Thread Content
Abstract: Many areas in our country have long cold seasons. Due to schedule constraints, winter concrete construction for many projects is inevitable. Research on the theories and methods for concrete construction in winter, both domestically and internationally, shows that when the ambient temperature drops to 4°C, appropriate construction methods can be employed to prevent the newly poured concrete from freezing prematurely and to maintain a small temperature difference between the exposed concrete and the winter temperatures; this approach can yield results similar to those achieved during construction in warmer weather. This article introduces the general principles of concrete construction in winter and the four commonly used construction methods. Keywords: concrete, construction, principles, methods. General principles of concrete construction in winter. The reason why concrete mixtures can gradually set and harden until they reach their final strength is due to the hydration of cement. The rate of cement hydration depends not only on the constituent materials and mix ratio of the concrete itself, but mainly on the temperature level. As the temperature rises, hydration accelerates, and strength increases rapidly as well ； When the temperature drops to 0°C, part of the water present in the concrete begins to freeze, gradually changing from a liquid state (water) to a solid state (ice). At this point, the amount of water involved in the cement hydration process decreases; as a result, hydration slows down, and the increase in strength is correspondingly slower. As the temperature continues to drop, when all the water present in the concrete turns into ice, that is, when it completely changes from a liquid state to a solid state, the hydration of cement essentially stops, and the strength no longer increases at this point. When water turns into ice, its volume increases by about 9%, and at the same time an ice expansion stress of around 2500 kilograms per square centimeter is generated. This stress value is often greater than the initial strength developed within the cement paste, causing the concrete to suffer varying degrees of damage (i.e., freeze-thaw damage) and resulting in a decrease in its strength. Furthermore, when water turns into ice, larger ice crystals form on the surfaces of the aggregates and rebar, reducing the adhesion between the cement paste and these materials, thereby affecting the compressive strength of the concrete. When the ice crystals melt, various voids are formed within the concrete, reducing its density and durability. It can be seen that in winter concrete construction, the change in the state of water is a key factor affecting the increase in concrete strength. Numerous experimental studies conducted by scholars at home and abroad on the state of water in concrete have shown that a curing period prior to freezing of freshly poured concrete can increase the amount of liquid phase within it, reduce the solid phase, and accelerate the hydration of cement. Experimental studies have also shown that the longer the pre-curing period before concrete is frozen, the less its strength is reduced. After thawing, when the concrete is back under normal temperature conditions and continues to be cured, its strength will increase further, although the extent of this increase varies. For concrete with a long curing period that achieves a high initial strength (such as 35% of R28), almost no loss in strength occurs after freezing. As for concrete with a short safety curing period and relatively low initial strength, its strength suffers varying degrees of loss after freezing. It can be seen that before concrete is frozen, it needs to undergo a pre-curing period at normal temperatures in order to accelerate the hydration of the cement, enabling the concrete to achieve a minimum strength that protects it from frost damage; this minimum strength is commonly referred to as the critical strength, and it is through this that the desired results can be attained. Regarding critical strength, different countries specify different values; in China, it is required to be no less than 30% of the design grade, and it must also not be less than 35 kilograms per square centimeter. Selection of methods for concrete construction in winter As can be seen from the above analysis, three main issues need to be addressed when constructing concrete in winter: first, how to determine the shortest curing period for the concrete; second, how to prevent early freezing damage to the concrete; and third, how to ensure that the concrete’s strength and durability meet the required standards over time. In actual engineering projects, a suitable construction method must be selected based on factors such as the temperature conditions during construction, the condition of the structural elements (volume of work, thickness of the structure, and degree of exposure), the urgency of the project timeline, the type and price of cement, the performance and cost of early-strength agents, retarding agents, and antifreeze agents, the performance and price of insulation materials, as well as the availability of heat sources. Generally speaking, for the same project, there can be several different winter construction plans. An ideal solution should achieve the highest quality of workmanship with the shortest construction time and lowest costs, that is, optimization of time, cost, and quality. Currently, basically the following 4 methods are used. Method for adjusting the mix ratio Mainly applicable to concrete construction at around 0°C. Specific methods: ① Choosing the appropriate type of cement is an important way to improve the frost resistance of concrete. The test results show that early-strength Portland cement should be used. This cement has a high heat of hydration, and it releases its strength at an early stage; generally, its compressive strength after 3 days is roughly equivalent to that of ordinary silica cement after 7 days, showing a significant effect. ②Try to reduce the water-cement ratio and slightly increase the amount of cement, thereby increasing the hydration heat and shortening the time required to reach the desired strength. ③Air-entraining agents are used. While keeping the concrete mix ratio unchanged, the bubbles generated by the addition of air-entraining agents increase the volume of the cement paste, thereby improving the workability of the mixture and enhancing its cohesion and water retention capacity. These bubbles also help to cushion the water pressure resulting from ice formation within the concrete, thus improving its frost resistance. ④The addition of early-strength admixtures shortens the setting time of concrete and improves its early strength. Commonly used ones include sodium sulfate (added at 2% of the cement amount) and MS-F composite early-strength water reducer (added at 5% of the cement amount). ⑤Select aggregates with high particle hardness and few gaps, so that their coefficient of thermal expansion is similar to that of the surrounding mortar. Heat storage method: Mainly used in projects with temperatures around -10°C and relatively thick structures. The method involves heating the raw materials (water, sand, gravel), so that the concrete retains a considerable amount of heat even after mixing, transportation, and pouring. This enables faster heat release during the hydration of cement, while also enhancing the concrete’s insulation properties; as a result, the newly poured concrete has sufficient frost resistance before the temperature drops to 0°C. This method is simple to implement and does not require high construction costs. However, it is necessary to pay attention to internal insulation to prevent the corners and exposed surfaces from freezing, as well as to extend the curing period. External heating method: Mainly used in projects where the temperature is above -10°C and the components are not very thick. Heat is transferred to the concrete by heating the air surrounding it, or by directly heating the concrete itself, so that it can harden properly under normal temperature conditions. ①Heated by a furnace. It is generally used in smaller construction sites; the method is simple. However, the indoor temperature is low and the environment is dry, and the carbon dioxide released causes carbonation of the surface of the freshly poured concrete, affecting its quality. ②Heated by steam. Use steam to harden concrete under humid and warm conditions. This method is easier to control, with uniform heating temperature. However, it requires specialized boiler equipment, resulting in high costs. Moreover, heat loss is high, and the working conditions are not ideal. ③Electric heating. By using rebar as electrodes or attaching electric heaters to the concrete surface, electrical energy can be converted into thermal energy to raise the temperature of the concrete. This method is simple and convenient, with low heat loss and easy control; its drawback is high power consumption. ④Infrared heating. The concrete is subjected to sealed radiant heating using a high-temperature electric heater or a gas infrared generator. Antifreeze admixtures: In temperatures above -10°C, a chemical agent that can lower the freezing point of water is added to the concrete mixture, allowing the concrete to remain in a liquid state at low temperatures. This enables hydration to continue, thereby facilitating an increase in the strength of the concrete. Currently, commonly used antifreeze agents include calcium oxide, sodium chloride, and other single-component antifreezes, as well as a composite antifreeze made of sodium nitrite and sodium chloride. The above four winter construction methods all have their advantages and disadvantages, and their applicability is restricted by certain conditions. Based on the existing conditions at the construction site, one or more construction methods should be used in combination.