What is the solution to the long setting time of concrete test blocks in cold weather?

The key is to assess the severity of freezing. Frozen water in concrete will destroy the structure of concrete, which is irreversible and must be removed.

Only the strength growth of concrete without freezing at low temperature is affected, and it can still grow to the design strength in the later reinforcement and maintenance. This part can be kept.

Connect the two with high-grade mortar, but pay attention to the position, which is the same as the construction joint treatment.

General principles of concrete construction in winter

The reason why concrete mixture can gradually set and harden after pouring until the final strength is obtained is due to the hydration of cement. The hydration speed of cement is not only related to the composition material and mixture ratio of concrete itself, but also changes with temperature. With the increase of temperature, hydration accelerates and strength increases rapidly. But when the temperature drops to 0℃, a part of water in concrete begins to freeze and gradually changes from liquid phase (water) to solid phase (water). At this time, the water involved in cement hydration decreases, so the hydration slows down and the strength growth slows down accordingly. When the temperature continues to drop and the water in concrete completely turns into ice, that is, completely changes from liquid phase to solid phase, the hydration of cement basically stops and the strength no longer increases.

When water turns into ice, its volume increases by about 9%, and at the same time, it produces an ice expansion stress of about 2500 kilograms per square centimeter. This stress value is often greater than the initial strength value formed inside the cement stone, which causes the concrete to be damaged to varying degrees (that is, freeze injury during drying period) and reduces the strength. In addition, when water turns into ice, it will also produce large particles of ice on the surface of aggregate and steel bar, which will weaken the bonding force between cement paste and aggregate and steel bar, thus affecting the compressive strength of concrete. When ice melts, it will form various voids in concrete, which will reduce the compactness and durability of concrete.

It can be seen that in winter concrete construction, the change of water form is the key to affect the strength growth of concrete. Many scholars at home and abroad have done a lot of experimental research on the existing 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 its solid phase and accelerate cement hydration. The experimental study also shows that the longer the pre-curing time of concrete before freezing, the smaller the strength loss.

After concrete is thawed (that is, at room temperature), its strength will be improved, but the improvement range is different. For the concrete with long curing period and high initial strength (such as 35% of R28), there is almost no loss of later strength after freezing. However, for concrete with short safe curing period and low initial strength, the later strength will be lost to varying degrees after freezing.

It can be seen that concrete should have a pre-curing period at room temperature before freezing, so as to accelerate the hydration of cement and make concrete obtain the lowest strength, commonly known as critical strength, and achieve the expected effect. The critical strength varies from country to country. In China, it is not less than 30% of the design label and not less than 35 kg per square centimeter.

Selection of concrete construction methods in winter

From the above analysis, it can be seen that in winter concrete construction, three problems are mainly solved: first, how to determine the shortest curing age of concrete, second, how to prevent the early freezing injury of concrete, and third, how to ensure the late strength and durability of concrete to meet the requirements. In practical engineering, we should choose a reasonable construction method according to the temperature during construction, the structural conditions of the project (engineering quantity, structural thickness, exposure), the urgency of the construction period, the variety and price of cement, the performance and price of early strength agent, reducing agent and antifreeze, the performance and price of thermal insulation materials, and the conditions of heat source. Generally speaking, there are several different winter construction schemes for the same project. An ideal scheme should be to obtain the best project quality with the shortest construction period and the lowest construction cost, that is, the optimization of construction period, cost and quality. At present, the following four methods are basically adopted.

Method for adjusting mixing ratio

It is mainly suitable for concrete construction at about 0℃. Specific methods: ① Choosing suitable cement is an important means to improve the frost resistance of concrete. The test results show that early strength portland cement should be used. This kind of cement has high hydration heat and the highest early release strength. Generally, the compressive strength of 3 days is almost the same as that of ordinary silicon cement in 7 days, and the effect is obvious. (2) Try to reduce the water-cement ratio and slightly increase the dosage of cement to increase the hydration heat and shorten the time to reach the age strength. (3) Adding air entraining agent. Under the condition of keeping the mixture ratio of concrete unchanged, the bubbles generated after adding air entraining agent correspondingly increase the volume of cement slurry, improve the fluidity of the mixture, improve its cohesiveness and water retention, buffer the water pressure generated by water freezing in concrete, and improve the frost resistance of concrete. ④ Adding early strength agent can shorten the setting time of concrete and improve the early strength. Sodium sulfate (2% of cement dosage) and MS-F composite early strength agent (5% of cement dosage) are widely used. ⑤ Choose aggregate with large particle hardness and few cracks to make its thermal expansion coefficient close to that of surrounding mortar.

Vacuum flask method

It is mainly used for projects with thick structures with air temperature around-10℃. The method is to heat the raw materials (water, sand, stone) to make the concrete still have considerable heat after mixing, transportation and pouring, so as to make the hydration of cement release heat faster, strengthen the thermal insulation of concrete, and ensure that the newly poured concrete has sufficient frost resistance before the temperature drops to 0℃. This method is simple in process and low in cost, but attention should be paid to internal heat preservation to prevent corners and exposed surfaces from freezing, and the curing age should be prolonged.

External heating method

It is mainly used in projects where the temperature is above-10℃ and the components are not too thick. By heating the air around the concrete member, the heat is transferred to the concrete, or the concrete is directly heated, so that the concrete can be hardened normally under the condition of positive temperature. (1) heating furnace. Generally used in smaller construction sites, the method is simple, but the indoor temperature is not high and it is relatively dry, and the released carbon dioxide will carbonize the surface of newly poured concrete, which will affect the quality. ② Steam heating. Steam is used to harden concrete under hot and humid conditions. This method is easy to control and the heating temperature is uniform. However, due to the need for special boiler equipment, the cost is higher. The heat loss is large and the working conditions are not ideal. ③ Electric heating. When steel bars are used as electrodes or electric heaters are attached to the surface of concrete, electric energy is converted into heat energy, which makes the temperature of concrete rise. This method is simple and convenient, with less heat loss and easy control. The disadvantage is that it consumes a lot of electricity. ④ Infrared heating. Seal and radiate concrete with high-temperature electric heater or gas infrared generator.

Antifreeze additive

At the temperature above-10℃, chemicals that can reduce the freezing point of water are added to the concrete mixture, so that the concrete is still in liquid state at negative temperature, and the hydration continues, thus improving the strength of concrete. At present, calcium oxide, sodium chloride and other single antifreeze and sodium nitrite plus sodium chloride composite antifreeze are commonly used.

The above four winter construction methods have their own advantages and disadvantages, and their scope of application is limited by certain conditions. According to the existing site conditions, one or more construction methods should be combined.

When the ambient temperature drops to about 0℃, some special technical measures should be taken in concrete construction, which is the common sense of the construction industry. How to formulate construction measures according to the causes of freezing injury is not a simple problem. The following interview combines some experience in specific work and refers to relevant literature, and puts forward some opinions. There are many measures to apply concrete in winter, and the comprehensive heat storage method is commonly used in the north. Combined with the causes of freezing injury, the anti-freezing mechanism of comprehensive heat storage method is emphatically introduced below.