General principles of foam concrete construction in winter

The reason why the concrete mixture in the foam concrete insulation material can gradually solidify and harden until it reaches the final strength after pouring is due to the hydration of the cement. The speed of cement hydration is not only related to the composition and mix ratio of the concrete itself, but also mainly changes with the temperature.

When the temperature rises, hydration accelerates and the strength increases faster; when the temperature drops to 0°C, part of the water present in the concrete It begins to freeze and gradually changes from liquid phase (water) to solid phase (water). At this time, less water participates in the hydration of the cement. Therefore, the hydration slows down and the strength growth is correspondingly slower. As the temperature continues to drop, when the water present in the concrete completely turns into ice, that is, completely changes from liquid phase to solid phase, the hydration of the cement basically stops, and the strength no longer increases at this time.

After water turns into ice, its volume increases by about 9%, and at the same time, an ice expansion stress of about 2500 kilograms per square centimeter is generated. This stress value is often greater than the initial strength value formed inside the cement stone, causing the concrete to be damaged to varying degrees (that is, frozen damage during dry periods) and reduce the strength. In addition, when water turns into ice, ice with larger particles will be produced on the surface of aggregates and steel bars, weakening the bonding force between cement slurry and aggregates and steel bars, thus affecting the compressive strength of concrete. When the ice melts, various voids will be formed inside the concrete, which will reduce the compactness and durability of the concrete.

It can be seen that during the construction of foam concrete insulation boards in winter, the change of water form is the key to the growth of concrete strength. Many scholars at home and abroad have conducted a large number of experimental studies on the form of water in concrete. The results show that newly poured concrete has a pre-curing period before freezing, which can increase its internal liquid phase, reduce the solid phase, and accelerate the hydration of cement. Experimental research also shows that the longer the pre-curing period of concrete before freezing, the smaller the strength loss.

If the foam concrete insulation board continues to be cured after thawing (that is, under normal temperature conditions), its strength will increase, but the magnitude of the increase will vary. For concrete with a long pre-curing period and high initial strength (for example, reaching 35% of R28), there will be almost no loss of later strength after freezing. For concrete with a short safety pre-curing period and relatively low initial strength, the later strength will be lost to varying degrees after freezing.

It can be seen that before the concrete freezes, it must have a pre-curing period at normal temperature to accelerate the hydration of the cement so that the concrete will not suffer The minimum intensity of freezing damage, generally called the critical intensity, can achieve the desired effect. As for the critical strength, the values ​​​​are stipulated by various countries. Our country stipulates that it should not be less than 30% of the design mark, nor less than 35 kilograms per square centimeter.

Selection of winter construction methods for foam concrete insulation boards

From the above analysis, we can know that in winter concrete During the construction, three main problems were solved: first, how to determine the shortest curing period of concrete, second, how to prevent early freezing damage of concrete, and third, how to ensure that the later strength and durability of concrete meet the requirements.

In actual projects, it should be based on the temperature conditions during construction, the condition of the project structure (the amount of work, the thickness of the structure and the exposure), the urgency of the construction period, the cement The varieties and prices, the performance and price of early strength agents, reducing agents, and antifreeze agents, the performance and price of insulation materials, the conditions of heat sources, etc., are used to select a reasonable construction method.

Generally speaking, for the same project, there can be several different winter construction plans. An ideal plan should use the shortest construction period and the lowest construction cost to obtain the best project quality, that is, the construction period, cost, and quality are optimized.

Four methods for constructing foam concrete insulation boards in winter:

1. Adjust the mix ratio

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It is mainly suitable for the construction of foam concrete insulation panels at around 0℃.

Specific methods:

①Selecting the appropriate type of cement is an important means to improve the frost resistance of concrete. The test results show that early strength Portland cement should be used. This cement has a large heat of hydration and releases the highest strength in the early stage. Generally, the 3-day compressive strength is approximately equivalent to the 7-day strength of ordinary silicon cement, and the effect is obvious.

② Reduce the water-cement ratio as much as possible and slightly increase the cement dosage, thereby increasing the heat of hydration and shortening the time to reach age strength.

③Add air-entraining agent. While keeping the concrete mix ratio unchanged, the bubbles generated after adding the air-entraining agent increase the volume of the cement slurry accordingly, improve the fluidity of the mixture, improve its cohesion and water retention, and buffer the freezing of water in the concrete. The water pressure generated increases the frost resistance of concrete.

④Add early strength admixture to shorten the setting time of concrete and improve early strength. The more commonly used ones are sodium sulfate (2% of the cement dosage) and MS?F composite early strength water test agent (5% of the cement dosage).

⑤Choose particles with high hardness.��The aggregate with few gaps makes its thermal expansion coefficient similar to that of the surrounding mortar.

2. Thermal storage method

Mainly used when the temperature is less than 10℃ and the structure is relatively thick engineering.

The method is to heat the raw materials (water, sand, stone) so that the concrete will retain considerable heat after mixing, transportation and pouring, so that the concrete can Cement hydrates and releases heat quickly, and the insulation of concrete is strengthened to ensure that the newly poured concrete has sufficient frost resistance before the temperature drops to 0°C. This method has a simple process and low construction cost, but attention should be paid to internal insulation to avoid freezing of corners and exposed surfaces, and the curing period should be extended.
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3. External heating method

Mainly used for projects where the temperature is above 10°C and the components are not thick. By heating the air around the concrete components, the heat is transferred to the concrete, or the concrete is heated directly, so that the concrete can harden normally under positive temperature conditions.

Specific method:

① Heating with stove. Generally used on smaller construction sites, the method is simple, but the indoor temperature is not high and it is relatively dry, and the carbon dioxide released will carbonize the surface of the newly poured concrete, affecting the quality.

②Steam heating. Concrete is hardened using steam under moist heat conditions. This method is easier to control and the heating temperature is uniform. However, since it requires specialized boiler equipment, the cost is relatively high. Moreover, the heat loss is large and the working conditions are not ideal.

③Electric heating. Use steel bars as electrodes or attach electric heaters to the concrete surface to convert electrical energy into thermal energy to increase the temperature of the concrete. This method is simple and convenient, has less heat loss and is easy to control. The disadvantage is that it consumes a lot of electricity.

④Infrared heating. Use high-temperature electric heaters or gas infrared generators to heat the concrete with sealed radiation.

4. Antifreeze admixture

At temperatures below 10°C, the effect on concrete A chemical agent that can lower the freezing point of water is added to the mixture, so that the concrete remains in a liquid phase at negative temperatures, and the hydration can continue, thereby continuing to increase the strength of the concrete. Currently, single antifreeze agents such as calcium oxide and sodium chloride and composite antifreeze agents such as sodium nitrite and sodium chloride are commonly used.

The above four winter construction methods have advantages and disadvantages, and their scope of application is restricted by certain conditions. One or a combination of two or more construction methods should be used based on the existing conditions of the construction site. </p