Problems and measures to be noticed in winter construction

Concrete is different from building materials such as steel bar and stone. Its strength and performance increase with age, and it is a growth material. In actual production, if the external weather factors or internal construction factors change, it may cause quality problems in the final concrete bridge. The northern regions have high winter winds, low temperatures, and low air relative humidity, which can easily cause adverse effects on the increase in strength and performance of concrete materials. In winter construction of concrete, if measures are not taken in time to adjust the construction process according to weather changes, it is easy to cause quality accidents and bring losses to the project. There are many types of concrete parts of highway bridges, and the construction techniques are different. Quality problems are more likely to occur in winter construction. This article focuses on the construction technology of concrete, and puts forward the safeguard measures for winter construction, and at the same time puts forward the corresponding precautions for different parts to ensure the smooth progress of winter construction.

  Common diseases in concrete construction in winter mainly include shrinkage cracking and frostbite damage. Concrete frostbite damage during winter construction refers to the damage of fresh concrete during freezing. Except for part of the water in the freshly mixed concrete participating in the hydration reaction of the cement, the remaining water only provides the workability of the concrete. This part of the water gradually evaporates and evaporates during the concrete setting process. When the air temperature is lower than the freezing point, the surface moisture begins to freeze, the volume of the frozen water expands, the gel and cement particles are expelled, and the gel skeleton formed by the hydration of the cement is destroyed. When the temperature rises above the freezing point, the ice aggregates begin to melt, leaving voids in the original location and increasing the porosity of the concrete. Therefore, the concrete damaged by frostbite not only reduces the compressive strength, but also significantly deteriorates the concrete durability indicators such as compactness and adhesion to steel bars. High wind and air drying will increase the water loss and shrinkage of the concrete surface, especially when the surface of the component is too thick, because the aggregate in the concrete itself does not shrink, the shrinkage mainly comes from the shrinkage of the cement slurry, so the shrinkage of the floating slurry is much greater than the concrete shrink. When the outside environment is windy and the air is dry, it is easy to cause shrinkage and cracking of the floating slurry layer. Once the float layer cracks, it will cause stress concentration, which will eventually lead to cracking of the concrete body.

Winter concrete diseases are often caused by many factors. Therefore, the prevention of diseases should be comprehensively considered from the aspects of design, construction, and maintenance.

Concrete ratio

(1) Strength. The early strength should be improved. The water-binder ratio can be reduced appropriately, and the amount of cement should be increased to increase the hydration heat of cement and improve the early strength of concrete. Or mixed with early strength admixture to shorten the setting time of concrete.

(2) Use antifreeze. During the construction, if the temperature is too low, or the construction period is tight, the formwork turnaround time is short, and an appropriate amount of antifreeze can be incorporated. Incorporating antifreeze can reduce the freezing point of water on the one hand and ensure the liquid water required for cement hydration under low temperature environment; , Can reduce the damage to the gel skeleton.

Concrete raw materials

(1) The raw material reserves must be sufficient to avoid untimely supply due to rain, snow, weather or road conditions, which will affect the construction progress. The consistency of some liquid additives will increase or crystallize when the temperature is too low, resulting in weighing errors. Therefore, during the winter application period, heat preservation measures should be taken outside the storage tank of the additive, and a stirring device should be added inside the barrel. When the ambient temperature is too low, aggregates, especially river sand, will agglomerate, and the free water in the river sand will freeze, affecting its weighing accuracy and causing fluctuations in concrete quality. Therefore, the aggregate silo should adopt a sealed structure, and cotton curtains can be hung at the entrance and exit to keep warm and prevent the rain and snow from polluting the aggregate. If the sealing performance of the silo can not meet the requirements, heating can be used in the silo to ensure that the aggregate does not cause lumps and icing due to too low temperature.

(2) During winter construction, in order to ensure the smooth progress of cement hydration and avoid freezing of freshly mixed concrete, the concrete must have a certain mold injection temperature. Generally, the method of heating and mixing water is preferred. There are many ways of on-site water heating, for example, steam heating method, hot water blending method, electric heating method, etc. For ordinary Portland cement with a strength level greater than or equal to 42.5, the maximum mixing water shall not exceed 60 ° C. When the heating water cannot meet the requirements, the aggregate can be heated, but the cement and other rubber materials cannot be heated, and only heat preservation measures can be taken.

Mixing station

During winter construction, the concrete mixing station can be set up with a warm shed to fully enclose the mixing station and reserve the entrance and exit of the tank truck. The erection material can use color steel plate, tarpaulin or military canvas. Install heating measures such as heating in the greenhouse to ensure the normal operation of the equipment.

 Concrete mixing and transportation

Before mixing the concrete, the water temperature and outdoor temperature should be measured and recorded. If the water temperature is too low, it should be heated to the specified temperature in time. When mixing concrete with hot water, the hot water cannot directly contact with the cementing material. Aggregate and hot water should be added during concrete mixing. After a little mixing, cement and other cementing materials should be added. Appropriate extension. The exit temperature of concrete should not be lower than 10 ℃, and the entry temperature should not be lower than 5 ℃.

The construction of concrete transportation in winter should do a good job of dispatching, try to avoid the phenomenon of car pickup and traffic jam, and shorten the transportation time to the maximum extent. Concrete tanks should adopt heat preservation measures, and the concrete tanks can be completely enclosed and wrapped with thermal insulation materials.

Concrete pouring

Winter construction should pay close attention to the weather forecast. Before the construction, you should understand the weather conditions of the day, try to avoid bad weather such as strong winds and snow, and try to cast when the temperature is high at noon. When pouring, the method of pouring and covering at the same time is used to reduce heat loss and water loss on the concrete surface. After the pouring is completed, it should be maintained in time according to the requirements of the specifications.

Concrete curing

The cast-in-situ components such as pier columns and cover beams are preferentially maintained by heat storage, that is, the insulation materials are wrapped around the components to reduce the loss of cement hydration heat. The heat generated by cement hydration inside the concrete provides the necessary temperature to prevent concrete frostbite. When the heat storage method cannot meet the requirements, it is necessary to set up a canopy outside the components, and steam should be maintained in the canopy for maintenance. Diseases of concrete construction in winter usually occur in early winter. Just entering the winter, the consciousness of the personnel did not change in time, and there was a fluke. Many factors, such as equipment not in place and untimely adjustment of construction technology, cause this period to be a high incidence period of winter-applied diseases. Often, a sudden weather cooling will cause diseases.

Pillars, cover beams and other large-volume components have relatively few diseases in winter. If the top floating slurry is too thick after pouring, the floating slurry layer should be removed in time, because the floating slurry layer shrinks greatly and it is easy to shrink under dry and strong wind Cracked.

In the construction of prefabricated beams, the time for removing the mold is very important. Many units like to take formwork for maintenance, and think that this can keep them warm. In fact, under normal maintenance conditions, the time for removing the mold should not be too late. Because the mold removal time is too late, the concrete shrinkage causes the formwork to be compressed, making it difficult to remove the mold, and even causing the beam to crack. In addition, the hole grouting of the precast beam after tension is also a high incidence of winter diseases. The grouting material is a cement-based material and has the same requirements as the beam maintenance. The quality of some pressed slurry materials fluctuated, and the final setting time became longer. The construction personnel still maintained according to the prescribed time. When the maintenance measures were canceled, freezing damage occurred, which seriously caused the beam to crack along the bellows.

The specific surface area of bridge deck pavement and guardrail is relatively large. In addition to preventing frostbite, the construction should also pay attention to cracks caused by water loss. When the float layer is too thick, it should be removed in time. Maintenance should adopt the method of pouring and covering while paying attention to the quality of the finishing work.

The construction cost of concrete works in winter is high, and diseases are easy to occur. The construction unit should make schedules to avoid the winter construction of concrete works as much as possible. When construction is really needed due to the construction period, sufficient preparations should be made, and analysis should be made from the aspects of man, machine, material, method, and environment to establish a perfect winter application system to ensure the smooth development of winter application.