1, when preparing concrete, portland cement and ordinary portland cement should be preferred. The strength grade of cement should not be lower than 42.5, and the water-cement ratio should not be greater than 0.5. When steam curing is adopted, slag portland cement should be preferred. Using heating method to cure concrete mixed with admixture, it is forbidden to use high alumina cement. When using other types of cement, we should pay attention to the influence of its additives on the strength, frost resistance and impermeability of concrete.
2. When pouring concrete, admixtures such as air entraining agent and air entraining water reducer should be added to improve the frost resistance of concrete. When chloride antifreeze is mixed into reinforced concrete, the chloride ion content shall not exceed the provisions in Chapter 1 1 and Chapter 12 of this specification, and steam curing is not suitable. When using plain concrete, the chloride content should not be greater than 3% of the cement quality. Prestressed concrete shall not be mixed with air entraining agent, air entraining water reducing agent and chlorine twist antifreeze. The admixture of air entraining agent, air entraining water reducer and antifreeze shall conform to the current national standard "Concrete Admixtures" (GB8076).
3. The temperature of various materials for mixing concrete shall meet the temperature required for mixing and synthesizing concrete mixture. When the original temperature of the material can't meet the needs, the mixing water should be heated first, and when it still can't meet the needs, the aggregate should be heated again. Cement is only heat preservation, not heating. The heating temperature of all materials shall be calculated and determined according to the thermodynamic calculation formula of winter construction in Appendix J, but it shall not exceed the provisions in Table 14.2.2.
4. When mixing concrete in winter, the aggregate shall not contain ice, snow and frozen blocks. Strictly control the mixture ratio and slump of concrete; Before feeding, use hot water or steam to clean the mixer. The feeding sequence is aggregate, water, stirring and then mixing with cement. Time is 50% longer than normal temperature. The outlet temperature of concrete mixture should not be lower than 10℃, and the inlet temperature should not be lower than 5℃.