Introduction
Temperature plays a very important role in the process of forming the strength of concrete after mixing and casting. Generally speaking, the maintenance temperature of concrete should be hot rather than cold, the temperature when casting into the mold should be cold rather than hot. For bulk structures, in order to prevent the mixture from freezing and slow setting, the temperature of casting should not be too low. For bulk structures, in order to prevent the mixture from freezing and slow setting, the temperature of casting should not be too low. To prevent concrete from freezing and to ensure strength growth, the construction site should be temporarily enclosed. The surface of the concrete and the formwork should be covered with insulating material for thermal preservation and maintenance, and heated by steam or electric heat if necessary. After casting, it is advisable to monitor the concrete strength in real time by the maturity method or other testing methods until the predetermined strength requirements (e.g. threshold anti-freeze strength, demoulding strength, prestressing tensioning strength) are reached before stopping the above measures. To keep the concrete casting temperature, the aggregates can be stored in a warm place and the aggregates and water can be heated if necessary.
Four Methods of Winter Construction for Concrete
It is necessary to choose a reasonable construction method according to the temperature during construction, the structural condition of the project (engineering quantity, structural thickness and exposure), the degree of urgency of construction period, the variety and price of cement, the performance and price of early strength admixture, water-reducing admixture and anti-freeze admixture, the performance and price of insulation material, the condition of heat source, etc. Generally speaking, for the same project, there can be several different winter construction solutions. An ideal solution should be to obtain the best project quality with the shortest construction period and the lowest construction cost, that is, to optimize the construction period, cost and quality. At present, the following methods are basically used.
- Method of Adjusting Proportion
It is mainly applied to the construction of concrete around 0 ℃. First, choose the appropriate variety of cement to improve the anti-freezing of concrete. Test results show that early strength silicate cement should be used. The cement heat of hydration is larger, and has the highest strength released in the early stage. In general, its 3d compressive strength is approximately equivalent to the strength of 7d of ordinary silicate cement, and the effect is more obvious.
Second, try to reduce the water cement ratio, slightly increase the amount of cement, thereby increasing the heat of hydration and shortening the time to reach the age strength.
Third, mixed with air-entraining admixture. In the case of keeping the concrete proportion unchanged, the bubbles generated by the addition of air-entraining admixture correspondingly increase the volume of cement paste, improve the fluidity of the mixture, improve its cohesiveness and water retention, buffer the water pressure generated by water freezing in concrete, improve the anti-freezing properties of concrete.
Fourth, mixed with early strength admixture. Shorten the setting time of concrete, improve the early strength. It is common to use sodium sulfate (mixed with 2% of the amount of cement) and compound early strength water test admixture (mixed with 5% of the amount of cement).
Fifth, choose aggregates with high particle hardness and few voids. Make its thermal expansion coefficient and surrounding mortar expansion coefficient similar.
- Method of Thermal Energy Storage
It is mainly used for projects with temperature around -10℃ and thicker and larger structure.
Heat the raw materials (water, sand, gravel). To make the concrete still reserve considerable heat after mixing, transportation and pouring. To make the cement hydration exothermic faster, and strengthen the insulation of concrete to ensure that before the temperature drops to 0 ℃, the newly cast concrete has sufficient anti-freezing ability. This method is a simple process and the construction cost is not much. However, attention should be paid to internal insulation. Avoid the corner and exposed surface frozen, and extend the maintenance age.
- Method of External Heating
It is mainly used for projects with temperature above -10℃ and no thick and large structure. Heat the air around concrete units to transfer heat to concrete. Or directly heating concrete so that it can harden properly under positive temperature conditions.
- Method of Using Admixtures
In the temperature above -10 ℃, mixed concrete mixture with a chemical agent that can reduce the freezing point of water, so that the concrete in the negative temperature is still in the liquid phase state and hydration can continue, thus making the concrete strength grow.
The above four winter construction methods all have their advantages and disadvantages. The scope of application is also restricted by certain conditions. According to the site conditions, one or two or more construction methods should be used in combination.