The concrete mix in foamed concrete insulation materials can gradually set and harden after pouring until the final strength is obtained due to cement hydration. The speed of cement hydration is related to the composition of the concrete itself and the mix ratio and mainly changes with the temperature.
When the temperature rises, the hydration is accelerated, and the strength increases rapidly. When the temperature is reduced to 0℃, a part of the water present in the concrete begins to freeze and gradually changes from the liquid phase (water) to the solid phase (water). At this time, the water involved in the hydration of the cement is reduced, so the hydration is slowed down, and the strength growth is correspondingly slower. The temperature continues to drop, and when the water in the concrete completely turns into ice, that is, entirely from the liquid phase to the solid phase, the hydration of the cement stops, and the strength will no longer increase.
When the water becomes ice, the volume increases by about 9%, generating an ice expansion stress of about 2,500 kg/mm2. This stress value is often more significant than the initial strength value formed inside the cement stone, so the concrete is damaged to different degrees (frozen damage during the drought period) and reduces the strength. In addition, when the water becomes ice, it will also produce large particles of ice on the surface of the aggregate and steel bars, weakening the bonding force between the cement slurry and the aggregate and steel bars, thus affecting the compressive strength of the concrete. When the ice melts, it will form a variety of voids inside the concrete and reduce its density and durability.
In 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, and the results show that there is a pre-conditioning period before freezing of newly poured concrete, which can increase its internal liquid phase, reduce the solid phase, and accelerate the hydration of cement. The experimental study also shows that the longer the pre-conditioning period of concrete before freezing, the smaller the strength loss.
1) The raw materials of antifreeze concrete must meet the requirements of winter application. The cement used should be preferred Portland cement or ordinary Portland cement; its label should not be lower than 425, and the use of high-aluminum cement is strictly prohibited.
2) When using antifreeze, pay attention to the mixing method. Salt containing insoluble matter or negligible solubility in antifreeze must be ground into powder and mixed with cement. When it is necessary to use the solution, it should be fully dissolved and stirred evenly, and the concentration and amount of each addition should be strictly controlled. If composite antifreeze is used, their co-solubility, if not co-soluble, should be separately mixed into a solution and then added to the concrete. To accelerate the dissolution, the solution can be prepared with hot water at 40℃- 60℃ and added to the concrete. The antifreeze added in powder form should be ground fine through the 0.63mm screen before use if there is moisture caking.
3) It is necessary to control the dosage strictly, according to the minimum temperature of -10℃, 15℃ and -20℃ during the construction period; antifreeze with the specified temperature of -5℃, -10℃ and -15℃ can be used respectively. The amount of different antifreeze is very different, and the inaccurate amount has a significant influence on the performance of concrete. Excessive will make the concrete set too fast, resulting in construction difficulties and serious salt out on the component's surface, affecting the quality of the exterior decoration. Adding too much will also reduce the strength of the concrete; the strength will not increase after the positive temperature curing. If the dosage is insufficient, the concrete structure will freeze. Now, it is gradually developing in the direction of low alkali and low content, which is conducive to preventing the harm of alkali-aggregate reaction to concrete.
4) The mixing time of concrete mixed with antifreeze should be 50% longer than that without antifreeze to ensure that the antifreeze is evenly distributed in the concrete so that the strength of the concrete is consistent. Minimize transportation and watering time. To improve concrete's early strength, the concrete's temperature entering the mold should not be below -5℃.
5) The concrete with compound antifreeze containing air entrainment agent should be cured at harmful temperatures and can not be cured by steam. The use of steam curing will not only reduce the strength of concrete but also reduce its durability.
After pouring the foam concrete, it should be covered immediately, not watered. The initial curing temperature shall not be lower than the prescribed antifreeze temperature. When the concrete temperature drops below the temperature specified by the antifreeze agent, the critical strength of the concrete antifreeze should reach 3.5MPa; otherwise, insulation measures must be adopted.
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