In the construction industry, some construction workers often refer to concrete constructed in winter and concrete with structural design requirements for frost resistance as "freeze-resistant concrete." Concrete constructed in winter mainly meets the technical requirements by taking technical measures to prevent frost heaving damage before reaching the critical strength of freezing after the concrete is poured. It should be called anti-freeze concrete. Structural design has frost resistance level requirements, and the concrete itself should have the ability to withstand freeze-thaw cycles for a long time, so it should be called frost-resistant concrete.
In the relevant standards and specifications, the term antifreeze concrete cannot be found. Only "concrete mixed with antifreeze" or "concrete constructed in winter" can be found. Word; the term for frost-resistant concrete in the specification is: concrete with a frost-resistant grade equal to or greater than F50. In fact, the technical requirements of these two types of concrete are completely different, but the standard specification does not give a definition of "antifreeze concrete", or some construction workers do not study the standard specification enough, so it is easy for some construction workers to misunderstand or confuse the two types of concrete. For example: Mistakenly believe that "antifreeze concrete is concrete mixed with antifreeze", or "Why is antifreeze concrete poured on hot days?" etc. These misunderstandings may result in insufficient attention to the production, pouring and maintenance of frost-resistant concrete and affect the quality of the project.
Anti-freeze concrete
The technical requirements for anti-freeze concrete are: during the winter construction process, adopt Reliable technical measures enable the concrete to solidify and harden as soon as possible after pouring, and prevent frost heaving damage from occurring before reaching the critical strength due to freezing.
When the temperature is between 0 and 4°C, the activity of water is low, the hydration reaction of cement is extremely slow, and the strength development of concrete cannot meet the requirements. When the temperature is below 0°C, most of the moisture inside the concrete freezes. When water freezes, it produces a volume expansion of 9%, which will cause permanent damage to the concrete structure. In addition, after water freezes, there is not enough liquid water in the concrete to participate in the hydration reaction of the cement, and the strength of the concrete increases very slowly or even stop. Therefore, concrete poured during winter construction should be mixed with early strength agent or antifreeze agent, and appropriate insulation or warming measures should be taken during the early stage of concrete setting and hardening to make full use of the concrete's own heat or external heat (such as electric heating method, greenhouse method etc.), ensure that the initial curing temperature after concrete pouring is: not lower than 10°C in severe cold areas; not lower than 5°C in cold areas, so that the strength of the concrete has the conditions for normal growth and the critical strength of freezing is obtained as soon as possible.
1. Technical measures taken during winter construction
Material preheating method: that is, first Water, sand and stone are preheated and then used to mix concrete;
Thermal storage and insulation method: that is, using insulation materials to cover the poured concrete, so that after the concrete is poured Maintain no cooling or slow cooling within a certain period of time;
Warm shed method: that is, build an insulated and heated greenhouse at the construction site, so that the concrete will be in the greenhouse after pouring Curing should be carried out under positive temperature conditions. When possible, it is best to stir and pour in a greenhouse;
Incorporate early strength agent or antifreeze.
In addition, the amount of cement used for concrete construction in winter should be appropriately increased, or early-strength cement should be used, as well as high-performance water-reducing agents or high-efficiency water-reducing agents to minimize Technical measures such as water consumption.
The purpose of taking the above technical measures is to control and increase the temperature of concrete coming out of the machine and the temperature after entering the mold, so as to ensure that the strength of the concrete has the conditions for normal growth after pouring , no freezing damage will occur before reaching the critical strength of freezing.
2. About early strength agents and antifreeze
The causes of concrete damage in today’s world are classified according to their importance. The order of severity is: corrosion of steel bars, freezing damage in cold climates, and physical and chemical effects of corrosive environments. Therefore, when concrete mixed with early-strength agent or antifreeze agent, care should be taken to limit the chloride content. Chloride salt is a very important factor in inducing corrosion of steel bars. Prestressed concrete and reinforced concrete should strictly follow the relevant standards and regulations to control the maximum content of chlorine salt in the concrete. Chloride ion content can avoid engineering quality accidents and causing huge waste. Many projects at home and abroad have paid a heavy price for this.
Concrete early strength additives and components
Admixtures that can accelerate the early strength development of concrete Called early strength agent. In actual use, most of them are compound early-strength agents. Inorganic salts are detrimental to the later strength of concrete; chloride early-strength agents can cause corrosion of steel bars; sulfate early-strength agents may cause volume expansion, reducing the durability of concrete; sodium salts Early strength agent will increase the alkali content in concrete and produce alkali-aggregate reaction with active silica aggregate. If the early strength agent is added excessively, although the early effect of the concrete is good, the later strength loss will be large, and the salt precipitation will be intensified, affecting the concrete finish; it will increase the conductivity of the concrete and increase the risk of shrinkage and cracking of the concrete. The main components of concrete early strength agent are: sodium chloride, calcium chloride, sodium sulfate, calcium sulfate, aluminum sulfate, potassium dichromate, triethanolamine, triisopropanolamine, methanol, ethanol, calcium formate, lithium oxalate , sodium acetate, etc.
<p style="texThe faster the freezing speed of �; the worse the bubble structure, the larger the average bubble spacing; the lower the concrete strength, the worse the ability to resist freezing and thawing. When the water-to-cement ratio changes in the range of 0.45 to 0.85, the frost resistance of concrete without air-entraining agents changes little. Only when the water-to-cement ratio is less than 0.45, the frost resistance increases significantly with the decrease of the water-to-cement ratio; water-to-cement ratio Concrete with a ratio of less than 0.35 has high frost resistance even without adding air-entraining agents.Air content
Within a certain range, the more air content, the better the frost resistance of concrete. The better the sex. However, when the air content exceeds a certain range, the frost resistance of concrete decreases instead. The reason is that the increase in air content reduces the average bubble spacing and at the same time reduces the strength of the concrete (for every 1% increase in air content in concrete, the compressive strength decreases by 3% to 5%. ). Generally, when the maximum particle size of the natural aggregate used is 10 to 40 mm, sufficient frost resistance can be obtained by making the air content in the newly poured concrete reach 4% to 7%.
Concrete strength
When the hydrostatic pressure and seepage pressure exceed the tensile strength of concrete, Concrete undergoes freeze-thaw damage. Therefore, the strength of concrete, which represents the ability to resist freeze-thaw damage, also has an impact on the frost resistance of concrete. When the air content or average cell spacing is the same, concrete with high strength has higher frost resistance than concrete with low strength. But relatively speaking, the impact of strength on the frost resistance of concrete is far less than that of bubble structures.
Aggregate
When the aggregate is saturated with water, it will form in the pores and Static pressure is generated at the aggregate-cement slurry interface, and when it exceeds the strength of the aggregate or interface, freezing damage occurs. Therefore, the main factors affecting the frost resistance of aggregates are aggregate water absorption and aggregate size. Preparing frost-resistant concrete with aggregates with high water absorption (such as lightweight aggregates) relies more on the incorporation of air-entraining agents; the larger the size of the aggregate, the easier it is to damage after being frozen, but the impact of fine aggregate on the frost resistance of concrete Not big. In addition, the solidity, weathering degree, clay content, impurity content, etc. of the aggregate also have an impact on the frost resistance of concrete.
Cement varieties and dosage
With the increase in the amount of mixed materials in cement, concrete The frost resistance is reduced, so the preparation of frost-resistant concrete with Portland cement is better than using other types of cement. For non-air-entrained concrete, cement type and dosage have a certain impact on the frost resistance of concrete, while for air-entrained concrete, this effect is not significant.
Mixed materials
The amount of fly ash is within a certain range, and the strength and content Under the same air volume conditions, the frost resistance of concrete with and without fly ash is basically the same. However, when the amount of fly ash exceeds a certain range, the frost resistance of concrete will be reduced. When the silica fume content does not exceed 10%, the frost resistance of concrete is improved; when it exceeds 15%, the frost resistance will be significantly reduced.
Maintenance
Early curing after concrete pouring has a significant impact on the physical strength of the concrete structure . The author used C30 to pump concrete and form 150mm cube specimens for the test. From the test results, the compressive strength of the specimens cured with watering for 14 days was on average 4.4MPa higher than that of the specimens cured without watering, and the carbonization thickness was 1.5 less at 28d. ~2.0mm; the unwatered specimen has a greater impact on the estimated rebound strength. It fully shows that the curing method also has a certain impact on the frost resistance of concrete. Therefore, effective moisturizing and maintenance measures should be taken promptly after concrete is poured to both strengthen and prevent cracking and improve the durability of the concrete.
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