Civil Engineering Reference
In-Depth Information
CHAPTER 13
Hot-Weather Concreting
Weather conditions at a jobsite—hot or cold, windy or
calm, dry or humid—may be vastly different from the
optimum conditions assumed at the time a concrete mix is
specified, designed, or selected, or from laboratory condi-
tions in which concrete specimens are stored and tested.
Hot weather conditions adversely influence concrete
quality primarily by accelerating the rate of moisture loss
and rate of cement hydration that occur at higher temper-
atures. Detrimental hot weather conditions include:
• high ambient temperature
• high concrete temperature
• low relative humidity
• high wind speed
• solar radiation
Hot weather conditions can create difficulties in fresh
concrete, such as:
• increased water demand
• accelerated slump loss leading to the addition of
water on the jobsite
• increased rate of setting resulting in placing and fin-
ishing difficulties
• increased tendency for plastic cracking
• critical need for prompt early curing
• difficulties in controlling entrained air
• increased concrete temperature resulting in long-term
strength loss
• increased potential for thermal cracking
Adding water to the concrete at the jobsite can
adversely affect properties and serviceability of the hard-
ened concrete, resulting in:
• decreased strength from higher water to cement ratio
• decreased durability due to cracking
• increased permeability
• nonuniform surface appearance
• increased tendency for drying shrinkage
• reduced abrasion resistance from tendency to
sprinkle water during finishing
Only by taking precautions to alleviate these difficul-
ties in anticipation of hot-weather conditions can concrete
work proceed smoothly. For more information on the
above topics, see
ACI Committee 305 (1999)
.
WHEN TO TAKE PRECAUTIONS
During hot weather the most favorable temperature for
achieving high quality freshly mixed concrete is usually
lower than can be obtained without artificial cooling. A
concrete temperature of 10°C to 15°C (50°F to 60°F) is
desirable to maximize beneficial mix properties, but such
temperature are not always practical. Many specifications
require only that concrete when placed should have a tem-
perature of less than 29°C to 32°C (85°F to 90°F). The
ASTM C 94 (AASHTO M 157) specifications for ready
Fig. 13-1. Liquid nitrogen added directly into a truck mixer
at the ready mix plant is an effective method of reducing
concrete temperature for mass concrete placements or
during hot-weather concreting. (69954)
