Our country stipulates that the average daily outdoor temperature for 5 days in a row stable below 5°C under the conditions of concrete construction, that is, the construction of concrete in winter.
Concrete construction in winter is characterized by fresh concrete is very sensitive to temperature, the temperature decreases, the cement hydration rate will be reduced by 5% -7%; when the temperature is below 0 ° C, the free water in concrete began to freeze, when temperature is below -15 °C, almost all free water is frozen. Hydration after the icing is about to end and produces a 9% volume expansion. Failure to take effective measures will result in permanent damage to the concrete structure.
Experiments show that concrete suffered damage caused by freezing, and the frozen time sooner or later, w/c ratio, the earlier the freezing time, the greater the w/c ratio, the more the strength loss, otherwise the less. To ensure the quality and duration of the project, effective antifreezing measures must be taken during construction.
1, the choice of concrete construction methods in winter
Concrete winter construction methods are: no heating during concrete curing period. The method comprises the following steps: adding admixtures, admixture as early strength agent and antifreeze agent.
2, the conservation of concrete construction in winter
Concrete winter construction additives used are: early strength agent, antifreeze agent, water reducer and air entraining agent, can play early strength, frost resistance, procoagulant, water reduction and reduce the role of freezing point. This is an effective method of concrete construction in winter.
3, the general requirements of winter construction of concrete
(1) When mixing concrete in winter, priority should be given to the method of heating water. When the heated water still can not meet the requirements, the aggregate is heated again. The heating temperature of water and aggregate should be calculated according to the calculation of thermal power, but it must not exceed the stipulation .
(2) For the construction of concrete for winter construction, Portland cement or ordinary portland cement should be given priority. Its strength grade shall not be lower than 42.5Mpa, concrete cement content per cubic meter shall not be less than 300kg, and w/c ratio shall not be greater than 0.6.
(3) Aggregates must be clean and free from frozen materials such as ice and snow.
(4) Before mixing hot water or steam rinse mixer, stirring time should be extended by 50% than at room temperature, feeding in a sequence of sequence aggregate, hot water, and then into the cement, admixture. Ensure that the concrete out of the machine temperature of not less than 15 ℃, the mold temperature of not less than 5 ℃.
(5) Conveyance of concrete should be as short as possible. The containers for transporting and pouring concrete should have insulation measures.
(6) Before concrete pouring, ice and dirt on the formwork and reinforcement should be removed. The containers used for transporting and pouring concrete should have thermal insulation measures. Concrete in transport, pouring process temperature should be consistent with the thermal calculation requirements, if the requirements do not meet, you should take measures to adjust.
(7) Strictly control the quality of commodity concrete, admixture and w/c ratio of concrete; shorten the waiting time for concrete to the construction site, so that it can be placed with the pouring.