Contents of this article
Why does the strength of concrete with antifreeze added in winter not increase?
What are the effects of adding antifreeze to the concrete when building a house?
Does the antifreeze admixture come with early strength?
What impact will freezing in the Northeast have on concrete?
What is the mechanism of action of concrete antifreeze?
Why does the strength of concrete with antifreeze added in winter not increase?
Adding antifreeze to concrete will increase the strength of the concrete in the early stage, and the strength will not decrease in the later stage. In addition, it is beneficial to add antifreeze to concrete. In winter, when the temperature is relatively low, it is easy to freeze when mixing concrete and mortar. Adding antifreeze can effectively reduce the freezing point of concrete and mortar and improve Frost resistance of concrete and mortar in low temperature environments.
What is the impact of adding antifreeze to the concrete when building a house?
If the temperature will not drop below zero (for more than a week after pouring), just insulate the concrete. If concrete is poured in a negative temperature environment (the lowest temperature is below minus 5°C), the main thing to pay attention to is the early anti-freeze of the concrete. Antifreeze needs to be operated from two aspects:
1) Use antifreeze: Antifreeze can greatly reduce the freezing point of the mixing water, preventing the moisture inside the concrete from freezing and expanding in the plastic state and early stage, and causing frostbite to the concrete. Antifreeze has no obvious harm to the strength performance of concrete, but some antifreeze contains urea, and the concrete will release ammonia smell for a long time, so special attention is required.
2) Insulation: Use plywood, wooden boards and other formwork with a certain insulation effect as much as possible. Use insulation materials such as quilts, straw curtains or foam insulation boards to cover the exposed concrete surface and wrap the steel formwork to make the concrete Strength develops quickly. For winter construction, it is best to use early-strength silicate or ordinary Portland cement because the strength develops quickly and the heat of hydration is relatively high. However, as long as the insulation is good, any cement can be used. The strength develops more slowly, which is better for preventing thermal stress cracks. In an environment with a minimum temperature of 2°C, antifreeze may not be used when pouring concrete, but insulation must be done and the temperature change trend must be paid close attention to. If the temperature continues to drop, concrete insulation needs to be strengthened. Personally, I think it’s okay to add antifreeze, it’s relatively straightforward and simple. You don’t need to add it, but you need to take antifreeze and heat preservation measures
Does the antifreeze admixture come with early strength?
No!
Admixtures that accelerate the early strength development of concrete become early strength agents. Admixtures that can harden concrete at negative temperatures and achieve sufficient antifreeze strength within a specified time are called antifreeze. Early strength agent is an admixture that improves the early strength of concrete. Antifreeze is an admixture that lowers the freezing point of water so that concrete can solidify and harden at negative temperatures.
Each has its own role. Nowadays, in general projects, composite antifreeze is used, that is, early-strength antifreeze.
What impact will freezing in the Northeast have on concrete?
The direct harm of concrete being frozen is the reduction of concrete strength. For example, C40 concrete becomes C30 after freezing. The degree of strength reduction varies depending on the degree of freezing. The worst is that it has no strength and the concrete thaws. It will become crispy and can be picked up by hand.
To prevent concrete from freezing, measures should be taken. Different measures should be taken according to the temperature. If the temperature is 5 degrees to minus 10 degrees, a comprehensive heat storage method can be adopted to heat the raw materials, heat the mixing water, and Add antifreeze to the concrete during mixing, and cover it with insulation material after pouring. If the temperature is below minus 10 degrees, or even colder, in addition to covering the insulation material, you must also take heating measures, such as a charcoal stove or electric heating equipment.
There are many hazards. The main one is the decrease in strength. Secondly, the surface will peel, and the concrete will freeze. When it gets warm in the coming year, the expansion stress inside will cause cracks, so the concrete must be prevented from freezing.
The measures currently taken are: materials for heating and insulation boxes should be prepared before construction to ensure that the temperature of the poured concrete entering the mold is not lower than 5°C. Thermal storage curing method and steam heating method can be used. Maintenance, electric blanket heating and maintenance.
What is the mechanism of action of concrete antifreeze?
According to Ural's law, the degree of reduction of the freezing point of the liquid phase in the concrete mixture is related to the molar concentration of the antifreeze, that is, to the type and dosage of the antifreeze. Lowering the freezing point ensures the liquid water required for cement hydration in concrete at a certain negative temperature. This is the basis for using antifreeze in winter concrete construction. With the deepening of research, ice crystal distortion theory, liquid-cement ratio balance theory, maturity theory, etc. have been proposed for the action mechanism of antifreeze in concrete.
Ice crystal distortion theory: Pure water freezes at 0°C. Due to the effect of hydrogen bonds, water molecules aggregate into huge molecular aggregates, and the ice expansion force is very strong. When the temperature of an aqueous solution mixed with antifreeze is lower than its freezing point, ice begins to precipitate. Due to the interference of the antifreeze molecules on the hydrogen bonds between water molecules, the precipitated ice crystals are small and have a flocculent structure, and appear very soft macroscopically. , the frost heave stress is greatly reduced.
For example, NaN02, Na2S04, Ca(N03)2, etc. have strong ability to reduce frost heave force. According to the liquid-cement ratio balance theory, the concrete mixture mixed with antifreeze begins to freeze when the temperature drops to the freezing point of the liquid phase in the concrete. Under a certain negative temperature,The concentration of the �� phase increases due to ice precipitation, the freezing point decreases, and a new dynamic equilibrium is reached at the new negative temperature, that is, the ice-liquid coexistence state. There is still liquid water in the concrete to ensure that the cement can continue to hydrate; antifreeze The amount in the concrete remains unchanged, and the concentration of the liquid phase does not change at a certain temperature. The water consumed by cement hydration is replenished by the melting of ice, that is, the liquid-cement ratio is balanced.
Maturity theory The hydration process of cement is a complex chemical change and physical and chemical change process, and is very significantly affected by temperature. The strength of concrete with a certain mix ratio is a function of curing temperature and curing time. When the curing temperature is low, the cement hydration rate is slow and the concrete strength is low. For example, the maturity coefficient is 1 at 20°C, and is only 0 at -10°C.
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