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 :
Rapid hydration of
cement
Increased evaporation of
mixing water
Greater mixing water
demand
Large volume changes in
concrete resulting in cracks.
Reduction in strength.
The climatic factors
affecting concrete in hot weather are:
High ambient temperature
Reduced relative
humidity
Increased wind velocity
Problems associated with
hot weather concreting shall be addressed as follows:
Controlling the
temperature of concrete ingredients
Suitable proportioning
of concrete mixes.
Controlling the
temperature of concrete as placed.
Controlling the
processes such as concrete production and delivery
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 concretes 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 400 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.
