The reason why the concrete mixture in the foamed concrete insulation material can gradually set and harden after pouring, until the final strength is obtained, is the result of cement hydration. The speed of cement hydration is mainly related to the composition and proportion of the concrete itself, and mainly changes with the temperature.
When the temperature rises, hydration speeds up, and the strength increases faster; when the temperature drops to 0°C, part of the water in the concrete begins to freeze and gradually changes from liquid (water) to solid ( water). At this time, the water involved in the hydration of cement is reduced, so the hydration slows down and the strength increase is correspondingly slower. The temperature continues to drop, and when the water in the concrete completely turns into ice, that is, when it completely changes from the liquid phase to the solid phase, the hydration of the cement basically stops and the strength no longer increases.
After the water turns into ice, the volume increases by about 9%, and at the same time an ice expansion stress of about 2500 kg/cm² 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 and reduce the strength. In addition, when the water turns into ice, it will also produce larger particles of ice on the surface of the aggregate and steel bars, weakening the bonding force of the cement paste with the aggregate and steel bars, thereby 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 in the construction of foamed concrete insulation board 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 fresh concrete has a pre-curing period before freezing, which can increase its internal liquid phase, reduce 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.
After the foamed concrete insulation board continues to be cured after thawing, its strength will increase, but the magnitude of the increase varies. For the long pre-curing period, the concrete with higher initial strength (for example, 35% of R28) is obtained after being frozen, there is almost no loss of later strength. For the concrete with a short safety pre-curing period and a relatively low initial strength, after being frozen, the later strength will lose to varying degrees.
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 obtains the lowest strength that does not suffer from freezing damage, generally called the critical strength, which can achieve the desired effect. Regarding the critical strength, the value varies from country to country. my country stipulates that it is not less than 30% of the design mark and not less than 35 kilograms per square centimeter.
Selection of winter construction method for foamed concrete insulation board
From the above analysis, we can know that in winter concrete construction, three problems are mainly solved: one is how to determine the shortest curing age of concrete, the other is how to prevent the early freezing damage of concrete, and the third is how to ensure that the later strength and durability of concrete meet the requirements.
In the actual project, according to the temperature during construction, the condition of the engineering structure (the amount of work, the degree of structure thickness and the exposure), the degree of urgency of the construction period, the type and price of cement, the early strength agent, reducing agent, and antifreeze agent Choose a reasonable construction method based on performance and price, performance and price of insulation materials, and heat source conditions.
Generally speaking, there can be several different winter construction plans for the same project. An ideal plan should use the shortest construction period and the lowest construction cost to obtain the best project quality, that is, the optimization of construction period, cost, and quality.
Four methods for construction of foamed concrete insulation board in winter:
1. Method of adjusting mix ratio
Mainly suitable for the construction of foamed concrete insulation board at about 0℃.
①Choosing appropriate types 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. The cement has a large heat of hydration and the highest strength in the early stage. Generally, the compressive strength of 3 days is approximately equivalent to the strength of ordinary silica cement for 7 days, and the effect is obvious.
② Reduce the water-cement ratio as much as possible and increase the amount of cement slightly, thereby increasing the heat of hydration and shortening the time to reach the age strength.
③Admixed with 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, improve the fluidity of the mixture, improve its cohesion and water retention, and buffer the water in the concrete from freezing The resulting water pressure improves the frost resistance of concrete.
④ Add early-strength admixtures to shorten the setting time of concrete and improve early strength.
⑤Choose aggregates with high particle hardness and few gaps to make the thermal expansion coefficient similar to that of the surrounding mortar.
2. Thermal storage method
It is mainly used for projects where the temperature is less than about 10℃ and the structure is relatively thick.
The method is: heating the raw materials (water, sand, stone), so that the concrete has considerable heat after mixing, transportation and pouring, so that the cement hydration heats up quickly, and strengthens the insulation of the concrete. Ensure that the fresh concrete has sufficient frost resistance before the temperature drops to 0°C. This method has simple process and low construction cost, but attention should be paid to internal heat preservation, to avoid freezing of corners and exposed surfaces, and to extend the maintenance period.
3. External heating method
It is mainly used for projects where the temperature is above 10℃ 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 directly heated, so that the concrete can be hardened normally under positive temperature conditions.
①Stove heating. Generally used in smaller construction sites, the method is simple, but the indoor temperature is not high, relatively dry, and the released carbon dioxide will carbonize the surface of the newly poured concrete and affect the quality.
②Steam heating. Steam is used to harden concrete in hot and humid conditions. This method is easier to control and the heating temperature is uniform. But because 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. Using steel bars as electrodes, or attaching an electric heater to the surface of the concrete, the electrical energy becomes heat energy to increase the temperature of the concrete. This method is simple and convenient, has less heat loss, is easy to control, and has the disadvantage of large power consumption.
④Infrared heating. Use high-temperature electric heaters or gas infrared generators to heat the concrete with sealed radiation.
4. Antifreeze admixture
In the temperature above 10℃, add a chemical agent that can lower the freezing point of water to the concrete mixture, so that the concrete is still in the liquid state at negative temperature, and the hydration can continue to make the concrete strength Continue to grow. Currently, there are commonly used monoclonal antifreeze agents such as calcium oxide and sodium chloride and sodium nitrite and sodium chloride composite antifreeze agents.
The above four winter construction methods have advantages and disadvantages, and their scope of application is restricted by certain conditions. According to the existing conditions of the construction site, one or two or more construction methods should be used in combination.
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