The reason why concrete mixes gradually set and harden after being poured until they obtain their final strength is due to the hydration of cement. And the rate of cement hydration, in addition to the concrete itself composition materials and ratio, mainly with the temperature of the high and low changes.
When the temperature rises, the hydration is accelerated, and the strength growth liquid is faster; and when the temperature decreases to 0 ℃, a part of the water present in the concrete begins to freeze, gradually from the liquid phase (water) into the solid phase (ice). At this time, the water involved in the hydration of cement is reduced. As a result, hydration slows down and strength growth is correspondingly slower. Temperature continues to fall, when the presence of water in the concrete completely into ice, that is, completely liquid phase into solid phase, cement hydration basically stopped, at this time the strength is not growing.
After the water becomes ice, the volume increases by about 9%, while generating an expansion stress of about 2500 kg/cm2. This stress value is often greater than the initial strength value formed inside the cement stone, so that the concrete receives different degrees of damage (i.e., early freezing damage) and reduce the strength.
In addition, when the water becomes ice, but also in the aggregate and reinforcing steel on the surface of the larger particles of ice, weakening the cement paste and aggregate and reinforcing steel bond, thus affecting the compressive strength of concrete. When the ice melts, and will form various voids inside the concrete, and reduce the compactness and durability of concrete.
It can be seen that in the winter concrete construction, the change in the form of water is the key to the growth of concrete strength, many scholars at home and abroad to conduct a large number of tests on the form of water in concrete. The research results show that newly poured concrete has a period of pre-curing before freezing, which can increase its internal liquid phase, reduce the solid phase and accelerate the hydration of cement. The experimental study also showed that the longer the pre-curing period before the concrete is frozen, the smaller the strength loss.
After the concrete is frozen (i.e., under normal temperature conditions) to continue to maintain, its strength will grow, but the magnitude of growth varies. For the long pre-curing period, to obtain the initial strength of high (such as up to 35% of R28) of the concrete after freezing, the late strength almost no loss. And for the safety of the short pre-curing period, the initial strength of the concrete to obtain a relatively low after freezing, the late strength are not used to the extent of the loss.
It can be seen that the concrete before freezing, to make it at normal temperature for a period of preconditioning to accelerate the hydration of cement, so that the concrete does not suffer from frost damage to the minimum strength, generally known as the critical strength, you can achieve the desired effect. For the critical strength, countries specify different values, China’s provisions for not less than 30% of the design strength level, and not less than 35 kg per square centimeter.
The harm of concrete frost damage
Reduce the strength of concrete
Cracking of concrete
Corrosion of reinforcing steel in concrete
Reducing the durability of concrete
Preparation work before construction
(1) The ice, snow and garbage on the formwork and reinforcement should be removed before the concrete is poured, especially at the junction of old and new concrete (such as the junction of beams and columns), but no water should be flushed.
(2) Before pouring, the concrete should be prepared to cover with insulation materials, such as plastic film, colored cloth, cotton felt and grass curtain, etc., and do the corresponding anti-freezing insulation measures. And take the necessary wind-blocking, closure measures to improve the insulation effect.
(3) shall not be concrete pouring on the permafrost layer, before pouring, must try to warm up so that the permafrost thaw. When the concrete hew, should preheat the old hew, and strengthen the insulation after pouring to prevent the hew from freezing.
(4) If the slump of concrete is too small to meet the construction requirements, it can be adjusted by using admixture under the guidance of the technical personnel of the concrete company, and it is strictly forbidden to adjust the slump of concrete by adding water.
(1) In order to ensure the quality of concrete placement and prevent the quality from being affected by temperature changes, concrete should be placed as soon as possible after it is transported to the construction unit where it is placed, and it is advisable to unload it within 90min; when a dump truck is used for transportation, it is advisable to unload it within 60min.
(2) During winter construction, the water from the pump truck shall not be put into the formwork; the mortar used in the pipe shall not be put into the formwork, and it shall not be concentrated in the structure of the component.
(3) During the pouring process, the construction unit should observe the uniformity and consistency change of the concrete mix at any time. When the concrete slump is found to change from the requirement at the casting site, the concrete company should be contacted in time so that timely adjustment can be made. It is strictly forbidden to add water to the concrete entering the pouring site, and the act of pumping and pouring while adding water should be eliminated.
(4) When pouring floor slab, beam, wall and column together, pour wall column and concrete first, then pour beam and floor slab. When pouring walls, columns and other higher elements, a pouring height to the concrete does not segregate, generally not more than 500mm per layer, pounding flat and then pour the upper layer, pouring more attention to vibrate in place, so that the concrete is full of test mold, not in significant sinking, no obvious bubble discharge.
(5) layered pouring of thick monolithic structural concrete, the concrete temperature of the poured layer should not be less than 2 ℃ before being covered by the previous layer of concrete. When using heating curing, the temperature before curing should not be lower than 2℃.
(6) The temperature of the concrete into the mold shall not be less than 5 ℃, after pouring, the concrete structure is easy to freeze parts, must strengthen the insulation to prevent frost damage.
Timely and reasonable smear pressure
(1) The initial setting time of winter concrete is generally 8~12 hours, and the final setting is 12~16 hours. Therefore, the timing of plastering should be properly grasped, and the second plastering should be carried out before the initial setting (finger marks can be left by lightly pressing the surface with hands), which can reduce the surface cracks. The demoulding time of concrete walls, columns and other side moulds should be extended appropriately to avoid the surface peeling and other effects on the appearance quality.
(2) concrete before the initial set with a scraper to drive flat, with a wooden trowel first smear, after the initial set to the final set before the iron trowel crush the surface several times, the surface of the uneven, irregular cracks closed, and finally the second smear with the closing trowel to close the water collection cracks, followed immediately by covering the concrete surface with plastic film, so that the free water evaporated within the concrete accumulates on the concrete surface for thermal insulation maintenance, and then cover the grass curtain on the film.
Curing of concrete
(1) After the concrete has been processed by the relevant construction process, it should be covered with plastic film and covered with straw curtains, cotton felt and other thermal insulation maintenance in time to ensure that the concrete is not frozen before the initial set. According to the construction project parts and temperature, the following methods of covering can be referred to.
When the temperature is in 0℃~5℃, cover a layer of cotton felt or straw curtain and a layer of plastic film.
When the temperature is in -5℃~0℃, cover two layers of cotton felt or grass curtain and one layer of plastic film.
When the temperature is -10℃~-5℃, cover with three layers of cotton felt or grass curtain and one layer of plastic film.
When the temperature is below -10℃, cover four layers of cotton felt or grass curtain and one layer of plastic film.
Below -15 ℃ should be used to heat and other materials (such as rock wool, benzene board, etc.) for insulation, the thickness of its insulation layer, the material should be determined according to the calculation.
(2) at the early stage of maintenance, send a person responsible for temperature measurement and detailed records of temperature changes throughout the maintenance period, at least four times per day and night to measure the concrete and environmental temperature in order to find problems and take timely measures to remedy.
(3) In the formwork external insulation, except for the foundation can be poured with the insulation, other structures must be set up after the insulation material before pouring concrete. The surface of the steel mold can first hang grass curtain, sacks and other insulation materials and tie then pouring concrete.
(4) Concrete should be covered immediately after the final set thermal insulation maintenance, according to national standards require maintenance time of not less than 14 days, if the early maintenance is not in place, its 28-day strength will be greatly affected.
(5) after the demoulding of the concrete should also be covered with thermal insulation material in time to prevent the surface temperature of concrete suddenly drop and produce cracks.
Formwork firm, timely loading and demoulding
(1). Demolition of the concrete must reach the specified demolition strength, premature demolition of the mold, load-bearing will lead to concrete surface tearing produce cracks and other quality problems. In the concrete has not reached 1.2mpa before, not allowed to step on top of the young concrete, support the mold and load. Do not prematurely carry out construction work or stack heavy objects on the floor slab to reduce or avoid shrinkage deformation cracks in the structure.
(2). Concrete demoulding should pay attention to the demolition mode time and order, especially for beams, wall panels and other structures should be properly extended demoulding time, after demoulding should continue to maintain.
(3). Formwork and insulation, should be cooled to 5 ℃ before the concrete can be removed. Uncooled concrete has a high brittleness, so the structure shall not be subjected to impact or dynamic loading before cooling. When the temperature difference between the concrete and the outside world is greater than 20 ℃, the surface of the concrete after demolition of the mold, should be temporarily covered, so that it slowly cools.
(4). According to the strength of the test block maintained under the same conditions to determine the time of mold removal.
The winter concrete construction methods
- Winter concrete construction method selection from the above analysis can be known, in the winter concrete construction, the main solution to three problems.
1) how to determine the shortest curing age of concrete.
2) how to prevent early concrete frost damage.
3) how to ensure that the late strength and durability of concrete meet the requirements
In the actual project, a reasonable construction method should be selected according to the temperature at the time of construction, the structural condition of the project (the volume of work, the thickness of the structure and the exposed situation), the degree of urgency of the construction period, the variety and price of cement, the performance and price of early strength agent, water reducing agent, anti-freeze agent, the performance and price of insulation materials, and the condition of heat source.
Generally speaking, for the same project, there can be several different winter construction programs. An ideal program should use the shortest construction period, the lowest construction cost, to get the best quality of the project, that is, the optimal duration, cost, quality. At present, four methods are basically used.
- Winter concrete construction method types
(1) adjust the ratio method is mainly applicable to the construction of concrete at about 0 ℃. Specific practices.
- Selecting the appropriate variety of cement is an important means of improving the frost resistance of concrete. Test results show that early-strength silicate cement should be used. The cement has a greater heat of hydration and releases the highest strength in the early stage, and the general 3-day compressive strength is approximately equivalent to the 7-day strength of ordinary silicate cement, which has a more obvious effect.
- Lower the water-cement ratio as much as possible, slightly increase the amount of cement, so as to increase the heat of hydration and shorten the time to reach the age strength.
- Mix with air-entraining agent. In the case of keeping the concrete ratio unchanged, the air bubbles generated by adding air-entraining agent, correspondingly increase the volume of cement paste, improve the fluidity of the mix, improve its cohesion and water retention, buffer the water pressure generated by water freezing in the concrete, and improve the frost resistance of concrete.
- Adding early strength admixture to shorten the setting time of concrete and improve the early strength. Application of more common are sodium sulfate (mixed with 2% of the amount of cement) and msf in line with the early strength water test agent (mixed with 5% of the amount select the aggregate with hardness of particles and few gaps, so that its coefficient of thermal expansion and the surrounding mortar expansion coefficient is the same.
(2) the heat storage method is mainly used for projects with temperature around -10℃ and thicker structure. The practice is: Heating of raw materials (water, sand, stone), so that the concrete in the mixing, transportation and pouring later, but also reserve considerable heat to make the cement hydration exothermic faster, and strengthen the insulation of concrete to ensure that the temperature drops to 0 ℃ before the newly cast concrete has sufficient frost resistance. This method is simple process, construction costs are not much, but we should pay attention to internal insulation, to avoid the corner and exposed surface frost, and to extend the maintenance period.
(3) anti-freeze admixture in the temperature above -10 ℃, the concrete mix mixed with a chemical agent to reduce the freezing point of water, so that the concrete is still in the liquid phase at negative temperatures, hydration can continue, so that the concrete strength continues to grow. Currently commonly used are calcium oxide, sodium chloride and other single antifreeze agent and sodium nitrite plus sodium chloride in line with the antifreeze agent.
(4) external heating method is mainly used in the temperature -10 ℃ below, and the members are not thick and large project. By heating the air around the concrete elements, the heat will be transferred to the concrete, or directly to the concrete heating, so that the concrete is in positive temperature conditions normal hardening.
- Furnace heating, generally used in smaller sites, the method is simple, but the room temperature is not high, relatively dry, and the release of carbon dioxide will make the surface of the newly cast concrete carbonation, affecting the quality.
- Steam heating. Use steam to harden the concrete in wet heat conditions. This method is easier to control, and the heating temperature is uniform. But because it requires special boiler equipment, the cost is higher. And heat loss is large, and labor conditions are not ideal.
- Electric heating. The steel as electrodes, or electric heaters attached to the surface of the concrete, then electricity for heat to raise the temperature of concrete. Sub-method is simple and convenient, less heat loss, easy to control, the shortcoming is the large power consumption.
- Infrared heating, high-temperature electric heating or gas infrared generator, sealed radiation heating of the concrete.
- Winter concrete construction technical measures
Winter construction of concrete composition of the material requirements.
Aggregate shall not have ice, snow mass and organic matter, should be clean, well-graded, hard texture.
Water: The use of drinkable tap water; admixtures: The choice of antifreeze, antifreeze mechanism of action is to significantly reduce the freezing point of the liquid phase of concrete at the specified negative temperature, so that the concrete in the liquid state does not freeze, to ensure the hydration of cement, in a certain period of time to obtain the expected strength.
Antifreeze should pass the technical appraisal, meet the quality standard, and master its performance by laboratory test.
Cement: Significant high activity, heat of hydration of ordinary silicate cement.
The requirements of winter concrete mixing and transportation of concrete mixing.
Concrete mixing selection of heated water, hot water above 80 ℃ shall not be in direct contact with the cement, the first hot water and aggregate mixing and then mixed into the cement mixing concrete to avoid cement pseudocondensation, concrete mixing time of not less than 3 minutes. In addition, if necessary, the mixer around the protection and ventilation heating insulation.
Concrete maintenance: Cover the concrete with plastic sheeting immediately after pouring to keep the water, while covering the outside of the plastic sheeting with insulation for heat preservation, the insulation should be neatly and tightly covered so that the concrete temperature does not fall too fast to avoid the occurrence of concrete frost damage.
Properly extend the time of concrete maintenance, to not less than 15 days is appropriate. Add two groups of concrete with the conditions of the test block placed in the field environment, in order to get the compressive strength of concrete under the same conditions at any time.
Winter concrete temperature measurement requirements of concrete temperature measurement work.
Buried in the concrete wire set up a person to measure the temperature, including atmospheric temperature, the concrete out of the tank, into the mold temperature, the internal temperature of concrete, such as abnormal and timely measures.
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