1. Concrete construction in winter
1. Regulations on the start and end time of winter construction
The construction of concrete structure projects should be carried out in winter For measures, the first day when the temperature is stably below 5℃ for 5 consecutive days can be taken as the starting date of winter construction. When the temperature rises, the last day that is stably above 5℃ for 5 consecutive days will be taken as the end date of winter construction. The date between the first day and the last day is the concrete winter construction period.
2. Effect of temperature on concrete properties
The ambient temperature is low, cement The hydration reaction is slow, which affects the growth of concrete strength. The test shows that every time the temperature decreases by 1°C, the hydration of cement decreases by about 5% to 7%. In the range of 1°C to 0°C, the hydration activity of cement decreases drastically and the hydration is slow. Generally, when the temperature is below a certain range of 0℃, free water will begin to freeze. When the temperature reaches about -15℃, almost all free water will freeze into ice, causing the hydration and hardening of cement to completely stop.
When water is converted into solid ice, its volume increases by about 9%, causing internal stress in the concrete, causing relative displacement of aggregates and cement particles and internal Moisture migrates to the negative temperature surface and forms ice aggregates in the concrete body, causing local structural damage
Premature freezing of newly poured concrete will greatly reduce the ultimate strength, and the strength loss rate may reach 50% of the design mark, and even cause damage to the overall structure; but when the concrete reaches the critical strength and is frozen, the ultimate strength of the concrete will The loss is small and no overall structural damage will occur.
A certain amount of antifreeze added to the concrete mixing water will lower the freezing point of the aqueous solution. The degree of reduction in the freezing point depends on the type and dosage of the antifreeze or solution. related to the concentration. The effectiveness of antifreeze depends largely on the concentration of the solution and the negative temperatures to which the concrete is subjected during hardening.
The main purpose of adding antifreeze to concrete is to maintain a sufficient liquid phase at negative temperatures so that the hydration of the cement can continue. After switching to positive temperature, the concrete strength can further increase and reach or exceed the design mark.
3. Precautions for winter construction of concrete
Concrete construction in winter requires positive temperature Pouring and positive temperature curing. The heating of raw materials, as well as the mixing, transportation, pouring and curing of concrete should be thermally calculated and constructed accordingly.
(1) Requirements for materials and material heating
①WinterImproves the ability of concrete to prevent early freezing damage.
Commonly used water-reducing agents include lignocalcium, lignin, naphthalene-based college water-reducing agents, melamine, sulfamate, etc.
(3) Air entrainment component
The volume of water increases when it freezes 9%. In severe cases, it can cause relative displacement of aggregates and cement particles in concrete, causing damage or even destruction of the concrete structure, resulting in irreversible strength loss. This air-entraining agent can introduce a large number of evenly distributed, stable and closed small bubbles during the mixing of concrete. These bubbles have four main effects on concrete:
(a) It can reduce the water consumption of concrete and further reduce the water-cement ratio;
(b) The introduced bubbles have a buffering and weakening effect on the expansion force of ice crystals in concrete, reducing the destructive effect of the expansion force of ice crystals;
(c) Improves the durability of concrete;
(d) Small bubbles block capillary pores, making them freezeable Water decreases.
When the number of bubbles introduced is small and the volume is large, they are usually visible bubbles, which will not achieve the above effect, but will have some adverse effects, and due to vibration When the air is not removed well, some bubbles will aggregate into larger bubbles, so these bubbles are called harmful bubbles, so be particularly careful when using air-entraining raw materials.
The air-entraining component can use air-entraining water-reducing agents such as lignocalcium, lignosodium, and anthracene-based water-reducing agents. Air-entraining agents such as rosin thermopolymers can also be used.
(4) Antifreeze component
The main function is under negative temperature conditions It lowers the freezing point of free water in concrete, reduces the ice-forming rate of water inside concrete, so that there is more liquid water for cement hydration, and more importantly, changes the ice-forming structure so that the ice crystals become loose and form a three-dimensional network structure. , thus having no destructive effect on concrete structures. The dosage of antifreeze components is generally relatively large. Considering the requirements for the durability of concrete, the introduction of K+ and Na+ should be minimized. This is also a prominent feature of antifreeze.
Most of the water in the concrete mixed with antifreeze remains in the liquid phase at negative temperatures, and the strength of the concrete still increases at negative temperatures. The growth rate of concrete at negative temperatures depends on the type of cement, type of antifreeze and negative temperature. The order of the growth rate of concrete strength is as follows:
K2CO3>CaCl2>NaCl>NaNO2>Ca(NO3)2
As long as the concrete is protected from early freezing before reaching critical strength, the strength of concrete mixed with antifreeze can continue to grow normally at subsequent positive temperatures.
3. Production and engineering application of antifreeze
1 Production of antifreeze
Compound antifreeze is widely used in northern China.
The key to the production of powdered antifreeze lies in the selection of formula and the uniformity of product quality. For liquid products, the compatibility between components in aqueous solution must be considered. No precipitation, crystallization, floc is allowed in liquid products, and the consistency must be suitable for pipeline transportation.
2 Selection of antifreeze in winter construction
Winter application of concrete is roughly divided into There are three categories: cold concrete, negative temperature concrete and early strength concrete.
Cold concrete means that except for the mixing water, which must remain liquid, other aggregates are not insulated, and no insulation measures are taken after pouring. It mainly relies on anti-freeze components. Ensure that a certain amount of supercooled liquid is slowly hydrated at negative temperature. This kind of concrete needs to be mixed with a large amount of inorganic salts to lower the freezing point. It should only be used when the construction conditions do not allow other measures to be taken.
Negative temperature concrete: When concrete is constructed at negative temperature, the sand, stone, and mixing water must be insulated and heat treated. During the concrete curing process, it must be combined with thermal insulation protection, Combined with comprehensive measures such as thermal storage method.
Low-temperature early-strength concrete: Construction at low temperatures around 0°C or not very low negative temperatures, using only heated mixing water and supplemented by comprehensive heat storage methods Keep the concrete hardening and strengthening at a positive temperature.
The function of adding antifreeze to cold concrete and negative temperature concrete is to lower the freezing point of the liquid phase in the mixed species so that the cement can continue to hydrate at negative temperature. Therefore, it is used Antifreeze type antifreeze, negative temperature concrete uses heating raw materials and heat storage and insulation maintenance to make the initial temperature of the concrete always higher than the lowest temperature outside, so the amount of admixture is less than that of cold concrete.
Low-temperature early-strength concrete only needs to add early-strength antifreeze.
Concrete construction is roughly divided into three categories: cold concrete, negative temperature concrete and early strength concrete. Cold concrete means that except for the mixing water, which must remain liquid, other aggregates are not insulated, and no insulation measures are taken after pouring. It mainly relies on anti-freeze components. Ensure that a certain amount of supercooled liquid is slowly hydrated at negative temperature. This kind of concrete needs to be mixed with a large amount of inorganic salts to lower the freezing point. It should only be used when the construction conditions do not allow other measures to be taken.
Negative temperature concrete: When concrete is constructed at negative temperature, the sand, stone, and mixing water must be insulated and heat treated. During the concrete curing process, it must be combined with thermal insulation protection, Combined with comprehensive measures such as thermal storage method.
Low-temperature early-strength concrete: Construction at low temperatures around 0°C or not very low negative temperatures, using only heated mixing water and supplemented by comprehensive heat storage methods Keep the concrete hardening and strengthening at a positive temperature.
The function of adding antifreeze to cold concrete and negative temperature concrete is to lower the freezing point of the liquid phase in the mixed species so that the cement can continue to hydrate at negative temperature. Therefore, it is used Antifreeze type antifreeze, negative temperature concrete uses heating raw materials and heat storage and insulation maintenance to make the initial temperature of the concrete always higher than the lowest temperature outside, so the amount of admixture is less than that of cold concrete.
Low-temperature early-strength concrete only needs to add early-strength antifreeze.
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