment of surface bleeding on concrete is one of the main causes of concrete surface defects. If surface bleeding is pressed into the concrete during the plastering process, the water-cement ratio of the surface part will increase, resulting in strength, Problems such as reduction in air content and surface impermeability. Concrete materials should be designed to minimize bleeding. If bleeding occurs during construction, it should be removed before plastering.
1.2 Low temperature conditions will reduce the hydration rate of cement, thereby affecting the strength development of concrete.
If fresh concrete is frozen and the temperature is maintained at about -10°C, the hydration and strength development of the cement will stop.
If the concrete is exposed to negative temperature after it has set but the tensile strength has not yet reached the point where it can resist the expansion force caused by freezing, the expansion and cracking of the concrete caused by freezing will lead to irrecoverable irregular cracks and strength loss. If fresh concrete suffers frost damage within 24 hours, its 28-day compressive strength will be reduced by about 50%, which will also cause spalling on the surface of the concrete and a reduction in durability.
1.3 For concrete structures under low temperature conditions, the surface temperature decreases more significantly than the interior, resulting in a large temperature gradient and resulting temperature stress. If the tensile strength of the concrete is not enough to resist the temperature The stress will cause irregular visible or invisible cracks on the concrete surface. Most of these cracks are irrecoverable and will gradually expand under load, slowly becoming a channel for corrosive components to enter the interior of the concrete. It is the existence of these cracks that greatly reduces the long-term durability of concrete.
1.4 The frost resistance durability of concrete is related to the age of the concrete when it is subjected to the first freeze-thaw cycle. However, there is a difference between the frost resistance of concrete at an early age and the frost resistance of mature concrete that has been subjected to multiple freeze-thaw cycles. There is no linear proportional relationship. What is really related to the frost resistance durability of concrete is the tensile strength and pore water saturation of concrete. If concrete is affected by negative temperatures within a short period of time after pouring, the performance degradation caused by a freeze-thaw cycle is irreversible because it has not yet reached sufficient tensile strength and the internal pores are in a highly saturated state.
2 Temperature control measures during construction
Although low temperature conditions have brought many negative effects to concrete pouring, in some areas in northern my country, low temperature conditions last for a long time every year. In order to extend the construction time of concrete structures as much as possible and avoid the impact of freezing on newly poured concrete, you can Take some precautionary measures.
2.1 Improving the concrete mix ratio When pouring concrete under low temperature conditions, in order to reduce the time for temporary protection in winter, the concrete needs to have higher early strength. The early strength of concrete can be appropriately improved through the following methods.
① Use early-strength cement. When the concrete structure is not corroded by sulfate, cement with higher C3S and C3A content can be used, because this type of cement hydrates faster and releases higher heat of hydration, which is conducive to the condensation and hardening of concrete. and early strength development.
② Increase the cement dosage appropriately, generally by 60-120kg/m3. It is generally believed that the heat generated by the hydration of every 45kg of Portland cement can increase the concrete temperature by 5-9℃. Appropriately reducing the water-cement ratio of concrete or using a mixture rich in cement slurry can shorten the setting time and accelerate early strength development.
2.2 Increase the temperature of certain components of concrete. When aggregates stacked in the open contain frozen particles or ice cubes, the ice cubes in the aggregates must be melted before mixing to avoid aggregate formation during mixing and pouring. group phenomenon.
If simply heating water is not enough to increase the concrete temperature, the aggregate can also be heated, but the aggregate temperature should not exceed 52°C. For example, when the temperature is lower than 4°C and the temperature of the mixing water has been heated to 60°C, the aggregate temperature should be heated to about 15°C; if the coarse aggregate is dry and has no freezing phenomenon, the temperature of the mixing water has been heated to 60°C. ℃, only need to heat the fine aggregate to about 40 ℃;
If there are no frozen particles or ice cubes in the aggregate, there is no need to heat the aggregate, and only the mixing water can be heated to make the concrete reach the appropriate mixing temperature. Although the combined mass of aggregates and cement in concrete is much greater than the mass of mixing water, the specific heat capacity of water is about five times that of aggregates and cement. Among concrete components, heating the mixing water is easy to operate and the temperature is easy to control, so it has become the most widely used method in actual projects. The temperature of the heated water should not exceed 60-80°C. Too high a temperature of the mixing water can easily cause cement flash and cement flash. Undesirable phenomena such as reunions. If the temperature of the mixing water exceeds 80°C, direct contact between the cement and the hot mixing water must be avoided during mixing. Therefore, the feeding order of each component must be reasonably arranged. Generally, the hot water and aggregates can be mixed first and then Put in the cement. The temperature of each component of the mixture must be controlled to ensure that the temperature of the concrete is conducive to the hydration and condensation of the cement without generating an excessively high internal temperature, otherwise it will affect the strength development of the concrete. In addition, if the temperature of the mixture is too high, it will easily cause an excessive temperature difference between the inside and outside of the concrete in a low temperature environment, which will be detrimental to the volume stability and long-term durability.
2.3 Adding concrete admixtures Adding early strength admixtures. Adding small doses of early strength admixtures under low temperature conditions can accelerate the setting of concrete and the development of early strength. However, chlorine-containing early-strength agents should not be used in concrete with potential corrosion risks, nor should they be used in concrete where alkali-aggregate reactions may occur. The use of early-strength agents cannot replace necessary maintenance and antifreeze measures.
2.4 Take reasonable maintenance and insulation measures to ensure that the concrete can hydrate and set at 7-21°C. The most important thing is to ensure that the temperature does not drop below 10°C within the first 3 days after the concrete is poured. It is best to maintain the temperature at 21°C. Keep it down for a longer time. Covering the concrete surface with an insulation blanket or other insulation material can retain the heat of hydration and mixing water within the concrete. Insulation should be kept dry and in close contact with the concrete or formwork. After the cement concrete structure is poured, the concrete can be insulated from the atmosphere and heated. The heating method should not accelerate the water loss on the concrete surface, make the local temperature too high, and not produce a high concentration of CO2. Practice has proved that steam curing is a good method.
3 Conclusion
Through the above analysis, the following preventive measures can be taken to avoid or reduce the impact on the quality of newly poured concrete under low temperature conditions: ① Carefully select the mix ratio; ② Do not use aggregates containing ice; ③ Improve certain components of the concrete mixture. ④ Reasonably select admixtures according to the concrete structure; ⑤ Take reasonable maintenance and insulation measures; ⑥ It is strictly forbidden to pour concrete when it is impossible to guarantee whether it can reach sufficient strength before freezing.
