1) Use antifreeze: antifreeze can greatly reduce the freezing point of mixed water, and prevent water in plastic state and early concrete from freezing and swelling, and frostbite concrete. Antifreeze has no obvious harm to the strength and performance of concrete, but some antifreeze contains urea, and long-term use of concrete will release ammonia smell, which needs special attention.
2) Thermal insulation: plywood, wooden boards and other formwork with certain thermal insulation function should be used as far as possible, and the exposed concrete surface and steel formwork should be covered with thermal insulation materials such as quilts, straw curtains or foam insulation boards, so as to make the concrete strength develop rapidly.
In winter construction, it is best to use early strength silicate or ordinary portland cement, because the strength develops rapidly and the hydration heat is relatively high. But as long as the heat preservation is done well, any cement can be used, and the strength development is slow, which is better for preventing thermal stress cracks.
In the environment with the lowest temperature of 2℃, concrete can be poured without antifreeze, but it must be kept warm and pay close attention to the temperature change trend. If the temperature will drop, it is necessary to strengthen the thermal insulation and cold preservation of concrete.
Attached:
Relevant regulations on temperature measurement of concrete in winter construction
Install a white shutter box with a specification of not less than 300*300*400 in a place with a height of 1.5m from the ground and good ventilation conditions. The starting and ending time of temperature measurement refers to the time when the outdoor average daily temperature is lower than 5℃ for five consecutive days, and the time when the outdoor average daily temperature is higher than 5℃ for five consecutive days is the applicable end time in winter;
1 concrete mixing temperature record
During concrete construction in winter, the temperature shall be measured by mixing (including on-site mixing and commercial concrete) and recorded. The temperature measurement records of concrete mixing in winter include atmospheric temperature, raw material temperature, tank temperature and mold entry temperature. The specific requirements of temperature measurement should be made public in writing, and the executor must operate in accordance with the regulations. Generally speaking, for the mixed concrete, it is necessary to regularly check the temperature of concrete leaving the tank and entering the mold (not less than 4 times per shift), and it is required that the temperature of concrete or mortar leaving the tank is not lower than 10℃ and the temperature of concrete entering the mold is not lower than 5℃.
After the original record is signed, it shall be submitted to the documenter for filing. Fill in the words "field mixing or commercial concrete" in the "Remarks" column. Each temperature value in the table should be marked with a symbol.
2, concrete curing temperature records
(1) The winter construction of concrete shall comply with the current national standard Code for Winter Construction of Building Engineering (JGJl04) and the provisions of the construction technical scheme.
(2) Before the concrete mixed with antifreeze reaches the critical strength of frost resistance (4MPa), it is measured every 2 hours. The temperature changes normally, and the temperature measurement interval can be adjusted from 2 hours to 6 hours.
Frequency control of temperature measurement: measure the temperature in time for the first time after concrete pouring, and then measure it every 2 hours for three consecutive days. After three days, measure every four hours (8: 00 am, 8: 00 pm and 2: 00 pm) until the concrete temperature reaches 0℃.
(3) Winter concrete curing temperature measurement should first draw the layout of the temperature measuring point (indicate the name of the specific part), including the location and depth of the temperature measuring point. The temperature measurement records shall include the atmospheric temperature, the measured temperature of each thermometer hole, the average temperature of each thermometer hole measured at the same time and the interval time, etc. In addition, due calculation (this time, cumulative) is required. Each temperature value in the table should be marked with a symbol.
(4) Temperature measuring items, temperature measuring frequency and setting of temperature measuring holes shall meet the requirements of current relevant standards.
(1) The location of the temperature measuring hole should be in the place where the temperature changes greatly and it is easy to lose heat and freeze. The northwest or the front should be set in a cool place, and the mouth of the temperature measuring hole should not face the wind and be temporarily closed.
Embedded temperature measuring tube: Before concrete covering, the temperature measuring tube should be embedded. The position and quantity of embedded temperature measuring tubes should be drawn in advance, and each temperature measuring point should be numbered.
② Setting of structural measuring holes
A beam (including simply supported and connected beams): the temperature measuring hole on the beam should be perpendicular to the axis of the beam, and the depth of the hole is 1/3 ~ 1/2 of the beam height.
B. Cast-in-place reinforced concrete constructional column: The lower end of each constructional column has a thermometer hole.
C. Bottom plate: The vertical and horizontal intervals of temperature measuring holes on the bottom plate are not more than 5m, and a room with an area of not more than 20m2 can be provided with a temperature measuring hole, which is vertical to the board surface and has a hole depth of 1/3 ~ 1/2.
D. Cast-in-place concrete wallboard: when the wall thickness is within 500 px ~ 500 px, a temperature measuring hole with a depth of 1/2 of the wall thickness is set on one side; When the wall thickness is greater than 500px, temperature measuring holes are set on both sides, the hole depth is 1/3 of the wall thickness, and it is not less than 250px, and the temperature measuring holes form an inclination angle of 30 degrees with the board surface. The vertical and horizontal spacing of temperature measuring holes in a large area wall is not more than 5m; When the area of each wall is less than 20m2, each wall can be provided with a thermometer hole.
3, mass concrete curing temperature records
During the construction of mass concrete, the atmospheric temperature, the temperature of each thermometer hole, the temperature difference between inside and outside and cracks should be checked and recorded. Mass concrete curing temperature measurement should be attached with the layout of temperature measurement points, including the location and depth of temperature measurement points. Each temperature value in the table should be marked with a symbol.
4, temperature measurement requirements
(1) When measuring, press the number of the measuring hole. After the thermometer is inserted into the measuring hole, plug the orifice and leave it in the hole for 3-5 minutes for reading;
(2) The temperature of concrete pouring, pouring and molding shall not be less than 4 times per shift;
(3) When the heat storage method is used for curing, it should be measured every 6 hours;
(4) Before the strength of concrete mixed with antifreeze reaches 4.0Mpa, it shall be tested every 2 hours 1 time, and then every 6 hours;
5] Winter construction has outdoor atmospheric measurement records;
[6] Prediction of concrete strength by maturity method.