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The normal procedure adopted for concreting in fair weather will not valid for concreting when the temperature is low or below the freezing point. In India, such areas are fairly small when compared to fair-weather regions.
1. Cold Weather Concreting
The production of concrete in cold weather introduces special and peculiar problems, such as delay in setting and hardening, and damage to concrete in plastic conditions when exposed to below freezing points owing to the formation of ice lenses. Therefore, it is essential to maintain the temperature of the concrete positively above 0ºC, possibly at a much higher temperature.
1. Effects of Cold Weather on Concrete
a. Delay in setting and hardening
The rate of hydration depends upon the temperature. If the temperature is low, concrete takes a long time to set and a longer time to harden i.e., for the development of strength. The delay in setting time makes concrete vulnerable to frost attack snd other disturbances. Delay in the hardening period doesn’t facilitate the removal of formwork in a short period. Also, the rate of progress of work will be very slow, all of which affects the economy.
b. Freezing of concrete at an early age:
When the temperature goes below the freezing point, the free water contained in the plastic concrete freezes. Freezing of water, not only prevents the hydration of cement but also makes the concrete expand. This expansion disrupts concrete due to which irreparable loss of strength and quality takes place.
c. Freezing and Thawing
It is likely that due to the varied behavior of climatic conditions in the cold weather regions, the fresh concrete or hardened concrete gets subjected to freezing and thawing cycles. The durability of concrete gets greatly impaired due to this alternate freezing and throwing. Freezing and thawing may also exert fatigue in the concrete.
In dealing with the aspect of cold weather concreting, the following conditions may be discussed.
a. Low temperature, but above 0ºC at the time of concreting and later during the hardening period.
b. Low temperature at the time of concreting but below 0ºC during the hardening period.
c. Temperatuyre below 0ºC at the time of concreting and during the hardening period.
d. Hardened concrete subjected to alternate freezing and thawing.
It is necessary to deal with the above four conditions to understand the behavior of concrete clearly and to take appropriate steps to offset the harmful effects of such conditions for the successful placing of concrete.
2. Low Temperature But Above 0ºC
If the temperature is only but always above the freezing point, it only retards the rate of development of strength as shown in the maturity equations stated earlier. There is no other bad effect on the fresh concrete or hardening concrete.
The ultimate strength of the concrete cured at 9ºC is of a higher order than that of the concrete cured at a higher temperature.
This may be due to the superiority of gel structure on account of slow growth. No other precautions are necessary except for recognition of the fact of delayed strength for stripping the formwork or for putting the concrete into service.
3. Low Temperature at the Time of Concreting But Below 0ºC After Concreting
The first condition has been discussed in the above paragraph. But after concreting, if the temperature falls below 0ºC, it is again necessary to view the conditions under the following two categories:
a. The Temperature falls below 0ºC when the concrete is still green.
b. Temperature prevailing below 0ºC after the concrete is sufficiently hardened.
Many times it may also happen that the concrete will have been mixed and placed when the ambient temperature is above freezing point. But before the concrete has attained sufficient strength, the temperature of the air and also the temperature within the concrete may fall below the freezing point, in which case the free water is still available in the concrete to freeze and form ice lenses on a microscopic scale.
These ice lenses formed in the capillary cavities may cause capillary suction of water from the ground if the ground is saturated, and become bigger to disrupt the mass which disturbs the compaction of concrete. Ice formations may also appear as ice needles in the contact surface between aggregate and cement paste.
After thawing these ice needles will melt forming cavities. Therefore, it can be concluded that freezing of freshly laid concrete seriously impairs the structural integrity of concrete and results in a considerable loss of strength.
In extreme cases, it may make the concrete sufficiently hardened when the freezing takes place, there will not be much harm to the structural integrity of the concrete. If the concrete has sufficiently hardened, the water that has been mixed for making concrete will have been lost either being used up in the hydration process or lost by evaporation. Due to the formation of cement gels, the capillary cavities also will have been very much reduced, with the result that there exists very little free water in the body of concrete to freezing.
Therefore, firstly the magnitude of the volume of this stage is strong enough to resist whatever osmotic pressure resulting from the freezing. Therefore, there is no immediate danger to the concrete.
4. Temperature Below 0ºC at the Time of Concreting and During Hardening Period
Certain precautions are necessary for concrete when the temperature is below 0ºC so that the fresh concrete ad the effect of sub-zero temperature on the hardened but not fully matured concrete is explained in the above paragraph.
5. Hardened Concrete Subjected to Alternate Freezing and Thawing
Concrete pavements constructed at high altitude is normally subjected to alternate freezing and thawing. The interval of cycles may be from season to season or between day and night or even a couple of times in a day. It has been found that the durability of hardened concrete is reduced to 1/3 to 1/7 when it is subjected to alternate freezing and thawing depending on the quality of concrete.
It is to be noted that concrete is a pervious material. The degree of porosity is depending upon the gel/space ratio. A concrete member is likely to get saturated due to the absorption of moisture from the surface or bed. The free water that has filled the capillary cavities of concrete will get frozen will the fall temperature. Subsequently when the temperature goes above 0C; the ice lens melts. Due to this alternate freezing and thawing, concrete is subjected to distress and surface scaling. The distress of concrete can be measured by the loss of weight against a number of cycles of freezing.
2. Precautions During Cold Weather Concreting
To summarise, the following general precautions may be observed in cold weather concreting.
a. Preparation for concreting in cold weather may be completed well in advance of severe conditions. Windbreakers shall be erected to shield the mixing and batching plants: tarpaulins, plastic sheets, and other covering and insulating materials may be made available may be installed and checked for correct functioning.
b. The concrete shall be delivered to the point of placing at not less than 5ºC. It is necessary to place the concrete quickly and cover the top of the concrete with an insulating material.
c. Before any concrete is placed, all ice, snow, and frost shall be completely removed, and the temperature is raised as close to the temperature of fresh concrete to be placed thereon, as practicable. No concrete shall be placed on a frozen subgrade or that contains frozen materials.
Where concrete is to be placed over permanently frozen ground, subgrade materials may be thawed deep enough to ensure that they will not freeze back up to the concrete or it may be covered with a sufficient depth of dry granular material.
d. During Cold weather, all concrete surfaces shall be covered as soon as the concrete has been placed in order to preserve the heat and to help prevent freezing. Clean straw blankets about 50 mm thick, sacks, tarpaulins, expanded polystyrene, plastic sheets, and waterproof paper can all be used in conjunction with the air gap as insulation. If possible insulating material shall be placed against any formwork before concreting and the same can be used as protection after the formwork has been stripped.
e. Heated enclosers are commonly used for protecting concrete when air temperatures are near or below freezing. Enclosures may be heated by steam, steam pipes, and other types of heaters. Enclosures may be made of wood, canvas, fiber insulation board, plywood, etc.
f. During placement of unformed concrete, tarpaulins or other readily movable coverings supported on framework shall follow closely the placing of the concrete so that only a small area of the finished slab is exposed to outside air at any time.
Such tarpaulins shall be used so that hot air can be circulated freely on the slab. layers of insulating materials placed directly on the concrete are also effective in protecting the concrete.
g. During periods of freezing or near-freezing conditions, water curing is not necessary as the loss of moisture placed directly on the concrete by evaporation will be greatly reduced in cold air conditions.
h. For concrete cast in insulated formwork, it is only necessary to cover the member completely in order to retain sufficient water for the hydration of cement. On removal of the formwork and insulation, the member shall immediately be covered with plastic sheet or tarpaulins, properly lapped handmade air-tight. On no account should such concrete, just released from insulated formwork, be saturated with cold water. When protective measures are to be discontinued, the surface temperature of the concrete shall be gradually adjusted to the air temperature.
i. Low-pressure wet steam provides the best means of both heatings the enclosures and moist curing the concrete. Early curing with liquid membrane-forming compounds may be followed on a concrete surface with heated enclosures. It is better to cure first with steam curing during the initial period of protection and then apply a curing compound after the protection is removed and the air temperature is above freezing.
j. Forms shall not be released until the concrete has achieved a strength of at least twice the stress to which the concrete may be subjected at the time of removal of formwork. In the normal circumstances where ambient temperature doesn’t fall below 15C and where OPC cement is used and adequate curing is done, the striking periodic a satisfactory guideline. For other types of cement and lower temperature, the stripping time recommended may be suitably modified.
k. In cold weather protection offered by formworks other than steel, is often of greater importance. With suitable insulations, the forms, including those of steel, in many cases will provide adequate protection without supplementary heating. Therefore, it is often advantageous not to remove forms until the end of a minimum period of protection or even later.
l. During cold weather, inspection personnel should keep a record of the date, time outside air temperature, the temperature of concrete at the time of placing, and general weather(calm, windy, clear, cloudy, etc. ). The record should also include the temperature at several points within the enclosures and on the concrete surface, corners, and edges in sufficient numbers to show the highest and lowest temperatures of concrete.
Thermometers should be inserted in those parts of the concrete where maximum stresses will appear at the removal of forms.
To control the hardening process, it is necessary to measure the temperature of concrete at placing, at the time of applying for the protection, and three times each day until resistance to freezing has been obtained.
Concreting in winter times requires that the quality control of the concrete is carried out with great care. The test results should be used for fixing the time of removal of insulations and form work. Control test specimens should be cast and cured in a standard way to indicate the potential strength properties of the mix.
In addition to the control test cubes, it is necessary to cast several specimens, the curing conditions of which are maintained in the same way as that of the actual structure. These specimens are tested before stripping the formwork to indicate the strength development of the actual structures.
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