How To Pour Concrete In Cold Weather | Recommended Practices And Precautions | Concrete Under Extreme Environmental Conditions
Any concreting operation
done at a temperature below 5°C is termed cold weather concreting. Most codes
do not advocate concreting to be done at an atmospheric temperature below 5°C
without special precautions.
Due to low temperature, the
problems are mainly due to the slower development of concrete strength; the
concrete in the plastic stage can be damaged if it is exposed to low
temperatures which cause ice lenses to form and expansion to occur within the
pore structure, and subsequent damage may occur due to alternate freezing and
thawing when the concrete has hardened.
EFFECTS OF POURING CONCRETE IN HOT WEATHER
The
effects of cold weather concreting may be summarized as follows.
1. Delayed setting At low
temperatures, the development of concrete strength is retarded
as compared with the strength development at normal temperatures. The setting period necessary
before the removal of formwork is thus increased. Although the initial strength of
concrete is lower, the ultimate strength will not be severely affected provided
the concrete has been prevented from freezing during its early life.
2. Early freezing of
concrete When plastic concrete is exposed to freezing temperature, it may
suffer permanent damage. If the concrete is allowed to freeze before a certain
prehardening period, it may suffer irreparable loss in its properties so much
so that even one cycle of freezing and thawing during the prehardening period
may reduce compressive strength to 50 per cent of what would be expected for
normal temperature concrete. The prehardening period depends upon the type of
cement and environmental conditions. It may be specified in terms of time
required to attain a compressive strength of the order of 3.5 to 7.0 MPa.
Alternatively, it can be specified in terms of period varying from 24 hours to
even three days depending upon the degree of saturation and water-cement ratio.
3. Stresses due to
temperature differential A large temperature differential within the concrete
member may promote cracking and has a harmful effect on durability. Such
situations are likely to occur in cold weather at the time of removal of
formwork.
SETTING TIME OF CONCRETE AT VARIOUS TEMPERATURE
RECOMMENDED PRACTICE WHILE POURING CONCRETE IN COLD WEATHER
As
per IS: 7861 (Part-II)–1981, the following measures should be taken:
Temperature
Control of Ingredients
The temperature at the time
of setting of concrete can be raised by heating the ingredients of the concrete
mix. It would be easier to heat the
mixing water. The temperature of the water should not exceed 65°C as the flash
set of cement will occur when the hot water and cement come in contact with the
mixers. Therefore, the heated water should come in direct contact with the
aggregate first, and not the cement. The aggregates are heated by passing steam
through pipes embedded in aggregate storage bins as shown in Fig. Another
precaution taken along with the heating of ingredients is to construct a
temporary shelter around the construction site. The air inside is heated by
electric or steam heating or central heating with circulating water. The
temperature of ingredients should be so decided that the resulting concrete
sets at a temperature of 10 to 20°C.
Use
of Insulating Formwork and Blanket Covers
A fair amount of heat is
generated during the hydration of cement. Such heat can be gainfully conserved by
having insulating formwork covers capable of maintaining a concrete temperature
above the desirable limit up to the first three days (or even seven days) even
though the ambient temperatures are lower. The formwork covers can be of
timber, clean straw, blankets, tarpaulins, plastic sheeting, etc., and are
used in conjunction with an air gap as insulation. The efficiency of the covers
depends upon the thermal conductivity of the medium as well as ambient
temperature conditions. For moderately cold weather, timber formwork alone is
sufficient.
Fig. Cold-weather
concreting
Proportioning
of Concrete Ingredients
The important factor for
cold-weather concreting is the attainment of suitable temperatures for fresh
concrete. Since the quantity of cement in the mix affects the rate of increase
in temperature, an additional quantity of cement may be used. It would be
preferable to use high alumina cement for concreting during frost conditions,
the main advantage is that a higher heat of hydration is generated during
the first 24 hours. During this period, sufficient strength (approximately 10 to
15 MPa) is developed to make the concrete safe against frost action. No
accelerator should be used if high alumina cement is used. Alternatively, the
rapid hardening of Portland cement or accelerating admixtures used with proper
precautions can help in getting the required strength in a shorter period. Air-entraining agents
are generally recommended for use in cold weather. Air-entrainment increases
the resistance of the hardened concrete to freezing and thawing and normally, at the same time, improves the
workability of fresh concrete. The calcium chloride used as an accelerating
admixture may cause corrosion of reinforcing steel. In any case, calcium
chloride should not be used in prestressed concrete construction.
Placement
and Curing
Before placing the
concrete, all ice, snow and frost should be completely removed. Care should be taken to see that
Delayed
Removal of Formwork
Because of a slower rate of
gain of strength during the cold weather, the formwork and props have to be
kept in place for a longer time than in usual concreting practice.
The problem of concreting
in cold weather can be minimized by adopting the precast construction of
structures. Precast members are manufactured in factories where adequate
precautions can be taken and concreting can be done in controlled conditions.
TIPS FOR POURING CONCRETE IN HOT WEATHER
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