I. Starting time of winter application:
When the average outdoor temperature is stable below 5℃ for five consecutive days, concrete engineering should take winter application measures.
Temperature determination: based on local years of meteorological data and current climate trends.

Ii. Freezing and allowable critical strength of concrete:
1. Freezing temperature of concrete:
-1.5 ~ -2℃ begins to freeze (free water -- accounting for 1/2 ~ 2/3) -2℃ ~ -4℃ all freeze (adsorb water).

2. The final strength loss caused by freezing of concrete:
The reason is that the ice swelling stress causes microcracks inside the concrete (the earlier the concrete is frozen, the more and bigger the cracks are), and the surface of the steel bar and coarse aggregate forms ice film, which affects the bonding force.
Characteristics - The earlier the freezing and the larger the water-cement ratio, the more strength loss.

3. The critical strength of concrete under freezing (the strength should be reached before freezing) :
Principle: After allowing the concrete to be frozen after the critical strength, the standard after thawing to 28d, the final strength loss ≯5%.
Critical strength of concrete under freezing: the minimum strength of concrete that can basically resist the ice heave stress (after freezing, the final strength loss does not exceed 5% of the pre-conditioned strength value).

Three, concrete winter application requirements
1. Materials:
(1) Cement -- ordinary Portland cement is preferred;
(Steam curing optional slag, pozzolanic cement; Antifreeze is strictly prohibited with high aluminum cement) marked 9:28 so 425#; The dosage is 300kg/m3 concrete at 9:28 a.m.
(2) Water-cement ratio ≤0.6.
(3) There will be no ice cube in the aggregate, the diameter of the block ≯20mm.
(4) The admixture should not use chlorine salt antifreeze.

2. Mixing system:
(1) The heating temperature of the material should be calculated according to the thermal engineering
(2) Stirring time: 50% longer than normal temperature.
(3) When doping antifreeze, the exit temperature is 9:28 ° C and the entry temperature is 9:28 ° C.

3. Transportation: shorten transportation distance and keep the container warm.

4. Casting:
(1) Remove ice, snow and dirt on formwork and steel bars;
(2) It shall not be poured on the foundation with strong frost heave;
③ When the bulk concrete is poured on the upper layer, the lower temperature is 9:28 ℃;
(4) When the concrete structure is heated and cured, the design consent should be obtained if the temperature is greater than 40℃ (to prevent greater temperature stress);
Using 45℃, the prefabricated structure joint should be preheated and then poured, curing to 75% more than the design strength.

5. Maintenance and quality inspection:
(1) Curing time: to ensure the concrete to achieve allowable freezing strength;
(2) Do a good job of concrete temperature measurement (leave the temperature hole, heat storage method every 6 hours to measure once, and the temperature isolation 9:28 so that 3 minutes);
(3) Add two groups of curing specimens under the same conditions (to test the strength of concrete before freezing and at room temperature for 28 days).

Four, the selection of concrete winter application method:
1. Heat storage method -- water and aggregate heating + hydration heat + thermal insulation covering
Principle: Concrete reaches critical strength before freezing.
Suitable for: outdoor minimum temperature 9:28-15 ℃ underground engineering;
Using 15m-1 structure surface coefficient (cooling area/total volume).

2, admixture method - add anti-freezing, early strength, catalysis, water reducing agent and other single or compound admixture
Principle: Concrete in the negative temperature does not freeze, continue to harden.
Suitable for: outdoor minimum temperature 9:28 -15℃, early winter, early spring.
Note: Strictly limit the amount of chloride admixture.

3, warm shed method -- build a shed enclosure, the shed heating to ≥5℃,
Features: same room temperature operation; It costs a lot of money.
Application: underground engineering, concrete concentrated engineering.

4, heating curing method:
Energy consumption, high costs; The strength of concrete increases rapidly; We will strictly control the rate at which temperatures rise and fall.
(1) Steam curing -- steam sleeve method, capillary method, internal steam method.
Using silicon cement using 85℃, slag cement using 85 ~ 95℃.
Low pressure (< 0.07MPa) saturated steam.
(2) Electric heating curing -- electrode method, electric heater heating method.
The effect is better at low intensity.