In countries which experience
extreme weather conditions special problems are encountered in preparation,
placement and curing of concrete. India has regions of extreme hot
weather (hot-humid and hot-arid) as well as cold weather. The Indian Standards
dealing with extreme weather concreting are: IS: 7861 (Part 1-1975 Reaff.
2007)-Hot weather concreting and IS: 7861 (Part 2-1981 Reaff. 2007) -Cold
weather concreting.
HOT WEATHER CONCRETING:
Special problems are encountered
in the preparation, placement and curing of concrete in hot weather. High
temperature result in :
Ø Rapid hydration of cement
Ø Increased evaporation of
mixing water
Ø Greater mixing water demand
Ø Large volume changes in
concrete resulting in cracks.
Ø Reduction in strength.
The climatic factors affecting
concrete in hot weather are:
Ø High ambient temperature
Ø Reduced relative humidity
Ø Increased wind velocity
Problems associated with hot
weather concreting shall be addressed as follows:
Ø Controlling the temperature
of concrete ingredients
Ø Suitable proportioning of
concrete mixes.
Ø Controlling the temperature
of concrete as placed.
Ø Controlling the processes
such as concrete production and delivery
Ø Carrying out effective
protection and curing of placed concrete.
Controlling the temperature of concrete ingredients:
The most direct approach to keep
concrete temperature down is by controlling the temperature of its ingredients.
The contribution of each ingredient to the temperature of concrete is a function
of the temperature, specific heat and quantity used of that ingredient. The
aggregates and mixing water exert the most pronounced effect on temperature of
concrete. Thus, in hot weather all available means shall be used for
maintaining these materials at as low temperatures as practicable.
Aggregates
Any one of the procedures or a
combination of the procedures given below may be used for lowering the
temperature or at least for preventing excessive heating of aggregates.
Shading stockpiles from direct rays of the
sun.
Sprinkling the stockpiles of
coarse aggregate with water and keeping them moist.
This results in cooling by
evaporation, and this procedure is specially effective when relative humidity
is low. Such sprinkling should not be done haphazardly because it leads to
excessive variation in surface moisture and thereby impairs uniformity of
workability. When coarse aggregates are stockpiled during hot weather,
successive layers should be sprinkled as the stockpile is-built up. If cold
water is available, heavy spraying of coarse aggregate immediately before use
may also be done to have a direct cooling action. Coarse aggregates may also be
cooled by methods, such as inundating them in cold water or by circulating
refrigerated air through pipes or by other suitable methods.
Water
The mixing water has the greatest
effect on temperature of concrete, since it has a specific heat of about 4.5 to
5 times that of cement or aggregate. The temperature of water is easier to
control than that of other ingredients and, even though water is used in
smaller quantities than the other ingredients, the use of cold mixing water
will effect a moderate reduction in concrete placing temperatures. For a
nominal concrete mixture containing 336 kg of cement, 170 kg water, 1850 kg of
aggregate per ma, a change in 2°C water temperature will effect a 0.5 º C
change in the concrete temperature.
Efforts shall be made to obtain
cold water, and to keep it cold by protecting pipes, water storage tanks, etc.
Tanks or trucks used for transporting water shall be insulated and/or coloured
and maintained white or yellow. Under certain circumstances, reduction in water
temperature may be most economically accomplished by mechanical’ refrigerator
or mixing with crushed ice. Use of ice as a part of the mixing water is highly
effective in reducing concrete temperature since, on melting alone, it takes up
heat at the rate of 80 kcal/kg. To take advantage of heat of fusion, the ice shall be incorporated directly into
the concrete as part of
the mixing water. Conditions shall be such that the ice is completely melted by
the time mixing is completed.
NOTE :- If the ice is not melted
completely by the time mixing is completed, there can be a possibility of Ice
melting after consolidation of concrete and thus leaving hollow pockets in
concrete, with detrimental effects.
Recommended procedure for
concreting during hot weather conditions is given below:
Ambient temperature shall be
below 40° C at the time of placement of
concrete. Concreting may be planned
during morning and evening hours.
The period between mixing and
delivery (placing) shall be kept an absolute minimum.
Keep aggregates under shade and
cool aggregates by sprinkling water.
Formwork, reinforcement shall be
sprinkled with cool water just prior to placement of concrete.
COLD WEATHER CONCRETING:
The production of concrete in cold weather introduces
special and peculiar problems which do not arise while concreting at normal
temperatures. Quite apart from the problems associated with setting and
hardening of cement concrete, severe damage may occur if concrete which is
still in the plastic state is exposed to low temperature, thus causing ice lenses
to form and expansion to occur within the pore structure. Hence it is essential
to keep the temperature of the concrete above a minimum value before it is
placed in the formwork. After placing, concrete may be kept above a certain
temperature with the help of proper insulating methods before the protection is
removed. During periods of low ambient temperature, special techniques are to
be adopted to cure the concrete while it is in the formwork or after its
removal.
The Precautions to be taken and methods adopted for
concreting in sub-zero temperature is listed below.
- Utilization
of the heat developed by the hydration of cement and practical methods of
insulation.
- Selection
of suitable type of cement
- Economical
heating of materials of concrete
(Heating of water is the easiest to be
adopted)
- Admixtures
of anti-freezing materials
- Electrical
heating of concrete mass
- Use
of air-entraining agents.
