During winter construction, when the average temperature is less than or equal to 5℃, the strength of concrete develops slowly. When the minimum temperature is between -1 and -2°C, if effective insulation measures are not taken, the newly poured concrete may suffer from freezing damage. Once it is frozen, it will cause great damage to the concrete. The use of antifreeze in winter, coupled with good insulation and maintenance, is a practical and effective measure to reduce early concrete diseases and improve the antifreeze effect of concrete. It can ensure that the durability and strength of concrete meet construction requirements and ensure the quality of concrete projects.
(1) Action mechanism of antifreeze
The main action mechanisms of antifreeze in concrete are as follows Several kinds.
(1) Ice crystal distortion theory
When pure water freezes at 0°C, due to hydrogen bonds The action of water molecules will cause water molecules to gather to form molecular aggregates with strong frost heaving force. After adding antifreeze to water, ice will slowly precipitate when the temperature drops below its freezing point. However, the hydrogen bonds between water molecules will be interfered by the antifreeze molecules and will precipitate into a flocculent structure, which is very soft macroscopically. Small ice crystals that significantly reduce frost heave stress. For example, NaNO2, Na2SO4, Ca(NO3)2, etc. have strong ability to reduce frost heave force.
(2) Ural’s Law
The concentration of antifreeze will reduce the liquid phase in the concrete mixture The freezing point plays a certain role. Only by lowering the freezing point of the liquid phase can the concrete have liquid water to participate in cement hydration under negative temperature conditions. This is concrete that uses antifreeze. The concrete can still hydrate under negative temperature conditions and improve the strength of the concrete.
(3) Liquid-cement ratio balance theory
When the temperature drops to the freezing point of the liquid phase in concrete , the concrete mixture containing antifreeze will gradually begin to freeze. In the liquid phase under a negative temperature environment, the freezing point decreases as the concentration of ice precipitation increases, and then reaches a state of coexistence of ice and liquid under this negative temperature condition. It is the new dynamic balance. At this time, the liquid water in the concrete can ensure that the cement continues to hydrate, the antifreeze concentration remains unchanged, and the liquid phase concentration remains unchanged. The water generated by melting ice supplements the water needed for cement hydration, which is the liquid-cement ratio balance.
(4) Maturity theory
The influence of temperature on cement hydration is extremely significant, and the mix ratio For a certain concrete, as the curing temperature decreases, the cement hydration rate slows down and the growth rate of concrete strength gradually decreases. Therefore, the development of concrete strength is not only related to curing time, but also has a great relationship with curing temperature. For example, the maturity coefficient is 1 at 20°C, but only 0.12 at -10°C. The concrete maturity during winter concrete construction acceptance is calculated based on 600℃·d, and is stipulated not to exceed 60 days. For many projects that do not take insulation measures, it is still difficult to reach the specified maturity of 600℃·d even if it exceeds 60 days. Springback detection The intensity is often low.
(2) Composition of antifreeze
In the construction of concrete projects in winter, in order to prevent the concrete from suffering For freezing damage, antifreeze is often added during production to reduce the freezing point temperature of liquid water in concrete, ensuring that there is liquid water inside the concrete that can satisfy hydration, and reducing damage caused by low temperature. Most of the currently widely used antifreezes are composite antifreezes, which have the advantages of small slump loss, high water reduction rate, high early strength, and moderate setting time. Its main components include the following.
(1) Antifreeze component
Its function is to destroy the crystal form of ice and reduce the liquid content of water. Phase freezing point, maintain the liquid water required for cement hydration reaction inside the concrete, avoid concrete suffering from freezing damage, and reach early strength as soon as possible. The use of antifreeze components can first lower the freezing point of water and ensure that the hydration reaction of concrete continues at negative temperatures, such as sodium chloride, sodium nitrite, etc.; secondly, it can also change the lattice shape of ice and reduce frost heave stress. , to avoid damage to concrete strength, such as urea, methanol, etc. In addition, although some antifreeze agents cannot reduce the freezing point of water in concrete, they can directly react with cement to accelerate the setting and hardening of concrete and improve the early strength of concrete, such as calcium chloride, potassium carbonate, etc.
(2) Early strength component
Early strength component plays a role in the hydration reaction of concrete. The catalytic effect accelerates the setting and hardening of concrete, making it reach the critical frost resistance strength as soon as possible and improving the frost resistance of concrete. At the same time, due to the progress of the cement hydration reaction, the generation of bound water is promoted. The hydration reaction can also generate a certain amount of hydration heat to reduce the freezing damage of concrete.
(3) Water-reducing component
Reduce the water consumption of concrete and eliminate the internal causes of frost heaving. Enhance concrete density. Water reducing agent can disperse cement into smaller particles and improve the internal pore structure of concrete. The water-reducing agent adsorbed on the surface of the cement can release the wrapped water, making the ice crystal size from large to small, reducing the freezing pressure on the concrete when it freezes.
(4) Air-entraining component
The air-entraining agent is introduced.�Small bubbles absorb the frost heave stress generated during the formation of ice crystals, reduce the crack expansion during concrete frost heave, and reduce the frost heave force on concrete. The increase in air content can also increase the internal lubricity of concrete, improve the workability of concrete, and thereby improve the internal structure of concrete. Small bubbles can cut harmful pores in concrete and improve the durability of concrete.
(3) Precautions during the compounding process of antifreeze
Preparation of antifreeze Rather than simply mixing several different components together, experiments should be conducted to verify whether the combined effects of the various components are superimposed or cancel each other out. At the same time, the effects of each component should be fully considered based on concrete raw materials, mix ratios, technical requirements, etc. Effects on concrete properties.
(1) The water consumption during compounding should not be too high, which may easily cause the antifreeze and water-reducing agent itself to freeze;
(2) Pay attention to the dosage of inorganic salts in the early stage and antifreeze components to prevent excessive dosage from causing a decrease in water reduction rate, slump retention ability, and impact on the later strength and durability of concrete;
(3) Select appropriate inorganic salt antifreeze and early strength components to avoid crystallization and precipitation at low temperatures, clogging pipelines, and affecting concrete production;
(4) When using organic matter as an antifreeze component, the dosage should not be too large to avoid affecting the later strength of the concrete;
( 5) Control the chloride ion content and alkali content in the antifreeze to prevent any impact on the durability of concrete.
(4) The dosage of antifreeze
How to properly and accurately determine the dosage of antifreeze , a problem that is of great concern during the use of antifreeze. From the perspective of practical application, the dosage of concrete antifreeze is affected by cement type, dosage, water-cement ratio, temperature, quality and dosage of water-reducing agent, curing conditions, engineering location, shape and formwork, etc. Among these, the water-cement ratio, temperature, surface coefficient of structural parts, etc. are directly proportional to the amount of antifreeze, while the amount of cement, water reduction by water reducing agent, etc. are inversely proportional to the amount of antifreeze. There is an argument based on the water-cement ratio that the frost resistance of concrete is related to the amount of liquid and ash. The corresponding antifreeze dosage is calculated as:
a＝Ct·(W/C)·I·dt (1)
Where: a——antifreeze content (%);
Ct——mass fraction of antifreeze solution at temperature t (%);
dt——density of original aqueous solution ( g/cm3);
I——Amount of ice generated (%);
W/ C——water-cement ratio of concrete.
From the formula of antifreeze dosage, many calculation parameters are not easy to determine, which increases the difficulty and inaccuracy of calculation. Relevant personnel have found that when the freezing amount I of the antifreeze aqueous solution in concrete reaches 40% to 50%, it will not have any adverse effect on the strength of the concrete. The antifreeze dosage under high temperature conditions is:
a=0.5Ct·(W/C)·dt (2)
(5) Does the concrete after adding antifreeze need thermal insulation and curing?
Some construction workers believe that the antifreeze added to the concrete will It already has anti-freeze capabilities and does not require maintenance. From the above analysis of the action mechanism of antifreeze, it is necessary and necessary to carry out thermal insulation maintenance during the construction process. Make concrete reach critical strength as early as possible to reduce the root cause of concrete freezing damage. According to national regulations, before the concrete poured under negative temperature conditions in winter is frozen, the concrete shall not be lower than "30% of the design concrete strength standard for concrete mixed with Portland cement or ordinary Portland cement, and slag Portland cement" The mixed concrete is 40% of the designed concrete strength standard. When the temperature of the concrete mixed with antifreeze drops below the specified temperature of the antifreeze, its strength should not be lower than 3.5~5.0MPa, that is, the concrete reaches a certain strength. value (critical strength), it can have a certain resistance strength. Low temperature will no longer cause damage to the concrete, and the strength will increase to the design strength after the temperature becomes normal.
Therefore, during the winter concrete project construction process, other winter construction measures should still be adopted after using antifreeze, such as the greenhouse method, comprehensive thermal storage method, etc.
Under the alternating positive and negative temperature conditions of -5℃~+5℃ daily temperature, early strength agent or early strength water reducing agent can be used, and cover should be used after concrete pouring Plastic film + one layer of straw bags or other substitutes for covering and maintenance; when the daily temperature is -5℃~-10℃, antifreeze with a specified temperature of -5℃ can be used. After concrete pouring, cover with plastic film + two layers of straw bags or Cover and maintain with other substitutes; when the lowest temperature is -15°C, use antifreeze with a specified temperature of -15°C, and use insulation measures in a timely manner for concrete curing after concrete pouring.
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