1. Winter construction of concrete
1. Regulations on the start and end time of winter construction
Winter construction measures should be adopted for the construction of concrete structure projects. 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
When the ambient temperature is low, the hydration reaction of cement is slow , affecting 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; when the concrete After reaching the critical strength and suffering from freezing, the ultimate strength loss of concrete will be small, and the overall structural damage will not 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 cement can continue; turn to positive After heating, the strength of concrete can further increase and reach or exceed the design label.
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
①For preparing concrete during winter construction For cement, Portland cement and ordinary Portland cement with high activity and high hydration heat should be given priority. The type of cement used in steam curing should be determined through experiments. The strength of cement should not be less than 42.5 grade, and the water-cement ratio should not be greater than 0.6. Cement must not be heated directly. It must be transported to a greenhouse for storage 1 to 2 days before use. The temperature of the greenhouse should be above 5°C.
② Because the specific heat of water is about 5 times that of sand and stone aggregates, the method of heating water should be given priority when mixing concrete in winter, but the heating temperature must meet the specified values.
③ Aggregates are required to be cleaned and stored in advance to ensure that the aggregates are clean and free of frozen blocks, ice and snow. All winter aggregate storage sites should be located on higher ground without water accumulation.
④The temperature of sand and stone for concrete mixing during winter construction must meet the temperature required by thermal calculations. The heating method can be adapted to local conditions, but steam heating is the best method.
⑤ No matter which method is used to heat the raw materials, when designing the heating equipment, the maximum daily material consumption and the required temperature must be found first. According to the raw materials From the initial temperature and specific heat, find the total heat required. Also consider the heat loss during the heating process. With the total heat required, the type, size and number of heat sources can be decided.
(2) Mixing
Concrete should not be mixed in the open air. A greenhouse should be set up as much as possible, and large concrete mixers should be used first. capacity of the mixer to reduce heat loss from the concrete. Before blending, rinse the blender with hot water or steam. The mixing time of concrete is 50% longer than the specified time at normal temperature. Since mixing cement with water at about 80°C will cause sudden solidification, the order of material feeding is to first add water and sand into the mix, and then add cement. If it can be ensured that the hot water does not come into direct contact with the cement, the water can be heated to 100°C.
(3) Transportation
The transportation time and distance of concrete should ensure that the concrete does not segregate or lose Plasticity. The main measures are to reduce transportation time and distance; use large-volume transportation tools and properly insulate them.
(4) Pouring and curing
Before pouring concrete, the formwork and steel bars should be removed from the Freezing snow and dirt, try to pour concrete as quickly as possible to prevent excessive heat loss. The exit temperature of the concrete mixture should not be lower than 10°C, and the mold entry temperature should not be lower than 5°C. When using heating curing, the temperature of concrete before curing shall not be lower than 2℃.
Heating and curingem”>(b) The introduced air 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 and reduce freezable water in capillary pores.
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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. Instead, it will have some adverse effects, and due to ineffective vibration elimination Sometimes, some bubbles will aggregate into larger bubbles, so these bubbles are called harmful bubbles, so be particularly careful when using air-entraining raw materials.
Air-entraining components You can use air-entraining water-reducing agents such as lignocalcium, lignosodium, anthracene-based water-reducing agents, etc. You can also use air-entraining agents, such as rosin thermopolymers, etc.
(4) Antifreeze component
The main function is to reduce the freezing point of free water in concrete under negative temperature conditions, reduce the ice formation rate of water inside concrete, and make In addition to having more liquid water for cement hydration, the more important thing is to change the ice structure, so that the ice crystals become loose and form a three-dimensional network structure, so there is no destructive effect on the concrete structure. The amount of antifreeze components is generally relatively large. Considering the requirements for the durability of concrete, the introduction of K+ and Na+ should be minimized, which is also the outstanding feature of antifreeze.
Add antifreeze at negative temperatures Most of the water in concrete still remains in liquid phase, and the strength of concrete is still growing at negative temperatures. The growth rate of concrete at negative temperatures depends on the type of cement, the type of antifreeze and the negative temperature. The order of the growth rate of concrete strength is as follows Arrangement:
K2CO3>CaCl2>NaCl>NaNO2>Ca(NO3)2
As long as the concrete It is not subject to early freezing before reaching the critical strength, and the strength of concrete mixed with antifreeze can continue to grow normally at subsequent positive temperatures.
3. Antifreeze Production and engineering application
1 Production of antifreeze
Compound antifreeze is widely used in northern China Use.
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 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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