Cold weather concrete

Cold Weather Concreting – Any operation of concreting done at about 5 O C atmospheric temperature or below.

EFFECTS OF COLD WEATHER ON CONCRETE
Delayed Setting – When the temperature is falling to about 5 o C or below, the development of concrete strength is retarded compared with the strength development at normal temperature.
Freezing of Concrete at Early Ages – When concrete is exposed to freezing temperature, there is a risk of concrete suffering irreparable loss and other qualities that is, permeability may increase and durability may be impaired.
Repeated Freezing and Thawing of Concrete – If the concrete is exposed to repeated freezing and thawing after final set and during hardening period, the final qualities of concrete may be impaired.
Stress Due to Temperature Differentials – It is a general experience that large temperature differentials within the concrete member may promote cracking and have harmful effect on the durability. Such differentials are likely to occur in cold weather at time of removal of form insulations.

Temperature Control of Concrete Ingredients

The most direct approach to keeping the concrete temperature up is by controlling the temperature of ingredients.
Aggregates – heating of aggregates shall be such that frozen lumps, ice and snow are eliminated and at the same time the over-heating is avoided. At no point the temp of aggregates shall exceeds 100 O C the average temp of aggregate of individual batch shall not exceeds 65 O C. The heating of aggregates of temperatures higher than 15 O C is rarely necessary with mixing water at 60 O C
Steam in embedded pipes is recommended for heating of aggregate.
Water – Mixing water shall be heated under such a control and in sufficient quantity as to avoid appreciable fluctuations in temperature from batch to batch.
Cement- During winter concreting, it is preferable to use rapid hardening Portland cement. Use of cement which gives earlier and higher heat of hydration is preferable. The cement content in mix shall preferably be not less than 300 kg/cum.

PRECAUTIONARY MEASURES

At low temperatures
Keep the formwork in position longer, or use rapid hardening cement
Cover the top of the concrete with insulating material
Insulate steel formwork and
Make sure that concrete is delivered to the point of placing at not less than 5 O C
When there is frost at night
Take all precautions as in at low temperatures
Insulate all formwork
Make sure that concrete is not placed against a frozen sub grade or against reinforcement or forms covered with snow , ice  and
Place concrete quickly and insulate.
When there is serve frost day and night
Take all precautions as in at low temperatures and when there is frost at night
Heat the water and if necessary , the aggregates and
Make sure that concrete is delivered to the point of placing at not less than 10 O C, place quickly and insulate , or make sure concrete is delivered to the point of placing not less than 5 O C , place quickly and provide continuous heating of concrete.

PRODUCTION AND DELIVERY

The concrete shall be delivered to the point of placing at not less than 5 O C. it’s necessary to place the concrete quickly and cover the top of concrete with insulating material.

PLACEMENT, PROTECTION AND CURING

Before any concrete is placed, all ice snow or frost shall be completely removed and the temperature of all surfaces to be in contact with new concrete shall be raised as close to the temperature of fresh concrete to be placed thereon as practicable.
During cold weather, all concrete surfaces shall be covered as soon as the concrete has placed in order to keep the heat in and to help prevent freezing.
During the periods of freezing or near freezing conditions, water curing is not necessary, as loss of moisture from concrete by evaporation is greatly reduced in cold air conditions.
For concrete cast in insulated formwork it is necessary to cover the member completely in order to retain sufficient water for the hydration of the cement.
Low pressure wet steam provides the best means of both heating and enclosures and moist curing the concrete.Early curing with liquid membrane forming compounds may be followed on concrete surface with heated enclosures.