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 and may cause accidents to the quality of the project. Admixtures that can harden concrete at negative temperatures and achieve sufficient antifreeze strength within a specified time are called antifreeze. Concrete mixed with antifreeze can be hardened at negative temperatures without the need for heating, ultimately reaching the same quality level as concrete cured at room temperature. 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. Therefore, antifreeze must be used correctly according to the mechanism and rules of concrete freezing damage, which is of great significance to ensuring the quality of the project.
1. Antifreeze action mechanism
Concrete antifreeze is constructed under negative temperature conditions in winter Admixtures used. Adding antifreeze can significantly reduce the freezing point of concrete at specified temperatures, refine ice crystals, prevent the liquid phase of concrete from freezing or maintain a certain amount of liquid phase water, ensure the hydration of cement, and achieve optimal results within a certain period of time. Expected strength, concrete will not be damaged due to frost heaving at low temperatures. The main mechanisms of action of antifreeze in concrete are as follows.
(1) Ice crystal distortion theory
When pure water freezes at 0℃, due to The effect of hydrogen bonds 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, due to the interference of the hydrogen bonds between water molecules, the antifreeze molecules will precipitate into a flocculent structure, which is macroscopically very soft and frost-heaving. Fine ice crystals with significantly reduced 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 has a significant impact on reducing the The freezing point of the liquid phase 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 equilibrium theory
When the temperature drops to the liquid phase in concrete At the freezing point, 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. , which 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. For concrete with a certain mix ratio, 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. Components of antifreeze
Antifreeze and antifreeze group are not the same concept. Antifreeze is a type of admixture, which consists of water-reducing components, antifreeze components, air-entraining components, and sometimes early-strength components. Its function is to not only harden the concrete at negative temperatures, but also enable it to eventually reach the quality level of concrete cured at normal temperature. The antifreeze component refers to a chemical substance that protects the concrete mixture from freezing damage in negative temperature environments. During the construction of concrete projects in winter, in order to prevent the concrete from freezing damage, antifreeze is often added during production to reduce the freezing point temperature of liquid water in the concrete, ensure that there is liquid water inside the concrete that can satisfy hydration, and reduce damage caused by low temperatures. 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 antifreeze components, water reducing agents, and air entrainment. Agents, etc., the percentage content of each component varies with the winter temperature change characteristics of the area where it is used.
(1) Antifreeze component
Its function is to destroy the crystal form of ice and reduce water content. The liquid phase freezing point maintains the liquid water required for the cement hydration reaction inside the concrete to prevent the concrete 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 the continuation of the hydration reaction of concrete at negative temperatures.�The amount of mixing water should be 7%; the amount of sodium nitrite, calcium nitrite, sodium nitrate, and calcium nitrate should not be greater than 5% of the cement weight; the amount of urea should not be greater than 4% of the cement weight; the amount of potassium carbonate should not be greater than 10% of cement weight.
4. The amount of antifreeze
How to properly and accurately determine the amount of antifreeze Quantity is a matter 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)
In the formula: a——antifreeze content (%);
p>
Ct——Mass fraction of antifreeze solution at temperature t (%);
dt—— Original aqueous solution density (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 concrete after adding antifreeze need thermal insulation maintenance?
Some construction workers believe that the antifreeze added to the concrete Agent, it already has anti-freeze ability 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. It is necessary to make the concrete reach critical strength as soon 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 heat storage method, etc.
Under the conditions of alternating positive and negative temperatures of -5℃～+5℃ in daily temperature, early strength agent or early strength water reducing agent can be used. After concrete pouring, Cover with plastic film + a layer of straw bags or other substitutes for maintenance; when the daily temperature is -5°C to -10°C, antifreeze with a specified temperature of -5°C can be used. After concrete pouring, cover with plastic film + two layers of straw bags Or other substitutes for covering and curing; when the lowest temperature is -15°C, use antifreeze with a specified temperature of -15°C, and adopt insulation measures in a timely manner after concrete pouring for concrete curing.
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