Hot
Weather
Concrete
Techniques
High
ambient
temperature
is
only
one
factor
to
consider
as
you
are
planning
a
“hot
weather”
concrete
pour.
Understanding
how
ambient
temperature,
relative
humidity,
direct
sunlight
and
wind
velocity
affect
the
curing
of
concrete
can
help
you
determine
the
proper
precautions
necessary
to
achieve
a
successful
concrete
pour.
Improper
placement
procedures
can
lead
to
excessive
plastic
shrinkage
cracking,
lower
ultimate
compressive
strengths
and
reduced
surface
freeze‐thaw
and
wear
durability.
Plastic
shrinkage
cracks,
the
most
common
result
of
hot
weather
concrete
placement,
occur
when
the
rate
of
evaporation
of
surface
moisture
exceeds
0.2
pounds
per
square
foot,
per
hour.
The
rate
of
surface
evaporation
can
be
adversely
affected
by
any
combination
of
factors,
including
high
ambient
temperature,
low
relative
humidity,
high
concrete
temperature
and
even
moderate
wind
velocity.
The
Portland
Cement
Association
(PCA)
has
published
a
chart
(Fig.
1)
that
correlates
the
factors
associated
with
hot
weather
concrete
placement
conditions
that
can
be
used
to
help
determine
if
special
concrete
placement
techniques
are
necessary.
The
chart
below
reflects
an
ambient
temperature
of
65
F,
relative
humidity
of
40%,
a
concrete
temperature
of
60
F
and
a
wind
velocity
of
20
miles
per
hour.
In
this
example,
the
rate
of
evaporation
falls
below
the
0.2
pounds
per
square
foot
per
hour
rate
which
would
indicate
that
ordinary
placement
procedures
are
acceptable.
However,
if
the
ambient
temperature
is
increased
to
80
F
with
a
relative
humidity
of
40%
and
a
concrete
temperature
of
90
F;
a
mind
velocity
of
20
miles
per
hour
would
cause
an
evaporation
rate
of
0.5
pound
per
square
foot
per
hour,
2
½
times
the
normal
evaporation
rate.
These
conditions
would
require
special
placement
procedures
to
prevent
shrinkage
cracking.
In
addition
to
the
potential
for
plastic
shrinkage
cracking,
hot
weather
conditions
will
have
an
effect
on
setting
time
and
concrete
placement
characteristics.
In
hot
weather,
a
concrete
mix
will
tend
to
set
sooner
and
also
lose
slump
at
a
more
rapid
rate.
There
will
typically
be
about
a
30
percent
decrease
in
set
time
for
each
10
F
increase
in
concrete
temperature
(Table
1).
It
can
also
be
anticipated
that
concrete
will
lose
about
1”
in
slump
for
every
20
F
change
in
concrete
temperature.
When
a
decrease
in
initial
set
time
is
combined
with
slump
loss,
handling,
consolidating
and
finishing
the
concrete
can
be
very
challenging.
Adjustments
to
the
water
content
to
accommodate
for
the
slump
loss
will
increase
the
water‐cement
ratio
and
have
an
adverse
effect
on
both
compressive
strength
and
shrinkage
characteristics.
Table
1
Temperature
Approximate
Initial
Set
Time
Degrees
F
Hours
100
1
hour
,
40
min
90
2
hours
,
40
min
80
4
hours
70
6
hours
60
8
hours
50
10
hours,
40
min
There
are
several
precautions
that
should
be
utilized
in
hot
weather
conditions
to
improve
placement
characteristics
and
assure
the
quality
and
durability
of
the
in‐place
concrete.
The
precautions
may
include
some
or
all
of
the
following:
1. Dampen
sub‐grade,
structural
steel
and
form
work
with
cool
water
prior
to
concrete
placement.
2. Construct
temporary
wind‐breaks
to
reduce
wind
velocity
and
sun
shades
to
reduce
concrete
temperatures.
3. Use
cool
mixing
water
or
ice
chips
to
reduce
initial
concrete
temperatures.
4. Store
packaged
cement
mixes
and
aggregates
in
shaded
areas.
5. Have
sufficient
labor
available
to
allow
for
the
reduced
placement
and
finishing
time
of
the
concrete
mix.
6. Consider
fogging
the
placement
area
with
a
fine
water
mist
to
increase
relative
humidity.
7. Consider
placing
the
concrete
either
early
or
late
in
the
day
when
ambient
temperatures
are
cooler.
8. Begin
curing
operations
with
QUIKRETE
Acrylic
Cure
&
Seal
Immediately
after
finishing
processes
are
completed
to
prevent
surface
moisture
loss
and
plastic
shrinkage
cracking.
With
proper
attention
to
these
hot
weather
placement
procedures,
concrete
can
be
successfully
placed
in
hot,
windy
and
low
humidity
conditions.
For
additional
information:
ACI
Committee
305,
Hot
Weather
Concreting,
ACI
305‐99
Portland
Cement
Association,
Hot
Weather
Concreting,
ISO14.05