Winter construction concrete is used for winter application, and appropriate amounts of admixtures such as antifreeze, early strength agent are added as needed. Aggregates must not contain ice, snow or frozen blocks. Cement with higher strength (42.5 and above) should be used, and a smaller water-cement ratio (less than 0.55).
Concrete mixing water must be heated, aggregate must be heated if necessary, cover and insulate, and add antifreeze with a freezing resistance of -15℃ to ensure that the concrete outgoing temperature is not lower than 10℃.
Try to arrange the mixing station nearest to supply materials. The transport tanker should be covered with insulation to ensure that the concrete tank temperature is not less than 10℃. The pump pipe should be wrapped with insulation to reduce temperature loss and ensure that the concrete molding temperature is not less than 5℃.
Concrete pouring should be arranged as much as possible during the period of time when the temperature is relatively high in the day, and construction should be carried out quickly and poured in layers. The layer thickness should not be greater than 400mm per layer, and the formwork should have corresponding insulation measures.
Concrete is covered with comprehensive heat storage method to cover insulation and maintenance. It is prohibited to sprinkle water to maintain . When the insulation material is removed, the difference between the concrete surface and the atmospheric temperature shall not exceed 15℃.
Timely temperature measurement, and the temperature measurement items include the concrete tank out temperature, mold entry temperature, atmospheric temperature, etc. Atmospheric temperature 4 times/day and night, concrete curing temperature, 12 times before 4Mpa/day and night, and 4 times after 4Mpa/day and night. The temperature measurement points shall be arranged according to the specifications.
Concrete test blocks are left: 2 groups of standard 28d test blocks, 5 groups of curing test blocks in the same condition (one group is used to test critical strength, one group is structural entity detection, one group is used to determine the strength basis before mold removal, one group is used to test the concrete strength after winter to room temperature curing for 28d, and one group is spare).
General Principles of Seasonal Concrete Winter Concrete Construction
General Principles of
Concrete mixture gradually solidifies and hardens until the final strength is obtained because of cement hydration. The speed of cement hydration is not only related to the material and mix ratio of the concrete itself, but mainly changes with the temperature. When the temperature rises, hydration accelerates, and the strength increases liquid faster; when the temperature drops to 0℃, part of the water present in the concrete begins to freeze, gradually turning from liquid phase (water) to solid phase (ice). At this time, the water involved in cement hydration decreased. Therefore, the hydration effect slows down and the intensity growth is correspondingly slower. The temperature continues to drop. When the water present in the concrete completely turns into ice, that is, when the liquid phase changes into solid phase, the cement hydration basically stops, and the strength does not increase at this time.
After water becomes ice, the volume increases by about 9%, and at the same time, it generates an expansion stress of about 2500 kg/ square centimeter . This stress value is often greater than the initial strength value formed inside cement stone , causing the concrete to undergo varying degrees of damage (i.e., caused by early freezing damage) and reduces the strength. In addition, when water turns into ice, ice 2 with larger particles will be generated on the surface of the aggregate and steel bars, weakening the adhesion between the cement slurry and the aggregate and steel bars, thereby affecting the compressive strength of the concrete. When the ice melts, various voids will form inside the concrete, reducing the compactness and durability of the concrete.
It can be seen that in winter concrete construction, the change of water morphology is the key to affecting the growth of concrete strength. Many scholars at home and abroad have conducted a large number of experiments on the form of water in concrete. The research results show that the newly poured concrete has a pre-cultivation period of before freezing, which can increase its internal liquid phase, reduce the solid phase, and accelerate the hydration of cement. Experimental studies also show that the longer the pre-herb period of concrete before freezing, the smaller the strength loss.
After concrete thaws (that is, under normal temperature conditions), its strength will increase, but the increase range varies. For concrete with a long pre-cultivation period, after obtaining a high initial strength (such as reaching 35% of R28) is frozen, there is almost no loss of strength in the later stage. For the short pre-curing period of safety and low initial strength, the later strength will be freezed at any time after obtaining concrete with low initial strength.It can be seen from this that before concrete freezes, it must have a pre-curing period at normal temperature to accelerate the hydration of cement and enable the concrete to obtain the lowest strength without frost damage. It is generally called critical strength, so that the expected effect can be achieved. For critical strength, the values stipulated by each country are different, and my country stipulates that the value shall not be lower than 30% of the designed strength level, nor shall it be lower than 35 kilograms per square centimeter.
Hazards of concrete frost damage:
1)Reduce concrete strength
2)Create concrete cracks
3)Rust of steel bars in concrete
4)Reduce concrete durability
Preparation before construction
(1) Before pouring concrete, ice, snow and garbage on the formwork and steel bars should be removed, especially at the junction of new and old concrete (such as the junction of beams and columns), but it should not be washed with water.
(2) Before pouring, insulation materials for concrete covering, such as plastic film, color strip cloth , cotton felt and straw curtains, etc., and corresponding anti-freeze and insulation measures should be taken. And take necessary windproof and sealing measures to improve the insulation effect.
(3) Concrete shall not be poured on the frozen soil layer. Before pouring, try to heat up to melt the frozen soil. When connecting concrete, the old connecting joints should be preheated and insulation should be strengthened after pouring to prevent the connecting joints from being frozen.
(4) If the slump of concrete is too small and cannot meet the construction requirements, you can use admixtures to adjust the concrete company's technical staff. It is strictly prohibited to use water to adjust the slump of concrete .
Concrete pouring
(1) In order to ensure the pouring quality of concrete and prevent temperature changes from affecting quality, concrete should be poured as soon as possible after it is transported to the pouring site of the construction unit. It is advisable to unload the material within 90 minutes; when transporting using the dump truck , unload the material within 60 minutes.
(2) During winter application, the water for pump trucks must not be placed in the template; the mortar used for pipe moistening is not allowed to be placed in the template, and it is not allowed to be poured in the component structure.
(3) During the pouring process, the construction unit should observe the changes in uniformity and consistency of the concrete mixture at any time. When concrete slump and requirements are found to change at the pouring site, contact the concrete company in time so that adjustments can be made in time. It is strictly forbidden to add water at will for concrete entering the pouring site, and the behavior of pumping pouring while adding water should be prevented.
(4) When the floor slabs, beams, walls and columns are poured together, the wall columns and concrete are solid, and then the beams and floor slabs are poured. When pouring higher components such as walls and columns, the pouring height of the one-time pouring shall be subject to the fact that the concrete does not separate. Generally, each layer shall not exceed 500mm. After pounding, the upper layer shall be poured. Pay more attention to vibrating in place during pouring, so that the concrete fills the test mold, does not sink significantly, and there are no obvious bubbles discharged.
(5) When casting thick monolithic structural concrete in layers, the concrete temperature of the cast layer should not be lower than 2℃ before it is covered by the previous layer of concrete. When heating maintenance is used, the temperature before maintenance shall not be lower than 2℃.
(6) The molding temperature of concrete must not be lower than 5℃. After pouring, insulation must be strengthened to prevent frost damage in parts of the concrete structure easily freezing.
Timely and reasonably smear
(1) The initial set time of concrete in winter is generally 8 to 12 hours, and the final set time of is 12 to 16 hours. Therefore, you should appropriately grasp the opportunity to smear the surface and perform a secondary smear before initial setting (lightly pressing the surface with your hands can leave finger marks), which can reduce surface cracks. The mold removal time of concrete walls, columns and other side molds should be appropriately extended to avoid surface peeling and other affecting appearance quality.
(2) Before the concrete is initially set, use a scraper to flatten the surface, apply the wooden trowel for the first time, and use an iron trowel to roll the surface several times before the final set, close the uneven and irregular cracks on the surface. Finally, use a scrubber to wipe the surface for the second time to close the water-collapse cracks. Then immediately cover the plastic film on the concrete surface, so that the free water evaporated in the concrete accumulates on the concrete surface for insulation and maintenance, and then cover the film with a straw curtain.
Current of concrete
(1) After the concrete has been processed by relevant construction technology, plastic film should be covered with straw curtains, cotton felt and other insulation to ensure that the concrete is not frozen before initial settling. According to the construction project location and temperature conditions, the following methods can be used for coverage: when the temperature is 0℃~5℃, cover a layer of cotton felt or straw curtain and a layer of plastic film; when the temperature is -5℃~0℃, cover two layers of cotton felt or straw curtain and a layer of plastic film; when the temperature is -10℃~-5℃, cover three layers of cotton felt or straw curtain and a layer of plastic film; when the temperature is lower than -10℃, cover four layers of cotton felt or straw curtain and a layer of plastic film. When the temperature is lower than -15℃, cover four layers of cotton felt or straw curtain and a layer of plastic film, and when it is lower than -15℃, use heating and other materials (such as rock wool , benzene board, etc.) for insulation, and the thickness of the insulation layer should be determined based on calculations.
(2) In the early stage of maintenance, a special person is assigned to take the temperature measurement and record the temperature changes in the entire maintenance period in detail. The concrete and ambient temperature should be measured at least four times a day and a night so that problems can be found to be taken to remedy.
(3) When insulation outside the formwork, except for the foundation that can be insulated as it is poured, concrete can be poured after the insulation material is installed. The surface of the steel mold can be hung with straw curtains, sacks and other insulation materials and tied firmly before pouring concrete.
(4) The concrete should be covered and insulated immediately after final solidification. The maintenance time should not be less than 14 days according to national standards. If the early maintenance is not in place, its strength of 28 days will be greatly affected.
(5) The concrete after dismantling the mold should also be covered with insulation materials in time to prevent cracks from falling sharply on the surface of the concrete.
template is firm, and the loading and mold removal are timely added and mold removal is carried out (1) When removing the mold, the concrete must reach the specified mold removal strength . Premature mold removal and load bearing will lead to quality problems such as tearing and cracks on the concrete surface. Before the concrete reaches 1.2MPa, trampling, supporting formwork or loading on young concrete is not allowed. Do not conduct construction work on the floor slabs too early to reduce or avoid shrinkage, deformation and cracks in the structure.
(2) When decomposing concrete molds, pay attention to the time and sequence of dismantling modes. In particular, for beams, wall panels and other structures, the demolition time should be appropriately extended, and maintenance should be continued after dismantling the molds.
(3) The formwork and insulation layer should be removed after the concrete is cooled to 5℃. Uncooled concrete has high brittleness, so the structure must not be affected by impact or dynamic load before cooling. When the temperature difference between the concrete and the outside is greater than 20℃, the concrete surface after the mold is removed should be temporarily covered to allow it to cool slowly.
(4) Determine the mold removal time based on the strength of the test blocks maintained in the same conditions.
1. Selection of winter concrete construction method
From the above analysis, we can know that in winter concrete construction, three main problems are solved:
1) How to determine the shortest curing age of concrete;
2) How to prevent early frost damage of concrete;
3) How to ensure the later stage of concrete In actual projects, the strength and durability meet the requirements. In actual projects, reasonable construction methods should be selected based on the temperature during construction, the construction structure status (the quantity, the degree of structural thickness and the exposure), the urgency of the construction period, the variety and price of cement, the performance and price of early strength agent, water reducer, and the performance and price of antifreeze agent, the performance and price of insulation materials, the conditions of heat source , etc. Generally speaking, for the same project, there can be several different winter construction plans . An ideal solution should use the shortest construction period and the lowest construction cost to obtain the best project quality, that is, the optimization of construction period, expense and quality. Currently, the following four methods are basically adopted.
2, winter concrete construction methods types
1) Adjusting mix ratio
is mainly suitable for concrete construction around 0℃. Specific practices: ① Selecting the appropriate variety of cement is an important means to improve the anti-freeze of concrete. The experimental results show that premature silicate cement should be used.The cement has a high hydration heat and the release strength is the highest in the early stage. Generally, the compressive strength of 3 days is approximately equivalent to the 7-day strength of ordinary silicate cement , and the effect is more obvious. ② Try to reduce the water-cement ratio and slightly increase the amount of cement, thereby increasing the hydration heat and shortening the time to reach the age intensity. ③ Use gas induction agent . While keeping the concrete mix ratio of unchanged, the bubbles generated after adding gas induction agents increase the volume of the cement slurry, improve the fluidity of the mixture, improve its conjugation and water retention, buffer the water pressure generated by the freezing of water in the concrete, and improve the frost resistance of the concrete. ④ Add early strength admixtures to shorten the settling time of concrete and improve early strength. The more common ones are sodium sulfate (2% of cement dosage) and MSF that meet the premature strength water reagent (5% of cement dosage). ⑤ Select aggregates with small particle hardness and gaps so that their thermal expansion coefficient is the same as the expansion coefficient of the surrounding mortar.
2) Thermal storage method
is mainly used in projects with a relatively thick structure around -10℃. The method is: heat the raw materials (water, sand, stone) so that after the concrete is mixed, transported and poured, it also reserves considerable heat to make the cement hydration and heat exothermic, and strengthens the insulation of the concrete to ensure that the newly poured concrete has sufficient freezing resistance before the temperature drops to 0℃. This method is simple in process and does not cost much construction, but you should pay attention to internal insulation to avoid freezing of corners and exposed surfaces, and extend the maintenance period.
3) Antifreeze admixture
In a temperature above -10℃, a chemical agent that reduces the freezing point of water is added to the concrete mixture, so that the concrete is still in the liquid phase state at a negative temperature, and the hydration can continue, thereby continuing to increase the strength of the concrete. Currently, there are commonly used monoclonal antifreeze agents such as calcium oxide , sodium chloride , and sodium nitrite and sodium chloride to meet the antifreeze agent.
4) External heating method
is mainly used in projects where the temperature is below -10℃ and the components are not thick and large. By heating the air around the concrete member, heat is transferred to the concrete, or directly heating the concrete, so that the concrete is normally hardened under positive temperature conditions.
① Heating of the stove is generally used on smaller construction sites. The method is simple, but the indoor temperature is not high and relatively dry. The emitted carbon dioxide will carbonize the surface of the newly poured concrete, affecting the quality.
②Steam heating. Steam to harden the concrete under humid and hot conditions. This method is easier to control and the heating temperature is even. However, because it requires special boiler equipment, the cost is high. Moreover, the heat loss is large and the working conditions are not ideal.
③Electric heating. Use the steel bar as the electrode, or paste the electric heater on the concrete surface to make the electric energy heat energy to increase the temperature of the concrete. The second method is simple and convenient, with less heat loss and easy to control. The disadvantage is that the electricity consumption is large.
④Infrared heating, sealed and radiated heating of concrete with high temperature electrical heating or gas infrared generator.
Technical measures for winter concrete construction
1. Requirements for the materials for concrete composition in winter construction
Aggregate: There must be no ice, snow balls and organic matter in the aggregate, it should be clean, well-graded, and hard texture;
Water: Use drinking tap water;
H l0 Admixture: Use antifreeze. The mechanism of action of antifreeze is to significantly reduce the liquid phase freezing point of concrete at the specified negative temperature, so that the concrete does not freeze in the liquid state, ensure the hydration of cement, and obtain the expected strength within a certain period of time. The antifreeze should pass technical identification, meet quality standards, and master its performance through laboratory tests;Cement: ordinary silicate cement with significant activity and high hydration heat.
2. Requirements for concrete mixing and transportation in winter
Concrete mixing:
Concrete mixing uses heating water. Hot water above 80°C shall not come into direct contact with cement. First mix the hot water with aggregate and then mix it into cement to mix concrete to avoid false settling of cement. The concrete mixing time shall not be less than 3 minutes.In addition, protect the surroundings of the mixer and keep them warm if necessary.
Concrete Curing: After the concrete is poured, the concrete is immediately covered with a plastic cloth to maintain moisture. At the same time, the outside of the plastic cloth is covered with insulation and for insulation. The insulation quilt should be neat and tight so that the concrete temperature will not drop too quickly, and avoid the occurrence of concrete frost damage. Appropriately extend the concrete curing time, which should be no less than 15 days.
Add two sets of concrete test blocks with the same conditions and placed in the on-site environment so that the compressive strength of concrete under the same conditions can be obtained at any time.
3. Requirements for concrete temperature measurement in winter
Temperature measurement of concrete:
Build wires in concrete to set up a special person to measure the temperature, including atmospheric temperature, concrete outlet, mold entry temperature, and concrete internal temperature. If there are any abnormalities, take timely measures.
