How to deal with floor concrete after freezing

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.

Critical frost resistance of 1

1. 1 In winter construction, when the strength of concrete reaches a certain limit value, because the structure has been initially formed and has the ability to resist frost heaving damage, the concrete will not be damaged by frost, and this strength is called the frost-resistant critical strength of concrete. The proposal of frost resistance critical strength is a major breakthrough in the theory of concrete construction in winter, and it is also an important basis for formulating concrete construction measures in winter. The key of concrete construction in winter is to make concrete reach the critical strength of frost resistance as soon as possible.

1.2 A large number of tests and practices show that the critical frost resistance strength of concrete is related to many factors such as cement type, water-cement ratio, cooling rate, etc. The critical frost resistance strength of plain concrete and concrete mixed with antifreeze is also different, and its value can be determined according to specifications. Generally speaking, the critical strength of concrete mixed with antifreeze is slightly lower (relatively blank), because the water content of concrete decreases, the ice crystals become more dispersed and weak, and the frost heaving effect is weakened.

2 antifreeze antifreeze principle

The antifreeze is based on the frost damage mechanism of concrete, combined with the theories of frost resistance critical strength, optimal ice formation rate and ice crystal morphological transformation, and summarized in long-term winter construction practice. It usually consists of four components, whose functions are described as follows:

2. 1 early strength member

Strength of the next working procedure before construction. In the initial stage of concrete final setting, it is necessary to avoid the construction load from causing great vibration to the floor. The main function is to accelerate the setting and hardening of concrete and make it reach the critical strength of frost resistance as soon as possible; After reaching the critical strength, the hardening speed of concrete can be accelerated and the slow strength growth caused by negative temperature and low temperature can be overcome.

2.2 air-entraining composition

Fine bubbles (beneficial bubbles) are introduced into concrete, and their functions are as follows:

1) cut off and close the connected channels (harmful channels) in concrete to reduce crack propagation during frost heaving;

2) A large number of bubbles introduced play the role of expansion "buffer", absorbing the expansion stress of ice crystals and reducing freezing damage. When 3. 5% gas is introduced into concrete, 6. It can absorb 6% volume expansion and improve the freeze-thaw resistance and durability at maturity.

2.3 Water-reducing components

Its function: 1) reduce the mixed water, thereby reducing the total amount of free water, fundamentally reducing the content of freezable ice (but maintaining a certain ice content), and eliminating the internal cause of frost heaving; 2) Through the dispersion of water reducing components, encapsulated water is released, inferior bubbles are eliminated, coarse ice crystals are transformed into fine ice crystals, the hydration environment of cement is optimized, and the frost heaving pressure is reduced.

2.4 antifreeze composition

Most of them are inorganic salts that can lower the freezing point, and their functions can be summarized as follows: the freezing point of aqueous solution (taking NaNO2 as an example) mixed with antifreeze components is about-1. 5℃. When the temperature drops to-1. At 5℃, the free water near the freezing surface in the pores began to freeze, some inorganic salts were precipitated in the ice body, and the salt concentration in the remaining free water increased. When the temperature continues to drop (such as -5℃), part of the free water near the freezing surface freezes, and the concentration of the remaining free water continues to increase until nitrite.

The lowest melting point of sodium appears, and all the free water in the pores forms ice. It can be seen that the function of antifreeze components is to keep a certain amount of liquid water (supercooled water) in concrete during the continuous cooling process, so that the hydration of cement can continue (although the hydration speed has been greatly slowed down compared with normal temperature).

It can be seen that the anti-freezing mechanism of anti-freezing liquid is comprehensive and embodies many effects. There is no such thing as instant antifreeze in the world. "Anti-freezing" is only the final effect, which is realized through the interaction of early strength, air entrainment, water reduction and anti-freezing. Moreover, the use effect of antifreeze is also related to the construction situation of the project, so the winter construction of concrete is a typical systematic project, which must be considered comprehensively.

3 Correct use of antifreeze

3. 1 Correct understanding of "working temperature"

Any standard antifreeze product has a clear "use temperature" (such as-15℃ and -20℃). It is not wrong to say that the service temperature is "the allowable temperature for concrete construction", but it should be understood in connection with the critical frost resistance strength of concrete, that is, the concrete must reach the critical frost resistance strength before the ambient temperature drops to the "service temperature" of the admixture, so that the concrete is safe. The lower the service temperature of concrete, the better the anti-freezing effect of antifreeze, and the more time it takes for concrete to increase its strength (including negative temperature zone), thus greatly increasing the possibility of reaching the critical anti-freezing strength.

At present, the use temperature of concrete antifreeze produced in China is mostly between-10℃ and-15℃ (which can be applied to the daily minimum of-15℃ to -20℃). The lower the temperature, the more difficult it is to design antifreeze formula, and the more uncertain factors there are. From this point of view, antifreeze is not required in most cases.

3.2 Take covering and heat preservation measures.

The basic method of comprehensive heat storage method is to cover and add antifreeze, and heat water and sand when necessary. The function of covering is to keep the hydration heat of cement in concrete and the heat of raw materials for a long time, and to prolong the hydration time of cement as much as possible, which is very important. The better the thermal insulation, the longer it takes for concrete to reach the "service temperature" of the admixture and the longer it takes to reach the critical frost resistance strength. In order to achieve this effect, the type and thickness of covering materials should be determined by thermal calculation in combination with the factors such as the use temperature of additives, critical frost resistance strength and environmental temperature.

3.3 do a good job in construction organization.

The use effect of antifreeze must be achieved through good mixing and vibration. The purpose of prolonging the stirring for 30 min is to make the admixture mix more fully, and the admixture mix unevenly, which may even lead to accidents. Furthermore, antifreeze has the problems of optimal stirring time and optimal vibration time. A large amount will reduce the air intake, and a small amount will cause uneven distribution of bubbles, and even produce rough and inferior bubbles, which is not good for antifreeze. In addition, measures such as shortening the transportation distance to the maximum extent, building a heat preservation shed in the mixing station, wrapping a heat preservation sleeve around the transportation pipeline, covering the heat preservation quilt with a shelf car, and closely connecting the working procedures are all aimed at increasing the concrete entering the mold, prolonging the curing time at room temperature, and reaching the critical strength of frost resistance as soon as possible.

3.4 Thermal calculation

Thermal calculation mainly refers to the "Wu Zhendong formula" put forward by Wu Zhendong of Hunan University, which has many uses and is mainly used for verification calculation in North China. The main points are:

1) According to the raw materials and ambient temperature, calculate the temperature of concrete entering the mold, t;

2) Calculate the time required for concrete to drop from this temperature to the specified temperature of antifreeze, h;

3) According to the maturity formula, calculate the concrete strength that can be achieved within the above curing time, MPa.

4) compare whether the strength is greater than the critical strength of frost resistance, and determine whether the winter application scheme is feasible;

5) During construction, the test blocks with the same conditions shall be kept, and their actual strength shall be tested 65438±0d before the ambient temperature drops to the use temperature of antifreeze to see whether the critical strength of antifreeze is reached.

3.5 Master the content of antifreeze.

Some people regard the dosage in the manual as benchmark dose, and it is risky to adjust it up and down according to the actual temperature during construction. Generally speaking, there is a one-to-one correspondence between the dosage and the use temperature in the formulation design of more formal antifreeze products, and there is no adjustment problem. Many components of antifreeze have the problem of optimal dosage, and the scope of application is very narrow, and the relationship between dosage and efficacy is not linear. For example, when 3% is added to 4%, all the components will increase by 33%, which is likely to lead to the decrease of strength (about 5% ~10%) due to the increase of air content; The increase of Na2SO4 and NaNO3 leads to the increase of alkali content, which is not good for durability. If it changes from 3% to 2%, the efficacy is definitely not a problem of reducing 33%, but more. Not only antifreeze, but also other additives.