Civil Engineering Reference
In-Depth Information
mixed concrete notes in some situations difficulty may be
encountered when concrete temperatures approach 32°C
(90°F). However, this specification does not mandate a
maximum concrete temperature unless heated aggregates
or heated water are used.
Precautions should be planned in advance to counter
the effects of a high concrete temperature when the con-
crete placed is somewhere between 25°C and 35°C (77°F
and 95°F). Last-minute attempts to prevent hot-weather
damage are rarely performed soon enough. If acceptable
field data is not available, the maximum temperature limit
should be established for conditions at the jobsite; this
should be based on trial-batch tests at the temperature and
for the typical concrete section thickness anticipated,
rather than on ideal temperatures of 20°C to 30°C (68°F to
86°F) cited in ASTM C 192 (AASHTO T 126). If possible,
large batches should be made to measure mix properties
at time intervals to establish the relationship for the prop-
erty of interest as a function of time at various batch tem-
peratures of interest. This process will establish the
maximum allowable time to discharge concrete after
batching for various concrete temperatures.
More than controlling the maximum temperature is
required to determine when to employ precautions to pro-
duce concrete with the required strength and durability.
For most work it is too complex to simply limit only the
maximum temperature of concrete as placed; circum-
stances and concrete requirements vary too widely. For
example, a temperature limit that would serve successfully
at one jobsite could be highly restrictive at another.
Atmospheric conditions, including air temperature, rela-
tive humidity and wind speed, in conjunction with site
conditions influence the precautions needed. For example,
flatwork done under a roof that blocks solar radiation with
exterior walls in place that screen the wind could be com-
pleted using a high temperature concrete; this concrete
would cause difficulty if placed outdoors on the same day
where it would be exposed to direct sun and wind.
Which precautions to use and when to use them will
depend on: the type of construction; characteristics of the
materials being used; and the experience of the placing
and finishing crew in dealing with the atmospheric con-
ditions on the site. The following list of precautions will
reduce or avoid the potential problems of hot-weather
concreting:
• use materials and mix proportions that have a good
record in hot-weather conditions
• cool the concrete or one or more of its ingredients
(Fig. 13-1)
• use a concrete consistency that allows rapid place-
ment and consolidation
• reduce the time of transport, placing and finishing as
much as possible
• schedule concrete placements to limit exposure to
atmospheric conditions, such as at night or during
favorable weather conditions
• consider methods to limit moisture loss during
placing and finishing, such as sunshades, wind-
screens, fogging, or spraying
• apply temporary moisture-retaining films after
screeding
• organize a preconstruction conference to discuss the
precautions required for the project
The above precautions are discussed in further detail
throughout this chapter.
EFFECTS OF HIGH CONCRETE
TEMPERATURES
As concrete temperature increases there is a loss in slump
that is often unadvisedly compensated for by adding
water to the concrete at the jobsite. At higher temperatures
a greater amount of water is required to hold slump con-
stant than is needed at lower temperatures. Adding water
without adding cement results in a higher water-cement
ratio, thereby lowering the strength at all ages and
adversely affecting other desirable properties of the hard-
ened concrete. This is in addition to the adverse effect on
strength at later ages due to the higher temperature, even
without the addition of water. Adding cement to compen-
sate for the use of additional mix water may not be
enough to achieve the desired concrete properties because
additional cement will further increase the concrete tem-
perature and water demand.
As shown in Fig. 13-2, if the temperature of freshly
mixed concrete is increased from 10°C to 38°C (50°F to
100°F), about 20 kg/m
3
(33 lb/yd
3
) of additional water is
needed to maintain the same 75-mm (3-in.) slump. This
additional water could reduce strength by 12% to 15% and
Concrete temperature,
F
°
40
60
80
100
310
180
300
290
170
280
160
Slump: 75 mm (3 in.)
Max. size agg: 37.5 mm (1
1
/
2
in.)
270
0
10
20
30
40
Concrete temperature,
°
C
Fig. 13-2. The water requirement of a concrete mixture
increases with an increase in concrete temperature
Bureau
of Reclamation (1981)
.
