Influence of Low Temperature Conditions on Concrete Performance
I. The influence of low temperature conditions on the early performance of concrete
It is proved by experiments that the lower the temperature of concrete is, the longer the initial setting time and final setting time will be, and the longer the final setting time will be. Under the condition of low temperature, the slump of concrete should generally not exceed 100mm, and the bleeding should be reduced as much as possible and set as soon as possible. At low temperature, bleeding water will stay on the concrete surface for a long time, which will affect the normal finishing process. Finishing the concrete surface without treatment is one of the main causes of concrete surface defects. If the surface bleeding water is pressed into the concrete during the plastering process, the water-cement ratio of the surface part will increase, resulting in the decrease of strength, 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. The influence of low temperature conditions on the strength of concrete
It is proved by experiments that the lower the temperature of concrete is, the longer the initial setting time and final setting time will be, and the longer the final setting time will be. Under the condition of low temperature, the slump of concrete should generally not exceed 100mm, and the bleeding should be reduced as much as possible and set as soon as possible. At low temperature, bleeding water will stay on the concrete surface for a long time, which will affect the normal finishing process. Finishing the concrete surface without treatment is one of the main causes of concrete surface defects. If the surface bleeding water is pressed into the concrete during the plastering process, the water-cement ratio of the surface part will increase, resulting in the decrease of strength, air content and surface impermeability. Concrete materials should be designed to minimize bleeding. If bleeding occurs during construction, it should be removed before plastering.
2. The influence of low temperature conditions on the strength of concrete
Low temperature conditions will reduce the hydration rate of cement, thus affecting the strength development of concrete. If the fresh concrete is frozen and the temperature is maintained at about -10℃, the hydration and strength development of cement will stop. If the concrete is affected by negative temperature after setting and the tensile strength has not yet reached the expansion force caused by icing, the concrete expansion and cracking caused by icing will lead to irreparable irregular cracks and strength loss. If the fresh concrete is frozen within 24 hours, its compressive strength at 28 days will be reduced by about 50%, and at the same time, it will cause concrete surface peeling and durability reduction.
3. The influence of low temperature conditions on the volume stability of concrete
For concrete structures at low temperature, the decrease rate of surface temperature is much more obvious than that inside, resulting in a larger temperature gradient and the resulting temperature stress. If the tensile strength of concrete is not enough to resist the stress at this temperature, irregular visible or invisible cracks will occur on the concrete surface. Most of these cracks are irrecoverable, and will gradually expand under the load, and gradually become a channel for corrosive components to enter the concrete, and it is these cracks that greatly reduce the long-term durability of concrete.
4. The influence of low temperature conditions on frost resistance and durability of concrete
The frost resistance durability of concrete is related to the age of the concrete when it is subjected to the first freeze-thaw cycle, but there is no linear proportional relationship between the frost resistance of concrete at early age and the frost resistance of mature concrete subjected to multiple freeze-thaw cycles. What is really related to the frost resistance and durability of concrete is the tensile strength and pore water saturation of concrete. If the concrete is affected by negative temperature in a short period after pouring, the performance degradation caused by a freeze-thaw cycle is irreversible because the tensile strength has not yet reached enough and the internal pores are in a highly saturated state.
II. Temperature control measures in construction process
Although low-temperature conditions have brought many negative effects on concrete pouring, in some areas of northern China, the time spent in low-temperature conditions is very long every year. In order to prolong the construction time of concrete structures as much as possible and avoid the influence of freezing on newly poured concrete, some preventive and protective measures can be taken.
1. Improve the concrete mixture ratio
When pouring concrete at low temperature, in order to reduce the time of temporary protection in winter, concrete needs to have high early strength, and the early strength of concrete can be appropriately improved by the following methods.
(1) Using early strength cement, when the concrete structure is not corroded by sulfate, cement with higher C3S and C3A content can be used, because this kind of cement hydrates faster and releases higher hydration heat, which is beneficial to the setting and hardening of concrete and the development of early strength.
(2) Properly increase the dosage of cement, generally 60-120kg/m3. It is generally believed that the heat generated by hydration of portland cement can increase the temperature of concrete by 5-9℃. Properly reducing the water-cement ratio of concrete or using a mixture rich in cement slurry can shorten the setting time and accelerate the early strength development.
2. Improve the temperature of some components of concrete
When the aggregate piled in the open air contains frozen particles or ice cubes, the ice cubes in the aggregate must be melted before mixing, so as to avoid aggregate agglomeration during mixing and pouring. If simply heating water is not enough to increase the temperature of concrete, aggregate can also be heated, but the aggregate temperature should not exceed 52℃. For example, when the temperature is lower than 4℃ and the temperature of mixing water has been heated to 60℃, the aggregate temperature should be heated to about 15℃. If the coarse aggregate is dry and there is no freezing phenomenon, when the mixing water temperature has been heated to 60℃, it is only necessary to heat the fine aggregate to about 40℃; If there are no frozen particles or ice cubes in the aggregate, the concrete can reach the appropriate mixing temperature only by heating the mixing water without heating the aggregate. Although the total mass of aggregate and cement in concrete is much larger than that of mixed water, the specific heat capacity of water is about five times that of aggregate and cement. Among concrete components, heating and mixing water is the most widely used method in practical engineering because of its convenient operation and easy temperature control. The water temperature after heating should not exceed 60-80℃, and too high mixing water temperature may easily lead to undesirable phenomena such as cement flash setting and cement agglomeration. If the temperature of mixing water exceeds 80℃, direct contact between cement and hot mixing water must be avoided during mixing, so the feeding sequence of each component must be arranged reasonably. Generally, hot water and aggregate can be mixed and stirred before putting into cement. The temperature of each component of the mixture must be controlled to ensure that the temperature of concrete is conducive to the hydration and condensation of cement without excessive internal temperature, otherwise it will affect the strength development of concrete. In addition, too high temperature of the mixture will easily lead to too large temperature difference between the inside and outside of concrete in low temperature environment, which will be unfavorable to volume stability and long-term durability.
3. Adding concrete admixture
Adding early strength agent and adding a small amount of early strength agent at low temperature can accelerate the setting and early strength development of concrete. However, chlorine-containing early strength agent should not be used in concrete with potential corrosion risk, nor should it be used in concrete where alkali-aggregate reaction may occur. The use of early strength agent cannot replace the necessary maintenance and anti-freezing measures.
4. Take reasonable maintenance and heat preservation measures
It is hoped that concrete can hydrate and coagulate at 7-21℃. The most important thing is to ensure that the temperature does not drop below 10℃ within the first three days after concrete pouring, and it is best to keep it at 21℃ for a long time. Covering the surface of concrete with a layer of thermal insulation blanket or other thermal insulation materials can keep the hydration heat and mixed water inside the concrete. Thermal insulation materials should be kept dry and in close contact with concrete or formwork. After the pouring of cement concrete structure is completed, the concrete can be isolated 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 produce high concentration of CO2. Practice has proved that steam curing is a good method.
II. Conclusion
Through the above analysis, the following preventive measures can be taken to avoid or reduce the influence of low temperature on the quality of newly poured concrete:
(1) carefully selected mixture ratio.
(2) Aggregates containing ice cubes cannot be used.
(3) increase the temperature of some components of concrete mixture.
(4) according to the concrete structure, reasonable selection of additives.
(5) Take reasonable maintenance and heat preservation measures.
(6) It is forbidden to pour concrete when there is no guarantee that it can reach enough strength before freezing.
